Introduced in RFC1323, the TCP timestamp option enables TCP hosts to better estimate the round-trip time by adding a timestamp to each transmitted TCP segment. These timestamps are also useful for high bandwidth transfer to protect against the reutilisation of the same TCP sequence number for different segments during the same MSL. These timestamps are supported by most TCP implementations today.

In the early 1990s, the T/TCP extension to TCP was proposed to allow hosts to exchange data without being forced to wait until the end of the three way handshake. Specifications ( RFC1379 and RFC1644) and implementations were written, but this TCP extension has never been largely used.

Despite its respectable age (the first specification was published in September 1981), TCP is still alive and researches and IETFers continue to develop new extensions to improve the protocol. Multipath TCP received a lot of attention during the last years. Another TCP extension to improve the security of TCP has been submitted for the next IETF meeting. TCP crypt aims at improving the security of TCP by allowing the utilisation of crypto mechanisms to protect the TCP segments.

The retransmission timeout plays an important role in allowing TCP to recover from segment losses. Over the years, extensions to TCP have been developed to reduce the reliance of TCP on the retransmission timer. Recently, a SIGCOMM paper proposed to use microsecond resolution timers in TCP. A recent IETF draft summarizes the discussions on TCP's retransmission timer, but surprisingly it does not discuss these microsecond resolution TCP timers.

First part

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Second part