Socket Programming

Socket Programming - Transcript

Socket Programming 101
Vivek Ramachandran

Why Socket Programming
To build any Networked Application
WWW Internet Explorer Firefox FTP WS FTP P2P Limewire Bitcomet

Pre Requisites
Basics of IP TCP UDP C programming Basics Data Structures basics Compilation of code on Linux Unix using gcc etc Same principles can be used to code on windows as well using the winsock library

Network Programming huh Telephone Analogy
A telephone call over a telephony network works as follows Both parties have a telephone installed A phone number is assigned to each telephone Turn on ringer to listen for a caller Caller lifts telephone and dials a number Telephone rings and the receiver of the call picks it up Both Parties talk and exchange data After conversation is over they hang up the phone

Dissecting the Analogy
A network application works as follows

An endpoint telephone for communication is created on both ends An address phone no is assigned to both ends to distinguish them from the rest of the network One of the endpoints caller initiate a connection to the other The other end receiver point waits for the communication to start Once a connection has been made data is exchanged talk Once data has been exchanged the endpoints are closed hang up

In the world of sockets
Socket Endpoint for communication Bind Assign a unique telephone number Listen Wait for a caller Connect Dial a number Accept Receive a call Send Recv Talk Close Hang up

The Client Server model
Server An entity which is a provider of information Client An entity which is a seeker of information Example Apache is a web server providing web pages information and Internet Explorer is a web client which requests those pages from the server In the socket programming world almost all communication is based on the Client Server model The Server starts up first and waits for a client to connect to it After a client successfully connects it requests some information The Server serves this information to the client The client then disconnects and the Server waits for more clients

A TCP Server Client Interaction

A UDP Server Client Interaction

Before we dive deeper Data Structures
Let us now look at the data structures used to hold all the address Information Struct sockaddr unsigned short sa family char sa data 14 Struct sockaddr in short sin family unsigned short sin port Port Number struct in addr sin addr IP Address char sin zero 8 Struct in addr unsigned long s addr 4 bytes long





Before we dive deeper Byte Ordering
Byte ordering or Endianess is the attribute of a system which indicates whether integers are stored represented left to right or right to left Example 1 short int x 0xAABB hex This can be stored in memory as 2 adjacent bytes as either 0xaa 0xbb or as 0xbb 0xaa Big Endian Byte Value Memory Little Endian Byte Value Memory 0xBB 0xAA 0 1 0xAA 0xBB 0 1

Before we dive deeper Byte Ordering
Example 2 int x 0xAABBCCDD This 4 byte long integer can be represented in the same 2 orderings Big Endian Byte Value 0xAA 0xBB 0xCC 0xDD Memory 0 1 2 3 Little Endian Byte Value 0xDD 0xCC 0xBB 0xAA Memory 0 1 2 3 All Network data is sent in Big Endian format In the networking world we call this representation as Network Byte Order and native representation on the host as Host Byte Order We convert all data into Network Byte Order before transmission

Some utility functions
Byte Ordering Host Byte Order to Network Byte Order htons htonl Network Byte Order to Host Byte Order ntohs ntohl IP Address format Ascii dotted to Binary inet aton Binary to Ascii dotted inet ntoa Many others exist explore the man pages D

Diving Deeper into the syscalls
We will now describe the following calls in detail Socket Bind Listen Accept Connect Read Send Sendto Write Recv Recvfrom Close

Socket A Connection Endpoint
This creates an endpoint for a network connection Int Socket int doman int type int protocol domain PF INET IPv4 communication type SOCK STREAM TCP SOCK DGRAM UDP protocol 0 for our discussion Example socket PF INET SOCK STREAM 0 This will create a TCP socket The call returns a socket descriptor on success and 1 on an error

Bind Attaching to an IP and Port
A server process calls bind to attach itself to a specific port and IP address Int Bind int sockfd struct sockaddr my addr socklen t addrlen sockfd socket descriptor returned by socket my addr pointer to a valid sockaddr in structure cast as a sockaddr pointer addrlen length of the sockaddr in structure Example struct sockaddr in my my sin family PF INET my sin port htons 80 my sin addr s addr INADDR ANY bzero my 8 bind sock struct sockaddr my sizeof my

Listen Wait for a connection
The server process calls listen to tell the kernel to initialize a wait queue of connections for this socket Int Listen int sock int backlog sock socket returned by socket backlog Maximum length of the pending connections queue Example Listen sock 10 This will allow a maximum of 10 connections to be in pending state

Accept A new connection
Accept is called by a Server process to accept new connections from new clients trying to connect to the server Int Accept int socket struct sockaddr client socklen t client len socket the socket in listen state client will hold the new client s information when accept returns client len pointer to size of the client structure Example struct sockaddr in client int len sizeof client Accept sock struct sockaddr client len

Connect connect to a service
Connect is called by a client to connect to a server port Int Connect int sock struct sockaddr server addr socklen t len sock a socket returned by socket server addr a sockaddr in struct pointer filled with all the remote server details and cast as a sockaddr struct pointer len size of the server addr struct Example connect sock struct sockaddr server addr len

Send Recv Finally Data
Send Recv Read Write etc calls are used to send and receive data Int send int sock void mesg size t len int flags Int recv int sock void mesg size t len int flags sock A connected socket mesg Pointer to a buffer to send receive data from in len Size of the message buffer flags 0 for our purpose The return value is the number of bytes actually sent received Example char send buffer 1024 char recv buffer 1024 int sent bytes int recvd bytes sent bytes send sock send buffer 1024 0 recvd bytes recv sock recv buffer 1024 0

Close Bye Bye
Close signals the end of communication between a server client pair This effectively closes the socket Int close int sock sock the socket to close Example close sock

Done Phew
This is all the theory you need to know about socket programming Finer points will be more clear when we being coding So let the games begin D