Hi,
RFC 4038 says a session refresh that uses INVITE should include a copy of the most recent SDP offer. The UAS can tell from the o-line that nothing has changed and respond with a copy of its original answer. This allows for session refresh without going through the trouble of re-negotiation the session parameters. But what happens if the current refresher happens to be the side that answered in the most recent SDP exchange? Its most recent SDP is an answer, not an offer. If the answer is well formed I don't think there is any problem using it as an offer, but the original offer may not be legal as the answer. For example, it might include m-lines or codecs that were rejected in the answer. A seemingly straightforward solution might be to send INVITE without SDP, and rely on the UAS to include its original SDP offer, then reply with the original SDP answer in ACK. But RFC 4038 says: "It is recommended that the UPDATE request not contain an offer, ***but a re-INVITE SHOULD contain one***, even if the details of the session have not changed.". This suggests INVITE without SDP is discouraged for session refresh. The alternative would be to send the original answer as the offer, and be prepared to receive a new SDP as the answer. Any thoughts? Thanks, Bram _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
