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Re: [homenet] DNCP/HNCP Revisited

Michael Richardson <mcr+ietf@sandelman.ca> Wed, 18 September 2019 19:32 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
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Date: Wed, 18 Sep 2019 15:32:33 -0400
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Subject: Re: [homenet] DNCP/HNCP Revisited
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Ray Hunter (v6ops) <v6ops@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+IETF@sandelman.ca>, Sandelman Software Works
 -= IPv6 IoT consulting =-

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