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Slide 1 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162
Slide 2 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec)
Slide 3 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted
Slide 4 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne
Slide 5 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks
Slide 6 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender)
Slide 7 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing
Slide 8 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send!
Slide 9 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch
Slide 10 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet.
Slide 11 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym
Slide 12 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3
Slide 13 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D
Slide 14 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc
Slide 15 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway
Slide 16 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches)
Slide 17 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets
Slide 18 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages
Slide 19 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries)
Slide 20 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales
Slide 21 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport
Slide 22 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere:
Slide 23 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port]
Slide 24 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port] Building a messaging service (con’t) UDP: The Unreliable Datagram Protocol Datagram: an unreliable packet sent from sourcedest UDP layered on top of basic IP (Call it UDP/IP) Unreliable, unordered, user-to-user communication Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered “anti-social” – none of the “well-behaved” aspects of (say) TCP/IP Ordered Messages Ordered messages on top of unordered ones IP can reorder packets! P0,P1 might arrive as P1,P0 How to fix this? Assign sequence numbers to packets 0,1,2,3,4….. If packets arrive out of order, reorder before delivering to user application For instance, hold onto #3 until #2 arrives, etc. Sequence numbers are specific to particular connection
Slide 25 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port] Building a messaging service (con’t) UDP: The Unreliable Datagram Protocol Datagram: an unreliable packet sent from sourcedest UDP layered on top of basic IP (Call it UDP/IP) Unreliable, unordered, user-to-user communication Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered “anti-social” – none of the “well-behaved” aspects of (say) TCP/IP Ordered Messages Ordered messages on top of unordered ones IP can reorder packets! P0,P1 might arrive as P1,P0 How to fix this? Assign sequence numbers to packets 0,1,2,3,4….. If packets arrive out of order, reorder before delivering to user application For instance, hold onto #3 until #2 arrives, etc. Sequence numbers are specific to particular connection Performance Considerations Before continue, need some performance metrics Overhead: CPU time to put packet on wire Throughput: Maximum number of bytes per second Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop Contributions to Latency Wire latency: depends on speed of light on wire about 1.5 ns/foot Router latency: depends on internals of router Could be < 1 ms (for a good router) Question: can router handle full wire throughput? Router Router
Slide 26 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port] Building a messaging service (con’t) UDP: The Unreliable Datagram Protocol Datagram: an unreliable packet sent from sourcedest UDP layered on top of basic IP (Call it UDP/IP) Unreliable, unordered, user-to-user communication Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered “anti-social” – none of the “well-behaved” aspects of (say) TCP/IP Ordered Messages Ordered messages on top of unordered ones IP can reorder packets! P0,P1 might arrive as P1,P0 How to fix this? Assign sequence numbers to packets 0,1,2,3,4….. If packets arrive out of order, reorder before delivering to user application For instance, hold onto #3 until #2 arrives, etc. Sequence numbers are specific to particular connection Performance Considerations Before continue, need some performance metrics Overhead: CPU time to put packet on wire Throughput: Maximum number of bytes per second Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop Contributions to Latency Wire latency: depends on speed of light on wire about 1.5 ns/foot Router latency: depends on internals of router Could be < 1 ms (for a good router) Question: can router handle full wire throughput? Router Router Sample Computations E.g.: Ethernet within Soda Latency: speed of light in wire is 1.5ns/foot, which implies latency in building < 1 μs (if no routers in path) Throughput: 10-1000Mb/s Throughput delay: packet doesn’t arrive until all bits So: 4KB/100Mb/s = 0.3 milliseconds (same order as disk!) E.g.: ATM within Soda Latency (same as above, assuming no routing) Throughput: 155Mb/s Throughput delay: 4KB/155Mb/s = 200μ E.g.: ATM cross-country Latency (assuming no routing): 3000miles * 5000ft/mile  15 milliseconds How many bits could be in transit at same time? 15ms * 155Mb/s = 290KB In fact, BerkeleyMIT Latency ~ 90ms Implies 1.7MB in flight if routers have wire-speed throughput Requirements for good performance: Local area: minimize overhead/improve bandwidth Wide area: keep pipeline full!
Slide 27 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port] Building a messaging service (con’t) UDP: The Unreliable Datagram Protocol Datagram: an unreliable packet sent from sourcedest UDP layered on top of basic IP (Call it UDP/IP) Unreliable, unordered, user-to-user communication Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered “anti-social” – none of the “well-behaved” aspects of (say) TCP/IP Ordered Messages Ordered messages on top of unordered ones IP can reorder packets! P0,P1 might arrive as P1,P0 How to fix this? Assign sequence numbers to packets 0,1,2,3,4….. If packets arrive out of order, reorder before delivering to user application For instance, hold onto #3 until #2 arrives, etc. Sequence numbers are specific to particular connection Performance Considerations Before continue, need some performance metrics Overhead: CPU time to put packet on wire Throughput: Maximum number of bytes per second Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop Contributions to Latency Wire latency: depends on speed of light on wire about 1.5 ns/foot Router latency: depends on internals of router Could be < 1 ms (for a good router) Question: can router handle full wire throughput? Router Router Sample Computations E.g.: Ethernet within Soda Latency: speed of light in wire is 1.5ns/foot, which implies latency in building < 1 μs (if no routers in path) Throughput: 10-1000Mb/s Throughput delay: packet doesn’t arrive until all bits So: 4KB/100Mb/s = 0.3 milliseconds (same order as disk!) E.g.: ATM within Soda Latency (same as above, assuming no routing) Throughput: 155Mb/s Throughput delay: 4KB/155Mb/s = 200μ E.g.: ATM cross-country Latency (assuming no routing): 3000miles * 5000ft/mile  15 milliseconds How many bits could be in transit at same time? 15ms * 155Mb/s = 290KB In fact, BerkeleyMIT Latency ~ 90ms Implies 1.7MB in flight if routers have wire-speed throughput Requirements for good performance: Local area: minimize overhead/improve bandwidth Wide area: keep pipeline full! Reliable Message Delivery: the Problem All physical networks can garble and/or drop packets Physical media: packet not transmitted/received If transmit close to maximum rate, get more throughput – even if some packets get lost If transmit at lowest voltage such that error correction just starts correcting errors, get best power/bit Congestion: no place to put incoming packet Point-to-point network: insufficient queue at switch/router Broadcast link: two host try to use same link In any network: insufficient buffer space at destination Rate mismatch: what if sender send faster than receiver can process? Reliable Message Delivery Reliable messages on top of unreliable packets Need some way to make sure that packets actually make it to receiver Every packet received at least once Every packet received only once Can combine with ordering: every packet received by process at destination once and in order
Slide 28 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port] Building a messaging service (con’t) UDP: The Unreliable Datagram Protocol Datagram: an unreliable packet sent from sourcedest UDP layered on top of basic IP (Call it UDP/IP) Unreliable, unordered, user-to-user communication Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered “anti-social” – none of the “well-behaved” aspects of (say) TCP/IP Ordered Messages Ordered messages on top of unordered ones IP can reorder packets! P0,P1 might arrive as P1,P0 How to fix this? Assign sequence numbers to packets 0,1,2,3,4….. If packets arrive out of order, reorder before delivering to user application For instance, hold onto #3 until #2 arrives, etc. Sequence numbers are specific to particular connection Performance Considerations Before continue, need some performance metrics Overhead: CPU time to put packet on wire Throughput: Maximum number of bytes per second Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop Contributions to Latency Wire latency: depends on speed of light on wire about 1.5 ns/foot Router latency: depends on internals of router Could be < 1 ms (for a good router) Question: can router handle full wire throughput? Router Router Sample Computations E.g.: Ethernet within Soda Latency: speed of light in wire is 1.5ns/foot, which implies latency in building < 1 μs (if no routers in path) Throughput: 10-1000Mb/s Throughput delay: packet doesn’t arrive until all bits So: 4KB/100Mb/s = 0.3 milliseconds (same order as disk!) E.g.: ATM within Soda Latency (same as above, assuming no routing) Throughput: 155Mb/s Throughput delay: 4KB/155Mb/s = 200μ E.g.: ATM cross-country Latency (assuming no routing): 3000miles * 5000ft/mile  15 milliseconds How many bits could be in transit at same time? 15ms * 155Mb/s = 290KB In fact, BerkeleyMIT Latency ~ 90ms Implies 1.7MB in flight if routers have wire-speed throughput Requirements for good performance: Local area: minimize overhead/improve bandwidth Wide area: keep pipeline full! Reliable Message Delivery: the Problem All physical networks can garble and/or drop packets Physical media: packet not transmitted/received If transmit close to maximum rate, get more throughput – even if some packets get lost If transmit at lowest voltage such that error correction just starts correcting errors, get best power/bit Congestion: no place to put incoming packet Point-to-point network: insufficient queue at switch/router Broadcast link: two host try to use same link In any network: insufficient buffer space at destination Rate mismatch: what if sender send faster than receiver can process? Reliable Message Delivery Reliable messages on top of unreliable packets Need some way to make sure that packets actually make it to receiver Every packet received at least once Every packet received only once Can combine with ordering: every packet received by process at destination once and in order Using Acknowledgements How to ensure transmission of packets? Detect garbling at receiver via checksum, discard if bad Receiver acknowledges (by sending “ack”) when packet received properly at destination Timeout at sender: if no ack, retransmit Some questions: If the sender doesn’t get an ack, does that mean the receiver didn’t get the original message? No What it ack gets dropped? Or if message gets delayed? Sender doesn’t get ack, retransmits. Receiver gets message twice, acks each. B A B A Timeout
Slide 29 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port] Building a messaging service (con’t) UDP: The Unreliable Datagram Protocol Datagram: an unreliable packet sent from sourcedest UDP layered on top of basic IP (Call it UDP/IP) Unreliable, unordered, user-to-user communication Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered “anti-social” – none of the “well-behaved” aspects of (say) TCP/IP Ordered Messages Ordered messages on top of unordered ones IP can reorder packets! P0,P1 might arrive as P1,P0 How to fix this? Assign sequence numbers to packets 0,1,2,3,4….. If packets arrive out of order, reorder before delivering to user application For instance, hold onto #3 until #2 arrives, etc. Sequence numbers are specific to particular connection Performance Considerations Before continue, need some performance metrics Overhead: CPU time to put packet on wire Throughput: Maximum number of bytes per second Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop Contributions to Latency Wire latency: depends on speed of light on wire about 1.5 ns/foot Router latency: depends on internals of router Could be < 1 ms (for a good router) Question: can router handle full wire throughput? Router Router Sample Computations E.g.: Ethernet within Soda Latency: speed of light in wire is 1.5ns/foot, which implies latency in building < 1 μs (if no routers in path) Throughput: 10-1000Mb/s Throughput delay: packet doesn’t arrive until all bits So: 4KB/100Mb/s = 0.3 milliseconds (same order as disk!) E.g.: ATM within Soda Latency (same as above, assuming no routing) Throughput: 155Mb/s Throughput delay: 4KB/155Mb/s = 200μ E.g.: ATM cross-country Latency (assuming no routing): 3000miles * 5000ft/mile  15 milliseconds How many bits could be in transit at same time? 15ms * 155Mb/s = 290KB In fact, BerkeleyMIT Latency ~ 90ms Implies 1.7MB in flight if routers have wire-speed throughput Requirements for good performance: Local area: minimize overhead/improve bandwidth Wide area: keep pipeline full! Reliable Message Delivery: the Problem All physical networks can garble and/or drop packets Physical media: packet not transmitted/received If transmit close to maximum rate, get more throughput – even if some packets get lost If transmit at lowest voltage such that error correction just starts correcting errors, get best power/bit Congestion: no place to put incoming packet Point-to-point network: insufficient queue at switch/router Broadcast link: two host try to use same link In any network: insufficient buffer space at destination Rate mismatch: what if sender send faster than receiver can process? Reliable Message Delivery Reliable messages on top of unreliable packets Need some way to make sure that packets actually make it to receiver Every packet received at least once Every packet received only once Can combine with ordering: every packet received by process at destination once and in order Using Acknowledgements How to ensure transmission of packets? Detect garbling at receiver via checksum, discard if bad Receiver acknowledges (by sending “ack”) when packet received properly at destination Timeout at sender: if no ack, retransmit Some questions: If the sender doesn’t get an ack, does that mean the receiver didn’t get the original message? No What it ack gets dropped? Or if message gets delayed? Sender doesn’t get ack, retransmits. Receiver gets message twice, acks each. B A B A Timeout How to deal with message duplication Solution: put sequence number in message to identify re-transmitted packets Receiver checks for duplicate #’s; Discard if detected Requirements: Sender keeps copy of unack’ed messages Easy: only need to buffer messages Receiver tracks possible duplicate messages Hard: when ok to forget about received message? Simple solution: Alternating-bit protocol Send one message at a time; don’t send next message until ack received Sender keeps last message; receiver tracks sequence # of last message received Pros: simple, small overhead Con: Poor performance Wire can hold multiple messages; want to fill up at (wire latency  throughput) Con: doesn’t work if network can delay or duplicate messages arbitrarily
Slide 30 - CS162 Operating Systems and Systems Programming Lecture 21 Networking November 14, 2005 Prof. John Kubiatowicz http://inst.eecs.berkeley.edu/~cs162 Review: File System Caching Key Idea: Exploit locality by caching data in memory Name translations: Mapping from pathsinodes Disk blocks: Mapping from block addressdisk content Buffer Cache: Memory used to cache kernel resources, including disk blocks and name translations Can contain “dirty” blocks (blocks yet on disk) Read Ahead Prefetching: fetch sequential blocks early Key Idea: exploit fact that most common file access is sequential by prefetching subsequent disk blocks ahead of current read request (if they are not already in memory) Delayed Writes: Writes to files not immediately sent out to disk Instead, write() copies data from user space buffer to kernel buffer (in cache) Flushed to disk periodically (e.g. in UNIX, every 30 sec) Review: Networking Definitions Network: physical connection that allows two computers to communicate Packet: unit of transfer, sequence of bits carried over the network Network carries packets from on CPU to another Destination gets interrupt when packet arrives Protocol: agreement between two parties as to how information is to be transmitted Goals for Today Networking Broadcast Networks Point-to-Point Networks Network Protocols Note: Some slides and/or pictures in the following are adapted from slides ©2005 Silberschatz, Galvin, and Gagne Broadcast Network: Shared Communication Medium Shared Medium can be a set of wires Inside a computer, this is called a bus All devices simultaneously connected to devices Originally, Ethernet was a broadcast network All computers on local subnet connected to one another More examples (wireless: medium is air): cellular phones, GSM GPRS, EDGE, CDMA 1xRTT, and 1evDO Broadcast Networks Broadcast Networks Details Delivery: When you broadcast a packet, how does a receiver know who it is for? (packet goes to everyone!) Put header on front of packet: [ Destination | Packet ] Everyone gets packet, discards if not the target In Ethernet, this check is done in hardware No OS interrupt if not for particular destination This is layering: we’re going to build complex network protocols by layering on top of the packet Header (Dest:2) Body (Data) Message ID:1 (ignore) ID:2 (receive) ID:4 (ignore) ID:3 (sender) Broadcast Network Arbitration Arbitration: Act of negotiating use of shared medium What if two senders try to broadcast at same time? Concurrent activity but can’t use shared memory to coordinate! Aloha network (70’s): packet radio within Hawaii Blind broadcast, with checksum at end of packet. If received correctly (not garbled), send back an acknowledgement. If not received correctly, discard. Need checksum anyway – in case airplane flies overhead Sender waits for a while, and if doesn’t get an acknowledgement, re-transmits. If two senders try to send at same time, both get garbled, both simply re-send later. Problem: Stability: what if load increases? More collisions  less gets through more resent  more load…  More collisions… Unfortunately: some sender may have started in clear, get scrambled without finishing Carrier Sense, Multiple Access/Collision Detection Ethernet (early 80’s): first practical local area network It is the most common LAN for UNIX, PC, and Mac Use wire instead of radio, but still broadcast medium Key advance was in arbitration called CSMA/CD: Carrier sense, multiple access/collision detection Carrier Sense: don’t send unless idle Don’t mess up communications already in process Collision Detect: sender checks if packet trampled. If so, abort, wait, and retry. Backoff Scheme: Choose wait time before trying again How long to wait after trying to send and failing? What if everyone waits the same length of time? Then, they all collide again at some time! Must find way to break up shared behavior with nothing more than shared communication channel Adaptive randomized waiting strategy: Adaptive and Random: First time, pick random wait time with some initial mean. If collide again, pick random value from bigger mean wait time. Etc. Randomness is important to decouple colliding senders Scheme figures out how many people are trying to send! Point-to-point networks Why have a shared bus at all? Why not simplify and only have point-to-point links + routers/switches? Didn’t used to be cost-effective Now, easy to make high-speed switches and routers that can forward packets from a sender to a receiver. Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Router: a device that acts as a junction between two networks to transfer data packets among them. Switch Point-to-Point Networks Discussion Advantages: Higher link performance Can drive point-to-point link faster than broadcast link since less capacitance/less echoes (from impedance mismatches) Greater aggregate bandwidth than broadcast link Can have multiple senders at once Can add capacity incrementally Add more links/switches to get more capacity Better fault tolerance (as in the Internet) Lower Latency No arbitration to send, although need buffer in the switch Disadvantages: More expensive than having everyone share broadcast link However, technology costs now much cheaper Examples ATM (asynchronous transfer mode) The first commercial point-to-point LAN Inspiration taken from telephone network Switched Ethernet Same packet format and signaling as broadcast Ethernet, but only two machines on each ethernet. Administrivia My office hours New office hour: Thursday 2:30-3:30 MIDTERM II: Wednesday November 30th 5:30-8:30pm, 10 Evans All material from last midterm and up to Monday 11/28 Includes virtual memory Final Exam December 17th, 12:30 – 3:30, 220 Hearst Gym Point-to-Point Network design Switches look like computers: inputs, memory, outputs In fact probably contains a processor Function of switch is to forward packet to output that gets it closer to destination Can build big crossbar by combining smaller switches Can perform broadcast if necessary Inputs Outputs Switch 1 Switch 2 Switch 3 Flow control options What if everyone sends to the same output? Congestion—packets don’t flow at full rate In general, what if buffers fill up? Need flow control policy Option 1: no flow control. Packets get dropped if they arrive and there’s no space If someone sends a lot, they are given buffers and packets from other senders are dropped Internet actually works this way Option 2: Flow control between switches When buffer fills, stop inflow of packets Problem: what if path from source to destination is completely unused, but goes through some switch that has buffers filled up with unrelated traffic? A,B B,C,D A C D Flow Control (con’t) Option 3: Per-flow flow control. Allocate a separate set of buffers to each end-to-end stream and use separate “don’t send me more” control on each end-to-end stream Problem: fairness Throughput of each stream is entirely dependent on topology, and relationship to bottleneck Automobile Analogy At traffic jam, one strategy is merge closest to the bottleneck Why people get off at one exit, drive 50 feet, merge back into flow Ends up slowing everybody else a huge emount Also why have control lights at on-ramps Try to keep from injecting more cars than capacity of road (and thus avoid congestion) aaaa bbbb cccc ababab dddd acbcac dadcdbdc The Internet Protocol: “IP” The Internet is a large network of computers spread across the globe According to the Internet Systems Consortium, there were over 353 million computers as of July 2005 In principle, every host can speak with every other one under the right circumstances IP Packet: a network packet on the internet IP Address: a 32-bit integer used as the destination of an IP packet Often written as four dot-separated integers, with each integer from 0—255 (thus representing 8x4=32 bits) Example CS file server is: 169.229.60.83  0xA9E53C53 Internet Host: a computer connected to the Internet Host has one or more IP addresses used for routing Some of these may be private and unavailable for routing Not every computer has a unique IP address Groups of machines may share a single IP address In this case, machines have private addresses behind a “Network Address Translation” (NAT) gateway Address ranges and subnets Address Ranges in IP IP address space divided into prefix-delimited ranges: NN.0.0.0/8: Class A (10.0.0.0 private) NN.NN.0.0/16: Class B (172.[16-31].0.0 private) NN.NN.NN.0/24: Class C (192.168.xx.0 private) Such ranges often owned by organizations Subnet: A network connecting a set of hosts with related destination addresses With IP, all the addresses in subnet are related by a prefix of bits The number of matching bits is called the “mask” and can be expressed as a single value (e.g. 24) or a set of ones in a 32-bit value (e.g. 255.255.255.0) A subnet is identified by 32-bit value, with the bits which differ set to zero, followed by a slash and a mask E.g.: 128.32.131.0/24 designates a subnet in which all the addresses look like 128.32.131.XX Same subnet: 128.32.131.0/255.255.255.0 Difference between subnet and complete network range Subnet is always a subset of address range Once, subnet meant single physical broadcast wire; now, less clear exactly what it means (virtualized by switches) Hierarchical Networking: The Internet How can we build a network with millions of hosts? Hierarchy! Not every host connected to every other one Use a network of Routers to connect subnets together Routing is often by prefix: e.g. first router matches first 8 bits of address, next router matches more, etc. subnet1 subnet2 Router Other subnets Router Router Transcontinental Link subnet3 Other subnets Simple Network Terminology Local-Area Network (LAN) – designed to cover small geographical area. Multiaccess bus, ring, or star network Speed  10 – 1000 megabits/second Broadcast is fast and cheap In small organization, a LAN could consist of a single subnet. In large organizations (like Berkeley), a LAN contains many subnets. Wide-Area Network (WAN) – links geographically separated sites Point-to-point connections over long-haul lines (often leased from a phone company) Speed  1.544 – 45 megbits/second Broadcast usually requires multiple messages Routing Routing: the process of forwarding packets hop-by-hop through routers to reach their destination Need more than just a destination address! Need a path Post Office Analogy: Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida, must route to local post office, sorted and sent on plane to somewhere in Florida, be routed to post office, sorted and sent with carrier who knows where street and house is… Internet routing mechanism: routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don’t need 4 billion entries in table: routing is by subnet Could packets be sent in a loop? Yes, if tables incorrect Routing table contains: Destination address range  output link closer to destination Default entry (for subnets without explicit entries) Setting up Routing Tables How do you set up routing tables? Internet has no centralized state! No single machine knows entire topology Topology constantly changing (faults, reconfiguration, etc) Need dynamic algorithm that acquires routing tables Ideally, have one entry per subnet or portion of address Could have “default” routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has “cost” for each entry Includes number of hops to destination, congestion, etc. Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet, replace your entry with neighbors entry (+1 for hop to neighbor) In reality: Internet has networks of many different scales Different algorithms run at different scales Network Protocols Protocol: Agreement between two parties as to how information is to be transmitted Example: system calls are the protocol between the operating system and application Networking examples: many levels Physical level: mechanical and electrical network (e.g. how are 0 and 1 represented) Link level: packet formats/error control (for instance, the CSMA/CD protocol) Network level: network routing, addressing Transport Level: reliable message delivery Protocols on today’s Internet: Physical/Link Network Transport Network Layering Layering: building complex services from simpler ones Each layer provides services needed by higher layers by utilizing services provided by lower layers The physical/link layer is pretty limited Packets are of limited size (called the “Maximum Transfer Unit or MTU: often 200-1500 bytes in size) Routing is limited to within a physical link (wire) or perhaps through a switch Our goal in the following is to show how to construct a secure, ordered, message service routed to anywhere: Building a messaging service Handling Arbitrary Sized Messages: Must deal with limited physical packet size Split big message into smaller ones (called fragments) Must be reassembled at destination Checksum computed on each fragment or whole message Internet Protocol (IP): Must find way to send packets to arbitrary destination in network Deliver messages unreliably (“best effort”) from one machine in Internet to another Since intermediate links may have limited size, must be able to fragment/reassemble packets on demand Includes 256 different “sub-protocols” build on top of IP Process to process communication Basic routing gets packets from machinemachine What we really want is routing from processprocess Example: ssh, email, ftp, web browsing Several IP protocols include notion of a “port”, which is a 16-bit identifier used in addition to IP address A communication channel (connection) defined by 4 items: [source address, source port, dest address, dest port] Building a messaging service (con’t) UDP: The Unreliable Datagram Protocol Datagram: an unreliable packet sent from sourcedest UDP layered on top of basic IP (Call it UDP/IP) Unreliable, unordered, user-to-user communication Important aspect: low overhead! Often used for high-bandwidth video streams Many uses of UDP considered “anti-social” – none of the “well-behaved” aspects of (say) TCP/IP Ordered Messages Ordered messages on top of unordered ones IP can reorder packets! P0,P1 might arrive as P1,P0 How to fix this? Assign sequence numbers to packets 0,1,2,3,4….. If packets arrive out of order, reorder before delivering to user application For instance, hold onto #3 until #2 arrives, etc. Sequence numbers are specific to particular connection Performance Considerations Before continue, need some performance metrics Overhead: CPU time to put packet on wire Throughput: Maximum number of bytes per second Depends on “wire speed”, but also limited by slowest router (routing delay) or by congestion at routers Latency: time until first bit of packet arrives at receiver Raw transfer time + overhead at each routing hop Contributions to Latency Wire latency: depends on speed of light on wire about 1.5 ns/foot Router latency: depends on internals of router Could be < 1 ms (for a good router) Question: can router handle full wire throughput? Router Router Sample Computations E.g.: Ethernet within Soda Latency: speed of light in wire is 1.5ns/foot, which implies latency in building < 1 μs (if no routers in path) Throughput: 10-1000Mb/s Throughput delay: packet doesn’t arrive until all bits So: 4KB/100Mb/s = 0.3 milliseconds (same order as disk!) E.g.: ATM within Soda Latency (same as above, assuming no routing) Throughput: 155Mb/s Throughput delay: 4KB/155Mb/s = 200μ E.g.: ATM cross-country Latency (assuming no routing): 3000miles * 5000ft/mile  15 milliseconds How many bits could be in transit at same time? 15ms * 155Mb/s = 290KB In fact, BerkeleyMIT Latency ~ 90ms Implies 1.7MB in flight if routers have wire-speed throughput Requirements for good performance: Local area: minimize overhead/improve bandwidth Wide area: keep pipeline full! Reliable Message Delivery: the Problem All physical networks can garble and/or drop packets Physical media: packet not transmitted/received If transmit close to maximum rate, get more throughput – even if some packets get lost If transmit at lowest voltage such that error correction just starts correcting errors, get best power/bit Congestion: no place to put incoming packet Point-to-point network: insufficient queue at switch/router Broadcast link: two host try to use same link In any network: insufficient buffer space at destination Rate mismatch: what if sender send faster than receiver can process? Reliable Message Delivery Reliable messages on top of unreliable packets Need some way to make sure that packets actually make it to receiver Every packet received at least once Every packet received only once Can combine with ordering: every packet received by process at destination once and in order Using Acknowledgements How to ensure transmission of packets? Detect garbling at receiver via checksum, discard if bad Receiver acknowledges (by sending “ack”) when packet received properly at destination Timeout at sender: if no ack, retransmit Some questions: If the sender doesn’t get an ack, does that mean the receiver didn’t get the original message? No What it ack gets dropped? Or if message gets delayed? Sender doesn’t get ack, retransmits. Receiver gets message twice, acks each. B A B A Timeout How to deal with message duplication Solution: put sequence number in message to identify re-transmitted packets Receiver checks for duplicate #’s; Discard if detected Requirements: Sender keeps copy of unack’ed messages Easy: only need to buffer messages Receiver tracks possible duplicate messages Hard: when ok to forget about received message? Simple solution: Alternating-bit protocol Send one message at a time; don’t send next message until ack received Sender keeps last message; receiver tracks sequence # of last message received Pros: simple, small overhead Con: Poor performance Wire can hold multiple messages; want to fill up at (wire latency  throughput) Con: doesn’t work if network can delay or duplicate messages arbitrarily Conclusion Network: physical connection that allows two computers to communicate Packet: sequence of bits carried over the network Broadcast Network: Shared Communication Medium Transmitted packets sent to all receivers Arbitration: act of negotiating use of shared medium Ethernet: Carrier Sense, Multiple Access, Collision Detect Point-to-point network: a network in which every physical wire is connected to only two computers Switch: a bridge that transforms a shared-bus (broadcast) configuration into a point-to-point network. Protocol: Agreement between two parties as to how information is to be transmitted Internet Protocol (IP) Used to route messages through routes across globe 32-bit addresses, 16-bit ports Reliable, Ordered, Arbitrary-sized Messaging: Built through protocol layering on top of unreliable, limited-sized, non-ordered packet transmission links