Subject: [MMUSIC] Appropriate usage of SIP and RTSP

From: "NAKAMURA, Hidefumi" <[email-masked]>
Date: Mon, 07 Jul 2003 16:49:18 +0900

Hi, all.

I have a question about the relationship between SIP and RTSP. 

In RFC3261, there is a description that says 
   "SIP is not a vertically integrated communications system.  SIP is
   rather a component that can be used with other IETF protocols to
   build a complete multimedia architecture.  Typically, these
   architectures will include protocols such as the Real-time Transport
   Protocol (RTP) (RFC 1889 [28]) for transporting real-time data and
   providing QoS feedback, the Real-Time streaming protocol (RTSP) (RFC
   2326 [29]) for controlling delivery of streaming media, ..."
   (RFC3261 Section 2)

However, my understanding is that both SIP and RTSP have the capability
for "discovery of UAs or media streams."
If I use SIP to establish a stream session and use RTSP to control
the stream, there will be redundant sequences, i.e. INVITE-OK-ACK and
SETUP-OK.

Was there any discussion on how to use these two protocols appropriately?

With SIP application server, network providers can provide various
services on contents delivery while we will not be able to do so only
with RTSP, I think.  On the other hand, RTSP has enough capability for
discovery of media streams and for negotiation of SDPs.
So, I think, for the simple delivery of contents, RTSP is appropriate to
use, and for various session control services regarding contents
delivery, SIP with RTSP is appropriate to use.

Any opinions?

I wonder why no contents delivery server seems to prepare SIP stack now. 
Is it because, up to now, there exists only a simple contents delivery
service?

best regards,

-- 
NAKAMURA, Hidefumi 
NTT ServiceIntegration /NetworkServiceSystems Labs.
tel: +81(422)59 3904; fax:+81(422)60 4012
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From: Magnus Westerlund <[email-masked]>
Date: Mon, 07 Jul 2003 10:35:00 +0200

Hi,

See comments inline:

> Was there any discussion on how to use these two protocols appropriately?

Not to my knowledge. The SIP and RTSP combination has not been explored, 
however it seem to contain a number of use cases. For example inviting a 
media server into a SIP session or a conference session.

There are significant overlap in SIP and RTSP, mostly in regards to 
session establishment that are done i different ways.

> delivery, SIP with RTSP is appropriate to use.

What do you mean with "session control services regarding contents 
delivery"?

> Is it because, up to now, there exists only a simple contents delivery

I believe that no one has looked into this and have had a need.

Best Regards

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: [email-masked]

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From: "NAKAMURA, Hidefumi" <[email-masked]>
Date: Mon, 07 Jul 2003 18:28:19 +0900

Dear Magnus,

Thank you for your quick response.
I wrote down my opinion after your comments.

> What do you mean with "session control services regarding contents 

For example, if we establish a stream session via SIP application server
owned by the network provider, the application server can redirect the
session to the appropriate contents delivery server from the standpoint
of location, bandwidth prepared between terminals and the server, or
condition of contents server (i.e. in congestion, failure, and so on.)
Say, we get a media stream URI from a portal server.  The terminal(or
set top box) requests a session initiation to the media stream URI to
the SIP application server.  The SIP application server routes the
request to the contents delivery server that stores the media.  However,
if the server is in congestion and returns 503, the SIP application
server can select and redirect the session to another contents delivery
server instead.
I do not think this is possible with only RTSP.
(though the interaction among the portal server, contents delivery
servers and the terminals might enable this type of service in some
extent.)

> I believe that no one has looked into this and have had a need.

I understand.  Thank you very much for your comments.
Maybe it is interesting to investigate the use case, anyway.

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From: Jonathan Rosenberg <[email-masked]>
Date: Tue, 08 Jul 2003 16:40:20 -0400

I agree that there is significant overlap between RTSP and SIP, and 
this is something that has been bothering more and more over time. I 
have had in mind to write a draft on the ways in which they could be 
better merged, but have not had the time.

The primary place where they overlap is session establishment. In this 
area, I think SIP is much richer, in terms of features. SIP would 
allow connections to media servers running on PCs and other end-user 
devices. Its rich routing facilities could allow nice policy-based 
connection to content. It has had a lot of work on firewall/nat 
traversal which is now only getting repeated for RTSP. Plus, in many 
cases, you might initiate a session, but not know, in advance, that 
the session is to be connected to a content server. As an example, I'd 
like to be able to make a sip call to a user, and instead get 
connected to a voicemail greeting that I can control with RTSP.

IMHO, the most obvious way to make this work is to use SIP to initiate 
a session. If the session is to a media server, then, using SDP 
negotiations, one can add an additional "media channel", which is 
actually RTSP, doing just media controls (play, fast-forward, etc.). 
Thus, SIP would set up an RTSP session to control the media that the 
SIP call connected to.

I havent really fleshed this out much, but it seemed like a start.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                600 Lanidex Plaza
Chief Technology Officer                    Parsippany, NJ 07054-2711
dynamicsoft
[email-masked]                     FAX:   (973) 952-5050
http://www.jdrosen.net                      PHONE: (973) 952-5000
http://www.dynamicsoft.com

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From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 09 Jul 2003 05:16:10 -0400

Paolo Asprino wrote:

> Hi all
> 
> I'm thinking a solution to join SIP and RTSP in the 3G IMS domain.
> In particular in the IMS, I think that to provide multilmedia
> service (i.e video on demand), one can estabilish a subscription to the service by SIP and then activate the service with RTSP by DIGEST autentication with the 
> parameters exchanged early during the subscription (username and password) 
> 
> What do you think about that?

I'm not sure I follow; it seems like you want SIP to be an enrollment 
protocol.

I believe the right path forward is for SIP to setup the session. If 
you want to execute controls, it would be possible to open a control 
session (using an offer/answer exchange), where the control protocol 
is RTSP. In that usage you would only use the PLAY, PAUSE, RECORD, 
GET-PARAMTER and SET-PARAMETER methods. I think the others would not 
be needed.

-Jonathan R.