Re: [MMUSIC] [rtcweb] Updating JSEP and BUNDLE: Semantics of same port in multiple m- lines

[Roman Shpount <[e-mail]>]

On Mon, Feb 1, 2021 at 12:27 PM Christer Holmberg <christer.holmberg=
[e-mail]> wrote:

> >>>> Sure, we could have done that. However, as has been discussed that in
> the past (in MMUSIC, SIPCORE etc), we typically don't describe situations
> that can occur when endpoints behave strangely, because it would easily
> become a slippery slope.
> >>>> Also note that, in addition to bundle-only there can also be other
> attributes that have been copy/pasted by the remote endpoint.
> >>>
> >>> This is not "endpoints behaving strangely". This is the expected
> behavior of an endpoint that does not support bundle.
> >>
> >> I don't think copy/pasting attributes you don't understand is expected
> behavior. That in general can cause lots of problems.
> >>
> >> But, I do buy the argument that some endpoints do that, and that it is
> not (as far as I remember) explicitly forbidden.
> >
> > Shooting yourself in the head isn't AFAIK explicitly forbidden. But
> unless you understand and desire the consequences it is generally an unwise
> thing to do.
> >
> > I don't think an IETF document is obligated to protect against people
> doing either.
> >
> > Also, if we have an implementation that doesn't support bundle that is
> copying an a=bundle-only it doesn't understand, won't it also likely be
> copying a=group:bundle and a=mid???
>
> My understanding was that it only affected media-level attributes.
>
> But, sure, in theory an endpoint could copy ANYTHING it doesn't
> understand...
>

The issue is how does an endpoint generate an m= line to refuse something
that the endpoint does not understand. For instance, the endpoint gets an
application webrtc-datachannel m= line but has no idea what any of the SDP
attributes mean and which ones are required to form a valid m= line. The
simplest way is to refuse such an m= line was to copy the entire m= line in
the answer and set the m= line port to 0. As far as the endpoint is
concerned, there is no difference between a=sctp-port or a=bundle-only, if
neither are supported.

If this is an invalid procedure to refuse an m= line, please point out
where it says this is invalid and let me know where the correct procedure
is described.

Session level attributes are different since this is something that is
negotiated, it would be logical not to include them if the endpoint does
not support them.
_____________
Roman Shpount