netdev - Re: TCP fast retransmit issues

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Date:   Fri, 28 Jul 2017 08:36:49 +0200
From:   Klavs Klavsen <kl@...n.dk>
To:     Willy Tarreau <w@....eu>
Cc:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Subject: Re: TCP fast retransmit issues

The network guys know what caused it.

Appearently on (atleast some) Cisco equipment the feature:

TCP Sequence Number Randomization

is enabled by default.


It would most definetely be beneficial if Linux handled SACK "not 
working" better than it does - but then I might never have found the 
culprit who destroyed SACK :)

Willy Tarreau wrote on 26-07-2017 16:38:
> On Wed, Jul 26, 2017 at 04:25:29PM +0200, Klavs Klavsen wrote:
>> Thank you very much guys for your insight.. its highly appreciated.
>>
>> Next up for me, is waiting till the network guys come back from summer
>> vacation, and convince them to sniff on the devices in between to pinpoint
>> the culprit :)
> That said, Eric, I'm a bit surprized that it completely stalls. Shouldn't
> the sender end up retransmitting unacked segments after seeing a certain
> number of ACKs not making progress ? Or maybe this is disabled when SACKs
> are in use but it seems to me that once invalid SACKs are ignored we should
> ideally fall back to the normal way to deal with losses. Here the server
> ACKed 3903858556 for the first time at 15:59:54.292743 and repeated this
> one 850 times till 16:01:17.296407 but the client kept sending past this
> point probably due to a huge window, so this looks suboptimal to me.
>
> Willy


-- 
Regards,
Klavs Klavsen, GSEC - kl@...n.dk - http://www.vsen.dk - Tlf. 61281200

"Those who do not understand Unix are condemned to reinvent it, poorly."
   --Henry Spencer