Re: [rtcweb] State of the Forking discussion

Cullen Jennings <fluffy@cisco.com> Tue, 08 November 2011 22:50 UTC

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From: Cullen Jennings <fluffy@cisco.com>
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Date: Tue, 08 Nov 2011 15:50:32 -0700
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To: Magnus Westerlund <magnus.westerlund@ericsson.com>
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Subject: Re: [rtcweb] State of the Forking discussion
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<in my individual contributor role>

Much of this I don't feel too strongly about but there is one thing that I do have a strong opinion on. I don't want to require PRACK for legacy SIP support because it is has many problems. 

Cullen

On Nov 3, 2011, at 4:13 AM, Magnus Westerlund wrote:

> WG,
> 
> I just reviewed the last weeks Forking discussion. This includes the
> threads "RTCWeb Forking usecase [was RE:
> draft-kaplan-rtcweb-sip-interworking-requirements-00]" and "Media
> forking solution for SIP interoperability (without a media gateway)"
> 
> As far as I can tell there is not yet even a rough consensus. Therefore
> I will attempt to summarize what I personally believe to be the
> important points and alternatives in this discussion. Keep in mind that
> my assumptions or understanding may be unclear or have errors. So don't
> hesitate to challenge what I write.
> 
> I think it is important that there are in fact at least two important
> questions here.
> 
> 1. Is forking needed to be supported at all?
> 
> 2. If it is supported in which form would it supported in.
> 
> so lets start looking into the arguments and possibilities for these two
> questions. And I do hope that you will read to the end of this mail
> which is quite long.
> 
> Lets start with the high level functionality part. Is forking needed and
> what usage does it have. So forking is all about sending out an
> invitation to a media session including an actual media configuration
> offer, i.e. SDP Offer, then get more than a single answer to that offer
> back. How you deal with these answers as they come in is the difference
> between serial and parallel forking. So lets define those.
> 
> Parallel forking: For each answer you receive you establish a new actual
> media session. Thus if you receive two answers you will have to
> different media sessions that are potentially in use at the same time.
> 
> Serial forking: The first answer is received and results the
> establishment of a media session. At a later point in time a second
> answer is received. At that point you take the decision if that second
> answer is going to be used to establish a new media session that
> replaces the first one. In other words at any given time you will only
> have a single media session established based on each offer.
> 
> So there has been a number of different views on how one can see on
> forking. And I think I will have to bring in a bit reflections on how
> this can be done with the current PeerConnection API.
> 
> A) No forking is needed: Between WebRTC end-points there is no need for
> forking. Instead the application can send out session invitations to the
> peers it wants to talk to. These are without any SDP Offer equivalent,
> instead end-points that want to communicate they create PeerConnections,
> which results in SDP Offers. Thus the communication initiating end-point
> becomes the ones that provides SDP answers and get one PeerConnection
> per remote end-point that actually want to communicate.
> 
> B) We need to have some interworking with SIP: So the fundamental here
> is that it needs to be reasonable to interwork with SIP, independent if
> one uses a SIP in JS in the application running on the WebRTC enabled
> browser, or have signalling gateway in the webserver, or as a remote
> WebRTC peer. The issue is that A)'s method of initiating call doesn't
> work well with SIP. There is a need to send a SIP Invite with an SDP
> Offer and that can result in multiple answers.
> 
> To resolve this one could deal with this in a couple of different ways:
> 
> B1) Use Iñaki's proposal which forces the WebRTC application to create a
> second PeerConnection and then forces an update in the SIP domain with
> the second peer-connections Offer. However, it was pointed out that this
> doesn't work with SIP Provisional answers, as used by ICE for example,
> unless PRACK is supported. The level of PRACK support is reasonable but
> far from universal so this would limit the SIP UAs one can interwork
> with. However, from WebRTC perspective no forking support is needed. A
> single PeerConnection results in one offer and a single answer is
> processed.
> 
> B2) Require WebRTC to handle replace Answers: So the idea here is that
> one changes the PeerConnection API and have underlying functionality so
> that at any point in time a new Answer can pushed onto a PeerConnection
> and that forces the media session to be reestablished if needed. So if
> the ICE candidate list is different an ICE restart happens. This clearly
> supports serial forking. It also can create some complexities in the
> underlying SDP handling logic if one desires to minimize the media impact.
> 
> B3) Local side shared parameters in multiple PeerConnections: The idea
> in this proposal is that all PeerConnections generated in a browser
> context, like a tab will implicit share the fundamental parameters like
> ICE candidates etc for the number of media streams added. So if one
> creates a second PeerConnection with the same audio+video MediaStream
> object added I will get an offer that is mostly identical to the the
> first PeerConnection, thus I can push in the answer from the first
> PeerConnection Offer into the second PC object and it will still work.
> The downside of this is that it is implicit and it becomes difficult to
> determine when it works and when it will fail. It will also be highly
> dependent on the application performing the right process to get it to
> work. It also causes a sharing of the parameters when not needed or
> desired, which primarily is an issue from a security point of view,
> especially with SDES keys (see below). The positive is this likely
> requires no API changes. This method would also support parallel forking.
> 
> B4) Cloning/Factory for PeerConnection: On the API level there will be
> explicit support for generating multiple PeerConnections from the same
> base. This could either be a factory for PeerConnections or some Object
> constructor that clones an existing PeerConnection but that is a W3C
> question. By being explicit some of the B3) issues goes away and the
> applications can choose when this should happen or not. This also
> support parallel forking as the application can deal with each media
> session independently. This clearly will have some impact on the API.
> 
> 
> Additional considerations:
> 
> Shared SDES keys: B2 to B4 will result in that SDES keys from the
> Offering party to be used towards all invited parties. This is security
> risk as any of the invited parties can spoof the offering side towards
> any of the other invited parties. This threat can be resolved by having
> the inviter rekey immediately after having received an answer.
> 
> Sharing ICE candidates: B3 and B4 and also B2 to some degree will share
> the ICE candidates. That has certain implications. One is the positive
> in that it minimizes the resource consumption as additional
> PeerConnections come at very little extra cost, no need for additional
> ICE gathering candidate phases, and also be very quick as no external
> communication is required. The downside of this is that the end-points
> candidates must always be kept alive as long as some PeerConnection
> instance exist. Because the browser never knows when the application may
> create an additional PC and expect them to have the same ICE candidates.
> It should also be noted that the answering WebRTC end-point will need to
> gather candidates for each offer. Otherwise it will become impossible to
> create multiple PeerConnections between the same end-points if that is
> desired by the application.
> 
> 
> I know the above doesn't list all of the pro and cons of the different
> alternatives. So please fill in additional arguments. And if I missed
> some proposal please add that also if relevant
> 
> As you may have noted I the two questions in the above have kind of
> floated together. The reason for this is that I think the majority are
> fine with having SIP support as long as it doesn't have to high cost or
> complexity associated with it. Thus, the question how becomes very
> intertwined with forking support yes or no.
> 
> So please continue the discussion
> 
> Cheers
> 
> Magnus Westerlund
> 
> ----------------------------------------------------------------------
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