Re: NAT stops forwarding ACKs after PMTU discovery

Corey Hickey
2013-08-19T18:22:30Z

On 2013-08-19 05:33, Christoph Paasch wrote:
> Hello,
>
> I would say, the problem is due to a sequence-number rewriting
> middlebox, who does not correctly handle the SACK-blocks.
>
> In remote.pcap, you see:
> Packet#10: A Dup-ACK: ACK 1005503816, SACK: 1005505184-1005505648
> The SACK actually covers Packet#9
>
> In tun0.pcap, you have:
> Packet#9: The one that is SACK'ed: SEQ: 3514869772
> Packet#11: The Dup-ACK: ACK: 3514868404, SACK: 3570452498-3570452962
>
> You can see that the SACK-block is not really aligned with the ACK-numbers.
>
> Netfilter is probably dropping the Dup-ACK, because the SACK-block is
> acknowledging unseen data.
>
>
> There are middleboxes out there that randomize the sequence numbers, due to
> an old bug in Windows, where the initial sequence number was not
> sufficiently randomized. There are some of these middleboxes, who simply do
> not support SACK, thus modify only the sequence numbers of the TCP-header
> (https://supportforums.cisco.com/docs/DOC-12668#TCP_Sequence_Number_Randomization_and_SACK).
>
> This results in a TCP retransmission timeout on the sender, because of
> the invalid SACK-blocks, the duplicate ACKs are not accounted. This
> completly kills the performance, as you can see in our presentation given at
> IETF87: http://tools.ietf.org/agenda/87/slides/slides-87-tcpm-11.pdf

This makes good sense. I normally look at these in wireshark with
relative sequence numbers turned on, and I see in bad/tun0.pcap that the
SACK values are very high, but are normal in bad/remote.pcap.

I see the same thing in good/tun0.pcap, though, so I don't fully
understand what is making it work sometimes and not others.

I will show this thread and the Cisco docs to the network engineer at
work, and maybe we can get the SEQ randomization turned off (or at least
test it).

> We have a patch that accounts DUP-ACKs with invalid SACK-blocks effectively
> as duplicate acknowledgments. I can send the patches, if the
> netdev-community is interested in accepting these upstream.

I'll keep my eye out and test them if they come up.

Thanks,
Corey