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

2017-04-26 13:35:56
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