Subject: Re: [Sip] Re: RFC 3264 An Offer/Answer Model Session Description Protocol

From: "Bob Penfield" <[email-masked]>
Date: Tue, 3 Sep 2002 09:49:22 -0400

----- Original Message -----
From: "Paul Kyzivat" <[email-masked]>

> I have a question of interpretation regarding section 8.3.2 (Changing the
Set of Media Formats).
>
> When describing how an offer is to be constructed for a change (a
reINVITE), this section says:
>
> "However, in the case of RTP, the mapping from a particular dynamic
payload type number to a particular codec within that media stream MUST NOT
change for the duration of a session.  For example, if A generates an offer
with G.711 assigned to dynamic payload type number 46, payload type number
46 MUST refer to G.711 from that point forward in any offers or answers for
that media stream within the session."
>
> What I question is the "or answers" at the end of this section. This seems
to be inconsistent with section 6.1, which says that the payload type
numbers for answers may be different than those for offers.

I believe it is talking about any offers or answers that a given endpoint
generates during the session since either side may re-INVITE. The two side
may use different payload numbers, but each has to use its same original
payload number in all SDP it generates.

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---

From: Paul Kyzivat <[email-masked]>
Date: Tue, 03 Sep 2002 11:12:20 -0400

Bob - OK, if you read it that way then all is ok.
It wasn't obvious to me that this is what that text
intended, but since it is only an example I guess it
is good enough.

	Thanks,
	Paul

---

From: Jonathan Rosenberg <[email-masked]>
Date: Fri, 06 Sep 2002 17:03:30 -0400

Bob's interpretation is correct. It means that each sid maintains its 
own mapping of payload type numbers to codecs, and that mapping applies 
to any offers and answers sent within the dialog by that UA.

Usage of the same payload type numbers for the same codec in each 
direction (ie. using PT 100 for g.723 from A to B and from B to A) is 
SHOULD strength, I believe. We allowed them to be asymmetric primarily 
to support H.323 interop.

-Jonathan R.

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