TCP Selective Acknowledgment Support

The TCP selective acknowledgment (TCP SACK) provides the support described in RFC 2018 to solve the problems related to congestion and multiple packet drops especially in applications making use of TCP large windows (RFC 1323) over satellite links or transcontinental links. See RFC 2018 for complete details on TCP SACK.

The configuration parameter is associated with the TCP device, /dev/tcp, and can be inspected or modified using ndd(1M). Normally, this parameter would be set in one of the shell scripts executed by init(1M) when the system is booted (see init.d(4) for information on how to add a new script).

The available parameter and its meaning is shown below.

tcp_sack_permitted

Specifies whether SACK is permitted. The default is 1. The available options are as follows:

0

TCP does not accept or send SACK information.

1

TCP does not initiate a connection with SACK_PERMITTED option. If the incoming request has SACK_PERMITTED option, TCP responds with SACK_PERMITTED option.

2

TCP initiates and accepts connections with SACK_PERMITTED option.

For additional information see the tcp(7P) man page.