netdev - Re: TCP fast retransmit issues

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Date:   Thu, 27 Jul 2017 23:53:35 -0700
From:   Christoph Paasch <christoph.paasch@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Willy Tarreau <w@....eu>, Klavs Klavsen <kl@...n.dk>,
        netdev <netdev@...r.kernel.org>
Subject: Re: TCP fast retransmit issues

Hello,

On Wed, Jul 26, 2017 at 7:32 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-07-26 at 15:42 +0200, Willy Tarreau wrote:
>> On Wed, Jul 26, 2017 at 06:31:21AM -0700, Eric Dumazet wrote:
>> > On Wed, 2017-07-26 at 14:18 +0200, Klavs Klavsen wrote:
>> > > the 192.168.32.44 is a Centos 7 box.
>> >
>> > Could you grab a capture on this box, to see if the bogus packets are
>> > sent by it, or later mangled by a middle box ?
>>
>> Given the huge difference between the window and the ranges of the
>> values in the SACK field, I'm pretty sure there's a firewall doing
>> some sequence numbers randomization in the middle, not aware of SACK
>> and not converting these ones. I've had to disable such broken
>> features more than once in field after similar observations! Probably
>> that the Mac doesn't advertise SACK support and doesn't experience the
>> problem.
>
> We need to check RFC if such invalid SACK blocks should be ignored (DUP
> ACK would be processed and trigger fast retransmit anyway), or strongly
> validated (as I suspect we currently do), leading to a total freeze.

quite some time ago this issue with sequence number randomizing
middleboxes came already up
(http://marc.info/?l=netfilter-devel&m=137691623129822&w=2). From what
I remember, the RFC does not say that invalid SACK blocks should  be
strongly validated. So, trigger dup-ack retransmission seems fine.

I had some patches at the time that ignored invalid sack-blocks and
allowed fast-retransmit to happen thanks to the duplicate acks:
https://patchwork.ozlabs.org/patch/268297/
https://patchwork.ozlabs.org/patch/268298/


Cheers,
Christoph