netdev - Re: [PATCH] net: sctp: Fix data chunk fragmentation for MTU values which are not multiple of 4

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Date:	Wed, 04 Sep 2013 13:20:58 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	alexander.sverdlin@....com
Cc:	linux-sctp@...r.kernel.org, vyasevich@...il.com,
	matija.glavinic-pecotic.ext@....com, netdev@...r.kernel.org
Subject: Re: [PATCH] net: sctp: Fix data chunk fragmentation for MTU values
 which are not multiple of 4

From: Alexander Sverdlin <alexander.sverdlin@....com>
Date: Mon, 02 Sep 2013 15:58:25 +0200

> From: Alexander Sverdlin <alexander.sverdlin@....com>
> 
> net: sctp: Fix data chunk fragmentation for MTU values which are not multiple of 4
> 
> Initially the problem was observed with ipsec, but later it became clear that
> SCTP data chunk fragmentation algorithm has problems with MTU values which are
> not multiple of 4. Test program was used which just transmits 2000 bytes long
> packets to other host. tcpdump was used to observe re-fragmentation in IP layer
> after SCTP already fragmented data chunks.
 ...
> Here problem in data portion limit calculation leads to re-fragmentation in IP,
> which is sub-optimal. The problem is max_data initial value, which doesn't take
> into account the fact, that data chunk must be padded to 4-bytes boundary.
> It's enough to correct max_data, because all later adjustments are correctly
> aligned to 4-bytes boundary.
> 
> After the fix is applied, everything is fragmented correctly for uneven MTUs:
 ...
> The bug was there for years already, but
>  - is a performance issue, the packets are still transmitted
>  - doesn't show up with default MTU 1500, but possibly with ipsec (MTU 1438)
> 
> Signed-off-by: Alexander Sverdlin <alexander.sverdlin@....com>

Applied, thanks.
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