Freebsd Tcp Bug

18 Jul 2007

We found an interesting bug in the FreeBSD 6.2 TCP stack at work this week. Full details are available in this thread on the freebsd-net mailing list, but essentially it caused the initial congestion window on the majority of connections to start at 1 segment size instead of 3 as recommended by RFC 3390.

With most hosts using delayed ACKs these days, a starting window of 1 segment means there will be a pause while the other end waits for a second packet or a timer expiry before sending an ACK. If the receiving host is FreeBSD the pause defaults to 100ms - it may differ on other systems.

For short flows (web, email, etc), 100ms is a significant amount of time for there to be no data transfer - particularly on a low RTT link like a LAN.

The bug was fixed a few months ago for FreeBSD 6.3 and 7.0, but from a quick check of the CVS logs it’s existed since at least the 4.X days.