Urg

urg is a term used to refer to the URG flag in the Transmission Control Protocol (TCP). The URG flag is one of the control bits in a TCP header. When the URG bit is set, it indicates that the urgent pointer field is meaningful and that the data up to the pointer should be treated as urgent by the receiver.

Urgent pointer and data: The urgent pointer is a 16-bit value that, if the URG flag is set, specifies the offset from the segment’s initial sequence number to the last urgent data octet. The data located up to and including that position is designated as urgent. The mechanism was designed to allow a TCP sender to signal that certain data within the stream should be processed with higher priority.

Historical usage and examples: The URG mechanism was historically used by some applications, notably Telnet, to signal control information within a TCP stream. In practice, however, the urgent data concept relies on end-to-end support and consistent handling by all intermediaries, which has proved unreliable.

Modern relevance and limitations: In contemporary networks, URG is rarely relied upon. Many operating systems, firewalls, and network devices do not preserve or honor the urgent pointer, leading to unpredictable behavior. As a result, URG-based signaling is often ignored or stripped by middleboxes, reducing its practical usefulness. Some systems provide separate out-of-band data support at the socket level, but this is a different mechanism from the TCP URG flag and is not universally portable.

See also: Transmission Control Protocol, urgent data, out-of-band data.