Re: [MMUSIC] SDPCapNeg, modified m-line issue

["Christer Holmberg" <[e-mail]>]

Hi Flemming,
 
I don't think having a 2nd offer/answer breaks the requirement (REQ-100). The capabilities ARE negotiated within a single offer/answer. The second offer/answer is then used to "synchronize" possible intermediates. And, if there are no intermediates, there is no need for a second offer/answer even with the changes proposed by Ingemar.
 
The big advantage is that intermediates not understanding SDPCapNeg will only see "normal" offer/answer transactions.
 
Regards,
 
Christer
 
 

________________________________

From: [e-mail] [mailto:[e-mail]] On Behalf Of Flemming Andreasen
Sent: 20. toukokuuta 2008 0:00
To: Ingemar Johansson S
Cc: [e-mail]
Subject: Re: [MMUSIC] SDPCapNeg, modified m-line issue




Ingemar Johansson S wrote: 

	Hi
	
	Thanks for the comments. To your questions/comments.
	
	Sofar e.g CSCF's are identified as nodes that may be problematic. In many cases it is likely that a B2BUA (e.g an SBC) may act as a "translator" (between e.g IMS and non-IMS) as it may anyway need to handle things such as encryption and transcoding but still a P-CSCF in the path may cause problems for different scenarios and may increase the number of interop scenarios and issues that must be dealt with. It is really difficult to say what might happen ( http://tools.ietf.org/html/draft-ietf-sipping-sip-offeranswer-08 does not consider unacceptable answers).
	
	It is true that some kind of a mandate for a 2nd offer/answer will very likely in the end lead to that media will not flow until the 2nd offer the earliest. Currently it is unclear if it will lead to increased setup times or if it is hidden by other signaling (e.g precondition)
	
	I tried to find some info about the modified "m=" line in the SDPCapNeg draft, more specifically if it is connected to a MUST. Sofar I have not found any such requirement. I may have missed it, please correct me.
	  

It follows from the description in Section 3.6.2 (Generating the answer), and in particular the following excerpt: 

Once the answerer has selected a valid and supported offered 
   potential configuration for all of the media streams (or has fallen 
   back to the actual configuration plus any added session attributes), 
   the answerer MUST generate a valid answer SDP based on the selected 
   potential configuration SDP, as "seen" by the answerer (see Section 
   3.6.2.1. for examples).



	The only more specific info I found is in section 3.4.2 that says that the receiver of the answer must be able to handle the case that an intermediary may rewrite the "m=" line. 
	  

I'm not sure where in Section 3.4.2 you see that. Answers are supposed to follow the rules in Section 3.6.2. and if an intermediary changes that, then things may or may not work (if it's a B2BUA, then obviously we are in a different situation). 



	This would then mean that the answer may leave the "m=" line unaltered ?. 

No - see Section 3.6.2


	The offer when it receives the answer will then see that the "a=acfg" and the "m=" line does not correlate well and will issue another offer/answer round (probably as an UPDATE). There may occur some transmission of "faulty" media but as the receivers MAY discard this media this should be a minor issue (could be some early media security issues though).
	  

Again, this is not what the draft specifies (see Section 3.6.3 as well)

-- Flemming 



	Regards
	Ingemar
	
	
	
	  

		-----Original Message-----
		From: Flemming Andreasen [mailto:[e-mail]] 
		Sent: den 18 maj 2008 22:19
		To: Ingemar Johansson S
		Cc: [e-mail]
		Subject: Re: [MMUSIC] SDPCapNeg, modified m-line issue
		
		Hi Ingemar
		
		Sorry for the delayed response - please see below.
		
		Ingemar Johansson S wrote:
		    

			Hi
			
			As you may know, SDP Capability Negotiation Framework 
			      

		(SDPCapNeg) is gaining interest in the 3GPP community 
		especially in the SA4 and CT1 working groups. 
		    

			The current plan is to propose SDPCapNeg to be included as 
			      

		a part of the 3GPP standard.
		    

			There are however concerns about a potential 
			      

		interoperability issue with SDPCapNeg. 
		    

			Namely that e.g. the m= line in the (first) SDP answer is 
			      

		modified in such a way that intermediaries may reject the SDP 
		with unpredictable/unknown consequences (also mentioned in 
		3.12 in the SDPCapNeg draft).
		    

			An example is the SDP
			Offer:
			   m=audio 1234 RTP/AVP 97
			   a=fmtp..
			   a=rtpmap..
			   a=tcap:1 RTP/AVPF RTP/AVP
			   a=pcfg:1 t=1|2
			
			The answerer supports AVPF and returns
			Answer:
			   m=audio 5678 RTP/AVPF 97
			   a=fmtp..
			   a=rtpmap..
			   a=acfg:1 t=1
			
			An intermediate may accept the offer but reject the answer 
			      

		for some reason and as it is the answer that is rejected the 
		consequences are worse than if the offer would be rejected.
		    

			  
			      

		Is this a general concern or are you looking at a specific 
		intermediary (e.g. the P-CSCF) ?
		
		    

			So far this is only a problem related to the m= line but as 
			      

		the framework allows for extensions that may elevate this 
		problem even more.
		    

			  
			      

		Agreed.
		    

			Our proposal is therefore that the SDPCapNeg answer is only 
			      

		allowed to do modifications to lines in the "conventional" 
		SDP parts that are well known to work or supported by 
		"conventional" SDP offer/answer exchange (the definition here 
		is yet unclear). The answer SDP above would then look like 
		    

			   m=audio 5678 RTP/AVP 97
			   a=fmtp..
			   a=rtpmap..
			   a=acfg:1 t=1
			In this example the preferred configuration is only 
			      

		indicated by the a=acfg line.
		    

			  
			      

		Would the media stream(s) have been established with the 
		indicated answer at this point in time (A) or would you now 
		require a subsequent offer/answer exchange before the media 
		stream(s) can be considered functional (B) ?
		    

			This could of course make it more necessary to do a 2nd 
			      

		offer/answer exchange, but we believe that mandating a 2nd 
		offer answer will be needed even with the current solution to 
		make sure things will work.
		    

			One could rightly argue, as is also hinted in the SDPCapNeg 
			      

		draft, that intermediaries should be upgraded. Problem is 
		however that depending on product cycles and other issues 
		among vendors and operators, this may take time. Also it is 
		worth notice that 3GPP works based on the principle that 
		things should be backward compatible. Furthermore we believe 
		that solutions based on the "products should be upgraded" 
		principle would cause problems even in non-3GPP networks 
		since intermediaries may be found in many network deployments.
		    

			Comments and suggestions on this issue are welcome.
			  
			      

		Taking a step back, I believe the essence of your comments 
		are, that if we want to use SDP Capability Negotiation to 
		establish media streams, then we cannot do that in a single 
		offer/answer exchange. Instead, you want to have a mechanism 
		whereby we first exchange capabilities between the offerer 
		and the answerer (without establishing a stream) followed by 
		another exchange where we actually negotiate the media stream 
		parameters. In other words, the current single-roundtrip O/A 
		exchange provided by RFC 3264 and supported by SDPCapNeg is 
		replaced by a two-roundtrip solution.
		
		While I understand the concern you are trying to address, we 
		have been around this issue several times and the sdpcapneg 
		document reflects the consensus solution. To recap, the 
		requirements document
		(draft-ietf-mmusic-sdp-capability-negotiation-reqts-01) has 
		the following requirement:
		
		   REQ-100: The mechanism MUST work within the context of the
		   offer/answer model [RFC3264]. Specifically, it MUST be possible to
		   negotiate alternatives within a single offer/answer exchange.
		
		That particular requirement was discussed in the design team, 
		in the Prague IETF, and subsequently on the MMUSIC mailing 
		list (see "SDPCapNeg Issue #2: Answer not getting through 
		middle-boxes if transport protocol in answer differs from 
		offer" on 6/25/07) and consensus was for the mechanism 
		specified currently.
		
		Note however, that the SDPCapNeg framework as currently 
		defined can be used more or less the way you seem to prefer 
		by merely including capabilities in the offer without the 
		actual potential configuration attributes (which are used to 
		trigger the extended O/A exchange). The thing that would be 
		missing is expressing valid combinations of capabilites as 
		well as preferences, however this is part of what the media 
		capabilities extension document is addressing by introducing 
		the "latent configuration" attribute, which you could then use.
		
		In lieu of the above, past history on requiring multiple O/A 
		exchanges, the significant changes to the overall scheme your 
		suggestion implies, and the fact that we completed WGLC a 
		while ago, I don't think we should make this change.
		
		Regards
		
		-- Flemming
		
		    

			Regards
			Ingemar
			*******************************************
			Ingemar Johansson
			Senior Research Engineer, IETF "nethead" 
			EAB/TVP - Multimedia Technologies
			Ericsson Research Ericsson AB
			Box 920 S-971 28 Luleå, Sweden
			Tel: [phone]
			ECN: 850-43042
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			Mobile: [phone]
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