> When codec negotiation is taking place,
> - Can the far-end send its codec capabilities in the Trying and
> Ringing response messages all the way upto 200(OK) message ?
> (There have been some discussions around not allowing specifying
> the codec preferences in the 18x message ??)
SDP is not really valid in 100 Trying since
it isn't end-to-end.
SDP is valid in other 1xx responses, and the
SDP values can change for each 1xx response.
However note that SDP in 1xx is no longer a
part of basic SIP. This functionality has
been moved back out into a yet to be updated
SIP 183 extension draft.
> - If it can send its preferences in any/all of these messages,
> can it change its codec preferences between any of the 3 messages
> as long as what it specified in the 200(OK) message is its
> final preference ?
Yes.
> - Can a different codec be specified in the ACK (that's different
> from what was sent in the INVITE ?)
If a complete SDP was supplied in the INVITE,
no SDP should be in the ACK.
If a complete SDP was supplied in both INVITE
and ACK, the expected result has/will not be
defined since it is discouraged.
http://www.cs.columbia.edu/sip/faq/cache/74.html
Note: the codecs in an SDP only signify the
listening codecs. They are not used to
identify what you plan to send unless maybe
a oneway media session is being established.
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