[SIPForum-discussion] Codec Prefrence Option

[Hadriel Kaplan]

No, you don't need to change the offerer - your SDP answer should contain only the one codec of those offered that you want to use, if you only want to use one (729 in this case).  There's no reason to answer both codecs if you don't want to use the second one.

-hadriel

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From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Shakil, Rashid
Sent: Friday, May 16, 2008 4:47 PM
To: Shakil, Rashid; Manpreet Singh; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Codec Prefrence Option

Sorry let me re-phrase it if the offerer preference takes precedence then sending a single or multiple choices in 200OK wouldn't matter. The only way I can avoid the RE-INVITE is by having offrere setting up only one codec instead of multiple choices. Please correct me if I phrase it wrong.

-----Original Message-----
From: Shakil, Rashid
Sent: Friday, May 16, 2008 4:41 PM
To: 'Manpreet Singh'; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] Codec Prefrence Option

Manpreet,

Thanks for the comprehensive reply. The goal which is to negotiate media over to G729 and by reading the specs I came to the conclusion that the only way I can avoid the RE-INVITE is by answering the specific codec in my case 729 only with 200 OK.

Regards,

Rashid Shakil.

-----Original Message-----
From: Manpreet Singh [mailto:msingh at ibasis.net]
Sent: Friday, May 16, 2008 4:13 PM
To: Shakil, Rashid; discussion at sipforum.org
Subject: RE: [SIPForum-discussion] Codec Prefrence Option

Rashid

The flow is valid and usually what the offerer do also. Check section 10.2 of rfc 3264 and you will get your answer. When the answer carries a list of codec, the original offerer would "usually" send a new offer with a single codec to negotiate down to one. This is usually done when the UAS wants to include all in the answer and let offerer decide what it wants/prefers. The preference in the 200OK doesnt drive the codec selection but what comes in INVITE or offer does. ( in your case ) Answer is the preference local to answerer but offerer picks the preference.

In SIP, the offerer preference takes precedence. In H323 world, the so called answerer preference takes precedence. Its the recommended way. In your case if you include all codecs, it would mean your switch should be ready to listen on those media streams anytime in the session. Not sure if thats what your intention is? So if you want to use your preferred one, I would say only include one media stream for simplicity in the answer.

>From spec something about preference.

 Although the answerer MAY list the formats in their desired order of
   preference, it is RECOMMENDED that unless there is a specific reason,
   the answerer list formats in the same relative order they were
   present in the offer.  In other words, if a stream in the offer lists
   audio codecs 8, 22 and 48, in that order, and the answerer only
   supports codecs 8 and 48, it is RECOMMENDED that, if the answerer has

   no reason to change it, the ordering of codecs in the answer be 8,
   48, and not 48, 8.  This helps assure that the same codec is used in
   both directions.

Hope it help
M

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From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Shakil, Rashid
Sent: Friday, May 16, 2008 3:57 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Codec Prefrence Option

I came across to this situation where we receive inbound call the codec preference set on the SIP INVITE is 711 first and 729 second (m=audio 50000 RTP/AVP 0 18 100) when our softswitch reply back with a 200 OK with SDP we set the preference as 729 first and 711 second (m=audio 3006 RTP/AVP 18 0 100). Instead of cutting the media over to 729 (means select this as a codec because the 200 OK has 729 set as preference and initial INVITE has both options) The peering partner sends a RE-INVITE with 729 as the only option (a=rtpmap:18 G729/8000).

My question is this legal in SIP. Is codec can be negotiated that way as well. I thought that carrier should negotiate the codec base on the preference my Softswitch set on 200 OK




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