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Slide 30 -
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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 pathsinodes
Disk blocks: Mapping from block addressdisk 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 machinemachine
What we really want is routing from processprocess
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 sourcedest
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, BerkeleyMIT 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
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