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Slide 109 -
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Networking
Alan L. Cox
alc@cs.rice.edu
Some slides adapted from CMU 15.213 slides Objectives Be exposed to the basic underpinnings of the Internet
Be able to use network socket interfaces effectively
Be exposed to the basic underpinnings of the World Wide Web Cox Networking 2 3 The 2004 A. M. Turing Award Goes to... Cox Networking 4 The 2004 A. M. Turing Award Goes to... "For pioneering work on internetworking, including the design and implementation of the Internet's basic communications protocols, TCP/IP, and for inspired leadership in networking."
The only Turing Award given to-date to recognize work in computer networking Bob Kahn Vint Cerf Cox Networking 5 But at the Same Time... Source: Akamai Technologies, Inc. Daily, e.g. 110 attacks from China; 26 attacks from USA 2/24/2008 YouTube traffic mis-routed to Pakistan Q2/2008 1000s of Netherlands DSL customers lost service due to network configuration error 11/10/2008 CTBC (Brazil) black-holed all Internet traffic in some parts of Brazil 2/16/2009 Supro (Czech) routing messages triggered a Cisco router bug world-wide 5/2009 AfNOG (Africa) routing messages triggered buffer overflow in open-source routing software Quagga world-wide Cox Networking 6 Telephony Interactive telecommunication between people
Analog voice
Transmitter/receiver continuously in contact with electronic circuit
Electric current varies with acoustic pressure
Over electrical circuits Analog/Continuous Signal Cox Networking 7 Telephony Milestones 1876: Alexander Bell invented telephone
1878: Public switches installed at New Haven and San Francisco, public switched telephone network is born
People can talk without being on the same wire! Without Switch With Switch Cox Networking 8 Telephony Milestones 1878: First telephone directory; White House line
1881: Insulated, balanced twisted pair as local loop
1885: AT&T formed
1892: First automatic commercial telephone switch
1903: 3 million telephones in U.S.
1915: First transcontinental telephone line
1927: First commercial transatlantic commercial service Cox Networking 9 Telephony Milestones 1937: Multiplexing introduced for inter-city calls
One link carries multiple conversations Without Multiplexing With Multiplexing Cox Networking 10 Data or Computer Networks Networks designed for computers to computers or devices
vs. communication between human beings
Digital information
vs. analog voice
Not a continuous stream of bits, rather, discrete “packets” with lots of silence in between
Dedicated circuit hugely inefficient
Packet switching invented
Digital/Discrete Signal Cox Networking 11 Major Internet Milestones 1960-1964 Basic concept of “packet switching” was independently developed by Baran (RAND), Kleinrock (MIT)
AT&T insisted that packet switching would never work!
1965 First time two computers talked to each other using packets (Roberts, MIT; Marill, SDC) Cox Networking 12 Major Internet Milestones 1968 BBN group proposed to use Honeywell 516 mini-computers for the Interface Message Processors (i.e. packet switches)
1969 The first ARPANET message transmitted between UCLA (Kleinrock) and SRI (Engelbart)
We sent an “L”, did you get the “L”? Yep!
We sent an “O”, did you get the “O”? Yep!
We sent a “G”, did you get the “G”? Crash! Cox Networking 13 Major Internet Milestones 1970 First packet radio network ALOHANET (Abramson, U Hawaii)
1973 Ethernet invented (Metcalfe, Xerox PARC)
Why is it called the “Inter-net”?
1974 “A protocol for Packet Network Interconnection” published by Cerf and Kahn
First internetworking protocol TCP
This paper was cited for their Turing Award
1977 First TCP operation over ARPANET, Packet Radio Net, and SATNET
1985 NSF commissions NSFNET backbone
1991 NSF opens Internet to commercial use
Cox Networking 14 Internet Hourglass Architecture Cox Networking Cox Networking 15 A Client-Server Transaction Most network applications are based on the client-server model:
A server process and one or more client processes
Server manages some resource
Server provides service by manipulating resource for clients Client
process Server
process 1. Client sends request 2. Server
handles
request 3. Server sends response 4. Client
handles
response Resource Note: clients and servers are processes running on hosts
(can be the same or different hosts) Cox Networking 16 Network Hardware main
memory I/O
bridge Bus Interface ALU register file CPU chip system bus memory bus disk
controller graphics
adapter USB
controller mouse keyboard monitor disk I/O bus Expansion slots network
adapter network Cox Networking 17 Computer Networks A network is a hierarchical system of boxes and wires organized by geographical proximity
Cluster network spans cluster or machine room
Switched Ethernet, Infiniband, Myrinet, …
LAN (local area network) spans a building or campus
Ethernet is most prominent example
WAN (wide-area network) spans very long distance
A high-speed point-to-point link
Leased line or SONET/SDH circuit, or MPLS/ATM circuit
An internetwork (internet) is an interconnected set of networks
The Global IP Internet (uppercase “I”) is the most famous example of an internet (lowercase “i”) Cox Networking 18 Lowest Level: Ethernet Segment Ethernet segment consists of a collection of hosts connected by wires (twisted pairs) to a hub
Operation
Each Ethernet adapter has a unique 48-bit address
Hosts send bits to any other host in chunks called frames
Hub slavishly copies each bit from each port to every other port
Every host sees every bit
Note: Hubs are largely obsolete
Bridges (switches, routers) became cheap enough to replace them (don’t broadcast all traffic)
host host host hub 100 Mb/s 100 Mb/s ports Cox Networking 19 Next Level: Bridged Ethernet Segment Spans room, building, or campus
Bridges cleverly learn which hosts are reachable from which ports and then selectively copy frames from port to port host host host host host hub hub bridge 100 Mb/s 100 Mb/s host host 1 Gb/s 1 Gb/s 1-10 Gb/s host host host bridge host host bridge 1 Gb/s Cox Networking 20 Conceptual View of LANs For simplicity, hubs, bridges, and wires are often shown as a collection of hosts attached to a single wire: host host host ... Cox Networking 21 Next Level: internets Multiple incompatible LANs can be physically connected by specialized computers called routers
The connected networks are called an internet host host host LAN 1 ... host host host LAN 2 ... router router router WAN WAN LAN 1 and LAN 2 might be completely different, totally incompatible LANs (e.g., Ethernet and WiFi, 802.11*, T1-links, DSL, …) Cox Networking 22 The Internet Circa 1986 Merit (Univ of Mich)
NCSA (Illinois)
Cornell Theory Center
Pittsburgh Supercomputing Center
San Diego Supercomputing Center
John von Neumann Center (Princeton) BARRNet (Palo Alto)
MidNet (Lincoln, NE)
WestNet (Salt Lake City)
NorthwestNet (Seattle)
SESQUINET (Rice)
SURANET (Georgia Tech) In 1986, the Internet consisted of one backbone
(NSFNET) that connected 13 sites via 45 Mbps T3 links
Connecting to the Internet involved connecting one of
your routers to a router at a backbone site, or to a
regional network that was already connected to the
backbone Cox Networking 23 NSFNET Internet Backbone source: www.eef.org Cox Networking 24 After NSFNET Early 90s
Commercial enterprises began building their own high-speed backbones
Backbone would connect to NSFNET, sell access to companies, ISPs, and individuals
1995
NSFNET decommissioned
NSF fostered the creation of network access points (NAPs) to interconnect the commercial backbones Cox Networking 25 Current Internet Architecture Cox Networking 26 Level 3 Backbone Cox Networking 27 AT&T Backbone Cox Networking 28 Submarine Cabling Cox Networking 29 The Notion of an internet Protocol How is it possible to send bits across incompatible LANs and WANs?
Solution: protocol software running on each host and router smoothes out the differences between the different networks
Implements an internet protocol (i.e., set of rules) that governs how hosts and routers should cooperate when they transfer data from network to network
TCP/IP is the protocol for the global IP Internet Cox Networking 30 What Does an internet Protocol Do? 1. Provides a naming scheme
An internet protocol defines a uniform format for host addresses
Each host (and router) is assigned at least one of these internet addresses that uniquely identifies it
2. Provides a delivery mechanism
An internet protocol defines a standard transfer unit (packet)
Packet consists of header and payload
Header: contains info such as packet size, source and destination addresses
Payload: contains data bits sent from source host
Cox Networking 31 Transferring Data Over an internet protocol
software LAN1
adapter Host A data data PH FH1 data PH data PH FH2 LAN1 LAN2 data data PH FH1 data PH FH2 (1) (2) (3) (4) (5) (6) (7) (8) internet packet LAN2 frame protocol
software LAN1
adapter LAN2
adapter Router FH1 LAN1 frame data PH FH2 protocol
software LAN2
adapter Host B client server Cox Networking 32 Other Issues We are glossing over a number of important questions:
What if different networks have different maximum frame sizes? (segmentation)
How do routers know where to forward frames?
How are routers informed when the network topology changes?
What if packets get lost?
We’ll leave the discussion of these question to computer networking classes (COMP 429) Cox Networking 33 Global IP Internet Based on the TCP/IP protocol family
IP (Internet protocol) :
Provides basic naming scheme and unreliable delivery capability of packets (datagrams) from host-to-host
UDP (Unreliable Datagram Protocol)
Uses IP to provide unreliable datagram delivery from process-to-process
TCP (Transmission Control Protocol)
Uses IP to provide reliable byte streams from process-to-process over connections
Accessed via a mix of Unix file I/O and functions from the sockets interface Cox Networking 34 Organization of an Internet Application TCP/IP Client Network
adapter Global IP Internet TCP/IP Server Network
adapter Internet client Internet server Sockets interface
(system calls) Hardware interface
(interrupts) User code Kernel code Hardware
and firmware Cox Networking 35 A Programmer’s View of the Internet Hosts are mapped to a set of 32-bit IP addresses
128.42.1.125 (4 * 8 bits)
A set of identifiers called Internet domain names are mapped to the set of IP addresses for convenience
www.cs.rice.edu is mapped to 128.42.1.125
A process on one Internet host can communicate with a process on another Internet host over a connection Cox Networking 36 IP Addresses 32-bit IP addresses are stored in an IP address struct
IP addresses are always stored in memory in network byte order (big-endian byte order)
True in general for any integer transferred in a packet header from one machine to another
E.g., the port number used to identify an Internet connection /* Internet address structure */
struct in_addr {
unsigned int s_addr; /* network byte order (big-endian) */
}; Handy network byte-order conversion functions:
htonl: convert long int from host to network byte order
htons: convert short int from host to network byte order
ntohl: convert long int from network to host byte order
ntohs: convert short int from network to host byte order Cox Networking 37 Dotted Decimal Notation By convention, each byte in a 32-bit IP address is represented by its decimal value and separated by a period
IP address 0x8002C2F2 = 128.2.194.242
Functions for converting between binary IP addresses and dotted decimal strings:
inet_pton: converts a dotted decimal string to an IP address in network byte order
inet_ntop: converts an IP address in network by order to its corresponding dotted decimal string
“n” denotes network representation, “p” denotes presentation representation Cox Networking 38 IP Address Structure IP (V4) Address space divided into classes:
Special Addresses for routers and gateways (all 0/1’s)
Loop-back address: 127.0.0.1
Unrouted (private) IP addresses:
10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16
Dynamic IP addresses (DHCP) Cox Networking 39 Internet Domain Names .net .edu .gov .com rice berkeley mit clear cs bell
128.42.151.14 unnamed root crystal
128.42.151.13 amazon www
72.21.210.11 First-level domain names Second-level domain names Third-level domain names Cox Networking 40 Domain Naming System (DNS) The Internet maintains a mapping between IP addresses and domain names in a huge worldwide distributed database called DNS
Conceptually, programmers can view the DNS database as a collection of millions of addrinfo structures:
Functions for retrieving host entries from DNS:
getaddrinfo: query DNS using domain name or IP
getnameinfo: query DNS using sockaddr struct struct addrinfo {
int ai_flags; /* flags for getaddrinfo */
int ai_family; /* address type (AF_INET or AF_INET6) */
int ai_socktype; /* the socket type */
int ai_protocol; /* the type of protocol */
size_t ai_addrlen; /* length of ai_addr */
struct sockaddr *ai_addr; /* pointer to a sockaddr struct */
char *ai_canonname;/* the canonical name */
struct addrinfo *ai_next; /* pointer to the next addrinfo struct */
};
Cox Networking 41 Properties of DNS Host Entries Each host entry is an equivalence class of domain names and IP addresses
Each host has a locally defined domain name localhost which always maps to the loopback address 127.0.0.1
Different kinds of mappings are possible:
Simple case: 1 domain name maps to one IP address:
forest.owlnet.rice.edu maps to 10.130.195.71
Multiple domain names mapped to the same IP address:
www.cs.rice.edu, ececs.rice.edu, and bianca.cs.rice.edu all map to 128.42.1.125
Multiple domain names mapped to multiple IP addresses:
aol.com and www.aol.com map to multiple IP addresses
Some valid domain names don’t map to any IP address:
for example: clear.rice.edu
Cox Networking 42 A Program That Queries DNS int main(int argc, char **argv) { /* argv[1] is a domain name */
char address[INET_ADDRSTRLEN]; /* or dotted decimal IP addr */
struct sockaddr_in *saddr;
struct addrinfo *addrp;
struct addrinfo *pp;
struct addrinfo hints;
hints.ai_flags = AI_CANONNAME;
hints.ai_family = AF_INET;
hints.ai_socktype = 0;
hints.ai_protocol = 0;
getaddrinfo(argv[1], NULL, &hints, &addrp);
printf("official hostname: %s\n", addrp->ai_canonname);
for (pp = addrp; pp != NULL; pp = pp->ai_next) {
saddr = (struct sockaddr_in *) pp->ai_addr;
inet_ntop(AF_INET, &saddr->sin_addr, address, INET_ADDRSTRLEN);
printf("address: %s\n", address);
}
freeaddrinfo(addrp);
return (0);
} Cox Networking 43 Querying DNS Domain Information Groper (dig) provides a scriptable command line interface to DNS
Available on CLEAR
Lots of web interfaces (google “domain information groper”)
unix> dig +short bianca.cs.rice.edu
128.42.1.125
unix> dig +short -x 128.42.1.125
bianca.cs.rice.edu.
unix> dig +short google.com
72.14.207.99
64.233.187.99
64.233.167.99
unix> dig +short -x 64.233.167.99
py-in-f99.google.com.
Cox Networking 44 Internet Connections Clients and servers communicate by sending streams of bytes over connections:
Point-to-point, full-duplex (2-way communication), and reliable
A socket is an endpoint of a connection
Socket address is an IP address, port pair
A port is a 16-bit integer that identifies a process:
Ephemeral port: Assigned automatically on client when client makes a connection request
Well-known port: Associated with some service provided by a server (e.g., port 80 is associated with Web servers)
A connection is uniquely identified by the socket addresses of its endpoints (socket pair)
(cliaddr:cliport, servaddr:servport)
Cox Networking 45 Putting it all Together: Anatomy of an Internet Connection Connection socket pair
(128.2.194.242:51213, 208.216.181.15:80) Server
(port 80) Client Client socket address
128.2.194.242:51213 Server socket address
208.216.181.15:80 Client host address
128.2.194.242 Server host address
208.216.181.15 Cox Networking 46 Clients Examples of client programs
Web browsers, ftp, telnet, ssh
How does a client find the server?
The IP address in the server socket address identifies the host (more precisely, an adapter on the host)
The (well-known) port in the server socket address identifies the service, and thus implicitly identifies the server process that performs that service
Examples of well known ports
Port 7: Echo server
Port 23: Telnet server
Port 25: Mail server
Port 80: Web server Cox Networking 47 Using Ports to Identify Services Web server
(port 80) Client host Server host 128.2.194.242 Echo server
(port 7) Service request for
128.2.194.242:80
(i.e., the Web server) Web server
(port 80) Echo server
(port 7) Service request for
128.2.194.242:7
(i.e., the echo server) Kernel Kernel Client Client Cox Networking 48 Servers Servers are long-running processes (daemons)
Created at boot-time (typically) by the init process (process 1)
Run continuously until the machine is turned off
Each server waits for requests to arrive on a well-known port associated with a particular service
Port 7: echo server
Port 23: telnet server
Port 25: mail server
Port 80: HTTP server
A machine that runs a server process is also often referred to as a “server” Cox Networking 49 Server Examples Web server (port 80)
Resource: files/compute cycles (CGI programs)
Service: retrieves files and runs CGI programs on behalf of the client
FTP server (20, 21)
Resource: files
Service: stores and retrieve files
Telnet server (23)
Resource: terminal
Service: proxies a terminal on the server machine
Mail server (25)
Resource: email “spool” file
Service: stores mail messages in spool file See /etc/services for a comprehensive list of the services available on a UNIX machine Cox Networking 50 Sockets Interface Created in the early 80’s as part of the original Berkeley distribution of Unix that contained an early version of the Internet protocols
Provides a user-level interface to the network
Underlying basis for all Internet applications
Based on client/server programming model
Cox Networking 51 Overview of the Sockets Interface Client Server socket socket bind listen accept rio_readlineb rio_readlineb rio_writen close rio_readlineb connect rio_writen close Connection
request EOF Await connection
request from
next client open_listenfd open_clientfd Cox Networking 52 Sockets What is a socket?
To the kernel, a socket is an endpoint of communication
To an application, a socket is a file descriptor that lets the application read/write from/to the network
Remember: all Unix I/O devices, including networks, are modeled as files
Clients and servers communicate with each other by reading from and writing to socket descriptors
The main distinction between regular file I/O and socket I/O is how the application “opens” the socket descriptors Cox Networking 53 Socket Address Structures Generic socket address:
Address for connect, bind, and accept
C did not have generic (void *) pointers when the sockets interface was designed
Internet-specific socket address:
Must cast (sockaddr_in *) to (sockaddr *) struct sockaddr {
unsigned short sa_family; /* protocol family */
char sa_data[14]; /* address data. */
}; struct sockaddr_in {
unsigned short sin_family; /* address family */
unsigned short sin_port; /* port num in network byte order */
struct in_addr sin_addr; /* IP addr in network byte order */
unsigned char sin_zero[8]; /* pad to sizeof(struct sockaddr) */
}; Cox Networking 54 Echo Client Main Routine int main(int argc, char **argv) /* usage: ./echoclient host port */
{
int clientfd, port;
char *host, buf[MAXLINE];
rio_t rio;
host = argv[1];
port = atoi(argv[2]);
clientfd = Open_clientfd(host, port);
Rio_readinitb(&rio, clientfd);
while (Fgets(buf, MAXLINE, stdin) != NULL) {
Rio_writen(clientfd, buf, strlen(buf));
Rio_readlineb(&rio, buf, MAXLINE);
Fputs(buf, stdout);
}
Close(clientfd);
exit(0);
} Cox Networking 55 Echo Client: open_clientfd int open_clientfd(char *hostname, int port) {
int clientfd;
struct hostent *hp;
struct sockaddr_in serveraddr;
if ((clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
return -1; /* check errno for cause of error */
/* Fill in the server's IP address and port */
if ((hp = gethostbyname(hostname)) == NULL)
return -2; /* check h_errno for cause of error */
bzero((char *) &serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
bcopy((char *)hp->h_addr,
(char *)&serveraddr.sin_addr.s_addr, hp->h_length);
serveraddr.sin_port = htons(port);
/* Establish a connection with the server */
if (connect(clientfd, (SA *) &serveraddr, sizeof(serveraddr)) < 0)
return -1;
return clientfd;
} This function opens a connection from the client to the server at hostname:port Cox Networking 56 open_clientfd (socket) socket creates a socket descriptor on the client
AF_INET: indicates that the socket is associated with Internet protocols
SOCK_STREAM: selects a reliable byte stream connection
int clientfd; /* socket descriptor */
if ((clientfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
return -1; /* check errno for cause of error */
... (more) Cox Networking 57 open_clientfd (gethostbyname) The client then builds the server’s Internet address int clientfd; /* socket descriptor */
struct hostent *hp; /* DNS host entry */
struct sockaddr_in serveraddr; /* server’s IP address */
...
/* fill in the server's IP address and port */
if ((hp = gethostbyname(hostname)) == NULL)
return -2; /* check h_errno for cause of error */
bzero((char *) &serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
bcopy((char *)hp->h_addr,
(char *)&serveraddr.sin_addr.s_addr, hp->h_length);
serveraddr.sin_port = htons(port);
Cox Networking 58 open_clientfd (connect) Finally the client creates a connection with the server
Client process suspends (blocks) until the connection is created
After resuming, the client is ready to begin exchanging messages with the server via Unix I/O calls on descriptor clientfd
int clientfd; /* socket descriptor */
struct sockaddr_in serveraddr; /* server address */
typedef struct sockaddr SA; /* generic sockaddr */
...
/* Establish a connection with the server */
if (connect(clientfd, (SA *)&serveraddr, sizeof(serveraddr)) < 0)
return -1;
return clientfd; Cox Networking 59 Echo Server: Main Routine int main(int argc, char **argv)
{
int listenfd, connfd, port;
socklen_t clientlen;
struct sockaddr_in clientaddr;
char host_name[NI_MAXHOST];
char haddrp[INET_ADDRSTRLEN];
port = atoi(argv[1]);
listenfd = open_listen(port);
while (1) {
clientlen = sizeof(clientaddr);
connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen);
/* determine the domain name and IP address of the client */
getnameinfo((struct sockaddr *)&clientaddr, sizeof(clientaddr),
host_name, sizeof(host_name), NULL, 0, 0);
inet_ntop(AF_INET, &clientaddr.sin_addr, haddrp, INET_ADDRSTRLEN);
printf("server connected to %s (%s)\n", host_name, haddrp);
echo(connfd);
Close(connfd);
}
} Cox Networking 60 Echo Server: open_listenfd int open_listenfd(int port)
{
int listenfd, optval=1;
struct sockaddr_in serveraddr;
/* Create a socket descriptor */
if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
return -1;
/* Eliminates "Address already in use" error from bind. */
if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR,
(const void *)&optval , sizeof(int)) < 0)
return -1;
... (more) Cox Networking 61 Echo Server: open_listenfd (cont) ...
/* Listenfd will be an endpoint for all requests to port
on any IP address for this host */
bzero((char *) &serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
serveraddr.sin_port = htons((unsigned short)port);
if (bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr)) < 0)
return -1;
/* Make it a listening socket ready to accept
connection requests */
if (listen(listenfd, LISTENQ) < 0)
return -1;
return listenfd;
} Cox Networking 62 open_listenfd (socket) socket creates a socket descriptor on the server
AF_INET: indicates that the socket is associated with Internet protocols
SOCK_STREAM: selects a reliable byte stream connection
int listenfd; /* listening socket descriptor */
/* Create a socket descriptor */
if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
return -1; Cox Networking 63 open_listenfd (setsockopt) The socket can be given some attributes
Handy trick that allows us to rerun the server immediately after we kill it
Otherwise we would have to wait about 30 secs
Eliminates “Address already in use” error from bind()
Strongly suggest you do this for all your servers to simplify debugging ...
/* Eliminates "Address already in use" error from bind(). */
if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR,
(const void *)&optval , sizeof(int)) < 0)
return -1; Cox Networking 64 open_listenfd (init socket address) Next, we initialize the socket with the server’s Internet address (IP address and port)
IP address and port stored in network (big-endian) byte order
htonl() converts longs from host byte order to network byte order
htons() convers shorts from host byte order to network byte order struct sockaddr_in serveraddr; /* server's socket addr */
...
/* listenfd will be an endpoint for all requests to port
on any IP address for this host */
bzero((char *) &serveraddr, sizeof(serveraddr));
serveraddr.sin_family = AF_INET;
serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);
serveraddr.sin_port = htons((unsigned short)port); Cox Networking 65 open_listenfd (bind) bind associates the socket with the socket address we just created int listenfd; /* listening socket */
struct sockaddr_in serveraddr; /* server’s socket addr */
...
/* listenfd will be an endpoint for all requests to port
on any IP address for this host */
if (bind(listenfd, (SA *)&serveraddr, sizeof(serveraddr)) < 0)
return -1; Cox Networking 66 open_listenfd (listen) listen indicates that this socket will accept connection (connect) requests from clients
We’re finally ready to enter the main server loop that accepts and processes client connection requests int listenfd; /* listening socket */
...
/* Make it a listening socket ready to accept connection requests */
if (listen(listenfd, LISTENQ) < 0)
return -1;
return listenfd;
} Cox Networking 67 Echo Server: Main Loop The server loops endlessly, waiting for connection requests, then reading input from the client, and echoing the input back to the client main() {
/* create and configure the listening socket */
while(1) {
/* Accept(): wait for a connection request */
/* echo(): read and echo input lines from client til EOF */
/* Close(): close the connection */
}
} Cox Networking 68 Echo Server: accept accept() blocks waiting for a connection request
accept returns a connected descriptor (connfd) with the same properties as the listening descriptor (listenfd)
Returns when the connection between client and server is created and ready for I/O transfers
All I/O with the client will be done via the connected socket
accept also fills in client’s IP address
int listenfd; /* listening descriptor */
int connfd; /* connected descriptor */
struct sockaddr_in clientaddr;
int clientlen;
clientlen = sizeof(clientaddr);
connfd = Accept(listenfd, (SA *)&clientaddr, &clientlen); Cox Networking 69 Echo Server: accept Illustrated listenfd(3) Client 1. Server blocks in accept, waiting for connection request on listening descriptor listenfd clientfd Server listenfd(3) Client clientfd Server 2. Client makes connection request by calling and blocking in connect Connection
request listenfd(3) Client clientfd Server 3. Server returns connfd from accept. Client returns from connect. Connection is now established between clientfd and connfd connfd(4) Cox Networking 70 Connected vs. Listening Descriptors Listening descriptor
End point for client connection requests
Created once and exists for lifetime of the server
Connected descriptor
End point of the connection between client and server
A new descriptor is created each time the server accepts a connection request from a client
Exists only as long as it takes to service client
Why the distinction?
Allows for concurrent servers that can communicate over many client connections simultaneously
E.g., Each time we receive a new request, we fork a child to handle the request
Cox Networking 71 Echo Server: Identifying the Client The server can determine the domain name and IP address of the client char host_name[NI_MAXHOST];
char haddrp[INET_ADDRSTRLEN];
getnameinfo((struct sockaddr *)&clientaddr, sizeof(clientaddr),
host_name, sizeof(host_name), NULL, 0, 0);
inet_ntop(AF_INET, &clientaddr.sin_addr, haddrp, INET_ADDRSTRLEN);
printf("server connected to %s (%s)\n", host_name, haddrp); Cox Networking 72 Echo Server: echo The server uses RIO to read and echo text lines until EOF (end-of-file) is encountered
EOF notification caused by client calling close(clientfd)
IMPORTANT: EOF is a condition, not a particular data byte void echo(int connfd) {
size_t n;
char buf[MAXLINE];
rio_t rio;
Rio_readinitb(&rio, connfd);
while((n = Rio_readlineb(&rio, buf, MAXLINE)) != 0) {
printf("server received %d bytes\n", n);
Rio_writen(connfd, buf, n);
}
} Cox Networking 73 Testing Servers Using telnet The telnet program is invaluable for testing servers that transmit ASCII strings over Internet connections
Our simple echo server
Web servers
Mail servers
Usage:
unix> telnet
Creates a connection with a server running on and listening on port Cox Networking 74 Testing the Echo Server With telnet bass> echoserver 5000
server established connection with KITTYHAWK.CMCL (128.2.194.242)
server received 5 bytes: 123
server established connection with KITTYHAWK.CMCL (128.2.194.242)
server received 8 bytes: 456789
kittyhawk> telnet bass 5000
Trying 128.2.222.85...
Connected to BASS.CMCL.CS.CMU.EDU.
Escape character is '^]'.
123
123
Connection closed by foreign host.
kittyhawk> telnet bass 5000
Trying 128.2.222.85...
Connected to BASS.CMCL.CS.CMU.EDU.
Escape character is '^]'.
456789
456789
Connection closed by foreign host.
kittyhawk> Cox Networking 75 Running the Echo Client and Server bass> echoserver 5000
server established connection with KITTYHAWK.CMCL (128.2.194.242)
server received 4 bytes: 123
server established connection with KITTYHAWK.CMCL (128.2.194.242)
server received 7 bytes: 456789
...
kittyhawk> echoclient bass 5000
Please enter msg: 123
Echo from server: 123
kittyhawk> echoclient bass 5000
Please enter msg: 456789
Echo from server: 456789
kittyhawk>
Cox Networking 76 For More Information W. Richard Stevens, “Unix Network Programming: Networking APIs: Sockets and XTI”, Volume 1, Second Edition, Prentice Hall, 1998.
THE network programming bible
Complete versions of the echo client and server are developed in the text
Available on the course web site
You should compile and run them for yourselves to see how they work
Feel free to borrow any of this code Cox Networking 77 Web History 1945:
Vannevar Bush, “As we may think”, Atlantic Monthly, July, 1945
Describes the idea of a distributed hypertext system
A “memex” that mimics the “web of trails” in our minds
1989:
Tim Berners-Lee (CERN) writes internal proposal to develop a distributed hypertext system
Connects “a web of notes with links”
Intended to help CERN physicists in large projects share and manage information
1990:
Tim Berners-Lee writes a graphical browser for Next machines Cox Networking 78 Web History (cont) 1992
NCSA server released
26 WWW servers worldwide
1993
Marc Andreessen releases first version of NCSA Mosaic browser
Mosaic version released for (Windows, Mac, Unix)
Web (port 80) traffic at 1% of NSFNET backbone traffic
Over 200 WWW servers worldwide
1994
Andreessen and colleagues leave NCSA to form "Mosaic Communications Corp" (became Netscape, then part of AOL) Cox Networking 79 Internet Hosts Source: www.isc.org Cox Networking 80 Web Servers Clients and servers communicate using the HyperText Transfer Protocol (HTTP)
Client and server establish TCP connection
Client requests content
Server responds with requested content
Client and server (may) close connection
Current version is HTTP/1.1
RFC 2616, June, 1999 Web
server HTTP request HTTP response
(content) Web
client
(browser) Cox Networking 81 Web Content Web servers return content to clients
content: a sequence of bytes with an associated MIME (Multipurpose Internet Mail Extensions) type
Example MIME types
text/html HTML document
text/plain Unformatted text
application/postscript Postcript document
image/gif Binary image (GIF format)
image/jpeg Binary image (JPEG format)
Cox Networking 82 Static and Dynamic Content The content returned in HTTP responses can be either static or dynamic
Static content: content stored in files and retrieved in response to an HTTP request
Examples: HTML files, images, audio clips
Dynamic content: content produced on-the-fly in response to an HTTP request
Example: content produced by a program executed by the server on behalf of the client (i.e., search results)
Bottom line: All Web content is associated with a file that is managed by the server Cox Networking 83 URLs Each file managed by a server has a unique name called a URL (Universal Resource Locator)
URLs for static content:
http://www.rice.edu:80/index.html
http://www.rice.edu/index.html
http://www.rice.edu
Identifies a file called index.html, managed by a Web server at www.rice.edu that is listening on port 80
URLs for dynamic content:
http://www.cs.cmu.edu:8000/cgi-bin/adder?15000&213
Identifies an executable file called adder, managed by a Web server at www.cs.cmu.edu that is listening on port 8000, that should be called with two argument strings: 15000 and 213 Cox Networking 84 How Clients and Servers Use URLs Example URL: http://www.aol.com:80/index.html
Clients use prefix (http://www.aol.com:80) to infer:
What kind of server to contact (http (Web) server)
Where the server is (www.aol.com)
What port the server is listening on (80)
Servers use suffix (/index.html) to:
Determine if request is for static or dynamic content
No hard and fast rules for this
Convention: executables reside in cgi-bin directory
Find file on file system
Initial “/” in suffix denotes home directory for requested content
Minimal suffix is “/”, which servers expand to some default home page (e.g., index.html) Cox Networking 85 Anatomy of an HTTP Transaction unix> telnet www.google.com 80 Client: open connection to server
Trying 209.85.164.104... Telnet prints 3 lines to the terminal
Connected to www.l.google.com.
Escape character is '^]'.
GET / HTTP/1.1 Client: request line
host: www.google.com Client: required HTTP/1.1 HOST header
Client: empty line terminates headers
HTTP/1.1 200 OK Server: response line
Cache-Control: private Server: followed by six response headers
Content-Type: text/html; charset=ISO-8859-1
Set-Cookie: PREF=ID=<..snip..>
Server: gws
Transfer-Encoding: chunked
Date: Tue, 13 Nov 2007 17:25:04 GMT
Server: empty line (“\r\n”) terminates hdrs
Server: first HTML line in response body
<..snip..> Server: HTML content not shown.
Server: last HTML line in response body
Connection closed by foreign host. Server: closes connection
unix> Client: closes connection and terminates Cox Networking 86 HTTP Requests HTTP request is a request line, followed by zero or more request headers
Request line:
is either GET, POST, OPTIONS, HEAD, PUT, DELETE, or TRACE
is typically URL for proxies, URL suffix for servers
is HTTP version of request (HTTP/1.0 or HTTP/1.1) Cox Networking 87 HTTP Requests (cont) HTTP methods:
GET: Retrieve static or dynamic content
Arguments for dynamic content are in URI
Workhorse method (99% of requests)
POST: Retrieve dynamic content
Arguments for dynamic content are in the request body
OPTIONS: Get server or file attributes
HEAD: Like GET but no data in response body
PUT: Write a file to the server!
DELETE: Delete a file on the server!
TRACE: Echo request in response body
Useful for debugging Cox Networking 88 HTTP Requests (cont) Request headers: :
Provide additional information to the server
Major differences between HTTP/1.1 and HTTP/1.0
HTTP/1.0 uses a new connection for each transaction
HTTP/1.1 also supports persistent connections
Multiple transactions over the same connection
Connection: Keep-Alive
HTTP/1.1 requires HOST header
Host: www.yahoo.com
HTTP/1.1 adds additional support for caching Cox Networking 89 HTTP Responses HTTP response is a response line followed by zero or more response headers
Response line:
is HTTP version of the response
is numeric status
is corresponding English text
200 OK Request was handled without error
403 Forbidden Server lacks permission to access file
404 Not found Server couldn’t find the file
Response headers: :
Provide additional information about response
Content-Type: MIME type of content in response body
Content-Length: Length of content in response body
Cox Networking 90 GET Request to Apache Server From Firefox Browser GET /~comp221/proxytest.html HTTP/1.1
Host: www.owlnet.rice.edu
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8.1.8) Gecko/20071022 Ubuntu/7.10 (gutsy) Firefox/2.0.0.8
Accept: text/xml,application/xml,application/xhtml+xml,text/html; q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Connection: keep-alive
Keep-Alive: 300
CRLF (\r\n) Cox Networking 91 GET Response From Apache Server HTTP/1.1 200 OK
Date: Tue, 13 Nov 2007 18:09:01 GMT
Server: Apache/1.3.26 (Unix) mod_ssl/2.8.9 OpenSSL/0.9.6e
Last-Modified: Tue, 13 Nov 2007 18:08:44 GMT
ETag: “ac330311-df-4739e82c"
Accept-Ranges: bytes
Content-Length: 212
Connection: close
Content-Type: text/html
CRLF
COMP221 Web Proxy Test Page
COMP221 Web Proxy Test Page
This page is a simple text-only HTML file that can be used to test your web proxy software.
Cox Networking 92 Serving Dynamic Content Client sends request to server
Server parses request URI to determine if request is for static or dynamic content
In the “old” days, this was as simple as the string “/cgi-bin” starting the URL
Web servers are far more flexible and configurable now Client Server GET /cgi-bin/env.pl HTTP/1.1 Cox Networking 93 Serving Dynamic Content (cont) The server creates a child process and runs the program identified by the URI in that process Client Server env.pl fork/exec Cox Networking 94 Serving Dynamic Content (cont) The child runs and generates the dynamic content
The server captures the content of the child and forwards it without modification to the client
Client Server env.pl Content Content Cox Networking 95 Issues in Serving Dynamic Content How does the client pass program arguments to the server?
How does the server pass these arguments to the child?
How does the server pass other info relevant to the request to the child?
How does the server capture the content produced by the child?
These issues are addressed by the Common Gateway Interface (CGI) specification
Client Server Content Content Request Create env.pl Cox Networking 96 CGI Because the children are written according to the CGI spec, they are often called CGI programs
Because many CGI programs are written in Perl, they are often called CGI scripts
However, CGI really defines a simple standard for transferring information between the client (browser), the server, and the child process Cox Networking 97 add.com: THE Internet addition portal! Ever need to add two numbers together and you just can’t find your calculator?
Try the addition service at “add.com: THE Internet addition portal!”
Takes as input the two numbers you want to add together
Returns their sum in a tasteful personalized message Cox Networking 98 The add.com Experience input URL Output page host port CGI program args Cox Networking 99 Serving Dynamic Content With GET Question: How does the client pass arguments to the server?
Answer: The arguments are appended to the URI
Can be encoded directly in a URL typed to a browser or a URL in an HTML link
http://add.com/cgi-bin/adder?1&2
adder is the CGI program on the server that will do the addition
argument list starts with “?”
arguments separated by “&”
spaces represented by “+” or “%20”
Can also be generated by an HTML form
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