Hi, We've been struggling with some of the interpretation used in RFC 3261 section 14.2 which has created a conflict between the software we use (FreeSWITCH) and another environment (based on Cisco previously Broadsoft equipment) both of which claim to be RFC 3261 compliant and claim that the other is not.
The problem occurs when the 3rd party sends an SDP with the media attribute 'a=sendonly' on an existing session then follow with a RE-INVITE with-out an SDP, they claim that our 2xx offer in response should contain an SDP with-out 'a=sendonly' (or replace with 'a=sendrecv') based on the interpretation of a "brand new call" used below. Anthony Minessale II (FreeSWITCH lead) claims that "brand new call" is only intended to refer to codecs (not all media attributes) and that the 3rd party (Broadsoft) invented this concept on their own. RFC 3261 14.2 UAS Behavior A UAS providing an offer in a 2xx (because the INVITE did not contain an offer) SHOULD construct the offer as if the UAS were making a brand new call, subject to the constraints of sending an offer that updates an existing session, as described in [13] in the case of SDP. Specifically, this means that it SHOULD include as many media formats and media types that the UA is willing to support. The UAS MUST ensure that the session description overlaps with its previous session description in media formats, transports, or other parameters that require support from the peer. This is to avoid the need for the peer to reject the session description. If, however, it is unacceptable to the UAC, the UAC SHOULD generate an answer with a valid session description, and then send a BYE to terminate the session. Source: https://tools.ietf.org/html/rfc3261#section-14.2 The 3rd party have also stated that this isn't a call going on hold as it's routing to an ACD group, according to RFC 6337 section 5.3 "the use of sendonly/recvonly is not limited to hold". In the following discussion on this subject involving the authors of RFC 3261 there is a clear indication that a RE-INVITE with-out an SDP should not modify 'a=sendonly', unfortunately this isn't enough to support our argument and our service providers protocol lead have determined that the 3rd party is acting correctly and have asked for more evidence. http://marc.info/?t=98738614300001&r=1&w=2 We have also pointed out that RFC 3264 section 8 states that the offer MAY be identical to the last SDP provided but are promptly referred back to "brand new call" in RFC 3261 section 14.2. RFC 3264 8 Modifying the Session At any point during the session, either participant MAY issue a new offer to modify characteristics of the session. It is fundamental to the operation of the offer/answer model that the exact same offer/answer procedure defined above is used for modifying parameters of an existing session. The offer MAY be identical to the last SDP provided to the other party (which may have been provided in an offer or an answer), or it MAY be different. We refer to the last SDP provided as the "previous SDP". If the offer is the same, the answer MAY be the same as the previous SDP from the answerer, or it MAY be different. If the offered SDP is different from the previous SDP, some constraints are placed on its construction, discussed below. Source: https://tools.ietf.org/html/rfc3264#section-8 I've been struggling to find anything more in RFC which can support our argument, on the contrary I had a response from Paul Kyzivat on IETF Dispatch who directed me towards RFC 6337. RFC 6337 section 5.1 refers us back to RFC 3261 in case of a RE-INVITE and "without regard for what the other party in the call may have indicated previously" would suggest we should be using 'a=sendrecv' in our offer, Paul's response was a follows. As one of the authors of 6337 I will agree that sendrecv is probably what the UAS should be offering given the circumstances. But it ultimately comes down to what it "wants" to be doing at that time. The folly comes when it offers something less than what *it* wants because it imagines (based on prior o/a) that the answerer wants less than it does. This can get you into "stuck on hold" scenarios or other trouble. This contradicts the previous discission on SIP Implementors ( http://marc.info/?t=98738614300001&r=1&w=2), which behaviour is correct and does the RFC need to be updated? Hope someone here can help. Regards, Shaun Shaun Stokes - T : 0800 0489300 E : [email protected] W : www.sysconfig.cloud SYSCONFIG is a trading name of ITEC Support LTD which is a limited company registered in England and Wales. Company Registered Number 06908001. Registered office: Suite 2, Prospect House, Bath Road Trading Estate, Stroud, GL5 3QF. 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