Original text:
If a data channel m= section has been offered, a m= section MUST also be generated for data.  The <media> field MUST be set to "application" and the <proto> and "fmt" fields MUST be set to exactly <del> match the fields in the offer. </del>  Within the data m= section, an "a=mid" line MUST be generated and included as described above, along with an "a=sctp-port" line

Reviewer comments:
attributes. As far as I can tell this can happen: On re-offers On any answer because nothing requires the offerer to generate bundle tags in the same order as m= lines (even though the data channel m= line is last) See: URL
NAME PTAL
Say you have a session where only a data channel is created. SDP looks like [session boilerplate] a=group:BUNDLE d1 m=X application UDP/DTLS/SCTP webrtc-datachannel a=mid:d1 ... Now, a media m= section is added (i.e. after first o/a). SDP becomes: [session boilerplate] a=group:BUNDLE d1 a1 m=X application UDP/DTLS/SCTP webrtc-datachannel a=mid:d1 a=ice-ufrag:ABCD ... m=X audio UDP/TLS/RTP/SAVPF 0 a=mid:a1 [no ice-ufrag or TRANSPORT attrs, since bundled] Now, where does a=rtcp-mux go? It can't go into because of the language that states Thoughts on how to resolve?
Could generating a session description be split into two parts? First, generate media-level attributes for each "m=" section, then transport-level attributes for the BUNDLE group and each non-bundled "m=" section.
That would be my preference, i.e., jam it into the BUNDLE group m= section, regardless of media type, as that's conceptually simplest, and easily handled here in JSEP. Either way, I suspect this will require changes upstream in the bundle spec.
So ... is it really not allowed in the d1 section. Unknown attributes are allowed in the d1 section, they are just ignored if they have no meaning for the data channel. Agree this sounds like something that will hit bundle.
Couldn't you do a=group:BUNDLE a1 d1 I would have thought this was prohibited, but is it really? URL
I think that's legal, but that approach doesn't work in the general case, where "m=" section A has TRANSPORT attributes that only it uses, and section B has TRANSPORT attributes that only it uses. BUNDLE just needs to allow an "m=" section to contain attributes that it normally wouldn't contain if it wasn't bundled. (If this isn't already allowed).
NAME any action needed here?
I think what we want in this scenario is to put "a=rtcp-mux" and "a=rtcp-rsize" in the data channel section. The simplest way would be to say: Which I guess could only happen in subsequent offers.
Let's go with what NAME suggests, although this makes it a bundle problem.
What would you say is needed on the BUNDLE side? Just clarification that the offerer BUNDLE-tag m= section should contain the union of all TRANSPORT and IDENTICAL attributes used by the BUNDLE group?
talked to chairs and advice from chairs on this and similar things that depend on mmusic outcomes. We put in JSEP now what our best guess is of how mmusic will end up and publish JSEP. At the same time we log a bug to check if we need to make any changes to line up with what mmusic decides. We can take changes to line up with mmusic as Last Call comments later on.
+1 with Taylor proposal to put RTP related things in data m= section if it is first and good with making this be bundles problem
NAME NAME I assume that you think that on initial offers this cannot happen because data comes last?
NAME Right. I can't think of a scenario where it wouldn't go last in an initial offer.
Agreed.
Oh, except that you aren't required to make the bundle group in order So you can have
I don't think the question whether you are allowed or not to place an RTP attribute in a non-RTP m= section is a pure BUNDLE issue. One needs to check what the specs defining those attributes say.
every SDP spec has to accept (and ignore) lines it does not understand ...
I looked into the a=rtcp-mux and a=rtcp-rsize specs, and I didn't see anything that prohibited their inclusion into a non-media section. Generally, I think BUNDLE can define the rules for this sort of thing, without fear that it will somehow contradict some other spec.
NAME NAME You may want to chime in on the current MMUSIC thread. I agree with you, but it seems to be going in the direction of "you can't do this".

I looked into the a=rtcp-mux and a=rtcp-rsize specs, and I didn't see anything that prohibited their >inclusion into a non-media section. It may not be explicitly stated, but since the semantics is about RTP and RTCP I think it is implicit. Having said that, I personally have no problem with allowing the attributes in non-RTP m- sections. I just want to verify that there is no text that has to be modified somewhere, and/or whether there is some text that has to be added somewhere :)
Resolved by
Looks good.

Revised text:
If a data channel m= section has been offered, a m= section MUST also be generated for data.  The <media> field MUST be set to "application" and the <proto> and "fmt" fields MUST be set to exactly  <ins> match the fields in the offer.  Note that if media m= sections are bundled into a data m= section, then certain TRANSPORT and IDENTICAL attributes may appear in the data m= section even if they would otherwise only be appropriate for a media m= section (e.g., "a=rtcp- mux"). </ins> Within the data m= section, an "a=mid" line MUST be generated and included as described above, along with an "a=sctp-port" line