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Internet Core Protocols: The Definitive Guide by Eric Hall

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Name

Urgent Pointer

Synopsis

Identifies the sequence number of the last byte of any urgent data that may be in this segment.

Size

Sixteen bits.

Notes

TCP offers the ability to flag certain bytes of data as “urgent.” This feature allows an application to process and forward any data that must be dealt with immediately, without the data having to sit in the send queue for processing. Instead, the data is packaged into a segment, the Urgent flag is set in the TCP header, and a byte off-set marking the end of urgent data is specified in the Urgent Pointer field.

It is important to note that the Urgent Pointer does not use a Sequence Number to specify the end of urgent data, but instead uses an off-set in the current stream, indicating the location where urgent data ends. A recipient of a TCP segment with the Urgent flag enabled must add the value provided in the Urgent Pointer to the Sequence Number field of the current segment, and use the resulting value to determine the ending sequence number. What this means is that the Urgent Pointer off-set can refer to a byte location in another TCP segment, allowing urgent data to span across multiple segments if needed.

This mechanism reflects a specific design as mandated in RFC 1122. Unfortunately, this design was not always clear, and many TCP implementations take a different interpretation. Most notably, many BSD-based systems use the Urgent Pointer to refer to the byte following the off-set specified in the Urgent Pointer, rather than the specified ...

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