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

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Date:	Wed, 25 Apr 2012 02:48:27 -0700
From:	Maciej Żenczykowski <maze@...gle.com>
To:	Tore Anderson <tore@....no>
Cc:	Eric Dumazet <eric.dumazet@...il.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

>> I get that we _choose_ to behave such, and I agree this adheres to
>> specs.
>
> "Chose" (past), not "choose" (present). ;-)

Details, but until we 'choose' to change it we continuously 'choose'
to have the current behaviour. ;-)

> This patch does not make this choice. This patch merely fixes a bug in
> the implementation of the choice that was made a long time ago.

yes, I wasn't saying this patch was bad, I was just wanting to point
that we should perhaps revisit the design choice.

>> But I'm not convinced that (even though this is allowed per RFC) this
>> is the right choice.
>
> That is a different issue entirely, but I don't disagree with you. A
> "min_pmtu" sysctl or something like that would be useful.

I don't really know what the default value should be?  Something around 500?
[to handle IPv4s min mtu of 576?]

Do we have any idea what values of small mtu actually show up in practice?

> Actually, in IPv6, fragmentation *must* be performed by end hosts,
> routers (including tunnel end points) *cannot* fragment.

Yes, I miss-phrased that, that's what I meant.

> However, the use case for the allfrag feature is not handling tunnels,
> but IPv4<->IPv6 translation. The issue is that a IPv6 host may very well
> receive an ICMPv6 Packet Too Big indicating a PMTU of <1280 that was
> originally transmitted by an IPv4 router (as an ICMPv4 Need To Fragment)
> and underwent translation to IPv6.

Very good point, although that's basically kind of like half a tunnel ;-)

> In case you're interested, I have a slide deck below that explains the
> use case for IPv4<->IPv6 translation. Slide 25 is about the particular
> corner case where the allfrag feature is necessary. URL:
>
> http://fud.no/talks/20120417-RIPE64-The_Case_for_IPv6_Only_Data_Centres.pdf

Yes, I've seen this slide set a couple days ago - very good.
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