Socket API

A network programmer gets to use the following API :

• socket() - Creates a new socket endpoint and returns an integer file descriptor.
• bind() -     Binds as local address to a socket (socket is passed as argument).
• listen() -  For a server socket, listens to incoming connections.
• connect() - Connects to a server from a client socket
• accept() - For a server socket, accepts new connections.
• send() - Writes data through a socket.
• sendto() - Writes data to a datagram socket.
• recv() - Reads data trough a socket
• recvfrom() -  Reads/receives data through a datagram socket
• getsockopt() - Get the socket options for the socket passed in argument.
• setsockopt() - Set options for the socket.
• close() -    Closes a socket endpoint.
• htons() -    Convert short to network byte order
• ntohs() -    Convert short to host byte order
• inet_addr() -    Converts a dotted string to a 32-bit network address
• inet_ntoa() -     Convert 32-bit network address to a dotted string (eg: 127.0.0.1)

An example of client server program shows the actual usage of the API in detail. Figure 1 shows the data flow between the client and the server.

Please refer server and client code examples.

Figure 1: Client - Server Connection Flow Diagram

Figure 2. Server Connection Queue

Socket Options

Setting and getting options for a socket is performed through two functions, setsockopt() and getsockopt(). These functions provide a single interface for getting and setting a variety of options using a number of different structures. The socket option prototypes are given below:

The level refers to the layer of protocol to which this option will be applied. The option name is given by optionname.  Various options exist depending on the protocol of interest. The optval argument specifies the value to be set or the location to store the option in a GET request. Finally, the optlength defines the length of the structure. As there are a number of different structures that can be used to set or get options, this parameter defines the length to avoid the call from overrunning the buffer.

Each option level has a number of options that can be manipulated.

The Sockets layer options are those defined within the context of level SOL_SOCKET and focus on the Sockets API layer. The typical options for the Sockets layer are defined below;

IP Layer Options

These options are those defined within the context of level IPPROTO_IP and focus on the IP layer.

Example : IP_TOS Option

The IP_TOS option permits an application to change the Type of Service (TOS) field in the IP header of a socket. The TOS field is used to specify service precedence within networks. The TOS field permits segmenting traffic using quality of service (QOS) parameters.

Figure: IP TOS fields

The Delay, Throughput, and Reliability bits indicate a requested quality of service.

The IP_TOS field is used in LAN environments to segment traffic based upon quality of service needs. Quality of service APIs manipulate this field.

Accessor / Mutator Function Implementation

In the kernel code the implementation of set option function can be found in net/ipv4/ip_sockglue.c

The get function  alonmg with various option names