Internet Core Protocols: The Definitive Guide

Notes on TCP Options

TCP options are conceptually similar to IP options, although their usages are quite different. For one thing, IP options are mostly used to define special-handling services for IP datagrams being sent across the Internet, while TCP options are mostly used to extend TCP’s native circuit-management services.

RFC 793 originally defined three option types, only one of which is required (the Maximum Segment Size option, used by the two endpoint systems during circuit setup to exchange information about their local MTU/MRU sizes). Over the years however, the list of options has grown dramatically.

RFC 1072 introduced the Window Scale, Selective Acknowledgment and Echo/Echo Reply options. However, the Window Scale option was redefined in RFC 1323, and the Echo/Echo Reply options were replaced with a single Timestamp option in RFC 1323 as well. Selective Acknowledgments were also redefined, although not until RFC 2018.

In addition, RFC 1106 defined an alternative to the Window Scale option called the Big Windows option, and also defined a Negative Acknowledgment option as a possible enhancement to TCP’s native cumulatively implicit acknowledgment scheme. However, RFC 1106 was never ratified, nor was it implemented on many systems.

Table 7.4 lists the current options that are commonly used and some notes on their usage. For a detailed listing of all of the TCP options that are currently registered, refer to the IANA’s online registry (accessible at http://www.iana.org/assignments/tcp-parameters ...