Subject: [Simple] New I-D on IM transport

From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 14 Nov 2001 15:17:00 -0500

Folks,

I've just submitted a new I-D to the archives which proposes a compromise
solution for the IM transport in the session model. Until it appears, you
can pick it up at:

http://www.jdrosen.net/papers/draft-rosenberg-simple-im-transport-00.txt

This draft was co-authored by Christian Huitema, Robert Osborne, Avshalom
Houri, and myself, in the hopes of making forward progress on this issue.
The compromise is to use a protocol called IMTP which is is a subset of SIP,
so that it can be forwarded through SIP proxies with proper configuration,
but at the same time, only uses reliable transports, and does not use things
like forking, record-routing, redirection, and so on.

The draft also proposes requirements, which I have gathered from the list
discussion. 

Thanks,
Jonathan R.

---
Jonathan D. Rosenberg, Ph.D.                72 Eagle Rock Ave.
Chief Scientist                             First Floor
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[email-masked]                     FAX:   (973) 952-5050
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From: Paul Kyzivat <[email-masked]>
Date: Wed, 14 Nov 2001 18:33:08 -0500

I just did a quick scan of this. This seems to balance out some of the
conflicting requirements that have been preventing closure on this
subject. 

I will probably have more comments after I think about it a bit, but I
see one problem now, with using the comedia draft for connection
management. I think you authors see it too, based on comments (from
various places) in the text:

   Streams that use IMTP are always connection oriented, and therefore
   use the SDP connection oriented media attributes [10] to allow for a
   single connection to be used for messaging in both directions.
   ...
        Need to get the port from somewhere too... maybe just
        specify a default port for IMTP? May not work with muli-
        user machines.
        ...
        Need a little more rigor in port handling and connection
        reuse, to ensure that the comedia rules and SIP transport
        rules are in sync.
   ...
   If A wants to send a message to B, it looks for an open TCP
   connection to 5.6.7.8. If one exists, that is used. If not, one is
   opened.
   ...
   R1 then looks up the request URI, and finds a binding...
   Since there is an existing TLS
   connection to that address, this connection is reused:

All of these say to me that what you want is not what is described in
the comedia draft. That draft:

- requires port numbers

- assumes that one (or two) connections will be established uniquely
  for each independently negotiated media stream (is that the 
  right word?). There is no provision for sharing a connection,
  except for sharing one for the two directions of a single stream.

This is clearly not what you want or assume. You would like to use
pretty much any connection of the proper type to the corresponding
endpoint. This is sufficient because you have the session identifier to
demultiplex messages sharing the connection. That is what you need, but
it isn't what comedia provides.

A related problem is that SDP requires the port number field of the m=
line to be a number. You have put the session identifier there, but it
isn't strictly numeric. I think SDP is much too rigid about this, but I
doubt if we are permitted to change it. You probably need to put the
session identifier somewhere else.

Rather than attempt to use comedia, I think it would be better to define
the IMTP transports so that they make/find connections in the same way
that SIP does, but drawing the information to do it from different
sources: address from c=, port from m= port field, transport (TCP/TLS)
imiplied from m= transport field. Perhaps something like the a=direction
field is still needed to force the connection to be opened one way even
when traffic will first flow the other way. If so, while similar to
comedia it is still a very different way of managing connections.

	Paul