TCP Socket Programming

An abstract interface provided to the application programmer File descriptor, allows apps to read/write to

the network Allows to processes on remotely

connected computers to talk to each other

What is a socket?

SOCK_STREAM TCP connection

oriented, bidirectional

reliable, in-order delivery

Two types of sockets

SOCK_DGRAM

UDP

no connection

unreliable delivery, no guarantee on the order

can send/receive

CPSC 441 - Application Layer 4

Socket-programming using TCP

Socket: a door between application process and end-end-transport protocol (UDP or TCP)

TCP service: reliable transfer of bytes from one process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet

CPSC 441 - Application Layer 5

Socket programming with TCPClient must contact server server process must first

be running server must have created

socket (door) that welcomes client’s contact

Client contacts server by: creating client-local TCP

socket specifying IP address, port

number of server process When client creates

socket: client TCP establishes connection to server TCP

When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk

with multiple clients source port numbers

used to distinguish clients

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint

Used to address processes on a host 0-1023 is usually reserved for known

service 1024-49151 are registered. They are

used for multiple purposes 49152-65535 are private ports

Ports

TransportLayer

FTP Server

Web Server

21 80

NetworkLayer

DLL/Physical

Socket Programming in C

CPSC 441 - Application Layer 7

Socket Programming - Flow

socket()

connect()

send()

recv()

close()

..

.

socket()

bind()

listen()

accept()

recv()

send()

close()

wait for connection request from next client

Client Server

int s_listen = socket(family, type, protocol);

family: AF_INET specifies Ipv4 type: SOCK_STREAM, SOCK_DGRAM protocol: 0 (pseudo, IP ). See /etc/protocols

socket()

connect() int connect(int sockfd, const struct sockaddr

*servaddr, socklen_t addrlen); Connect to server. sockfd is socket descriptor from socket() servaddr is a pointer to a structure with:

port number and IP address must be specified (unlike bind())

addrlen is length of structure client doesn’t need bind()

OS will pick ephemeral port

returns socket descriptor if ok, -1 on error

CPSC 441 - Application Layer 10

bind(s_listen, localAdd, addLength) Server specifies which port and address it will

be listening to s_listen: our listening socket descriptor localAdd: socket address structure addLength: length of localAdd

bind()

struct in_addr { in_addr_t s_addr; /* 32-bit IPv4 addresses */};

struct sockaddr_in { u_char sin_len; /* length of address */ u_char sin_family; /* family of address, Ex. AF_INET */ u_short sin_port; /* Protocol (TCP/UDP) Port num */ struct in_addr sin_addr; /* IPv4 address (above) */ char sin_zero[8]; /* set to zero, used for padding */};

Socket Address Structure

Declare address structure struct sockaddr_in sockAdd;

Set family sockAdd.sin_family = AF_INET;

Set IP address (2 ways)//specify address to listen to inet_pton(AF_INET, “127.0.0.1”, &sockAdd.sin_addr.s_addr)

//listen to any local address sockAdd.sin_addr.s_addr = htonl(INADDR_ANY)

Set port sockAdd.sin_port = htons(9999);

Address Structure

int status = listen(s_listen, queuelength); status: -1 if error, 0 otherwise s_listen: socket descriptor queuelength: Number of clients that can “wait” for a

connection listen is non-blocking: returns immediately

listen()

int s_new = accept(s_listen, &clientAddress, &addLength); s_new: new socket for communication with client s_listen: the listening socket clientAddress: struct sockaddr, address of client addLength: size of client address structure accept is blocking: waits for connection before returning

accept()

int send(int s_new, const void *buf, int len, int flags);

• s_new – socket descriptor• buf – pointer to buffer• len – size of buffer• flags – can be safely set to 0

int recv(int s_new, void *buf, int len, unsigned int flags);

• similar to send• buf holds the data to be transferred

Talking

fork() is a C system call used to spawn child processes Execution for both child and parent process

continues at the next instruction fork() returns

• 0 if this is the child process• PID (>0) of the child process if this is the parent• <0 if fork() fails

Used to keep listening on socket and talking on another socket

System calls - fork()

Demo

A simple client – server example:Echo Server

Socket Programming, Dan Rubinstein, http://www1.cs.columbia.edu/~danr/courses/6761/Fall00/intro/6761-1b-sockets.ppt

15-441 Socket Programming, www.cs.cmu.edu/afs/cs/academic/class/15441-f01/www/lectures/lecture03.ppt

Network Programming, Geoff Kuenning, www.cs.hmc.edu/~geoff/classes/hmc.cs105.200701/slides/class21_net2.ppt

Socket Programming, Abhinav Jain, www.cs.purdue.edu/homes/jain8/cs422/pso3.ppt

References