Multiplexing happens at the sender:

• Ingest data from multiple sockets + add transport header for later demultiplexing

Demultiplexing happens at the receiver:

• Header info from multiplexing is used to deliver received segments to correct socket

Demultiplexing

• Host receives IP datagrams

  • Datagrams have source IP address + destination IP address

  • Each datagram carries one transport-layer segment - segment has source and destination port number

    

• Host uses IP addresses & port numbers to direct segment to the appropriate socket

Connectionless Demultiplexing

On creation of a UDP socket, a host-local port number must be specified - this is included in transport layer.

UDP sockets on destination are uniquely specified by pair of (destination IP address, destination port number).

On host receipt of UDP segment, the host checks the destination port number in the segment and directs the UDP segment to the socket with the matching port number.

• IP/UDP datagrams with the same destination port but different source IP address/source port numbers are directed to the same socket

Note in this example, the messages from different hosts have their messages written to the same receiver.

Connection-Oriented Demultiplexing

TCP sockets are uniquely identified by the 4-tuple of:

• Source IP address
• Source port number