What is the Urgent Pointer in TCP and is it commonly used?

• A. It marks urgent data within a stream; rarely used in modern protocols and often ignored by applications.
• B. It indicates window size
• C. It handles congestion control
• D. It marks segment order

Answer: (A)

In TCP, the Urgent Pointer is a header field that marks a portion of the data within a segment as urgent when the URG flag is set. It provides an offset from the segment’s initial sequence number to indicate the last urgent byte, signaling the receiver to deliver that urgent data to the application immediately and potentially bypass the normal in-order delivery of the stream. This mechanism was designed for small, urgent control data in the byte stream, such as interrupt signals in older protocols like Telnet.

However, in modern networks this feature is rarely used and many implementations or middleboxes ignore it, so urgent data isn’t relied upon by today’s applications. The Urgent Pointer is not about window size, congestion control, or segment order—the window size is a separate field, congestion control is handled by algorithms like Reno or Cubic, and segment order is determined by sequence numbers.