What is demultiplexing?

Demultiplexing is a process at the transport layer, in which incoming data from the network is distributed to the right applications.

More simply: demultiplexing is when the OS figures out which application to hand the received data to.

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#Why demultiplexing is needed

When a packet arrives at a computer:

• it arrives at a single IP address
• through a single network interface

But there are many applications. Demultiplexing solves the task of:

which program to pass this data to

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#How demultiplexing works

#1. A segment/datagram arrives

The transport layer receives data from the network.

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#2. The header fields are analyzed

The OS looks at:

• the protocol (TCP or UDP)
• the destination port
• (for TCP, also the IP addresses and the source port)

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#3. The data is delivered to the right socket

Based on this information:

• a socket is selected
• the data is delivered to a specific process

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#Demultiplexing in TCP and UDP

#UDP

• the choice is made mainly by:
  • the destination port
  • the protocol
• one socket can receive messages from different senders

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#TCP

• the connection is defined by the exact pair:

  Copy

  (source IP, source port, destination IP, destination port)

• this lets a server:

  • serve thousands of clients
  • on the same port

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#Relationship to multiplexing

Important for the interview.

• Multiplexing - combining data from different applications when sending
• Demultiplexing - parsing incoming data and delivering it to the right applications

These are two sides of the same mechanism.

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#What is important to remember

• demultiplexing is a function of the transport layer
• it is based on ports and sockets
• without it, applications could not work in parallel