[SIPForum-discussion] FW: [Sip] MUST 200 OK contain SDP? Thanks

[Jean-Sébastien Decubber]

Hello,

please find some answers below in orange ... hope it helps ...



2008/5/21 Yuantao Zhang <yuantao.zhang at ericsson.com>:

>  Dear all
>
> I am really confused by the following part in RFC_3261. Could you please
> help us to explain it? Thank you very much.
>
> "
>
> 13.2.1 Creating the Initial INVITE
>
> .
>
> .
>
> .
>
>       o  If the initial offer is in an INVITE, the answer MUST be in a
>
>          reliable non-failure message from UAS back to UAC which is
>
>          correlated to that INVITE.  For this specification, that is
>
>          only the final 2xx response to that INVITE.  That same exact
>
>          answer MAY also be placed in any provisional responses sent
>
>          prior to the answer.  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.
>
> .
>
> .
>
> ."
>
> Let's break it into 2 parts, in part 1,
>
>          "If the initial offer is in an INVITE, the answer MUST be in a
>
>          reliable non-failure message from UAS back to UAC which is
>
>          correlated to that INVITE.  For this specification, that is
>
>          only the final 2xx response to that INVITE. "
>
> Does that mean if "Invite" contains SDP offer, "200 OK" MUST contain SDP
> answer. ==> yes!!! if the invite contains an SDP the 200 OK msut also
> contains an SDP
>
> In part 2,
>
>          "That same exact answer MAY also be placed in any provisional
> responses sent
>
>          prior to the answer.  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."
>
> Does that mean if "Invite" contains SDP offer, the answer SDP MAY be in "180
> ring" and not in "200 OK"? I think part 1 and part 2 are inconsistent. ==>
> Not really. In this case, the 180 ringing AND the 200 OK contain both SDP
> (So the part 1 is still correct). This way of proceeding is used to sends
> early media (as ringing tone)
>
> The attached sequence chart is the real problem we meet. Sx1 sends "bye"to sx2 after the "200
> OK" because there is no SDP answer in "200 OK". But the producer of sx2
> argues that the SDP answer is in "180 ring" already. No need to repeat it
> in "200 OK". Could you please tell me who is right? MUST 200 OK contain
> SDP? Thanks you very much.
>
>
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