[Sipping] Question wrt transcoding

["Jose Rey" <Jose.Rey at eu.panasonic.com>]

[I am new to the list, sorry if question is already answered]

Hi,

I have a (basic) question regarding O/A in the transcoding scenario; it refers to http://www.ietf.org/internet-drafts/draft-ietf-sipping-transc-3pcc-02.txt

If an answerer B can receive audio but cannot understand any of the codecs that the offerer A includes in the INVITE, it usually sets the port=0 in the corresponding m line in the answer.  This is understood.  But how does the SIP UA acting as controller know that this means "No, I don't want that m line" OR "I want it but I cannot support it"??

Is it by including an additional m line with the proposed formats in the answer as mentioned in RFC 3264:

"Rosenberg & Schulzrinne     Standards Track                     [Page 9]

RFC 3264  An Offer/Answer Model Session Description Protocol   June 2002


   For streams marked as recvonly in the answer, the "m=" line MUST
   contain at least one media format the answerer is willing to receive
-->   with from amongst those listed in the offer.  The stream MAY indicate
-->   additional media formats, not listed in the corresponding stream in
-->   the offer, that the answerer is willing to receive.  For streams
   marked as sendonly in the answer, the "m=" line MUST contain at least
   one media format the answerer is willing to send from amongst those
   listed in the offer.  For streams marked as sendrecv in the answer,
   the "m=" line MUST contain at least one codec the answerer is willing
   to both send and receive, from amongst those listed in the offer.
   The stream MAY indicate additional media formats, not listed in the
   corresponding stream in the offer, that the answerer is willing to
   send or receive (of course, it will not be able to send them at this
   time, since it was not listed in the offer). "


Is this how it is usually detected and what every SIP client does?


Thanks,

José


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