What people should be doing and what people are doing as short cuts are two different things. If a B2BUA (aka 3PCC) attempts to manipulate the SDP in any way shape or form, then it has to become responsible for the integrity of the SDP that results. It cannot just merge media lines and hope it gets away with it. And I would not phrase that as patching up the SDP - the 3PCC has to become the SDP protocol entity. After all, RFC 3725 that you reference for this scenario states: From here, new parties can be added, removed, transferred, and so on, as the controller sees fit. In many cases, the controller will be required to modify the SDP exchanged between the participants in order to affect these changes. In particular, the version number in the SDP will need to be changed by the controller in certain cases. If the controller should issue an SDP offer on its own (for example, to place a call on hold), it will need to increment the version number in the SDP offer. The other participant in the call will not know that the controller has done this, and any subsequent offer it generates will have the wrong version number as far as its peer is concerned. As a result, the controller will be required to modify the version number in SDP messages to match what the recipient is expecting. To me this clearly indicates that the controller has become responsible for the SDP contents. Regards Keith > -----Original Message----- > From: sipping-bounces@xxxxxxxx [mailto:sipping-bounces@xxxxxxxx] On Behalf > Of Elwell, John > Sent: 27 April 2011 08:42 > To: Paul Kyzivat; sipping@xxxxxxxx > Subject: Re: [Sipping] Late comment on draft-ietf-sippping-sip- > offeranswer-14 > > > > > -----Original Message----- > > From: sipping-bounces@xxxxxxxx > > [mailto:sipping-bounces@xxxxxxxx] On Behalf Of Paul Kyzivat > > Sent: 26 April 2011 22:42 > > To: sipping@xxxxxxxx > > Subject: Re: [Sipping] Late comment on > > draft-ietf-sippping-sip-offeranswer-14 > > > > John, > > > > On 4/26/2011 9:37 AM, Elwell, John wrote: > > > I know last call finished already, but the following has > > just been brought to my attention: > > > > > > In section 5.2.5 > > > "Changing the o-line, > > > except version number value, during the session is > > an error case. > > > The behavior when receiving such a non-compliant > > offer/answer SDP > > > body is implementation dependent. " > > > I would content this is NOT an error situation, or at least > > not an error in the case where a NEW session is being signalled. > > > > > > Consider a 3PCC situation along the lines described in > > section 7 of RFC 3725. The controlling B2BUA converts a > > session between UA A and UA B into a session between UA B and > > UA C. Prior to this conversion, UA B has received UA A's SDP > > (SDP A). As a result of the conversion, UA B receives UA C's > > SDP (SDP C). > > > > > > SDP C is likely to be completely different from SDP A. > > Therefore just a change of version number in the o-line is > > insufficient and would probably violate RFC 3264. In > > particular, if SDP A has 2 m-lines and SDP C has only one > > m-line, the change from 2 m-lines to 1 m-line is not > > permitted according to RFC 3264. So although RFC 3725 talks > > about the controlling B2BUA adjusting version numbers, that > > is insufficient in some cases. > > > > It was precisely issues like this that led to the statements you are > > taking issue with. > > > > As I understand it, what you describe is not permitted - you can't > > reduce the number of m-lines, even by changing the o-line. > [JRE] Where is that stated normatively? > > > > > > This does put a burden on the 3pcc device - to patch up the SDP. > [JRE] This MIGHT be feasible, but it goes way beyond just manipulating > version numbers. Basically the B2BUA would have to retain state about > which m-lines are in use in each leg of the call (i.e., to B and to C) and > perform mappings each time a SDP is passed through (e.g., the second m- > line from B becomes the third m-line to C and vice versa). I wonder how > many implementations do this today? > > > I would actually prefer to have a change that would loosen up > > what can > > be done in this regard but it would be a normative change with pretty > > severe backward compatibility issues. > > > > > The text of 5.2.5 then goes on to say: > > > "The behavior when receiving such a non-compliant offer/answer SDP > > > body is implementation dependent." > > > It is not clear what this fails to comply with. I can find > > nothing in RFC 3264 that stops you sending a new o-line if > > there is a new session. Yes, it would be non-compliant if > > only modifying an existing session, but how does the > > recipient know whether or not it is a new session, and > > therefore whether or not it is valid? > > > > I think you are describing "SessionS Initiation Protocol", not the > > "Session Initiation Protocol". AFAIK you only get one session per > > invite-dialog-usage. > [JRE] Again, where is that stated normatively? > > > > > > It then goes on to recommend use of Replaces in this > > situation (i.e. change of session): > > > "If a UA needs to negotiate a > > > 'new' SDP session, it should use the INVITE/Replaces method." > > > But Replaces is not feasible if the UA concerned does not > > support it (and hence "should", presumably). So there will > > still be cases where a controlling B2BUA is forced to change > > the o-line (not just the version) in order to comply with RFC 3264. > > > > > > So there needs to be a mechanism for changing to a 'new' > > session without relying on Replaces. As far as I can see, > > there is no standards track RFC that forbids changing the > > o-line to achieve this, so this new Informational draft > > should not attempt to make that change, and in particular > > should not do so without proposing an alternative solution. > > > > I think the mechanism requires a normative change to SIP. > [JRE] That depends - it is unclear to me what normative statements are > broken by starting a new session with a new o-line. > > John > > > > > > But I'm interested to hear what others think about this. > > > > > A simple fix would be to delete the entire bullet beginning > > "In the o-line, only the version number may change". > > > > Its awfully late - in the category of the "it ain't happening > > unless you > > lodge a complaint with the IESG". > > > > But regardless of that, it seems we have a difference of opinion here > > about what the standard is, and should discuss it. > > > > Thanks, > > Paul > > _______________________________________________ > > Sipping mailing list https://www.ietf.org/mailman/listinfo/sipping > > This list is for NEW development of the application of SIP > > Use sip-implementors@xxxxxxxxxxxxxxx for questions on current sip > > Use sip@xxxxxxxx for new developments of core SIP > > > _______________________________________________ > Sipping mailing list https://www.ietf.org/mailman/listinfo/sipping > This list is for NEW development of the application of SIP > Use sip-implementors@xxxxxxxxxxxxxxx for questions on current sip > Use sip@xxxxxxxx for new developments of core SIP _______________________________________________ Sipping mailing list https://www.ietf.org/mailman/listinfo/sipping This list is for NEW development of the application of SIP Use sip-implementors@xxxxxxxxxxxxxxx for questions on current sip Use sip@xxxxxxxx for new developments of core SIP
