HP-UX TCP/IP Performance White Paper, March 2008

preventing TCP from being able to back-off its retransmission timer far enough to be at or above the actual

round-trip time of the network.

tcp_rexmit_interval_min:

Lower limit for computed round trip time-out.

Unless you know that all TCP connections from the system are going

through links where the RTT is greater than 500 milliseconds, and are

also _highly_ variable in their RTTs you should not increase this

value. If you increase this value, and there are actual packet losses,

you could have a drop in throughput as TCP could sit longer waiting

for a TCP retransmission timer to expire.

Similarly, unless you have concrete proof that the RTT is never more

than 500 milliseconds, you should not decrease this value as it could

result in spurious TCP retransmissions which could also decrease

throughput by keeping the TCP congestion window artificially small.

[1,7200000] Default: 500ms

tcp_sack_enable:

Allows recipients to selectively acknowledge out-of-sequence

data. The TCP sender can then retransmit only the lost segments

and adjust its send window to reflect the actual amount of

received data.

The use of TCP SACK is controlled by the ndd parameter

tcp_sack_enable. Supported parameter values are:

0: Local system never uses SACK.

1: Local system sends the SACK option in SYN packet

2: Local system enables SACK if remote system negotiates

the use of SACK in SYN packet (default)

Negotiation of the use of SACK is done by sending TCP SYN packets

with an Option Kind value of 4 to indicate that the system can

receive (and process) SACKs.

TCP packets with SACK information will have an Option Kind value

of 5. [0-2] Default: 2

SACK or "Selective ACKnowledgment" is a TCP feature intended to increase performance for data transfers

over lossy links. It accomplishes this by extending TCP's original, simple "ACK to the first hole in the data"

algorithm with one that can describe which segments past the first lost segment are missing. This

information, sent from the receiver to the sender, allows the sender to both retransmit lost segments sooner,

and avoid retransmission of segments which were not lost (those segments received, coming after the first

hole in the sequence space).

The default value is a somewhat conservative value of two (2) - the system will not initiate the use of SACK.

However, if the remote initiates the connection to us, and asks for SACK, we will honour that request.

A value of one (1) should be used if you want the system to use SACK for those connections initiated from

the system itself (i.e. applications on the system calling connect() or t_connect()).