All About Network Sockets

What is as Socket?

A socket is one endpoint of a two-way communication link between two programs running on the network.

Types of Sockets

Stream Socket

• Also called connection-oriented sockets
• Sends and receives messages through streams (much like reading/writing data from files)
• Doesn't care about message boundaries
• Commonly used for TCP

Datagram Socket

• Also called connectionless sockets
• Sends and receives messages as independent packets with defined boundaries (i.e. one receive = one message)
• Commonly used for UDP

Raw Socket

• Allows direct sending and receiving of IP packets without any protocol-specific transport layer formatting (basically skips the transport layer)
• Has no concept of ports
• Can be used if you want to implement your own transport protocol
• Also used by network utility programs such as nmap, ping, etc.

Socket API

A collection of functions/methods/commands usually provided by the operating system to enable programs to use sockets and communicate with other programs.

Berkeley Sockets

• Also called connectionless sockets
• Sends and receives messages as independent packets with defined boundaries (i.e. one receive = one message)
• Commonly used for UDP

List of Common Berkeley Socket API Functions

socket()

Instantiates a socket object

bind()

Binds an IP address and port number to a socket object

connect()

Establishes a connection to a remote host

listen()

Used by servers to listen for incoming connections

accept()

Accepts a connection request from the listen() function and returns a new socket associated with the new connection

send()

Sends data through the socket

recv()

Receives data from a socket

close()

Closes the socket

Other common socket functions include:

• read(), recvfrom(), or recvmsg()
• write(), sendto(), or sendmsg()
• gethostbyname() and gethostbyaddr()

TCP Socket API Call Sequence

UDP Socket API Call Sequence

Socket Protocols

• IPPROTO_TCP for TCP
• IPPROTO_UDP for UDP
• Other values include IPPROTO_SCTP and IPPROTO_DCCP

Note that you can use the same port number for different protocols.

Socket Domains (aka Protocol Family/Address Family)

AF_INET/PF_INET

• Corresponds to IPv4
• AddressFamily.InterNetwork in the .NET implementation

AF_INET6/PF_INET6

• Corresponds to IPv6
• AddressFamily.InterNetworkV6 in the .NET implementation

AF_UNIX/PF_UNIX

• Corresponds to local sockets (using a file)
• Used in UNIX for inter-process communication (IPC)

Socket Operating Modes

Blocking Mode

• When operating in Blocking mode, some socket functions such as accept(), send(), and recv() will block execution until the operation is successful. The accept() function for instance, will wait until a client initiates a connection to the server.
• In .NET, this can be set by assigning true to the Socket.Blocking property. This is the default value in .NET.

Non-Blocking Mode

• In non-blocking mode, socket functions will not block execution.
• In .NET, this can be set by assigning false to the Socket.Blocking property.