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.

A recent Internet draft draft-scheffenegger-tcpm-timestamp-negotiation-00 proposes an extension that allows the TCP hosts to negotiate the clock that they use to encode these timestamps. This common clock could be useful for congestion control purposes, but also for other purposes. For example, an MPTCP implementation could use the timestamps to estimate the delay over the different subflows and select the best performing subflow.