Subject: [Simple] MSRP O/A refresh question

From: Paul Kyzivat <[email-masked]>
Date: Tue, 30 Oct 2007 13:24:41 -0400

While reviewing the latest version of the file transfer draft I 
developed some doubts about MSRP that I want to check:

RFC4975, section 8.4 talks about updated SDP offers. I want to double 
check my interpretation of this section.

Suppose Alice makes an initial offer to Bob for an MSRP session, and Bob 
accepts. At that time Alice is required to do the Connect to Bob.

Later, Bob has need to refresh the sip session with a reinvite, but has 
no desire to change anything about the MSRP session. So Bob sends an 
offer with an unchanged o-line. Alice doesn't want to change anything 
either, and so answers with an unchanged o-line.

By my reading, its required that Bob now connect to Alice. Even though 
neither wants to change anything there is need to establish a new 
connection, and presumably to drop the old one.

Is that right???

If not, where is some other behavior called out?

	Thanks,
	Paul

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From: Miguel Garcia <[email-masked]>
Date: Wed, 31 Oct 2007 10:53:25 +0200

Paul:

I think you are assuming that a successful O/A always implies a TCP 
connection. I don't think this interpretation is correct. For example, 
it is possible that several MSRP sessions share the same TCP connection. 
There is some text below Figure 3 in RFC 4975.

So, if it is possible to have different MSRP sessions multiplexed over 
the same TCP connection, then it implies that not every O/A implies a 
new TCP connection.

In the case you expose, a new SDP O/A on an existing MSRP session does 
not imply a new TCP connection if the TCP connection still exists.

/Miguel

-- 
Miguel A. Garcia           tel:+358-50-4804586
Nokia Siemens Networks     Espoo, Finland

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From: Ruud Klaver <[email-masked]>
Date: Wed, 31 Oct 2007 10:57:41 +0100

Hi Paul,

On Oct 30, 2007, at 6:24 PM, Paul Kyzivat wrote:

> RFC4975, section 8.4 talks about updated SDP offers. I want to  
> double check my interpretation of this section.
>
> Suppose Alice makes an initial offer to Bob for an MSRP session,  
> and Bob accepts. At that time Alice is required to do the Connect  
> to Bob.
>
> Later, Bob has need to refresh the sip session with a reinvite, but  
> has no desire to change anything about the MSRP session. So Bob  
> sends an offer with an unchanged o-line. Alice doesn't want to  
> change anything either, and so answers with an unchanged o-line.
>
> By my reading, its required that Bob now connect to Alice. Even  
> though neither wants to change anything there is need to establish  
> a new connection, and presumably to drop the old one.
>
> Is that right???
>

If I understand the RFC correctly, I think this is what should happen:

If Bob wishes to refresh an already existing MSRP session, I think he  
should include exactly the same MSRP path attribute in the SDP as he  
did for the initial session negotiation. As this includes session  
identifiers unique to this session, Alice will be able to determine  
that this session is already established and no action should be  
taken, besides replying with a SIP OK with her own MSRP path in the SDP.

Best Regards,
Ruud Klaver

---

From: Paul Kyzivat <[email-masked]>
Date: Wed, 31 Oct 2007 12:00:40 -0400

Miguel,

The devil is in the details. IMO the desired result is that the 
connection remains and continues to be used. I'm just not finding the 
text that says to do that.

	Paul

Miguel Garcia wrote:
>> Suppose Alice makes an initial offer to Bob for an MSRP session, and 
>> Bob accepts. At that time Alice is required to do the Connect to Bob.
>>
>> has no desire to change anything about the MSRP session. So Bob sends 
>> an offer with an unchanged o-line. Alice doesn't want to change 
>> anything either, and so answers with an unchanged o-line.
>>