[Simple] COMEDIA vs MSRP Relays: Shared Connections

[Ben Campbell <ben at nostrum.com>]

(As individual)

We've had quite a bit of discussion about how the Transport Connection  
Addressing Section (draft-ietf-simple-msrp-acm-00 section 4.2) may or  
may not work with MSRP relays. I'd like to make sure we're not  
neglecting how the COMEDIA Usage section (Section 4.1 and children)  
interact with MSRP relays.

I think there are some open questions. Since each one seems likely to  
generate a thread of discussion, I'm sending them separately:

How does the COMEDIA negotiation interact with the TCP-connection- 
sharing features of MSRP? We negotiate sessions in the SDP offer/ 
answer, not connections per se. COMEDIA implicitly assumes a one-to- 
one correspondence between these, but MSRP explicitly allows multiple  
sessions to use the same TCP connection.

I can see two approaches. One would be that the COMEDIA attributes are  
only relevant if you have to create a new connection. If a reusable  
connection already exists, you just use it. Alternatively, we could  
state that a connection is only reuseable if it's direction matches  
the COMEDIA attributes, and if devices wanted to reuse an existing  
connection, they would either omit COMEDIA entirely, or choose COMEDIA  
attributes that match the connection.

Neither of those approaches seems obviously wrong to me. I suspect  
there may be some traps here, though--we should analyze this pretty  
closely.