netdev - Re: TCP IPv4 strange retransmits

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Date:	Wed, 05 Mar 2008 14:04:39 +0100
From:	Arnd Hannemann <hannemann@...s.rwth-aachen.de>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Netdev <netdev@...r.kernel.org>
Subject: Re: TCP IPv4 strange retransmits

Ilpo Järvinen wrote:
> On Wed, 5 Mar 2008, Arnd Hannemann wrote:
> 
>> Ilpo Järvinen wrote:
>>
>>> No, if there's any skb which is more than fackets_out-tp->reordering from 
>>> the highest SACKed skb, it will be marked TCPCB_LOST (see 
>>> tcp_mark_head_lost & it's caller), and all LOST segments are retransmitted 
>>> by the earlier loop (for a while still as I'm going to very likely change 
>>> that in net-2.6.26, commits for consolidating both, nearly identical loops 
>>> are already in my local git and await some testing).
>>>
>>> Forwardretrans is only incremented when there isn't TCPCB_LOST set for a 
>>> segment and it doesn't apply in this case anyway because you have new data 
>>> to send (see the decision making for forward retransmits, it's well 
>>> commented btw).
>> Ah, I see. Thank you for clarifying.
>> However fackets_out is not so well documented ;-)
> 
> I think I've fixed this for 2.6.25... :-) :
> 
> ...
> /* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
>  * counter when SACK is enabled (without SACK, sacked_out is used for
>  * that purpose).
>  *
>  * Instead, with FACK TCP uses fackets_out that includes both SACKed
>  * segments up to the highest received SACK block so far and holes in
>  * between them.
>  *
>  * With reordering, holes may still be in flight, so RFC3517 recovery
>  * uses pure sacked_out (total number of SACKed segments) even though
>  * it violates the RFC that uses duplicate ACKs, often these are equal
>  * but when e.g. out-of-window ACKs or packet duplication occurs,
>  * they differ. Since neither occurs due to loss, TCP should really
>  * ignore them.
>  */
> static inline int tcp_dupack_heurestics(struct tcp_sock *tp)
> ...

Great :-) But shouldn't it read "heuristics" ?

> ...Though some FACK comments seem to be saying something else still.
> 
>> But it now makes all sense (with dump order):
>> An ACK 19225 arrives with SACK block {27745:29165}, so fackets_out becomes 
>> ~6 ((27745-19225)/1450)
>> tp->reordering is 3 at this time so he starts to retransmit.
>> However some SACK ACK comes early enough so he stops at 4 retransmits.
>> Or something like that...
> 
> Another thing you should consider is reordering detection which hopefully 
> worked at 13:08:20.667529 through the newly discored SACK block which is 
> _lower_ than the highestmost SACK block received so far. That results in 
> FACK -> RFC3517, FACK is built on inorder assumptions and whenever we find 
> that untrue, e.g., due to SACK/ACK for non-rexmit when something larger 
> has been confirmed received we disable it. Ah, but this was 2.6.24.y? It 

Yes, it was 2.6.24.2. Actually you can see reordering detection at work here[3],
the tool[4] we are using to measure TCP throughput samples the tcp_info struct and the
column #reor should reflect tp->reordering.
First it is 3 then it grows up to 16. Off course this is only a hint because
tcp_info is only sampled every 50ms in this example, but at least it shows that some
reordering detection took place...

> doesn't yet do RFC3517 IIRC, but has something remotely resembling 
> newreno, but only for the first packet because the next cumulative ACK may 
> often trigger timedout loop which basically marks everything lost (I don't 
> remember if the latter was changed to occur only with FACK ages ago or 
> not).

Not sure if I understood this. Will have to look into this some more.

> 
>>>> Tcpdump:
>> Sorry, this was just bogus. Just wanted to point out the timestamp 
>> differences and made a wrong example. Screen full of numbers... ;-)
> 
> I thought so :-).
> 
> ...Large, nearly equal numbers in two dimensions, maybe at some day 
> I wake up and notice I've read them too long noticing that capturing 
> this kind of things is no longer a problem to me... :-/
> 

[3] http://www.umic-mesh.net/~hannemann/strange-reorder/flowgrind.output
[4] http://www.umic-mesh.net/research/tcp/flowgrind.html
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