Subject: [core] Which IPv6 multicast scopes for the "All CoAP Nodes" address?

From: "Dijk, Esko" <[email-masked]>
Date: Wed, 27 Nov 2013 13:17:25 +0000

Dear all,

in coap-18 it mentions for the IPv6 "All CoAP Nodes" address:

Note that there is a distinct multicast address
      for each scope that interested CoAP nodes should listen to; CoAP
      needs the Link-Local and Site-Local scopes only.

which could raise some interoperability concerns:

-          CoAP nodes are not obliged to join any of these addresses; so multicast discovery may not work in practice.
A client may be tempted to instead use the "All Nodes" link-local IPv6 multicast address to send out a CoAP request, since IPv6 / 6LoWPAN-ND requires joining this address.

-          CoAP nodes only join the link-local and/or site-local scoped address, not others (admin-local, or the new "realm-local" defined in draft-droms-6man-multicast-scopes-02)
So a discovering client using e.g. admin-local may not find any CoAP nodes.

My goal was to write some guidelines in groupcomm what IPv6 / IPv4 multicast addresses a CoAP server should join, and which addresses a discovering client should use.
For now I will base on the information that core-coap provides; adding that a server SHOULD join both the link-local and site-local "All CoAP nodes" IPv6 multicast addresses.

regards,
Esko

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