netdev - Re: high rate injection and fragmentation

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Date:	Mon, 9 Feb 2009 18:18:50 +0100
From:	Raúl Hernández <rauhersu@...il.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	netdev@...r.kernel.org
Subject: Re: high rate injection and fragmentation

Hi Ilpo,

Thanks for your answer, I did not know about that feature. Looking at
my settings:

# ethtool -k eth2
...
tcp segmentation offload: on

I think ethereal hooks up before reaching the TCP offload which is
handled by the NIC.

Best regards,
Raul

On Mon, Feb 9, 2009 at 5:09 PM, Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> wrote:
> On Mon, 9 Feb 2009, Raúl Hernández wrote:
>
>> I am trying to understand the behavior of the IP stack regarding
>> fragmentation when injecting at high speed ratio : so far I am seeing
>> that TCP starts multiplexing the application level protocol (in my
>> case, diameter: previously 1 diameter message = 1 TCP message, when
>> the rate increases, multiple diameter messages are coalesced in one
>> TCP messages - similar to a Nagle control -).
>>
>> At that very moment is when we start finding issues regarding packet
>> length checking with ethereal: frames start growing due the multiplex
>> above and ethereal shows captured packets >3000 bytes ! (IP total
>> lenght makes sense as well according to that figure). The thing is
>> that I am working under ethernet law (1500 bytes) and I have checked
>> through iproute this is the MTU set for the link (neither
>> DONT_FRAGMENT ip options for the socket regarding nor MTU
>> modifications have been introduced) so my understanding is that no
>> packet > 1500 bytes should be allowed with my configuration (would
>> need fragmentation).
>>
>> Checking the IP header with ethereal, "dont fragment" is set and
>> "fragment offset"=0. IOW, It seems that IP is not fragmenting the
>> packet although > MTU which I am not able to understand.
>>
>> Any idea why Linux tries to work with MTU > 1500 bytes when not
>> configured explicitly to do so (may I have missed more
>> options/checkings) ? Hope not to be cheated by Ethereal ...
>
> I suppose you refer here to what GSO is doing though the explanation
> is so fuzzy that I lost track of it in the middle. ...You needs
> ethtool to configure it.
>
> --
>  i.



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