tcp over slow links broken?
Bakul Shah
bakul at bitblocks.com
Mon May 12 06:56:27 UTC 2008
On Sun, 11 May 2008 12:07:34 PDT Matthew Dillon <dillon at apollo.backplane.com> wrote:
> Hmm. It looks like C has gone deaf, not B. B is retransmitting from
> sequence 4744 which is the last sequence that C acked. C is then not
> acking any further packets.
Yes indeed.
> 14:22:42.411144 IP B.55535 > C.ssh: . 7664:9124(1460) ack 2016 win 65535
> 14:22:42.411259 IP B.55535 > C.ssh: . 9124:10584(1460) ack 2016 win 65535
> 14:22:42.468350 IP C.ssh > B.55535: . ack 4744 win 65535
> 14:22:42.490556 IP C.ssh > B.55535: . ack 4744 win 65535
> 14:22:42.830171 IP B.55535 > C.ssh: . 4744:6204(1460) ack 2016 win 65535
...
>
> This sounds like a packet filter state issue. My guess is that
> PF running on B is getting confused. Either PF is getting confused,
> or the packet is getting munged somehow to the point where PF refuses
> to bridge it.
I had already tried this.
> The A->C path (the one that is working) is going through PF's NAT rules.
> The B->C path is probably going through a different set of PF rules.
>
> I suggest capturing a trace on C to see if C is actually receiving
> B's retransmissions.
Finally this evening thanks to my friend Rob Warnock's help
this got narrowed down quite a bit. We captured a trace on C
and saw that it was not seeing the [4744:6204) data range
packet or any of its retransmits. But this was a perfectly
valid packet on B (verified with tcpdump -v + manual header
checksumming). Then Rob recalled having run across mbuf
alignment issues in the past so to check for that I swapped
NICs around and the problem stayed with the NIC, an old DEC
21140 card!
So this was not related to pf or a slow link but most likely
due to mbuf misalignment (IIRC de requires aligned mbufs).
There is just one commit on if_de.c during past April.
Perhaps this is due to a side effect of that (bpf is not
given a packet during device attach) or perhaps some change
elsewhere.
Thanks for your & Julian's help!
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