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