Web Simple Request
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Communication
Sockets (Haviland Ch. 10)
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How do we find the server?
Every computer on the Internet has an Internet address. Called an IP address (Internet Protocol) An IP address is four 8-bit numbers separated by dots. www.eecg.toronto.edu = 128.100.10.235
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Protocols
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TCP/IP
Transmission Control Protocol. Tells us how to package up the data.
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How many messages?
It depends on the size of the web page we retrieve. If the web page is 75 Kbytes (small!) it will be broken up into 103 IP packets. Remember DNS took 10 messages [KRSV PHVVDJHV
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The Big Picture
network Client Process Server Process
Client-Server model: a client process wants to talk to a server process Client must find server - DNS lookup Client must find process on server - ports Finally establish a connection so two processes can talk
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Sockets
One form of communication between processes. Similar to pipes, except sockets can be used between processes on different machines. Use file descriptors to refer to sockets. Built on top of TCP layer
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TCP: Three-way handshake
sequence number = J
client
server
sequence number = K
socket connect (blocks) connect returns
SYN J
socket,bind,listen accept(blocks)
SYN K, ack J+1 ack K+1
accept returns
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TCP Server socket()
TCP Client socket()
Connection-Oriented
Server
Create a socket: socket() Assign a name to a socket: bind() Establish a queue for connections: listen() Get a connection from the queue: accept()
bind() listen() accept()
block until connection from client data transfer Connection establishment (3-way handshake)
connect()
write()
read() write() read() close()
end-of-file notification
Client
close()
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Create a socket: socket() Initiate a connection: connect()
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try nslookup
Socket Types
Two main categories of sockets
UNIX domain: both processes on the same machine INET domain: processes on different machines
Addresses and Ports
A socket pair is the two endpoints of the connection. An endpoint is identified by an IP address and a port. IPv4 addresses are 4 8-bit numbers:
128.100.31.200 = werewolf 128.100.31.201 = seawolf 128.100.31.202 = skywolf
Three main types of sockets:
SOCK_STREAM: the one we will use SOCK_DGRAM: for connectionless sockets SOCK_RAW
Ports
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because multiple processes can communicate with a single machine we need another identifier.
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More on Ports
Well-known ports: 0-1023
80 = web 22 = ssh 23 = telnet 2709 = supermon 26000 = quake
www.iana.org
TCP Server socket()
TCP Client socket()
bind() listen() accept()
block until connection from client data transfer
21 = ftp 25 = smtp (mail) 194 = irc
Connection establishment (3-way handshake)
connect()
Registered ports: 1024-49151
read() write()
write()
Dynamic (private) ports: 49152-65535
You should pick ports this in range to avoid overlap
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read() close()
end-of-file notification
close()
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Server side
int socket(int family, int type, int protocol); family specifies protocol family:
PF_INET IPv4 PF_LOCAL Unix domain
bind to a name
int bind(int sockfd, const struct sockaddr *servaddr, socklen_t addrlen); sockfd returned by socket struct sockaddr_in { short sin_family; /*AF_INET */ u_short sin_port; struct in_addr sin_addr; char sin_zero[8]; /*filling*/ }; sin_addr can be set to INADDR_ANY to communicate with any host
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type
SOCK_STREAM, SOCK_DGRAM, SOCK_RAW
protocol
set to 0 except for RAW sockets
returns a socket descriptor
Set up queue in kernel
int listen(int sockfd, int backlog) after calling listen, a socket is ready to accept connections prepares a queue in the kernel where partially completed connections wait to be accepted. backlog is the maximum number of partially completed connections that the kernel should queue.
Complete the connection
int accept(int sockfd, struct sockaddr *cliaddr, socklen_t *addrlen); blocks waiting for a connection (from the queue) returns a new descriptor which refers to the TCP connection with the client sockfd is the listening socket cliaddr is the address of the client reads and writes on the connection will use the socket returned by accept
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Client side
socket() same as server, to say how we are going to talk int connect(int sockfd, const struct sockaddr *servaddr, socklen_t addrlen); the kernel will choose a dynamic port and source IP address. returns 0 on success and -1 on failure setting errno. initiates the three-way handshake.
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inetclient.c
int soc; struct hostent *hp; struct sockaddr_in peer;
most error checking is omitted in this example
peer.sin_family = AF_INET; peer.sin_port = htons(PORT); /* fill in peer address */ hp = gethostbyname(argv[1]); peer.sin_addr = *((struct in_addr *)hp->h_addr); /* create socket */ soc = socket(PF_INET, SOCK_STREAM, 0); /* request connection to server */ if (connect(soc, (struct sockaddr *)&peer, sizeof(peer)) == -1) { perror("client:connect"); close(soc); exit(1); } write(soc, "Hello Internet\n", 16); read(soc, buf, sizeof(buf)); printf("SERVER SAID: %s\n", buf); close(soc);
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inetserver.c
struct sockaddr_in peer; struct sockaddr_in self; int soc, ns, k; int peer_len = sizeof(peer); self.sin_family = AF_INET; self.sin_port = htons(PORT); self.sin_addr.s_addr = INADDR_ANY; bzero(&(self.sin_zero), 8); peer.sin_family = AF_INET; /* set up listening socket soc */ soc = socket(PF_INET, SOCK_...