Socket

• application process sends/receives messages to/from its socket (the socket is created by application)

Two socket types for two transport services:

• UDP socket at host [IP@ (host), Port#(host)]
• TCP socket [IP@ source, IP@ dest, Port# s, Port# d]
  • possible that two TCP sockets → OS chose everything the same, but the IP@ of the source is what tells them apart.

Addressing processes

• to receive messages, processes must have identifier
• host device interface has unique 32-bit IP address

Does IP address of host on which process runs suffice for identifying the process?

No, many app processes can be running on the same host.

• identifier includes both IP address and port numbers associated with process on host.
• Ex port numbers:
  • HTTP server: 80
  • Mail server: 25
• To send HTTP message to gaia.cs.umass.edu web server:
  • IP address: 128.119.245.12
  • port number: 80

Multiplexing/demultiplexing at Transport Layer

How demultiplexing works

• host receives IP datagrams
  • each datagram has source IP address, destination IP address
  • each datagram carries at most one transport-layer segment
  • each segment has source port number, destination port number
• host uses IP addresses & port numbers to direct segment to appropriate socket (different for UDP and TCP)

Socket programming with TCP

Client must contact server

• server process must first be running
• server must have created socket (door) that welcomes client’s contact

Client contacts server by:

• Creating TCP socket, specifying IP address, port number of server process
• when client creates socket: client TCP establishes connection to server TCP

Connection-oriented demultiplexing

• TCP socket identified by 4-tuple:
  • source IP address
  • source port number
  • dest IP address
  • dest port number
• demux: receiver uses all four values (4-tuple) to direct segment to appropriate socket
• server may support many simultaneous TCP sockets:
  • each socket identified by its own 4-tuple
  • each socket associated with a different connecting client

• Three segments, all destined to IP address: B, dest port: 80 are demultiplexed to different sockets
• trying to connect to server via TCP sockets
• TCP socket: remembers the past → connection oriented

Summary

• Multiplexing, demultiplexing: based on segment, datagram header field values
• UDP: demultiplexing using destination port number (only)
• TCP: demultiplexing using 4-tuple: source and destination IP addresses, and port numbers
• Note: Multiplexing/demultiplexing happen at all layers!!!