bin/112882: systat -tcp does not show tcp rexmit on sack enabled
sockets (patch included)
pr at isprime.com
Wed May 23 03:20:02 UTC 2007
>Synopsis: systat -tcp does not show tcp rexmit on sack enabled sockets (patch included)
>Arrival-Date: Wed May 23 03:20:01 GMT 2007
>Originator: Phil Rosenthal
>Release: FreeBSD 6.2-STABLE amd64
FreeBSD bmw2.isprime.com 6.2-STABLE FreeBSD 6.2-STABLE #0: Tue May 22 22:20:23 EDT 2007 root at bmw2.isprime.com:/usr/obj/usr/src/sys/BMW2 amd64
FreeBSD's "systat" tool has a mode to display statistics for all tcp
sessions on the system. This mode is called by running "systat -tcp".
When a packet is retransmitted due to duplicate ack packets, or a timeout
waiting for an ack, tcps_sndrexmitpack is incremented.
When a packet is retransmitted due to a selective acknowledgement (sack),
tcps_sack_rexmits is incremented instead.
systat -tcp only shows tcps_sndrexmitpack, and given that most clients today
support sack, the user is falsely informed that there is a much lower amount
of packet loss/retransmits.
Additionally, the 9 digit values are easily overflowed on a server serving
hundreds of megabits of traffic which is becoming more commonplace given
the performance and wide availability of gig-e chipsets.
The patch referenced below fixes both of these issues.
1) Enable sack on both a client and a server, and set up a tcp socket.
2) Cause packet loss (you can use dummynet for this)
3) Watch systat -tcp
patch < systat-sack-rexmit.diff
This patch is built against FreeBSD 6.2-STABLE. Earlier builds will fail with
More information about the freebsd-bugs