Re: Genart telechat review of draft-ietf-6man-rfc1981bis-06

[Joe Touch <[e-mail]>]

Hi, Stewart,


On 4/26/2017 1:48 AM, Stewart Bryant wrote:
>
>
> On 25/04/2017 19:26, Joe Touch wrote:
>> Hi, Stewart,
>>
>> ...
>>
>>> SB>
>>> SB> Otherwise I would have thought that this was entirely a matter
>>> SB> for the host whether it wanted to use a Path MTU below the IPv6
>>> SB> link minimum. Nothing breaks if the host takes a more conservative
>>> SB> decision.
>> I don't agree; the host at that point is violating RFC2460. It should
>> never think that an IPv6 link or path with an MTU below what RFC2460
>> requires is valid.
>>
>> Joe
>>
>
> That is as maybe, but a host can do more or less what it wants, so
> this is surely an
> unenforceable constraint, or are you telling me that the receiving
> host MUST drop a
> fragment that is shorter than this? In which case the question whether
> in practice
> they do, and whether such a constraint is reasonable.

A "path MTU" is a value calculated from information from various sources
(attached links, ICMP messages, and perhaps other information), but IMO
it's never appropriate to set a "path MTU" smaller than the limit
established by IPv6 for a single link.

Individual packets and fragments can be smaller than the MTU, of course.
Nothing forces fragments to push up against any MTU limit at all. But I
would not describe that has a host changing its path MTU; it's just
sending packets.

Joe