Hi In draft-ietf-mmusic-sdp-capability-negotiation it is proposed that the transport in the m-line is modified in the SDP answer. While this is a good idea in order to reduce the number of offer/answer exchanges it can in fact cause problems with intermediaries that for instance compare offer and answer SDP's. The problem may be exacerbated further with the addition of extensions to the framework. Section 3.12 in the draft says: "The solution to this problem is to upgrade the intermediary to support the SDP Capability Negotiation framework". We find the conclusion unsatisfactory as the problem is that such upgrades are not done overnight and there will therefore, for a foreseeable future, exist middleboxes in the network that does not understand the SDP Capability Negotiation Framework. Our proposal to solve this issue is to introduce an indicator in the SDP that tells that the actual configuration MUST only be indicated on the a=acfg line. A proposed SDP attribute for this may be "a=spdcapneg-acfg-indication-only". In essence it means that (if the indicator is included in the SDP) the first offer/answer exchange will only be done to get the actual configuration (a=acfg...). This would affect section 3.6.2 in the draft. If the indicator is included in the offer SDP it makes a 2nd offer/answer exchange a MUST in order to complete the offer/answer exchange. Section 3.6.3 specifies a "SHOULD" regarding the behavior if the actual cofiguration and the SDP does not match. It is possible that "SHOULD" must be changed to "MUST" here. A UA that for some reason knows that intermediaries don't understand the new framework it will add the said SDP attribute at the session level in the offer-SDP. Indications that intermediaries don't understand the new framework may for instance in the 3GPP IMS case be that for a specific 3GPP release the said attribute is mandated, a possible location for such a text in this particular case is 3GPP TS26.114. We believe that the addition of the new functionality to the framework will increase acceptance for the SDP Capability Negotiation framework greatly and this without breaking the framework. PS. This issue has been raised previously in the MMUSIC WG, still we want to bring this up again for your consideration. Regards Ingemar Johansson Christer Holmberg _______________________________________________ Ietf@xxxxxxxx https://www.ietf.org/mailman/listinfo/ietf