Subject: [MMUSIC] RFC3264: answer indicate a diff PT number

From: "kkuei" <[email-masked]>
Date: Wed, 2 Mar 2005 23:56:55 +0800

Hello all,

I'm a little bit confusing in rfc3264 (offer/answer model with SDP) when 
answer indicate a diff payload type number case. Is there anybody can help 
me?? 

in 5.1, it says: 
  For sendrecv RTP 
  streams, the payload type numbers indicate the value of the payload 
  type field the offerer expects to receive, and would prefer to send. 
  However, for sendonly and sendrecv streams, the answer might indicate 
  different payload type numbers for the same codecs, in which case, 
  the offerer MUST send with the payload type numbers from the answer. 

in 6.1, it says: 
  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.  The 
  media formats in the "m=" line MUST be listed in order of preference, 
  with the first format listed being preferred.  In this case, 
  preferred means that the offerer SHOULD use the format with the 
  highest preference from the answer. 

My question is, the payload type number in answer may be different than in 
offer. Here is the case: 

1. Offer 
PCM-WB: 98 
ISAC: 101 
iLBC: 102 

2. Answer 
iLBC: 98 

This means both parties agreed to use iLBC. 
My questionis , what payload type number should be use in constructing RTP 
pkts. 

In sec5.1, (in my understanding) it says offerer MUST respect the answer. It 
MUST put "98" in 
payload type number in RTP. 

But what about in the answerer?? should it respect the offer? which payload 
type 
number (102 or 98) it should put in RTP?? 

--
Kevin Kuei

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From: Paul Kyzivat <[email-masked]>
Date: Wed, 02 Mar 2005 12:16:56 -0500

This was just discussed on the sip-implementors list. The answer to your
question at the end is "102".

	Paul