Subject: [Sip] Who Should Initiate Re-invite??

From: murali <[email-masked]>
Date: Mon Jan 19 17:26:39 2004

Hi,

I have a question regarding the SDP negotiation.
Consider the following scenario,

	INVITE(w/o SDP)
     -------------------->
	100 trying
     <--------------------
	180 Ringing
     <--------------------	
	200(with SDP, a=inactive)
     <--------------------	
	Ack(with SDP)
     -------------------->

As a=inactive has been used in SDP the RTP session is not established.
As RFC 2327 does not take care of a=inactive, some older implementations
respond with one codec in ACK with sendrecv mode(default). 

1. In this scenario, who should initiate Re-       Invite? 
2. What if both parties do not initiate
   a Re-invite?
3. What if both parties initiate a Re-invite? 
4. When does the call gets terminated?
5. Does this have some interop issues(between     3264,2327 implementations)?

Thanks & Rgds,
Murali

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---

From: Paul Kyzivat <[email-masked]>
Date: Tue, 20 Jan 2004 09:52:09 -0500

> Consider the following scenario,

What do you mean by "not established"? There should be valid ports, 
though no packets should be flowing. And RTCP should be flowing between 
them.

> respond with one codec in ACK with sendrecv mode(default). 

Yes, the issue of backward compatibility hasn't been well addressed. 
Presumably we should expect the party sending the a=inactive to be 
prepared for a peer that doesn't understand and do the right thing.

In this case, in addition to returning sendrecv, the caller will most 
likely begin to send media. The callee had better be prepared to deal 
with that.

> 1. In this scenario, who should initiate Re-       Invite? 

You can't expect much from the caller since it is a now obsolete device. 
If you are going to impose new requirements on it, the first one should 
be to support a=inactive.

It isn't clear that a reinvite is required in this case. The caller is 
happy, and the callee is presumably dealing with the incoming audio it 
doesn't want. But if it wants to stop that, it could send a reinvite 
with c=0.0.0.0 to kill the audio.

> 2. What if both parties do not initiate

See above.

> 3. What if both parties initiate a Re-invite? 

If they happen concurrently, then it is a glare condition, and is 
covered by 3261. If they don't happen concurrently there is no problem - 
they both get acted upon.

> 4. When does the call gets terminated?

When somebody sends a BYE.

> 5. Does this have some interop issues(between     3264,2327 implementations)?

Yes, clearly there are some backward compatibility issues.

This has been discussed somewhat. If you use a=sendonly, recvonly, or 
inactive, you probably ought to be prepared for the other party not 
understanding. If it doesn't return a corresponding a-line in accord 
with the rules in 3264 then you should probably conclude it doesn't 
understand and act accordingly.

	Paul