On 19/08/13 - 06:24:17, Eric Dumazet wrote: > On Mon, 2013-08-19 at 14:33 +0200, 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 > > > > 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. > > > > You mean a netfilter patch, a tcp patch, or both ? It's a TCP-patch, that interprets duplicate-acks with invalid SACK-blocks as duplicate acks in tcp_sock->sacked_out. Christoph -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
