TCP - Urgent pointer field

The Urgent Pointer field in the TCP header plays a role in data transmission when urgent data needs to be processed. Here are its key characteristics:

  1. Purpose: The Urgent Pointer is used when the TCP segment contains urgent data that should be processed immediately at the destination, bypassing some of the usual processing queues.

  2. Size: It is a 16-bit field in the TCP header.

  3. Functioning with the URG Flag: The Urgent Pointer field is only relevant and interpreted when the URG flag (one of the control flags in the TCP header) is set to 1. If the URG flag is not set, the Urgent Pointer field is often ignored.

  4. Offset Value: The Urgent Pointer is an offset from the current sequence number indicating the end of the urgent data. So, the urgent data is located between the sequence number of the segment and the sequence number plus the urgent pointer value.

  5. Use Cases: A common use case for the Urgent Pointer is when a user hits a key to interrupt or abort a process, such as the Ctrl+C interrupt signal in many command-line interfaces. This signal can be sent as urgent data to ensure it is processed immediately.

  6. Limitations: In modern network applications, the use of the Urgent Pointer is rare. Many TCP implementations do not support urgent data distinctly, or they handle it in the same queue as normal data, which makes its utility limited in contemporary networking.

  7. Compatibility Issues: Because of different interpretations of the RFC (Request for Comments) standards and inconsistent implementation in various TCP stacks, the behavior of the Urgent Pointer can be unpredictable across different systems.

In summary, while the Urgent Pointer field in the TCP header was designed to transmit urgent data, its practical use and support in modern network environments are limited, and it is often handled in the same way as non-urgent data.

https://networklessons.com/cisco/ccie-routing-switching-written/tcp-header https://datatracker.ietf.org/doc/html/rfc793#page-17 https://datatracker.ietf.org/doc/html/rfc6093

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