Ray Hunter (v6ops) <v6...@globis.net> wrote:
> First observation: Running HNCP over L2TPv3 breaks HCNP because L2TPv3
breaks
> UDP fragmentation (works as designed).
> The L2TPv3 tunnel has a lower MTU than the local LAN, and does not report
> ICMP PTB, so HNCP packets in one direction get through, but replies get
> dropped.
> Early drafts of HNCP stated that UDP fragmentation would not be broken in
the
> Homenet for the foreseeable future. Well I managed to break that ;)
(Why are we using a L2 tunnel? So that a few of us, including Ray, can hack on
stuff together)
> Changing the MTU on the LAN interfaces of my routers to 1280 brought
> everything back to normal, as expected.
I think that the right answer is for the software to assume a 1280 MTU.
Someone could write an HNCP extension to do PLPMTUD inside afterwards to
increase the size if desired.
> Question: If Homenets are moving to flat L2 meshes over foo, as some have
> said, will this impact HNCP?
Unlikely, in my opinion, because "good" L2 meshes will preserve the 1500 byte
MTU.
> Question: As a simple mitigation, is there any way of manually signalling
to
> the kernel that ALL UDP packets on port 8231 should assume an PMTU of 1280
> octets?
> That wouldn't require any specification change and would allow HNCP to
work
> reliably in the presence of tunnels and varying MTU's that don't match the
> local interface MTU.
I think that the HNCP code can do this.
--
Michael Richardson <mcr+i...@sandelman.ca>, Sandelman Software Works
-= IPv6 IoT consulting =-
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