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

Joe Touch <touch@isi.edu> Thu, 16 February 2017 21:10 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>, "Templin, Fred L" <Fred.L.Templin@boeing.com>, "gen-art@ietf.org" <gen-art@ietf.org>
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From: Joe Touch <touch@isi.edu>
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Date: Thu, 16 Feb 2017 13:09:40 -0800
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On 2/16/2017 11:59 AM, Brian E Carpenter wrote:
> On 17/02/2017 04:59, Stewart Bryant wrote:
>>
>> On 14/02/2017 23:00, Templin, Fred L wrote:
>>> Unless there is operational assurance of
>>> some size X>1280, however, tunnels have to use fragmentation to
>>> guarantee that - at a minimum - packets up to 1280 will get through.
>> In that case there really needs to be a note about MPLS.
>>
>> You can fragment into an IP tunnel, but not an MPLS tunnel, because you 
>> cannot fragment the payload as you can in IPv4 and you cannot fragment MPLS.
> I'm confused. A tunnel end point that accepts IPv6 packets MUST accept packets
> of 1280 bytes (or shorter) and MUST emit them. How it gets them through the
> tunnel is irrelevant - if it's an ATM tunnel it has to chop them into 48 byte
> fragments and re-assemble them at the other end - if it's an avian carrier tunnel
> it might have to use several pigeons per packet*. None of this matters to the IPv6
> nodes concerned; the physical MTU of the tunnel technology is irrelevant except
> to the tunnel end points.
There are too many systems that try to optimize the link MTU to
represent the size of the payload of the link source, rather than the
reassembly limit of the link receiver.

It's trying to make PMTUD work multiple layers down rather than stopping
at the IP layer (which I think is where it should stop).

Joe