Please see inline. On Tue, Sep 9, 2008 at 2:19 PM, Bram Verburg <[EMAIL PROTECTED]> wrote:
> 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. > > Once an offer answer completes, you are left with some state of session. > You can not say that I have answer or offer now. There is nothing in the SDP > that says that I am answer or offer. Therefore in case of session-timers, if > the most recent answerer is sending session-refresh INVITE, it will have SDP > same as the answer that it sent which would be its present session state. > > 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 reason why re-INVITE without SDP is discouraged is because the UAS may > send back its full capabilities instead of just its present state of > session. In this case, you need to again re-negiotiate session along with > session refresh. For example, if you are in audio media session and you send > re-INVITE without SDP to UAS which supports video session too, the offer may > contain both audio and video media descriptions instead of just audio. > > The alternative would be to send the original answer as the offer, and > be prepared to receive a new SDP as the answer. > > Yes, this SDP will be exactly same as your answer assuming you are doing > only session refresh. > > Any thoughts? > > > > Thanks, > > > > Bram > > > > _______________________________________________ > Sip-implementors mailing list > [email protected] > https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors > _______________________________________________ Sip-implementors mailing list [email protected] https://lists.cs.columbia.edu/cucslists/listinfo/sip-implementors
