The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Overview, Functions and Characteristics

TCP Functions: What TCP Does 1 2 TCP Fundamentals and General Operation

The Robustness Principle

There's one more thing about TCP that is more an indication of the general maxim behind its creation than a particular characteristic of its operation. The TCP standard says that TCP follows the robustness principle, which is described thusly: “be conservative in what you do; be liberal in what you accept from others”. It means that every TCP implementation tries to avoid doing anything that would cause a problem for another device's TCP layer, while at the same time also trying to anticipate problems another TCP may cause and deal with them gracefully.

This principle represents a “belt and suspenders” approach that helps provide extra protection against unusual conditions in TCP operation. In fact, this general principle is applied to many other protocols in the TCP/IP suite, which is part of the reason why it has proven so capable over the years. It allows TCP and other protocols to often deal with even unanticipated problems that might show up in the difficult environment of a large internetwork such as the Internet.

Also, while we are on the subject of TCP’s characteristics, I want to reinforce one other thing I said in the overview of TCP and UDP: TCP has many attributes but one of them is not “slow”. It is true that UDP is usually used by applications for performance reasons when they don't want to deal with the overhead TCP incorporates for connections and reliability. That, however, should not lead one to conclude that TCP is glacial, by any means. It is in fact quite efficient—were it not, it's unlikely it would have ever achieved such widespread use.

TCP Functions: What TCP Does 1 2 TCP Fundamentals and General Operation