netdev - high rate injection and fragmentation

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Date:	Mon, 9 Feb 2009 16:20:34 +0100
From:	Raúl Hernández <rauhersu@...il.com>
To:	netdev@...r.kernel.org
Subject: high rate injection and fragmentation

Hi there,

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 ...

Thx !

Best regards,
Raul

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