Subject: [Sip] SDP in unreliable 18x

From: Christer Holmberg <[email-masked]>
Date: Wed, 06 Mar 2002 21:15:54 +0200

Hi,

I am a little confused...

Chapter 13.2.1 in bis-09 says:

"If the initial offer is in an INVITE, the answer MUST be in a
reliable non-failure message from UAS back to UAC which is
correlated to that INVITE. For this specification, that is
only the final 2xx response to that INVITE. That same exact
answer MAY also be placed in any provisional responses sent
prior to the answer. The UAC MUST treat the first session
description it receives as the answer, and MUST ignore any
session descriptions in subsequent responses to the initial
INVITE."

Now, does this mean I'm not allowed to send an SDP in an UN-reliable
18x? And, if I do, is it then not regardes as an answer?

Eg the latest version of the isup draft uses unreliable (the PRACK has
been removed completely from the draft) 18x messages, with SDP bodies
(eg chapter 6.1.1. in draft-ietf-sipping-isup-01). They are needed for
early media, remote ringback tones etc etc. But, according to bis they
are not "valid" answers to the offer in INVITE.

Have I missunderstood something?

Regards,

Christer Holmberg
Ericsson Finland

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---

From: "Bob Penfield" <[email-masked]>
Date: Wed, 6 Mar 2002 16:03:18 -0500

----- Original Message -----
From: "Christer Holmberg" <[email-masked]>

> 18x? And, if I do, is it then not regardes as an answer?

You are allowed to send SDP in an unreliable 1xx. However, it MUST be the
same as the SDP you will send in a 2xx. The key sentence is "That same exact
answer MAY also be placed in any provisional responses sent prior to the
answer."

---

From: Christer Holmberg <[email-masked]>
Date: Thu, 07 Mar 2002 07:47:49 +0200

Hi Bob,

> answer MAY also be placed in any provisional responses sent prior to the

True. But, in this case the UAC is not allowed to send a new offer (eg using
UPDATE) before it has received the final response.

Thanks for your answer!

Regards,

Christer Holmberg
Ericsson Finland

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From: Jonathan Rosenberg <[email-masked]>
Date: Thu, 07 Mar 2002 22:01:24 -0500

> UPDATE) before it has received the final response.

Right. The idea was to try and add a semblance of compatibility with
existing implementations. We want everything to be mapped to a set of
non-overlapping O/A exchanges. So, the idea is that the SDP in an
unrealiable 18x, and the SDP in a 2xx, are just the same answer. THus,
we maintain the property of everything being a series of O/A exchanges,
but allow a single answer to be present in multiple sip responses.

However, it is NOT allowed, to send an offer in INVITE, an answer in a
reliable 183, and then repeat the answer in the next reliable 183. Thats
because the answer has to be in the first reliable response sent back,
which is the first 183. The SDP in the second 183 would be illegal,
since it would be an offer, and those can't go in responses after the
first SDP.

-Jonathan R.

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From: "Salva Rey Calatayud" <[email-masked]>
Date: Fri, 15 Mar 2002 10:37:31 +0100

Hi, 

	please see inline at end,

> since it would be an offer, and those can't go in responses after the

	I don't quite understand why these new offer on th
e reliable 183 
would be illegal. Isn't it conveyed on a reliable delivered message. I 
thought that was the only condition for O/A exchanges. or is it that 
it's only allowed one O/A exchange per transaction?

Thanks
Salva

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From: Gonzalo Camarillo <[email-masked]>
Date: Fri, 15 Mar 2002 19:53:57 +0200

Hi,

> it's only allowed one O/A exchange per transaction?

It is illegal to avoid glare conditions, where you could not refuse the
offer in the provisional response even though you would have sent an
UPDATE.

The update draft outlines when you can send an offer and how.

Regards,

Gonzalo

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