Re: [MMUSIC] Bundle rejection oddities

[Christer Holmberg <[e-mail]>]

Hi,

>>What about adding the following note to section 11.1:
>>
>>   “NOTE: The ICE-related media-level SDP attributes might end up being associated with
>>   with an "m=" line in the answer that does not correspond to the "m=" line 
>>   to which the ICE-related SDP attributes were associated in the associated offer.”
>
>I think the key point here is not that but that the attributes may be attached in the
>offer to an m= line which ends up being defunct.

That could e.g. be covered by adding the following sentence to the note:

"This could happen e.g. if the "m=" line in the offer is rejected and moved out 
of the bundle group by the answerer."

>I'd also like to observe that this paragraph exemplifies an infelicity in the current
>draft, which is the overloading of the term "associated", which appears three times
>in this paragraph alone with two different meanings. I think it would be clearer if
>instead different terms were used. I suggest "member of" for being in a bundle group,
>"appears in" or "attached to" for being in a given m= section, and "corresponding"
>for "the SDP offer/answer that this answer/offer" goes with

I agree that "associated" is used in many places. But, I have had to change the terminology so many times already, because every time someone has reviewed the document he/she has had ideas on how to modify the terminology.

The current terminology is what we have been able to agreed upon, so I do not want to change it again. I really think it's time to ship BUNDLE.

Regards,

Christer






From: mmusic [mailto:[e-mail]] On Behalf Of Christer Holmberg
Sent: 4. maaliskuuta 2016 5:53
To: Eric Rescorla; mmusic WG
Subject: Re: [MMUSIC] Bundle rejection oddities
 
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
Sent: ‎04/‎03/‎2016 02:53
To: mmusic WG
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