An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 85
TCP Timestamps and PAWS
The Timestamp Option [JBB92] provides for timing of every packet inflight,
rather than only one per RTT as defined in the original specifications
[Pos81b]. This improves the RTT estimation and enables keeping a more
dynamic and accurate value for RTO and hence slightly quicker response to
TCP timeouts.
TCP Timestamps occupy 10-bytes and require the sender to place a current
timestamp into each packet sent out (including retransmissions) which
is then echoed by the TCP receiver in the ack.
The implementation of the Timestamps option is especially useful for
environments with very variable latencies such as wireless networks [LK02].
However, especially in wireless environments, the implementation has to be
balanced against the extra overhead of header information.
The implementation of a timestamp also enables extra protection against
potential wrap around due to the reuse of sequence numbers for long duration
transfers. [JBB92] defines the Protection Against Wraparound Sequence
Numbers (PAWS) algorithm that enables the checking to ensure that arriving
packets are valid in time for the connection.
The inclusion of the Timestamp option also enables low-level improvements
in processing TCP data packets at the receiver. This is referred to
as ‘Header Prediction’ [Jac90] and enables the receiver to quickly process a
TCP segment provided that the timestamp is larger than the previous one
recorded and that the sequence number is the next in sequence.