Re: Last Call: <draft-ietf-6man-rfc1981bis-04.txt> (Path MTU Discovery for IP version 6) to Internet Standard

Joe Touch <> Tue, 07 February 2017 20:19 UTC

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Subject: Re: Last Call: <draft-ietf-6man-rfc1981bis-04.txt> (Path MTU Discovery for IP version 6) to Internet Standard
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From: Joe Touch <>
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Date: Tue, 7 Feb 2017 12:18:16 -0800
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On 2/7/2017 12:06 PM, wrote:
> Joe,
>>> I'm very much in favour of working on better ways of doing Path MTU discovery.
>>> A blanket statement of "use "PLMTUD" seems very premature though.
>> The point is that this document fails to indicate the current state of PMTUD. It correctly notes that:
>>    An extension to Path MTU Discovery defined in this document can be
>>    found in [
>> RFC4821
>> ].  It defines a method for Packetization Layer Path
>>    MTU Discovery (PLPMTUD) designed for use over paths where delivery of
>>    ICMP messages to a host is not assured.
>> IMO, it fails to note that this case - where ICMP messages are assured along a path - is effectively a unicorn except within systems maintained by a single entity.
>>> RFC1981 has 70 citations:
>>> Could you expand on your view of how this pertains to advancing RFC1981?
>> It's called last call input. My input is that this document needs to be more realistic in noting that, for all intents, ICMP-based MTU discovery isn't viable and that other methods need to be *expected*, not just that they're available.
> Right, but if you are correct that ICMP-based MTU discovery is not viable then this document should not be advanced.
Not necessarily. It's fine to update a protocol that works inside an
enterprise or other controlled environment.

> At the same time for many protocols we have nothing else. An operator can break any protocol if that's their policy. And that's the breakage we're talking about here, not any issues with the protocol specification.

Well, it's a problem with the protocol spec that it's susceptible to
operator interference.

> There is a philosophical aspect of this. (Which I'm not the best person to represent as I skipped my University studies in philosophy and used the student loan to buy a motorcycle... (and only read the art of motorcycle maintenance years later) )
> This is a tussle. The IETF specifies protocols under the assumption that operators treat those protocols largely as specified. The 5-10% failure of PMTUD messages may be caused by misconfiguration, misunderstanding or mis-intent... Many of our protocols are suffering from the same fate. Should the IETF adjust all its protocols to be as middlebox friendly as possible? You can make this argument about IPv6 fragments, any packet with IPv6 extension headers, IPv4 fragments. Or anything but TCP port 443/80 and UDP port 53 for that matter. Are we as the IETF going to continue standardising protocols to work as best as they possible can, ignoring protocol abuse, or are we going to bend over and do whatever it takes to make it work for those 5-10% who've actively broken the protocol? What about the 90-90% where the protocols work as expected?

What I'm suggesting is a lot simpler - just text that makes it clear
what the current state of deployment of support (or interference) for
this protocol is.

I appreciate that you want to not point at PLPMTUD because it's not
widely supported, but **for the same reason** this doc should not hold
up this solution without pointing out very clearly that it basically
isn't going to be work.