[asterisk-dev] SDP negotation problem

[Olle E. Johansson]

8 feb 2010 kl. 02.18 skrev Kevin P. Fleming:

> Salvatore Frandina wrote:
> 
>> I'm using SIPp program to make a call toward Asterisk PBX. The SIP call
>> works well with Asterisk 1.6 (1.6.0.x, 1.6.1.x and 1.6.2.x) but not with
>> Asterisk 1.4.x. After I have analyzed the sip message between SIPp and
>> Asterisk 1.4.29 I have discovered a discrepancy in SDP message.
>> The RTP payload type is different: SIPp sends 96 dynamic payload type
>> for H263-1998 - H264 while Asterisk response 103 dynamic type for
>> H263-1998 - H264. Why? With the other versions there are not problems.
> 
> SDP offer/answer does not require that both ends use the same payload
> type numbers for matching payloads; they can differ and it's perfectly
> acceptable. This is not an error.
> 
RFC 3264:

" 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.
"

As a developer, we should tread SHOULD treat SHOULD as a MUST, remember :-)

/O