Re: [MMUSIC] Bundle rejection oddities

[Christer Holmberg <[e-mail]>]

Hi Ekr,

I think I presented exactly the same use-case when it was first suggested to not associate shared candidates with each bundled m- line.

I'll look if I can add some additional text somewhere, to point out that the candidates can be associated with different m- lines in offers and answers, e.g. due to the use-case you described.

Regards,

Christer


Sent from my Windows Phone
________________________________
From: Eric Rescorla<mailto:[e-mail]>
Sent: ‎04/‎03/‎2016 02:53
To: mmusic WG<mailto:[e-mail]>
Subject: [MMUSIC] Bundle rejection oddities

Hi folks,

I've been working through the bundle draft and I've come across a sort
of surprising consequence that I wanted to share with the group. Sorry
for the extended example, but I think it's hard to get without that.

Consider the case where an offerer has three m= lines, alpha, bravo,
and charlie. This creates an initial offer that's (schematically) like
this:

   a=group:BUNDLE alpha bravo charlie
   m=audio 1111
   a=mid:alpha
   a=candidate

   m=video 2222
   a=mid:bravo
   a=candidate

   m=video 3333
   a=mid:charlie
   a=candidate

Per 8.2, and because this isn't bundle-only, each m= line has a unique
address and a set of candidates (I'm ignoring the ufrag and ice-pwd
for now).

Assume that the answerer rejects the first m= line, so per 8.3, it
pulls alpha out of the bundle group and sets is port number to 0.  Per
11.2, it only associates candidates with the answerer bundle tag,
which is bravo:

   a=group:BUNDLE bravo charlie
   m=audio 0
   a=mid:alpha

   m=video 4444
   a=mid:bravo
   a=candidate

   m=video 4444
   a=mid:charlie


So far so good. Now consider what happens if the offerer does an
ICE restart and (for instance) changes some property about the
second m= line. Per S 11.1 bullet 1, it only uses a shared address
for the second two m= lines and only attaches a candidate to the
second one (bravo). I.e.,

   a=group:BUNDLE bravo charlie
   m=audio 0
   a=mid:alpha

   m=video 5555
   a=mid:bravo
   a=candidate

   m=video 5555
   a=mid:charlie


If the answerer decides to reject bravo (leaving only charlie),
it now sets its port to 0 and pulls it out of the bundle group,
giving us:

   a=group:BUNDLE charlie
   m=audio 0
   a=mid:alpha

   m=video 0
   a=mid:bravo

   m=video 4444
   a=mid:charlie
   a=candidate     <-- See here


Note that the candidate here is on the third m= line but it needs to
be paired with the candidates on the second (rejected) m= line from
the offerer. I'm not saying that this is fatal: the logical
interpretation is that the candidates (and other ICE parameters) are
associated with the whole bundle group (and implicitly duplicated) and
just appear with one m= line. However, it is a bit odd and probably
should be called out in the spec lest people get confused.

-Ekr