Request For Comments 1323 suggested some TCP extensions for performance improvement over very high speed links. Two TCP options are introduced by RFC 1323: Window Scale and TCP Timestamps.
The Window Scale option allows the TCP receive window size to be larger than the present limit of 64K bytes, by defining an implicit scale factor, which is used to multiply the window size value found in the TCP header to obtain the true window size. The Window Scale option is carried in SYN segments during the connection setup phase.
If the TCP Timestamp option is enabled, the sender places a time stamp in every TCP segment, and then the receiver sends the time stamp back in the acknowledgment, allowing the sender to calculate the Round Trip Time (RTT) for each acknowledgment. This RTT estimation helps TCP to discard received duplicate segments, to calculate the retransmission timer, and to decide whether to start the slow-start process. The presence of the TCP Timestamp option also allows TCP to perform Protection Against Wrapped Sequences (PAWS). PAWS assumes that every received TCP segment contains a TCP time stamp whose values are monotonically non-decreasing in time. A segment can be discarded as an old duplicate if it is received with a time stamp less than a time stamp recently received on this connection.
The TCP Timestamp option can be enabled for a system-wide effect through the INET configuration utility 'inetcfg -set timestmp 1'. It can also be retrieved and set for each socket through the TCP_TIMESTMP socket option. Similarly, the Window Scale option can be enabled system-wide through 'inetcfg -set winscale 1,' and can also be retrieved and set for each socket through the TCP_WINSCALE socket option.
By default, the system-wide Window Scaling option is enabled and the system-wide TCP Timestamp option is disabled. Both options, if set, can be traced through the IPTRACE utility and can be displayed in the IPFORMAT dump.
For more details on these two TCP extensions, refer to RFC 1323 TCP Extensions for High Performance.