Subject: [MMUSIC] ICE: Usage of non-reliable provisional responses to send SDP answer

From: "Christer Holmberg \(JO/LMF\)" <[email-masked]>
Date: Wed, 6 Sep 2006 14:04:56 +0200

Hi,

Chapter 12.1 of ICE-10 says:

"When reliable provisional responses are not used, the SDP in the
provisional response is the answer, and that exact same answer reappears
in the 200 OK. To deal with possible losses of the provisional response,
it SHOULD be retransmitted until some indication of receipt. This
indication can either be through PRACK [9], or through the receipt of a
successful STUN Binding Request.  Even if PRACK is not used, the
provisional response SHOULD be retransmitted using the exponential
backoff described in [9]."

I guess this means that the offer/answer transaction is complete once
this "indication" has been received, even if the provisional response
was sent without 100rel. That would mean that new offer/answer
transactions could be initiated after that.

Now, is this really something we want to do??? 

I think it sounds like a hack, and could cause problems, e.g. if there
are intermediate entities that don't support ICE (and therfore do not
regard the SDP in the unreliable provisional response as an answer). 

If you want to send information reliably in a provisional response - use
100rel.

Regards,

Christer

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From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 07 Sep 2006 12:43:48 -0400

inline.

> transactions could be initiated after that.

No. You cannot initiate new offer/answer transactions. All this is doing 
is adding a few retransmits of the unreliable 18x so that we have 
confidence it was delivered. It changes nothing about any other SIP 
offer/answer rules.

> regard the SDP in the unreliable provisional response as an answer). 

Exactly. Thats why I'm not suggesting that the offer/answer rules are 
changed.

It is somewhat of a hack but it solves a lot of problems and was the 
result of lots of concerns and grief over basically requiring 100rel for 
ICE to work.

-Jonathan R.
-- 
Jonathan D. Rosenberg, Ph.D.                   600 Lanidex Plaza
Cisco Fellow                                   Parsippany, NJ 07054-2711
Cisco Systems
[email-masked]                              FAX:   (973) 952-5050
http://www.jdrosen.net                         PHONE: (973) 952-5000
http://www.cisco.com

---

From: "Christer Holmberg \(JO/LMF\)" <[email-masked]>
Date: Thu, 7 Sep 2006 21:20:24 +0200

Hi, 

>>provisional response is the answer, and that exact same answer 
>>reappears in the 200 OK. To deal with possible losses of the 
>>provisional response, it SHOULD be retransmitted until some 
>>indication of receipt. This indication can either be through PRACK
[9], or 
>>through the receipt of a successful STUN Binding Request.  Even if 
>>PRACK is not used, the provisional response SHOULD be retransmitted 
>>using the exponential backoff described in [9]."
>> 
>No. You cannot initiate new offer/answer transactions. All 
>this is doing is adding a few retransmits of the unreliable 
>18x so that we have confidence it was delivered. It changes 
>nothing about any other SIP offer/answer rules.

What is "a few retransmits"? What if something goes wrong in the
signalling path and the 18x never reaches to other end, and/or the STUN
request (or whatever indication is used) doesn't reach the one sending
the 18x? How long are you going to retransmit?

>>I think it sounds like a hack, and could cause problems, 
>>e.g. if there are intermediate entities that don't support ICE (and 
>>therfore do not regard the SDP in the unreliable provisional response
as an 
>>answer). 
>result of lots of concerns and grief over basically requiring 
>100rel for ICE to work.

I don't understand. We define a "tool" for providing reliable transport
of provisional responses in SIP, and now we define a hack because people
don't want to use it???

Regards,

Christer