Subject: RE: Question on RTP

From: "Fairlie-Cuninghame, Robert" <[email-masked]>
Date: Tue, 21 Mar 2000 01:44:50 -0800

Ahhh .. sorry, the message lost some quotation marks in the Microsoft
Outlook translation.

Just to clarify: The "No" at the top of the message was quoted from Colin
Perkins reply.

Robert.

		-----Original Message-----
		From:	Fairlie-Cuninghame, Robert
[mailto:[email-masked]]
		Sent:	Tuesday, March 21, 2000 5:37 PM
		To:	'Colin Perkins'; [email-masked]
		Cc:	[email-masked]
		Subject:	RE: Question on RTP

				--> [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.