SACK problems

[Sam Jansen]

During some testing on an isolated network we have, I found some 
interesting behaviour from a FreeBSD 5.3 host using TCP SACK.

I've detailed this problem fully at:

    http://www.wand.net.nz/~stj2/nsc/emu_freebsd.html

PCAP traces and some screenshots from tcptrace graphs can be found at 
the above link to show what is happening. It looks to me like SACK 
blocks are being incorrectly generated in this example. I can't think of 
any valid reason why a SACK block would SACK from below the current ACK 
value to above it (which is the problem here).

Thoughts, anyone? Am I just wrong here and this is valid, expected 
behaviour?

Cheers,
--
Sam Jansen                                           sam at wand.net.nz
Wand Network Research Group             http://www.wand.net.nz/~stj2