Installing and Administering LAN/9000 Software

21
RFC 1323 (TCP Extensions for High Performance) specifies TCP
extensions intended to benefit high-speed networks and networks
with both large bandwidths and long delays, such as high capacity
satellite networks or long-distance fiber-optic networks.
Scaled windows and timestamps were available beginning in release
10.30, but these features were not documented, and the ndd
parameters to disable them were not supported.
Scaled Windows
Scaled windows increases the window size from 2**16 (65K) bytes to
2**32 bytes by allowing a scale factor (multiplier) to be applied to the
window field.
TCP will always offer to use Window Scaling in SYN and SYN ACK
packets. The scale factor will be 1 (window size remains 2**16) unless
the application is using a receive buffer that is larger than 2**16.
Window Scale options have the Option Kind value of 3.
Timestamps
RFC 1323 defines a timestamps option that can be sent with every
segment. The timestamps in the option are used for two purposes:
— More accurate RTTM (Round Trip Time Measurement), or the
interval between time a TCP segment is sent and the time return
acknowledgment arrives.
— PAWS (Protect Against Wrapped Sequences) on very high-speed
networks. On connections with large transmission rates where the
sequence number may wrap, the timestamps are used to detect old
packets.
The use of timestamps is controlled by the ndd parameter
tcp_ts_enable.
Supported parameter values are:
0: Never timestamp
1: Always initiate
2: Allow but don't initiate (Default)
Use of timestamps is requested by the initiator of a TCP connection
by sending a timestamps option (Option Kind 8) in the initial TCP
SYN packet.