Transport OSI Layer Protocols

This post summarizes transport layer protocols.

List of Transport Layer Protocols

Transport Layer Protocols:

  • TCP
  • UDP
  • SCTP

TCP and UDP are the main transport protocols of the TCP/IP framework.

You can find a list of popular TCP and UDP ports on this post.


Transmission Control Protocol (TCP) is a full-duplex connection-oriented protocol.

TCP uses a port for each connection. As port numbers are 16-digit binary numbers, there are 2^16 = 65,536 ports available, from 0 to 65,535.

A socket is the combination of an IP and a port.

Ports are classified as follows:

  • Well-known ports (0-1,023).
  • Registered software ports (1,024-49,151).
  • Random, dynamic or ephemeral (49,152-65,535).

TCP uses communication flags. They can be reviewed on this post.

A TCP connection is established through the three-way handshake process:

  1. The client sends a SYN (synchronize) flagged packet to the server
  2. The server responds with a SYN/ACK (synchronize and acknowledge) flagged packet back to the client.
  3. The client responds with an ACK (acknowledge) flagged packet back to the server

Disconnection is done using either the FIN (finish) or RST (reset) flags.


User datagram protocol (UDP) is a connectionless protocol.

As it is unreliable, it is mostly used on video or audio streaming.

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External References

  • M. Chapple et al, “CISSP Official Study Guide, Ninth Edition”, section “Transport Layer Protocols”, pp. 508-509; Wiley, 2021

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