Chapter 5 Socket API

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Chapter 5 Socket API

• 1980, a group at UC Berkeley create a interface
  that applications use to communicate.
• Known as socket API or socket interface
• System is known as Berkeley UNIX or BSD UNIC

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Specifying a protocol interface

• Two broad approaches
• Define functions specifically to support TCP/IP
  communication
• Define functions that support network
  communication in general, and use parameters to
  make TCP/IP communication a special case.
• Because the designer at Berkeley wanted to
  accommodate multiple sets of communication
  protocol, they used the second approach.

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• In fact, they provides for generality far beyond
  TCP/IP. They allowed for multiple families of
  protocols, with all TCP/IP protocols represented
  as single family(family PF_INET)
• The socket API provides generalized functions
  that support network communication using many
  possible protocols as a single protocol family.
  The calls allow the programmer to specify the
  type of service required rather than the name of
  specific protocol.

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Socket abstraction

• File descriptors
• Open function create a file descriptor
• The OS maintains a separate file descriptor as an
  array of pointers to internal data structure
• Application program only need to remember the
  descriptor and use it

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• Socket API adds a new abstraction for network
  communication, the socket.
• Each active socket is identified by a small
  integer called its socket descriptor
• OS provides a separate system function, socket,
  that applications call to create a socket
• Application only use open to crate file
  descriptor
• Once the socket has been created, an application
  must make additional system calls to specify the
  details of its exact use.

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System data structure for sockets
Operating system
Descriptor table ( one per process)
Family PF_INET
0
ServiceSOCK_STREAM
1
Local IP
2
Remote IP
3
Local Port
4
Remote Port
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Using sockets

• A socket used by a server to wait for an incoming
  connection is called a passive socket
• A socket used by a client to initiate a
  connection is called an active socket.
• The sockets are created the same way initially

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Endpoint address

• When a socket is created, it does not contain
  detailed information about how it will be used.
  (port number, IP address)
• ICP/IP protocols define a communication endpoint
  to consist of an IP address and a protocol port
  number.
• It allows each protocol family to specify
  endpoints however it likes
• TCP/IP protocols all use a single address
  representation, with address family AF_INET

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Generic address structure

• Struct sockaddr
• u_char sa_len // total length
• U_short sa_family // type of address
• char sa_data14 // value of address