netdev - Re: [PATCH net-next] ipv6: RTAX_FEATURE_ALLFRAG causes inefficient TCP segment sizing

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Date:	Fri, 27 Apr 2012 00:03:53 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	tore@....no, netdev@...r.kernel.org, maze@...gle.com,
	therbert@...gle.com
Subject: Re: [PATCH net-next] ipv6: RTAX_FEATURE_ALLFRAG causes inefficient
 TCP segment sizing

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 24 Apr 2012 19:37:38 +0200

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Quoting Tore Anderson from :
> https://bugzilla.kernel.org/show_bug.cgi?id=42572
> 
> When RTAX_FEATURE_ALLFRAG is set on a route, the effective TCP segment
> size does not take into account the size of the IPv6 Fragmentation
> header that needs to be included in outbound packets, causing every
> transmitted TCP segment to be fragmented across two IPv6 packets, the
> latter of which will only contain 8 bytes of actual payload.
> 
> RTAX_FEATURE_ALLFRAG is typically set on a route in response to
> receving a ICMPv6 Packet Too Big message indicating a Path MTU of less
> than 1280 bytes. 1280 bytes is the minimum IPv6 MTU, however ICMPv6
> PTBs with MTU < 1280 are still valid, in particular when an IPv6
> packet is sent to an IPv4 destination through a stateless translator.
> Any ICMPv4 Need To Fragment packets originated from the IPv4 part of
> the path will be translated to ICMPv6 PTB which may then indicate an
> MTU of less than 1280.
> 
> The Linux kernel refuses to reduce the effective MTU to anything below
> 1280 bytes, instead it sets it to exactly 1280 bytes, and
> RTAX_FEATURE_ALLFRAG is also set. However, the TCP segment size appears
> to be set to 1240 bytes (1280 Path MTU - 40 bytes of IPv6 header),
> instead of 1232 (additionally taking into account the 8 bytes required
> by the IPv6 Fragmentation extension header).
> 
> This in turn results in rather inefficient transmission, as every 
> transmitted TCP segment now is split in two fragments containing
> 1232+8 bytes of payload.
> 
> After this patch, all the outgoing packets that includes a
> Fragmentation header all are "atomic" or "non-fragmented" fragments,
> i.e., they both have Offset=0 and More Fragments=0.
> 
> With help from David S. Miller
> 
> Reported-by: Tore Anderson <tore@....no>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Maciej Żenczykowski <maze@...gle.com>
> Cc: Tom Herbert <therbert@...gle.com>
> Tested-by: Tore Anderson <tore@....no>

Applied.
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