What is multiplexing at the transport layer?

Multiplexing at the transport layer is a mechanism that lets several applications simultaneously use a single network connection (a single IP address).

More simply: multiplexing is when the transport layer gathers data from different applications and sends it over the network, distinguishing them by port.

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#Why multiplexing is needed

On one computer, the following run at the same time:

• a browser
• mail
• messengers
• system updates

All of them:

• use a single IP address
• go out onto the network in parallel

Multiplexing makes this possible.

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#How multiplexing works

#On the sender's side

The transport layer:

1. accepts data from different applications
2. for each data stream:

• assigns a source port

3. packs the data into segments
4. sends them onto the network

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#On the receiver's side (demultiplexing)

The transport layer:

1. receives segments from the network
2. looks at:

• the IP address
• the destination port
• the protocol (TCP/UDP)

3. delivers the data to the right application

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#Multiplexing and demultiplexing

Important for the interview.

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

These two processes are usually mentioned together.

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#The role of ports

Ports are the key element of multiplexing:

• they let applications be distinguished
• they enable parallel connections
• they make end-to-end communication possible

Without ports, multiplexing would be impossible.

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#Example

• browser → port 54321 → HTTPS
• mail → port 54322 → SMTP
• messenger → port 54323 → its own server

All the data:

• goes through a single network interface
• but is not mixed up

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#What is important to remember

• multiplexing is a function of the transport layer
• it works on top of IP
• it is not tied to physical channels
• TCP and UDP have separate multiplexing