Re: [Gen-art] Review of draft-ietf-6man-rfc1981bis-04

Joe Touch <touch@isi.edu> Wed, 15 February 2017 21:32 UTC

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Subject: Re: [Gen-art] Review of draft-ietf-6man-rfc1981bis-04
To: Brian E Carpenter <brian.e.carpenter@gmail.com>, Stewart Bryant <stewart.bryant@gmail.com>, Stewart Bryant <stewart@g3ysx.org.uk>, gen-art@ietf.org
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From: Joe Touch <touch@isi.edu>
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Date: Wed, 15 Feb 2017 13:32:20 -0800
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Hi, Brian,


On 2/15/2017 1:26 PM, Brian E Carpenter wrote:
> On 16/02/2017 10:12, Joe Touch wrote:
>> Brian (et al.),
>>
>>
>> On 2/10/2017 11:45 AM, Brian E Carpenter wrote:
>>>> practice the
>>>> Internet breaks the mechanism. However it breaks it is a way that seems
>>>> disruptive to some user traffic. The document is really guidance
>>>> one how hosts might use  ICMP for optimization, and arguable need
>>>> not be a standard at all.
>>> I think that's a mischaracterisation of the mechanism (and the draft).
>>> PMTUD is not an optimisation. Without it, you get black holes
>> PMTUD is an optimization to avoid fragmentation.
>>
>> Without it, you use fragmentation (which has overheads and other
>> consequences, notably for IPv4).
> In IPv6, you don't even know you *need* fragmentation without PMTUD,
> since only the source is allowed to fragment. (That was one of the
> many failure modes for 6to4, which is why the only safe approach was
> to use 1280 always.) 
A compliant IPv6 source can send 1500 byte packets that a source without
PMTUD, which would need to be fragmented based on the assumption that
the path MTU is at its required minimum of 1280.

A compliant IPv6 source can send larger packets, which would work if the
receiver supported larger reassembly limits than 1500. There is no
currently standard mechanism to determine the receiver reassembly limit
in IPv6, but this IS supported in TCP.

Joe