Re: [rtcweb] State of the Forking discussion
Christer Holmberg <christer.holmberg@ericsson.com> Thu, 03 November 2011 11:53 UTC
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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Magnus Westerlund <magnus.westerlund@ericsson.com>, "rtcweb@ietf.org" <rtcweb@ietf.org>
Date: Thu, 03 Nov 2011 12:53:39 +0100
Thread-Topic: [rtcweb] State of the Forking discussion
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Subject: Re: [rtcweb] State of the Forking discussion
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Hi Magnus, Question #1: Is forking needed to be supported at all? Answer: Yes Question #2: If it is supported in which form would it supported in? Answer: Serial (ie one "active" media session at any given time) forking is enough. Solution B2, B3 or B4 - whatever is possible and least complex from a PeerConnection perspective :) Regards, Christer > -----Original Message----- > From: rtcweb-bounces@ietf.org > [mailto:rtcweb-bounces@ietf.org] On Behalf Of Magnus Westerlund > Sent: 3. marraskuuta 2011 12:13 > To: rtcweb@ietf.org > Subject: [rtcweb] State of the Forking discussion > > 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 > > ---------------------------------------------------------------------- > Multimedia Technologies, Ericsson Research EAB/TVM > ---------------------------------------------------------------------- > Ericsson AB | Phone +46 10 7148287 > Färögatan 6 | Mobile +46 73 0949079 > SE-164 80 Stockholm, Sweden| mailto: magnus.westerlund@ericsson.com > ---------------------------------------------------------------------- > > _______________________________________________ > rtcweb mailing list > rtcweb@ietf.org > https://www.ietf.org/mailman/listinfo/rtcweb >
- Re: [rtcweb] State of the Forking discussion Randell Jesup
- [rtcweb] State of the Forking discussion Magnus Westerlund
- Re: [rtcweb] State of the Forking discussion Christer Holmberg
- Re: [rtcweb] State of the Forking discussion Ravindran Parthasarathi
- Re: [rtcweb] State of the Forking discussion Iñaki Baz Castillo
- Re: [rtcweb] State of the Forking discussion Hadriel Kaplan
- Re: [rtcweb] State of the Forking discussion Christer Holmberg
- Re: [rtcweb] State of the Forking discussion Cullen Jennings
- Re: [rtcweb] SRTP mandatory to implement versus m… Bernard Aboba
- Re: [rtcweb] State of the Forking discussion Christer Holmberg
- Re: [rtcweb] SRTP mandatory to implement versus m… Bernard Aboba
- Re: [rtcweb] State of the Forking discussion Cullen Jennings
- Re: [rtcweb] State of the Forking discussion Christer Holmberg
- Re: [rtcweb] SRTP mandatory to implement versus m… Magnus Westerlund