Subject: [SIP] Re: Question on RTP

From: S_S_Dinesh/India/[email-masked]
Date: Mon, 8 May 2000 14:10:19 +0530

Dohoon Kim wrote:
>
> The caller lists the codecs avaiable in the session description in an
> INIVTE,
> then the callee lists the codecs which can be supported among them.
>
> If multiple codec is available in a media stream, such as
>
> m=audio 1 RTP/AVP 0 1
>
> which codec should be selected by caller,

Any one they want.

>
> and how can the callee know which codec is selected?

By the payload type number in the RTP packets.

-Jonathan R.

--
Jonathan D. Rosenberg                       200 Executive Drive
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[email-masked]                     FAX:   (732) 741-4778
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From: S_S_Dinesh/India/[email-masked]
Date: Mon, 8 May 2000 14:10:12 +0530

--> [email-masked] writes:
          >I like to add one more question:
          >Is it absolutely necessary that both direction use the same
type of
          >codec ?

          No.

          This is rather undesirable for vocoders or implementations
employing advanced echo cancellation, intelligent jitter buffer management
or a host of other DSP or voice-related properties.

          From my reading of the RFC, it is also equally possible for
either side to chop-and-change between vocoders at any point in time
(without SIP message exchange). Supporting this mixing and splicing of
vocoders significantly complicates the playback engine - the ability will
most likely be unsupported on simpler implementations or at best may cause
small audio glitches.

          It would be nice if the caller could force the callee to
agree on a single common vocoder if it doesn't support the use of multiple
vocoders. [Making the SDP session closer to a true two-way media exchange
rather than simply two disjoint vocoder packet flows in each direction.]

          One approach might be to place an updated SDP body in the
ACK where the SDP body has only one media format selected. The RFC
currently
prohibits changing the SDP body in the ACK (unless the INVITE had no SDP
body). Does changing the body introduce any major problems or
inconsistencies ?

          Furthermore, IMO it would be desirable if the callee could
indicate their vocoder preference by ordering the list of RTP media format
values in the 183/200 response.

          How have other people tackled this problem ? Are there any
drafts or FAQ on these subjects.

          Cheers,

          Robert.