Re: [Sip] SDP telephone-event (DTMF) payload negotiation
Paul Kyzivat <pkyzivat@alum.mit.edu> Wed, 23 November 2011 17:36 UTC
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Date: Thu, 24 Nov 2011 01:36:35 +0800
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [Sip] SDP telephone-event (DTMF) payload negotiation
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On 11/22/11 6:25 PM, RUOFF, LARS (LARS)** CTR ** wrote: > Thanks, Paul. > >> It doesn't quite say that the offerer must send with a pt listed in the answer, but its clear for consistency that it should. That is widely understood to be the intent. > > That's how I understood it, too. But it seems that opinions diverge on this. > Maybe the RFC authors should insist on the above a bit more explicitely. In retrospect, yes. But the RFC has been published already. You can file an errata report. That will probably be queued pending a revision to the RFC. Thanks, Paul > Regards, > Lars > > -----Original Message----- > From: sip-bounces@ietf.org [mailto:sip-bounces@ietf.org] On Behalf Of Paul Kyzivat > Sent: vendredi 18 novembre 2011 13:04 > To: sip@ietf.org > Subject: Re: [Sip] SDP telephone-event (DTMF) payload negotiation > > On 11/18/11 4:22 PM, RUOFF, LARS (LARS)** CTR ** wrote: >> Hi all, >> >> New to this list. Apologies if this question has been asked many times before (my searches showed that it has, but i was still unable to find a definitive answer). >> >> The question is about SDP telephone-event (DTMF) payload negotiation. >> >> Imagine the following call setup between A and B: >> INVITE A->B >> SDP: >> (among other media formats) >> a=sendrecv >> a=rtpmap:101 telephone-event/8000 >> >> 200 OK B->A >> SDP: >> (among other media formats) >> a=sendrecv >> a=rtpmap:97 telephone-event/8000 >> >> The question is: >> Is the above legal? I.e. is B allowed to choose a different PT than proposed y A. >> In case yes, what PT should the telephone-events be sent... >> from A to B? >> from B to A? >> >> Please corroborate your answers by providing normative references if possible. >> >> I studied RFC 3264 (SDP Offer/Answer Model) intensively, but could not conclude for a definitive answer in that particular case. > > From section 6.1: > > In the case of RTP, if a particular codec was referenced with a > specific payload type number in the offer, that same payload type > number SHOULD be used for that codec in the answer. Even if the same > payload type number is used, the answer MUST contain rtpmap > attributes to define the payload type mappings for dynamic payload > types, and SHOULD contain mappings for static payload types. The > media formats in the "m=" line MUST be listed in order of preference, > with the first format listed being preferred. In this case, > preferred means that the offerer SHOULD use the format with the > highest preference from the answer. > ... > Once the answerer has sent the answer, it MUST be prepared to receive > media for any recvonly streams described by that answer. It MUST be > prepared to send and receive media for any sendrecv streams in the > answer, and it MAY send media immediately. The answerer MUST be > prepared to receive media for recvonly or sendrecv streams using any > media formats listed for those streams in the answer, and it MAY send > media immediately. When sending media, it SHOULD use a packetization > interval equal to the value of the ptime attribute in the offer, if > any was present. It SHOULD send media using a bandwidth no higher > than the value of the bandwidth attribute in the offer, if any was > present. The answerer MUST send using a media format in the offer > that is also listed in the answer, and SHOULD send using the most > preferred media format in the offer that is also listed in the > answer. In the case of RTP, it MUST use the payload type numbers > from the offer, even if they differ from those in the answer. > > From section 7: > > When the offerer receives the answer, it MAY send media on the > accepted stream(s) (assuming it is listed as sendrecv or recvonly in > the answer). It MUST send using a media format listed in the answer, > > From the above, "the same payload type number SHOULD be used", so its possible that a different payload type is used. And if so, > > - the answerer must send with the pt listed in the offer > - the offerer must send with a media format listed in the answer > > It doesn't quite say that the offerer must send with a pt listed in the answer, but its clear for consistency that it should. That is widely understood to be the intent. > > Thanks, > Paul > _______________________________________________ > Sip mailing list https://www.ietf.org/mailman/listinfo/sip > This list is essentially closed and only used for finishing old business. > Use sip-implementors@cs.columbia.edu for questions on how to develop a SIP implementation. > Use dispatch@ietf.org for new developments on the application of sip. > Use sipcore@ietf.org for issues related to maintenance of the core SIP specifications. > _______________________________________________ > Sip mailing list https://www.ietf.org/mailman/listinfo/sip > This list is essentially closed and only used for finishing old business. > Use sip-implementors@cs.columbia.edu for questions on how to develop a SIP implementation. > Use dispatch@ietf.org for new developments on the application of sip. > Use sipcore@ietf.org for issues related to maintenance of the core SIP specifications. >
- [Sip] SDP telephone-event (DTMF) payload negotiat… RUOFF, LARS (LARS)** CTR **
- Re: [Sip] SDP telephone-event (DTMF) payload nego… Paul Kyzivat
- Re: [Sip] SDP telephone-event (DTMF) payload nego… RUOFF, LARS (LARS)** CTR **
- Re: [Sip] SDP telephone-event (DTMF) payload nego… Paul Kyzivat