Re: [core] RFC 7252 - 8.2 - Multicast - Request / Response Layer, page 67, top

[Klaus Hartke <[e-mail]>]

Thomas Fossati wrote:
> I don't have an opinion yet, but I'd need some clarification about the
> above before feeling comfortable with the direction...

I guess there is some room for debate on what a CoAP endpoint with a
multicast IP address precisely means... RFC 7252 defines 224.0.1.187
as the "All CoAP Nodes" address. So I see at least two possible
interpretations for the endpoint 224.0.1.187:9999:

1. All CoAP nodes that are listening on port 9999 on their unicast IP address.

2. All CoAP nodes that are "subscribed" to mailing list 9999.

Both seem to have advantages and disadvantages. Maybe there are even
more interpretations?

> I have the same trouble as you understanding the port mapping part.
> Where is that supposed to happen (kernel, CoAP stack, application)?
> And why is it needed?

I always imagined multicast CoAP to work roughly like this:

+---------------+                +-----------------+
|               |    request    _|_                |
|               |        .---> /   \   224.0.1.187 |
|              _|_      /      \___/ --.   :9999   |
| 192.168.0.1 /   \ ---´         |      \          |
|   :54321    \___/ <---.       _|_     /  rewrite |
|               |        \     /   \ <-´           |
|               |         `--- \___/ 192.168.0.100 |
|               |    response    |         :5683   |
+---------------+                +-----------------+
      Client                           Server

So, a server is listening for request messages on both a unicast IP
address/port and a multicast IP address/port. If a request message
arrives on the multicast IP address/port, it "rewrites" the UDP
destination to its unicast IP address/port and then processes the
message as if it had received it on its unicast IP address/port.

The "rewrite" is at least needed for the IP address, because the
server cannot send its response from the multicast IP address.

Klaus