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

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Date:	Tue, 24 Apr 2012 14:50:54 -0700
From:	Maciej Żenczykowski <maze@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.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, Apr 24, 2012 at 1:10 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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

... is not required to reduce...

but doesn't that mean it _may_ reduce anyway?
and thus make life easier on the guy who will have to fragment (and
thus he won't have to fragment).

[of course you'd still need to lose 8 bytes for the frag header...]

>   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.

I just don't see what not decreasing mtu below 1280 buys us.
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