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

[Thomas Fossati <[e-mail]>]

Hey Klaus,

On Wed, Apr 1, 2020 at 11:56 AM Klaus Hartke <[e-mail]> wrote:
> 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.

This interpretation doesn't look right to me.

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

This is how I interpret it.

> > 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.

Not sure why you would also want to rewrite the transport endpoint?

cheers, thanks!