Add section on multicast via Proxy and its limitations

[esko.dijk@…]

Add a section to explain the issues & limitations of doing CoAP multicast requests via a CoAP Proxy. Goal is to warn and guide implementers in this subject. Also such a section would provide a placeholder/reminder of the problems that might be addressed further in the future.

(Based on IETF 85 CoRE WG meeting discussion on Multicast CoAP requests via a Proxy.)

Discussion summary, for reference

• aggregation of responses in a proxy is difficult, since it doesn't know when to stop aggregating responses.
• SIP tried to deal with this problem but didn't solve it.
• There may be an expectancy of CoAP client to get single response back from proxy, not multiple. The client in this case may not even know what it could expect. It may not even know the Proxy-URI contains a multicast target.
• presented via-proxy use case exposes gap in the core-coap specification regarding multicast via proxy. (Note: Expectation is that core-coap won’t address it in the first planned RFC release.)

[esko.dijk@…]

Done in [1276]