Subject: Re: [rtcweb] Review of draft-ietf-rtcweb-jsep-07

From: Kiran Kumar Guduru <[email-masked]>
Date: Sat, 19 Jul 2014 02:41:13 +0000 (GMT)

Dear Paul,

My comment is only regarding the comments on section 6 of the JSEP draft.

The sdp mangling described in section 6 may result in various error scenarios.

In that section, it is described like most of the parameters can be modified thorugh constraints.

The required modification, I found, which cannot not be done by the constraints is "Remove or reorder of codecs (m=)"

In order to avoid the SDP mangling at Java Script application level, I submitted a draft for changing the codec preferences http://tools.ietf.org/html/draft-guduru-rtcweb-codec-preferences-01. (Please review it).

If this proposal is acceptable to the group, I hope we can remove section 6 from JSEP draft.

(Expecting that we can find alternatives for adding extra attributes like CLUE as identified by you, for example by extending the proposed RTCRTPSender/Receiver by adding a generic setSdPAttribute (key, value) method).

Thanks,

Kiran.

-------Original Message--------
Sent: Paul Kyzivat
Date: Fri, 18 Jul 2014 14:32:24 -0400
Subject: [rtcweb] Review of draft-ietf-rtcweb-jsep-07

I just did my first thorough read-through of JSEP in quite some time. I
came away with a number of concerns:

Section 1.1:

When describing offers, modification by the application is mentioned:

JSEP's handling of session descriptions is simple and
straightforward.  Whenever an offer/answer exchange is needed, the
initiating side creates an offer by calling a createOffer() API.  The
application *optionally modifies that offer*, and then uses it to set
up its local config via the setLocalDescription() API.

but when describing answers it is not:

When the call is accepted, the callee uses the createAnswer() API to
generate an appropriate answer, applies it using
setLocalDescription(), and sends the answer back to the initiator
over the signaling channel.  When the offerer gets that answer, it
installs it using setRemoteDescription(), and initial setup is
complete.

And Section 6 only talks about changing offers, not answers. It is
equally important to be able to modify answers. (More on this in later
comment on section 6.)

Section 4.1.4:

The only difference between a provisional and final answer is that
the final answer results in the freeing of any unused resources that
were allocated as a result of the offer.  As such, the application
can use some discretion on whether an answer should be applied as
provisional or final, and can change the type of the session
description as needed.  For example, in a serial forking scenario, an
application may receive multiple "final" answers, one from each
remote endpoint.  The application could choose to accept the initial
answers as provisional answers, and only apply an answer as final
when it receives one that meets its criteria (e.g. a live user
instead of voicemail).

Issue: in this forking case, until the final selection is made it is
unclear which one will need ICE completed. Only when a setlocal is done
on one of the answers will ICE begin to be performed. Then, if another
answer is provisionally set, won't that stop ICE for the prior one? And
won't it require new candidates? What if one of the earlier ones is
eventually chosen as final?

And what if ICE completes for one of the candidates? Can't media start
to flow? Then what if a different candidate is set as the final answer?

Section 4.1.4.1:

I question the following:

...  While it is good practice to send an immediate
response to an "offer", in order to warm up the session transport and
prevent media clipping, the preferred handling for a web application
would be to create and send an "inactive" final answer immediately
after receiving the offer.  Later, when the called user actually
accepts the call, the application can create a new "sendrecv" offer
to update the previous offer/answer pair and start the media flow.

This is very unfriendly when receiving calls that might be forked. By
immediately "answering" a call before knowing if the user will accept
it, you preempt the possibility that it will be answered on some other fork.

For instance, if a call could come to your browser, or be sent to an
answering service, and your broswer is on but you aren't present to
accept the call, then the call will be accepted and then dropped rather
than sent to the answering machine.

So this technique should not be used if there is any possibility that
the request could be coming from a source that might try other
possibilities.

Section 5.2.1:

When createOffer is called for the first time, the result is known as
the initial offer.

By this definition, if a peer connection initially *received* an offer
and sent an answer, and then it later sends an offer then that offer
would be considered an initial offer.

I think perhaps what is intended is:

When createOffer is called before setLocal has been called with
an offer,  the result is known as the initial offer.

The following doesn't support the "balanced" BUNDLE policy:

Once all m= sections have been generated, a session-level "a=group"
attribute MUST be added as specified in [RFC5888].  This attribute
MUST have semantics "BUNDLE", and MUST include the mid identifiers of
each m= section.  The effect of this is that the browser offers all
m= sections as one BUNDLE group.  However, whether the m= sections
are bundle-only or not depends on the BUNDLE policy.

Section 5.2.2 says:

o  If any MediaStreamTracks have been added, and there exist recvonly
m= sections of the appropriate media type with no associated
MediaStreamTracks, or rejected m= sections of any media type,
those m= sections MUST be recycled, and a local MediaStreamTrack
associated with these recycled m= sections until all such existing
m= sections have been used.  This includes any recvonly or
rejected m= sections created by the preceding paragraph.

This fails to say anything about codec compatibility. SDP O/A requires
the answer to be a subset of the offer in terms of PT/codec
configuration. You should not use the same m-line unless there is a
desire to use the same settings for the new track as are currently in
use in the other direction.

Section 5.3.1:

When createAnswer is called for the first time after a remote
description has been provided, the result is known as the initial
answer.

By this definition, if a peer connection initially sent an offer and
received an answer, and then it later receives an offer then the answer
to that first *received* offer would be considered an Initial Answer.

I think perhaps what is intended is:

When createAnswer is called before setLocal has been called with
an offer,  the result is known as the initial answer.

When specifying the mapping of local tracks to m-lines in a received
offer, there is again no discussion of codec compatibility. It is quite
possible that the m-line chosen by the algorithm in this section won't
have compatible codec attributes, but some other m-line will.

Sections 5.3.2, 5.3.3, and 5.4-5.7:

Are these empty because the content is yet to be written?

Section 6:

Other likely changes are addition of extra attributes and maybe other
parameters. For instance, CLUE will want to add another grouping construct.

Thanks,
Paul