Introduction to Sockets

Objectives

1. Gain a basic understanding of the socket IPC mechanism

Notes

• This is chapter 56
  • There are about 6 chapters in this section.
  • And you might consider it a mini course in practical datacomm.
• Sockets allow two way communications between processes
  • On the same machine or across the network.
  • Unix domain sockets are on a single computer
  • Internet domain sockets are across a network.
  • Both have the same interface.
  • And guess what, both will give us a file descriptor.
• We will soon have a set of machines on which we can build internet socket software.
• Unlike pipes, there is no need for socket applications to come from the same ancestor.
• Sockets employ communications domains
  • AF_UNIX
    • Communications are provided by the kernel.
    • Address is in the form of a path name.
    • sockaddr_un is the address structure
  • AF_INET, AF_INET6
    • via IP4 or IP6
    • Address is in the form of an ip (or ipv6) address
    • sockaddr_in or sockaddr_in6
• Sockets can have three properties:
  • Reliable: we are guaranteed that the data will arrive intact, exactly as transmitted or we will receive an error. (all data correct in order)
  • Boundaries preserved: there is a way to distinguish between message, as opposed to just a string of bits.
  • Connection oriented: Operates in pairs (sender - receiver) and expects the pair to communicate until the session/connection ends.
    • Think phone call not chat group.
• There are two types of sockets
  • Stream : Connection oriented, reliable, but no message boundaries, TCP
  • Datagram: Connectionless, unreliable, but message boundaries., UDP
• In general, it takes the following calls to make a socket program work
  • socket - allocate the basic socket infrastructure: client and server
  • bind - associate the socket with an address: server
  • listen - allow for incoming connections: server
  • accept - select one connection and begin "servicing" it: server
  • connect - connect to a server : client
  • read/write - both
  • send, recv, sendto, recvfrom - both
• Look at the pictures on page 1156 and 1157