G723.1H and G723.1L SDP negotiation

[Elison Niven]

I apologize for posting to this list as I already posted this on SIP
implementers list but somehow this seems to be the better place to ask.

Hi List,

I know this has been discussed and asked a lot of times in the past but
still I find no affirmative solution.

For a UA that supports both G7231-H and G7231-L, SHOULD it specify both the
codecs in the offer that it sends?

If the UA is willing to specify both the codecs in the offer that it sends
(because it specifies two different codecs in its user interface), what
should be the method of specifying the codecs in the SDP body?

I find two possible ways of specifying both the codecs in the offer:

Case 1: Both codecs use different payload types, one of them uses a dynamic
payload type.

m = audio 8000 RTP/AVP 4 97
m = rtpmap:4 G723/8000
m = fmtpmap:4 bitrate=5.3
m = rtpmap:97 G723/8000
m = fmtpmap:97 bitrate=6.3

Case 2: Both codecs use the same static payload type.

m = audio 8000 RTP/AVP 4 4
m = rtpmap:4 G723/8000
m = fmtpmap:4 bitrate=5.3
m = rtpmap:4 G723/8000
m = fmtpmap:4 bitrate=6.3

Which of the above SDP offers seems valid?

Suppose I receive an answer from UA2 to the offer in Case 1 as:

m = audio 8000 RTP/AVP 4
m = rtpmap:4 G723/8000
m = fmtpmap:4 bitrate=5.3

What should be the payload of the packets that UA2 sends? Should it be 4 or
97?

Best Regards,
Elison




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[Kevin P. Fleming]

Post by Elison Niven
I know this has been discussed and asked a lot of times in the past but
still I find no affirmative solution.
For a UA that supports both G7231-H and G7231-L, SHOULD it specify both the
codecs in the offer that it sends?
If the UA is willing to specify both the codecs in the offer that it sends
(because it specifies two different codecs in its user interface), what
should be the method of specifying the codecs in the SDP body?

The G.723.1 recommendation mandates that a receiver be willing and able
to accept frames in both bitrates at any time, without any signaling
required to switch between them. Given that requirement, there is no
need to indicate both bitrates in the SDP and use separate payload
numbers; an offer can include that endpoint's *preferred* bitrate (using
the bitrate= parameter), but the endpoint must be prepared to accept
frames in both bitrates regardless.
--
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: ***@digium.com
Check us out at www.digium.com & www.asterisk.org
_______________________________________________
Sip mailing list https://www.ietf.org/mailman/listinfo/sip
This list is essentially closed and only used for finishing old business.
Use sip-***@cs.columbia.edu for questions on how to develop a SIP implementation.
Use ***@ietf.org for new developments on the application of sip.
Use ***@ietf.org for issues related to maintenance of the core SIP specifications.