Subject: Re: Anyone working on TCP SACK?
To: Dave Huang <>
From: Pavel Cahyna <>
List: tech-net
Date: 05/20/2003 11:13:19
> There has been talk for at least 7 years about how it'd be good if
> NetBSD supported TCP selective ACKs... but is anyone actively working
> on it? Sorry, I'm not volunteering; just curious :)


SACK is supported in OpenBSD. But, at least in 3.1, it has some bugs,
because uploading a large file from an openbsd machine through a link
with mild packet loss was very slow. Doing the same from linux was as
fast as without the packetloss. From netbsd, it was faster than from
openbsd, but still much slower than in the case without the packetloss.
Turning off SACK in openbsd made it as fast as netbsd. 

If I turned off SACK in linux, the speed remained the same, and if there
was no packetloss, all three systems were roughly equal.

I get those conclusions from this:
1. SACK in openbsd (3.1) is buggy and makes things only worse.
2. linux is far better than *BSD on links with packetloss, and the cause
   is not its SACK support.

I can post exact numbers and tcpdump traces, if someone is interested.

Bye	Pavel