Hi folks,
* As I understand it the endpoint B is getting an offer of
2 codecs (8 and 119) and must respond from choosing one
or more of these 2 codecs that were present in the offer.
Giving a third codec (97) in the answer is not allowed.
If the endpoint B understands that 97 and 119 are dynamic
payload numbers for the same codec (as in this case
telephone-event/8000), it should respond with 119 in the
answer. If endpoint B understands that they are different
codecs (with the help of the rtpmap line), then it should
reject 119. Quoting from page 10 of the offer/answer-02
draft:
"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."
So, the correct SDP answer from the endpoint B would be:
m=audio 10022 RTP/AVP 8 119
a=rtpmap:8 pcma/8000
a=rtpmap:119 telephone-event/8000
and both parties SHOULD use the offerer's payload type.
* Secondly, the SDP default direction attribute is 'sendrecv'
and both parties should be explicit only if they don't
support bi-directional transfer for the media they are
describing in the SDP. So, this is not really a deficiency
of SDP, right? Or I missed something here?
Regards,
Siddharth.
----------------
Siddharth Toshniwal @ Hughes Software Systems
http://www.hssworld.com
"Bert Culpepper" <[EMAIL PROTECTED]> on 05/21/2002
09:03:49 PM
To: "'Attila Sipos'" <[EMAIL PROTECTED]>,
[EMAIL PROTECTED]
cc: (bcc: Siddharth J Toshniwal/HSSBLR)
Subject: RE: [Sip-implementors] Quick SDP dynamic payload question
comment below...
Attila Sipos wrote:
>
>
> Hi,
>
> If an endpoint A sends SDP containing "m=" and "a=" lines
> like this:
>
> m=audio 10036 RTP/AVP 8 119
> a=rtpmap:8 PCMA/8000
> a=rtpmap:119 telephone-event/8000
>
> I understand this to mean that the sender of this SDP
> wishes to receive DTMF in an RFC2833 RTP stream with
> an RTP payload of 119.
>
>
> If the replying endpoint B, send an SDP with the
> following "m=" and "a=" lines:
>
> m=audio 10022 RTP/AVP 8 97
> a=rtpmap:8 pcma/8000
> a=rtpmap:97 telephone-event/8000
>
> Then I understand this to mean that this person wishes
> to receive an RFC2833 RTP stream with an RTP payload
> of 97.
>
> So, if I am right, endpoint A should send RFC2833 RTP
> with an RTP payload of 97.
>
> And endpoint B should send RFC2833 RTP with an
> RTP payload of 119.
>
> Is my understanding correct?
IMO, this is one main "deficiency" with SDP (RFC 2327) in SIP. Since there
are no direction attributes in the above, it would seem that neither
endpoint will "communicate" telephone-events to the other. Neither
endpoint
has agreed on the payload type that they will both send and receive with.
If the endpoints want to use a different payload type for a specific
encoding, they need to be explicit IMHO about which value they'll use in
each direction.
Regards,
Bert
>
> Any help is greatly appreciated.
>
> Thanks,
>
> Attila
>
>
>
> Attila Sipos
> Software Engineer
>
> <mailto:[EMAIL PROTECTED]>
> <http://www.vegastream.com>
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