24.9 Summary

This chapter has looked at five new TCP features: path MTU discovery, window scale option, timestamp option, protection against wrapped sequence numbers , and improved transactional processing using TCP. We saw that the middle three features are required for optimal performance on long fat pipes ” networks with a large bandwidth-delay product.

Path MTU discovery allows TCP to use windows larger than the default of 536 for nonlocal connections, when the path MTU is larger. This can improve performance.

The window scale option takes the maximum TCP window size from 65535 bytes to just over 1 gigabyte. The timestamp option lets more segments be accurately timed, and also lets the receiver provide protection against wrapped sequence numbers (PAWS). This is essential for high-speed connections. These new TCP options are negotiated at connection establishment, and ignored by older systems that don't understand them, allowing newer systems to interoperate with older systems.

The TCP extensions for transactions, T/TCP, allow a client-server request-reply sequence to be completed using only three segments in the usual case. It avoids the three-way handshake and shortens the TIME_WAIT state by caching a small amount of information for each host with which it has established a connection. It also overloads the data segments with the SYN and FIN flags.

We finished the chapter with a look at TCP performance, since there is still much inaccurate folklore about how fast TCP can run. For a well- tuned implementation using the newer features described in this chapter, TCP performance is limited only by the maximum 1-gigabyte window and the speed of light (i.e., the round-trip time).