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

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Date:	Wed, 25 Apr 2012 12:45:02 +0200
From:	Tore Anderson <tore@....no>
To:	Maciej Żenczykowski <maze@...gle.com>
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

* Maciej Żenczykowski

>> The sensible default would be either 1280 (and keep allfrag 
>> feature), or
>> the minimum IPv4 PMTU currently enforced by the kernel + 20 bytes 
>> (to
>> compensate for the larger IPv6 header size). I don't know what the 
>> current
>> minimum PMTU is.
>
> I'd actually go with min IPv4 PMTU - 20, not + 20, see below for why.

I think you forgot to include the explanation why. :-)

> Hmm, it may be best to honour PMTUs below 1280 to some small but not
> too small value (512?), and below that give up, say mtu remains 1280
> but still add the frag header.

That is also possible, yes.

> Hmm, I thought one way to implement an IPv6 over IPv4 tunnel was to 
> basically
> rely on IPv4 fragmentation, hence you would actually be using the 
> same mechanism
> as for an IPv6-IPv4 translator, then you would be tunneling the IPv6 
> packet over IPv4,
> so your IPv6 mtu would be 20 less than the v4 one, not 20 more which 
> you get if you
> replace the v6 header with a smaller v4 one.
>
> Anyway, this certainly seems like an ipv6-in-ipv4 tunneling mechanism
> which would currently work, wouldn't it?

I suppose. This would be invisible to IPv6, though - the fragmentation 
and reassembly
happens at a lower layer than IPv6. Same as ATM for example. Situation 
is described
by RFC 2460:

«On any link that cannot convey a 1280-octet packet in one piece, 
link-specific
fragmentation and reassembly must be provided at a layer below IPv6.»

> (re: Eric's patch, I think it should protect itself against malicious
> PMTU messages with too small MTUs, like 0 or 1 or 68 [not enough for
> timestamped ipv6/tcp)

Does this happen for IPv4, I wonder? IMHO, it makes sense to keep the 
the minimum
PMTUDs allowed in sync. If PMTUD=1 is allowed in IPv4, and this is not 
problematic,
I don't see why it couldn't be allowed in IPv6 either.

Tore


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