Re: [MMUSIC] Offer/Answer PT Questions
Paul Kyzivat <pkyzivat@alum.mit.edu> Sat, 20 February 2016 17:16 UTC
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From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Date: Sat, 20 Feb 2016 12:16:54 -0500
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Subject: Re: [MMUSIC] Offer/Answer PT Questions
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On 2/19/16 11:30 PM, DOLLY, MARTIN C wrote: > Hello O/A lovers, > > Section 8.3.2 of RFC 3264 says the following: > > “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.” As you know, this area is not well defined. > Now, assume the following case: > > 1.Alice sends an Offer, with the following PT values: > > PT 100: AMR-WB(mode-set=2) > > PT 111: AMR-WB(mode-set=2, OA) > > PT 0: PCMU > > 2.Bob sends the associated Answer: > > PT 100: AMR-WB (mode-set=2) > > PT 0: PCMU At this point, I think there is an open question of whether the "must not change" even applies for a PT that was offered but not accepted in the answer. One might argue that the real point of the requirement is that it can't be reused once there might be packets in flight using it. But that isn't true in this case. So IMO it is uncharted territory. > 3.Later Bob sends the following subsequent Offer: > > PT 111: AMR(mode-set=0,2,5,7) You mean the new offer no longer mentions 0 and 100, only 111 with this new mapping? (Though I don't think the answer to that affects your questions.) > QUESTION 1: Is it allowed for Bob to offer PT 111 for AMR in the > subsequent Offer, as Alice used PT 111 for AMR-WG in the initial Offer? > In other words, does the > pt-value-must-only-refer-to-a-specific-payload-for-the-stream-within-the-session > text in section 8.3.2 apply to user A only, or to **all** users within > the session? While it is *recommended* that the answer use the same PT as the offer for a given codec configuration, that isn't *required*. The answer may accept the codec from the offer using a different PT. So, in the first answer Bob *could* have accepted the offer for AMR-WB(mode-set=2, OA) using some other PT (say 112). If so, when Bob subsequently sent an offer reusing the same codecs it would have used 100, 112, and 0. (111 would not have been used.) In that case it seems like Bob would be free to use 111 for something else. Alice, in her answer, would be free to map it to a different PT in order to avoid a clash with 111 on her end. All of that seems equally applicable in your example, where Bob never accepted AMR-WB(mode-set=2, OA). > QUESTION 2: Assuming the text applies to all users, is it allowed to > re-use a PT value for the same codec, but for a different CODEC > CONFIGURATION? The text only talks about codec. I think use of "codec" was just sloppy, and that it always meant codec configuration. > Now, I agree that the easiest way to avoid trouble is simply to make > sure PT values aren’t re-used by any party. But, the text still needs to > be clear, because this is causing problems in real deployments. Agree. The spec is far from clear on this subject. A starting point for clarifying it to ascertain as best we can the original intent. But then we also need to try to understand common practice. Where this has been unclear there may well be conflicting interpretations in the wild. *Somebody* is likely to be negatively affected by whatever decision we come to. Thanks, Paul P.S. Martin: maybe you have found a subject where we can agree.
- [MMUSIC] Offer/Answer PT Questions DOLLY, MARTIN C
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Christer Holmberg
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount
- Re: [MMUSIC] Offer/Answer PT Questions Christer Holmberg
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount
- Re: [MMUSIC] Offer/Answer PT Questions DOLLY, MARTIN C
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount
- Re: [MMUSIC] Offer/Answer PT Questions DOLLY, MARTIN C
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount
- Re: [MMUSIC] Offer/Answer PT Questions Colin Perkins
- Re: [MMUSIC] Offer/Answer PT Questions Drage, Keith (Nokia - GB)
- Re: [MMUSIC] Offer/Answer PT Questions Christer Holmberg
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Ben Campbell
- Re: [MMUSIC] Offer/Answer PT Questions Christer Holmberg
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Ben Campbell
- Re: [MMUSIC] Offer/Answer PT Questions Makaraju, Raju (Nokia - US)
- Re: [MMUSIC] Offer/Answer PT Questions Colin Perkins
- Re: [MMUSIC] Offer/Answer PT Questions Christer Holmberg
- Re: [MMUSIC] Offer/Answer PT Questions Dale R. Worley
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Drage, Keith (Nokia - GB)
- Re: [MMUSIC] Offer/Answer PT Questions Paul Kyzivat
- Re: [MMUSIC] Offer/Answer PT Questions Christer Holmberg
- Re: [MMUSIC] Offer/Answer PT Questions Christer Holmberg
- Re: [MMUSIC] Offer/Answer PT Questions Roman Shpount