Re: [Simple] COMEDIA vs MSRP Relays: Shared Connections

[Ben Campbell <ben at nostrum.com>]

On Jul 27, 2009, at 3:27 PM, Christer Holmberg wrote:

> Hi,
>
> More on this.
>
> > > > > I'm not sure I follow you - Comedia has a "connection:existing"
> > > > > attribute setting to re-use the same connection (e.g., in a new
> > > > > offer).  Or do you mean of two separate media m= lines?
> > > >
> > > > You are right of course. Brain fart on my part. However, the draft
> > > > should probably mention how to apply this--the only reference I see
> to
> > > > connection:existing is in the ICE section.
> > >
> > > Wait, I remember why I brought this up--if a peer is behind an MSRP
> > > relay, how is it to know whether a reuseable connection already  
> > > exists
> > > downstream of that relay?
> >
> > Even if it knows, how would it tell the relay whether to actually  
> > reuse
> that connection? I guess that would require some
> > new information element.
>
> We also need to think about the meaning of a=connection.
>
> According to my understanding (I guess we can verify that with the
> authors) of comedia, a=connection is not used to indicate that one can
> use "any existing TCP connection" towards a specific destination.
>
> My understanding is that a=connection is used during a session
> modification, to indicate whether the existing TCP connection for that
> specific session should be re-used (if possible) or not. So, for a
> specific session, the change of address would always trigger a new TCP
> connection - even if there exist TCP connections to the same address  
> for
> OTHER sessions.
>
> Or?
>
> > Note, however, that according to section 4.1.3, connection re-use is
> not used for initial offer/answers, only for re-
> > negotiations.

My read of COMEDIA agrees with yours. It's not clear to me if the  
connection attribute applies at all--RFC4975 says if there is already  
an connection associated with the same host, port, and URI scheme, it  
SHOULD reuse it.  COMEDIA doesn't appear to have been written with  
that sort of thing in mind. I am leaning (not strongly) towards  
suggesting we ignore the COMEDIA connection attribute and just do what  
an MSRP client would normally do, at least for this purpose.

> So, for session establishment, there is currently no difference  
> between
> ACM and 4975 - if there is a TCP connection to a specific address,  
> then
> use it.
>
> For 4975, I assume the same applies for session modifications
> (eventhough 4975 doesn't explicitly says much about session
> modifications)...

You are right, it doesn't say much. I agree that the same would apply  
here. Also, I think all of this so far is the same with or without  
relays.

> ACM currently specifies the usage of a=connection for session
> modifications, in accordance with comedia.
>
> So, IF we a=connection to work with relays, I guess we would need to
> provide the information to the relay inside the MSRP message.

I'm not sure I follow--what would you conceptually be telling the relay?

> Again, I will add something about this to my presentation.
>
> Regards,
>
> Christer
>
>
> > > > -----Original Message-----
> > > > From: simple-bounces at ietf.org [mailto:simple-bounces at ietf.org] On
> > > > Behalf Of Ben Campbell
> > > >
> > > > 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.
> > > >
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