Subject: [Simple] MSRP: SDP usage concern

From: Ben Campbell <[email-masked]>
Date: Fri, 3 Dec 2004 19:35:10 -0600

In the DC meeting, we discussed that MSRP's SDP usage should be 
reviewed in MMUSIC, or at least by people who are more expert in SDP.  
Colin Perkins and Joerg Ott looked at this, and suggested a change. 
(Many thanks to them for taking the time, and I hope they will jump in 
and correct me if I mischaracterize anything.)

The upshot of the proposal is that the fact that MSRP ignores the 
address in the sdp c-line, and the port and proto fields in the m-line. 
Instead, MSRP gets all of this from the first URL in the path a-line 
attribute. This deviates from the traditional usage in SDP, and causes 
confusion.

The proposed change is that the destination URL be broken into the 
appropriate parts and encoded into the c-line and m-line. For example 
(leaving out lines that don't change)

  c=IN IP4 alice.example.com
  m=message 9 msrp *
  a=path:msrp://a.example.com:7394/2s93i;tcp

would become:

  c=IN IP4 a.example.com
  m=message 7394 msrp/tcp *
  a=session:2s93i

The change would only be made for the destination URL. URLs for any 
relays would be expressed the same way they are now.

The advantage of this approach is that it more closely follows the 
spirit of SDP. This will be easier to understand for people who already 
know SDP. Existing SDP code could be reused more effectively.

The disadvantage of the proposed change is, it means the way an MSRP 
client determines the destination URL would be different depending on 
whether the URL represented the final destination or a relay. Instead 
of the current approach of simply copying the path attribute value into 
the To-Path header in MSRP messages,  a client would copy the path 
header value if exists, reconstitute the destination URL from the m and 
c lines, and append the destination URL to the path header value.

So, what do people think? (I note that we actually did something 
similar to this before we added the "path" attribute. )

Thanks!

Ben.

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---

From: Paul Kyzivat <[email-masked]>
Date: Mon, 06 Dec 2004 11:22:10 -0500

Ben Campbell wrote:
> In the DC meeting, we discussed that MSRP's SDP usage should be reviewed 
> in MMUSIC, or at least by people who are more expert in SDP.  Colin 
> Perkins and Joerg Ott looked at this, and suggested a change. (Many 
> thanks to them for taking the time, and I hope they will jump in and 
> correct me if I mischaracterize anything.)
> 
> whether the URL represented the final destination or a relay. Instead of 
> the current approach of simply copying the path attribute value into the 
> To-Path header in MSRP messages,  a client would copy the path header 
> value if exists, reconstitute the destination URL from the m and c 
> lines, and append the destination URL to the path header value.
> 
> So, what do people think? (I note that we actually did something similar 
> to this before we added the "path" attribute. )

I find the decomposition to be unspeakably ugly. Its bad enough in the 
historic usage where you need to gather things from several places to 
figure out where the RTP is going. But it is MUCH worse when there is a 
familiar format (e.g. a url format) but you have to rip it apart for the 
sake of adhering to the old format that doesn't suit the situation.

The need for two formats - one for the Path and another for the next hop 
is very confusing. Maybe very careful specification could perfume this 
pig so it doesn't smell so bad, but why? Maybe it would be worthwhile if 
it could result in unification with comedia, but I think that is 
generally impossible because we support connection sharing and it doesn't.

     Paul