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

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Date:	Thu, 26 Apr 2012 12:32:17 +0200
From:	Tore Anderson <tore@....no>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Maciej Żenczykowski <maze@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH net-next] ipv6: RTAX_FEATURE_ALLFRAG causes inefficient TCP segment sizing

* Eric Dumazet

> Yes, in fact we are going to keep the allfrag feature, and arm it if 
> we
> receive an mtu < 512.

One thing to think about if you go down this path is to keep track of 
the PMTU received in the ICMPv6 PTB. If you activate the allfrag feature 
on a destination because you received a PTB with PMTU=400, it's not 
useful to include the Fragmentation header in packets <400 bytes in 
size, for example.

With the current code, even tiny packets without any L4 payload (e.g. 
TCP SYN/ACK) will have the Fragmentation header added. That's not 
optimal, but necessary since the current code does not keep track of the 
actual PMTU if it is <1280 as far as I can tell (instead it discards the 
received PMTU value, clamps the effective PMTU to 1280, and enables 
allfrag).

Tore
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