The TCP/IP Guide - TCP Overview, History and Standards

RFC 793 is the defining standard for TCP, but it doesn't include all the details of how modern TCP operates. Several other standards include additional information about how the protocol works, and describe enhancements to the basic TCP mechanisms that were developed over the years. Some of these are fairly “esoteric” and not widely known, but they are useful in gaining a more complete understanding of TCP. I have listed some of them in Table 149

**Table 149: Supplementary TCP Standards**
RFC Number	Name	Description
813	Window and Acknowledgment Strategy in TCP	Discusses the TCP sliding window acknowledgment system, describing certain problems that can occur with it and methods to correct them.
879	The TCP Maximum Segment Size and Related Topics	Discusses the important Maximum Segment Size (MSS) parameter that controls the size of TCP messages, and relates this parameter to IP datagram size.
896	Congestion Control in IP/TCP Internetworks	Talks about congestion problems and how TCP can be used to handle them. Note the interesting inversion of the normal protocol suite name: “IP/TCP”.
1122	Requirements for Internet Hosts — Communication Layers	Describes important details of how TCP should be implemented on hosts.
1146	TCP Alternate Checksum Options	Specifies a mechanism for having TCP devices use an alternative method of checksum generation.
1323	TCP Extensions for High Performance	Defines extensions to TCP for high-speed links, and new TCP options.
2018	TCP Selective Acknowledgment Options	An enhancement to basic TCP functionality that allows TCP devices to selectively specify specific segments for retransmission.
2581	TCP Congestion Control	Describes four algorithms used for congestion control in TCP networks: slow start, congestion avoidance, fast retransmit and fast recovery.
2988	Computing TCP's Retransmission Timer	Discusses issues related to setting the TCP retransmission timer, which controls how long a device waits for acknowledgment of sent data before retransmitting it.

Of course, there are hundreds of higher-layer application protocols that use TCP, and whose defining standards therefore make at least glancing reference to it.

TCP is of course designed to use the Internet Protocol, since they were developed together and as we have seen, were even once part of the same specification. At the same time, they were split up for the specific reason of respect the principles of architectural layering. For this reason, TCP tries to make as few assumptions as possible regarding the underlying protocol over which it runs. It is not as strictly tied to the use of IP as one might imagine, and can even be adapted for use over other network-layer protocols. For our purposes, however, this should be considered mainly an "interesting aside". We will be assuming TCP works over IP in our discussions, since that is almost always how it is used.

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