Romel Khan wrote:
>
> If an INVITE contains an initial offer, per RFC3261 "the answer MUST be
> in a reliable non-failure message from UAS back to UAC.... The UAC MUST
> treat the first session description it receives as the answer, and MUST
> ignore any session descriptions in subsequent responses to the initial
> INVITE."
>
> Say a UAS for basic telephony sends back 180 Ringing without SDP when it
> receives INVITE with SDP to cause local ringing. Then the UAS would send
> 200 OK with SDP for the INVITE when call answers.
> So does this mean that if an INVITE contains an SDP offer with support
> for reliable provisional response, the UAS cannot send SIP 180 response
> without SDP as a reliable provisional response (RFC3262)? This implies
> that first reliable response must carry the SDP answer.
No, it need not in this case.
> Or does it mean that 180 can still be sent reliably, and the lack of SDP
> in 180 will cause UAC to wait for SDP answer in some subsequent
> response, and PRACK must not contain SDP? Thus implying that first
> reliable response does not have to carry the SDP answer.
Right.
OTOH, if the INVITE has no offer, then there MUST be an offer in the
first reliable provisional response. So until the UAS is ready to send
an answer, it may only send *unreliable* 1xx responses. If the UAC had
specified Require:100rel, then the UAS may not send any non-100
provisional responses until it is ready to send an answer.
Paul
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