Hi Siddharth,
Thanks again for the feedback.
Despite your answer, I'm still not convinced
that you are right (and I'm not convinced that
my proposed theory is right).
We agree that the answerer must transmit RTP with
a payload field set as specified in the offerer's SDP:
"In the case of RTP, it [the answerer] MUST use
the payload type numbers from the offer, even
if they differ from those in the answer."
However, I cannot see any explicit statement
regarding the payload field in the offerer's
RTP packets.
The reason I posed this question is that different
vendors are doing different things. I was just trying
to find the definitive answer so that the issues caused
by the different implementations could be resolved.
Thanks to everyone for their input so far.
Regards,
Attila
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
> Sent: 22 May 2002 12:09
> To: Attila Sipos
> Cc: Bert Culpepper; [EMAIL PROTECTED]
> Subject: RE: [Sip-implementors] Quick SDP dynamic payload question
>
>
>
>
>
> Hi Atilla,
>
> > Then, given that the draft says
> > (end of section "6.1 Unicast Streams"):
> > "In the case of RTP, it MUST use the payload type numbers
> > from the offer, even if they differ from those in the
> > answer."
> >
> > I believe "it" is referring to the answerer.
>
> True. I believe so too :)
>
> > So, therefore, from the example scenario, endpoint B
> > (the answerer) must send to port 10036 using an RTP payload
> > of 119.
> >
> > And, though not explicitly stated, I believe endpoint A
> > (the offerer) must send to port 10022 using an RTP payload
> > of 97.
>
> Right about the ports, incorrect about the RTP payload I feel.
>
> As per the draft:
>
> * offer SDP -
> > m=audio 10036 RTP/AVP 8 119
> > a=rtpmap:8 PCMA/8000
> > a=rtpmap:119 telephone-event/8000
>
> * a proper SDP answer SHOULD be -
> > m=audio 10022 RTP/AVP 8 119
> > a=rtpmap:8 pcma/8000
> > a=rtpmap:119 telephone-event/8000
>
> * It MAY (if the endpoint B does not form the SDP
> correctly) be -
> > m=audio 10022 RTP/AVP 8 97
> > a=rtpmap:8 pcma/8000
> > a=rtpmap:97 telephone-event/8000
>
> *However* both parties must use 119 as the RTP payload
> (and port numbers as you said).
>
> Let me elaborate a bit. Again, there's one slight
> misunderstanding here. What each party considers as
> the SDP "offer" is the same SDP description:
> The SDP sent by endpoint A to endpoint B.
>
> So if both endpoints obey the snippet you sent in your mail,
> "it (answerer) MUST use the payload type numbers
> from the offer, even if they differ from those in the
> answer."
>
> Even if the answerer sends payload "97" in the SDP answer,
> the RTP payload MUST use 119 as that was the payload number
> in the offer SDP. Please correct me if I missed something
> here as this is my understanding as of now.
>
> Regards,
> Siddharth.
>
> ------------------
> Siddharth Toshniwal @ Hughes Software Systems
> http://www.hssworld.com
>
>
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