[SIPForum-discussion] Early Media

[srinidhi n]

In the scenario of PSTN-SIP gateways, the SDP is
likely to remain the same for both 183 and 200 OK. In
the case, of SIP-B2BUA-SIP, its unlikely that the
SDP's in both 183 and 200 OK will be the same, if the
b2BUA is responsible for playing some
tones/announcements.  If the SDP is different in
183(reliable Tx) and 200 OK, RFC3261 section 13
indicates that only the first SDP answer will be
considered and the   other offers ignored. 
I think, in this case, we need to send the 200 Ok with
the same SDP as the 183 and once, the dialog is
established, send an Re-INVITE with the required SDP.
Not sure, if an UPDATE can be sent to acheive this
before sending the 200 OK, as the purpose of UPDATE is
just to do this.

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