amd64 tcp weirdness

[Jacco Braat]

Hello,

i'm using RELENG_6 on amd64 and have trouble with downloads.

I think this is related to the window scaling feature.

fetch http://www.google.com/intl/en_com/images/logo_plain.png
generates 5 packets for the HTTP GET request, and lots of DUP ACK packets.
there is no Window Scale option in the SYN ACK reply.

fetch http://www.freebsd.org/layout/images/logo-red.png
generates only 1 packet for the HTTP GET request.
Window Scale = 0 in the SYN ACK reply

on i386 also RELENG_6 no such differences.

setting net.inet.tcp.recvspace to a smaller value also gives normal results.

I noticed this behavior because my Realtek 8168B got stuck on some
websites, a Intel Pro/1000 did not. With both cards the tcpdumps are
similar.

Thanks,
Jacco