Re: [MMUSIC] Making b=CT session level only - draft-ietf-mmusic-rfc4566bis

[Suhas Nandakumar <[e-mail]>]

>From Magnus  on Mux Attributes usage of b=CT:

6.1.  RFC4566 - SDP: Session Description Protocol

   [RFC4566] defines the Session Description Protocol (SDP) that is
   intended for describing multimedia sessions for the purposes of
   session announcement, session invitation, and other forms of
   multimedia session initiation.

   +------------+-----------------------------------+-------+----------+
   | Name       | Notes                             | Level | Mux      |
   |            |                                   |       | Category |
   +------------+-----------------------------------+-------+----------+
   | bwtype:CT  | Not Impacted                      | S     | NORMAL   |
   |            |                                   |       |          |
   | bwtype:AS  | For the media level usage, the    | B     | SUM      |
   |            | aggregate of individual bandwidth |       |          |
   |            | values is considered              |       |          |
   |            |                                   |       |          |
   +------------+-----------------------------------+-------+----------+

                          RFC4566 bwtype Analysis


So here Level needs to be changed to B (Both). Then I would state that the
Category should be IDENTICAL. I don't know what it would mean to have
different CT values for different media sources in a bundle group.

It might be worth adding a Note about CT, and why CT is much more useful in
a bundle-group than on a non-bundled m= line.


[Suhas] : I am fine with changing the level to B and category to IDENTICAL
as suggested ,

I do think need to update the SDP IANA registry to make that change. I am
assuming that RFC4566bis will be doing that change.

On the note about usage of CT, does the below text sounds good

Note:
 Specifying the SDP b=CT attribute at the media level represents
approximate aggregate bandwidth for all the media streams multiplexed and
the value must be identical across all the media streams multiplexed.
However in non-bundled scenarios, a b=CT media level attribute's
interpretation is not clearly identified and MUST NOT be specified.



Please let me know if the text suggestion looks fine and I can submit a new
version to match the same.


Thanks
Suhas


On Thu, Sep 10, 2015 at 2:50 AM, Magnus Westerlund <
[e-mail]> wrote:

> Hi,
>
> The chairs have been nagging me about some action points that I got
> assigned in my absence at the IETF92 MMUSIC session. So lets try to settle
> them.
>
> The main question for one of them is if SDPbis should make the b=CT
> bandwidth modifier into a session level only parameter?
>
> So, it is clear that session level CT value is the total upper limit for
> bandwidth consumption, independent on direction incoming or outgoing from
> an endpoint for this Multimedia session.
>
> Note that I believe it to be Multi-media session rather than communication
> session. This is due to that SDP is used to describe a multi-media session.
>
> I would also note that for Offer/Answer it is not clear if the offer
> applies to the offerer or the answerer. This is due to the
> non-directionality declaration of CT. My personal interpretation is that
> the offer applies to the answerer and the answer applies to the offerer.
> But, this is one thing that should be clarified if anyone dares revise RFC
> 3264.
>
> Regarding CT on media level (m=) I think the interpretation of it also
> here is not that unclear. It is just that it has no meaning in the
> pre-bundle Offer/Answer context, where one had only one Media source in
> that RTP session the m= created. But if one looks at SDP/SAP usage then CT
> do make much more sense. In multi-party usages like multicast, defining an
> AS for one media source a CT value for the aggregate across all session
> endpoints do make sense.
>
> Which leads me to want to clarify that CT on media level is possible and
> has meaning but only in contexts where one m= line (or a bundle group of m=
> lines) may contain multiple media sources and thus media streams and
> therefore an aggregate limitation makes sense.
>
> If people agree with this I think we need to make some amendment both to
> SDPbis and to Mux-attributes to make this clear.
>
> SDPbis (-15) says:
>
> 5.8.  Bandwidth ("b=")
>
>       b=<bwtype>:<bandwidth>
>
>    This OPTIONAL line denotes the proposed bandwidth to be used by the
>    session or media.  The <bwtype> is an alphanumeric modifier giving
>    the meaning of the <bandwidth> figure.  Two values are defined in
>    this specification, but other values MAY be registered in the future
>    (see Section 8 and [RFC3556], [RFC3890]):
>
>    CT If the bandwidth of a session or media in a session is different
>       from the bandwidth implicit from the scope, a "b=CT:..." line
>       SHOULD be supplied for the session giving the proposed upper limit
>       to the bandwidth used (the "conference total" bandwidth).  The
>       primary purpose of this is to give an approximate idea as to
>       whether two or more sessions can coexist simultaneously.
>
>    AS The bandwidth is interpreted to be application specific (it will
>       be the application's concept of maximum bandwidth).  Normally,
>       this will coincide with what is set on the application's "maximum
>       bandwidth" control if applicable.  For RTP-based applications, AS
>       gives the RTP "session bandwidth" as defined in Section 6.2 of
>       [RFC3550].
>
>    Note that CT gives a total bandwidth figure for all the media at all
>    sites.  AS gives a bandwidth figure for a single media at a single
>    site, although there may be many sites sending simultaneously.
>
>    A prefix "X-" is defined for <bwtype> names.  This is intended for
>    experimental purposes only.  For example:
>
>       b=X-YZ:128
>
>    Use of the "X-" prefix is NOT RECOMMENDED: instead new modifiers
>    SHOULD be registered with IANA in the standard namespace.  SDP
>    parsers MUST ignore bandwidth fields with unknown modifiers.
>    Modifiers MUST be alphanumeric and, although no length limit is
>    given, it is recommended that they be short.
>
>    The <bandwidth> is interpreted as kilobits per second by default.
>    The definition of a new <bwtype> modifier MAY specify that the
>    bandwidth is to be interpreted in some alternative unit (the "CT" and
>    "AS" modifiers defined in this memo use the default units).
>
>
> I think this needs to be modified in a couple of different places.
>
> First I think the CT explanation text should be expanded to discuss this
> context of possible multiple streams in the context of one m= line, and
> that in that and only that case does media level make sense.
>
> Then I would like to move up the text in the note about CT into the main
> definition and change "sites" to "endpoints".
>
>
> And Mux-attributes (-10) says:
>
> 6.1.  RFC4566 - SDP: Session Description Protocol
>
>    [RFC4566] defines the Session Description Protocol (SDP) that is
>    intended for describing multimedia sessions for the purposes of
>    session announcement, session invitation, and other forms of
>    multimedia session initiation.
>
>    +------------+-----------------------------------+-------+----------+
>    | Name       | Notes                             | Level | Mux      |
>    |            |                                   |       | Category |
>    +------------+-----------------------------------+-------+----------+
>    | bwtype:CT  | Not Impacted                      | S     | NORMAL   |
>    |            |                                   |       |          |
>    | bwtype:AS  | For the media level usage, the    | B     | SUM      |
>    |            | aggregate of individual bandwidth |       |          |
>    |            | values is considered              |       |          |
>    |            |                                   |       |          |
>    +------------+-----------------------------------+-------+----------+
>
>                           RFC4566 bwtype Analysis
>
>
> So here Level needs to be changed to B (Both). Then I would state that the
> Category should be IDENTICAL. I don't know what it would mean to have
> different CT values for different media sources in a bundle group.
>
> It might be worth adding a Note about CT, and why CT is much more useful
> in a bundle-group than on a non-bundled m= line.
>
> Comments on this?
>
> Magnus Westerlund
>
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