Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets

Iljitsch van Beijnum <> Mon, 08 February 2010 20:26 UTC

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From: Iljitsch van Beijnum <>
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Date: Mon, 08 Feb 2010 21:27:00 +0100
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To: Dan Wing <>
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Subject: Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets
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On 8 feb 2010, at 18:46, Dan Wing wrote:

> For stateful translation, it is _possible_ for the translator
> to include MTU in its IPv4 and IPv6 tables, and do provide end-to-end
> PMTUD even for links below 1280.  To do this, a stateful translator
> would maintain the MTU for all IPv4 hosts that currently have a mapping,
> and go ahead and translate with DF=1.

So in this case the translator receives IPv6 packets with or without a fragment header that are 1280 bytes but the IPv4 destination has a path MTU of (say) 576 bytes. This means the translator has to fragment.

So essentially this means doing more work (keeping PMTUD state for the IPv4 path) in order to do even more work later (fragment). Not sure how that makes sense... If you simply set DF=0 for packets < 1280 bytes then the people who thought it was a good idea to set an unreasonably small MTU are the ones who get to fragment, which to me seems how it should be.

(Although in practice few fragments will happen because in IPv4 PMTUD is only supported for TCP and TCP has the MSS option.)

> Then, if an IPv4 ICMP PTB is
> received and describes an MTU smaller than 1280, go ahead and translate
> that to ICMPv6 (in the hopes of hitting the ~45% of hosts that will
> successfully process it),

You can always do that, yes.