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

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Date:	Tue, 24 Apr 2012 22:10:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Maciej Żenczykowski <maze@...gle.com>
Cc:	Tore Anderson <tore@....no>, 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

On Tue, 2012-04-24 at 12:49 -0700, Maciej Żenczykowski wrote:
> Why do we refuse to set ipv6 mtu's below 1280?
> how is what we do any better?

I guess you didnt read Tore use case.

Thats the standard : http://tools.ietf.org/html/rfc2460#section-5

   In response to an IPv6 packet that is sent to an IPv4 destination
   (i.e., a packet that undergoes translation from IPv6 to IPv4), the
   originating IPv6 node may receive an ICMP Packet Too Big message
   reporting a Next-Hop MTU less than 1280.  In that case, the IPv6 node
   is not required to reduce the size of subsequent packets to less than
   1280, but must include a Fragment header in those packets so that the
   IPv6-to-IPv4 translating router can obtain a suitable Identification
   value to use in resulting IPv4 fragments.  Note that this means the
   payload may have to be reduced to 1232 octets (1280 minus 40 for the
   IPv6 header and 8 for the Fragment header), and smaller still if
   additional extension headers are used.

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