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
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