Network Working Group                                          J. Uberti
Internet-Draft                                                    Google
Intended status: Standards Track                             C. Jennings
Expires: September 4, December 6, 2012                            Cisco Systems, Inc.
                                                           March 3,
                                                            June 4, 2012

               Javascript Session Establishment Protocol
                       draft-ietf-rtcweb-jsep-00
                       draft-ietf-rtcweb-jsep-01

Abstract

   This document proposes a mechanism for allowing a Javascript
   application to fully control the signaling plane of a multimedia
   session, and discusses how this would work with existing signaling
   protocols.

   This document is an input document for discussion.  It should be
   discussed in the RTCWEB WG list, rtcweb@ietf.org.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 26, 2012.

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   document authors.  All rights reserved.

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Table of Contents

   1. Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2. JSEP Approach . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3. Other Approaches Considered . . . . . . . . . . . . . . . . . .  6
   4. Semantics and Syntax  . . . . . . . . . . . . . . . . . . . . .  7
     4.1. Signaling Model . . . . . . . . . . . . . . . . . . . . . .  7
     4.2. Session Descriptions  . . . . . . . . . and State Machine  . . . . . . . . . .  7
     4.3. Session Description Format  . . . . . . . . . . . . . . . .  8  9
     4.4. Separation of Signaling and ICE State Machines  . . . . . .  9 10
     4.5. ICE Candidate Trickling . . . . . . . . . . . . . . . . . .  9 10
     4.6. ICE Candidate Format  . . . . . . . . . . . . . . . . . . . 10
   5. Media Setup Overview 11
     4.7. Interactions With Forking . . . . . . . . . . . . . . . . . 11
       4.7.1. Serial Forking  . . . . 10
     5.1. Initiating the Session . . . . . . . . . . . . . . . . 11
       4.7.2. Parallel Forking  . . 10
       5.1.1. Generating An Offer . . . . . . . . . . . . . . . . . 12
     4.8. Session Rehydration . 11
       5.1.2. Applying the Offer . . . . . . . . . . . . . . . . . . 11
       5.1.3. Initiating ICE . 12
   5. Interface . . . . . . . . . . . . . . . . . . . 11
       5.1.4. Serializing the Offer and Candidates . . . . . . . . 13
     5.1. Methods . 11
     5.2. Receiving the Session . . . . . . . . . . . . . . . . . . . 12
       5.2.1. Receiving the Offer . . . . . . 13
       5.1.1. createOffer . . . . . . . . . . . . 12
       5.2.2. Initiating ICE . . . . . . . . . . 13
       5.1.2. createAnswer  . . . . . . . . . . 12
       5.2.3. Handling ICE Messages . . . . . . . . . . . 14
       5.1.3. SessionDescriptionType  . . . . . . 12
       5.2.4. Generating the Answer . . . . . . . . . . 14
       5.1.4. setLocalDescription . . . . . . . 12
       5.2.5. Applying the Answer . . . . . . . . . . . 15
       5.1.5. setRemoteDescription  . . . . . . . 13
       5.2.6. Serializing the Answer . . . . . . . . . . 15
       5.1.6. localDescription  . . . . . . 13
     5.3. Completing the Session . . . . . . . . . . . . . 16
       5.1.7. remoteDescription . . . . . 13
       5.3.1. Receiving the Answer . . . . . . . . . . . . . . 16
       5.1.8. updateIce . . . 13
     5.4. Updates to the Session . . . . . . . . . . . . . . . . . . 13
   6. Proposed WebRTC API changes . . 16
       5.1.9. addIceCandidate . . . . . . . . . . . . . . . . 14
     6.1. PeerConnection API . . . . 17
     5.2. Configurable SDP Parameters . . . . . . . . . . . . . . . . 14
       6.1.1 MediaHints 17
   6. Media Setup Overview  . . . . . . . . . . . . . . . . . . . . . 17
     6.1. Initiating the Session  . . 15
       6.1.2 createOffer . . . . . . . . . . . . . . . . 18
       6.1.1. Generating An Offer . . . . . . 16
       6.1.3 createAnswer . . . . . . . . . . . . 18
       6.1.2. Applying the Offer  . . . . . . . . . . 16
       6.1.4 SDP_OFFER, SDP_PRANSWER, and SDP_ANSWER . . . . . . . . 17
       6.1.5 setLocalDescription 18
       6.1.3. Handling ICE Callbacks  . . . . . . . . . . . . . . . . 18
       6.1.4. Serializing the Offer and Candidates  . . . 17
       6.1.6 setRemoteDescription . . . . . . 19
     6.2. Receiving the Session . . . . . . . . . . . . 18
       6.1.7 localDescription . . . . . . . 19
       6.2.1. Receiving the Offer . . . . . . . . . . . . . 18
       6.1.8 remoteDescription . . . . . 19
       6.2.2. Handling ICE Messages . . . . . . . . . . . . . . 18
       6.1.9 IceOptions . . . 19
       6.2.3. Generating the Answer . . . . . . . . . . . . . . . . . 20
       6.2.4. Applying the Answer . . . . 19
       6.1.10 startIce . . . . . . . . . . . . . . 20
       6.2.5. Serializing the Answer  . . . . . . . . . 19
       6.1.11 processIceMessage . . . . . . . 20
     6.3. Completing the Session  . . . . . . . . . . . . 19
   7. Example API Flows . . . . . . 20
       6.3.1. Receiving the Answer  . . . . . . . . . . . . . . . . . 20
     7.1. Call using ROAP

     6.4. Updates to the Session  . . . . . . . . . . . . . . . . . . 20
   7. Security Considerations . . . . 20
     7.2. Call using XMPP . . . . . . . . . . . . . . . . 21
   8. IANA Considerations . . . . . . 20
     7.3. Adding video to a call, using XMPP . . . . . . . . . . . . 22
     7.4. Simultaneous add of video streams, using XMPP . . . . 21
   9. Acknowledgements  . . . 22
     7.5. Call using SIP . . . . . . . . . . . . . . . . . . . . 21
   10. References . . 23
     7.6. Handling early media (e.g. 1-800-FEDEX), using SIP . . . . 24
   8. Example Application . . . . . . . . . . . . . . . . . . . . 21
     10.1. Normative References . . 24
   9. Security Considerations . . . . . . . . . . . . . . . . . 21
     10.2. Informative References . . . . . 26
   10. IANA Considerations . . . . . . . . . . . . . 21
   Appendix A. JSEP Implementation Examples . . . . . . . . 26
   11. Acknowledgements . . . . . 22
     A.1. Example API . . . . . . . . . . . . . . . . . . 26
   12. References . . . . . . 22
     A.2. Example API Flows . . . . . . . . . . . . . . . . . . . . 26
     12.1. Normative References . 23
       A.2.1. Call using ROAP . . . . . . . . . . . . . . . . . . 26
     12.2. Informative References . . 23
       A.2.2 Call using XMPP  . . . . . . . . . . . . . . . . . 27
   Appendix A. Open Issues . . . 24
       A.2.3. Adding video to a call, using XMPP  . . . . . . . . . . 25
       A.2.4. Simultaneous add of video streams, using XMPP . . . . . 26
       A.2.5. Call using SIP  . . . . . . . . . . . . . . . . . . . . 27
       A.2.6. Handling early media (e.g. 1-800-FEDEX), using SIP  . . 28
     A.3. Full Example Application  . . . . . . . . . . . . . . . . . 28
   Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . . 27 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27 30

1. Introduction

   The general thinking behind WebRTC call setup has been to fully specify and
   control the media plane, but to leave the signaling plane up to the
   application as much as possible. The rationale is that different
   applications may prefer to use different protocols, such as the
   existing SIP or Jingle call signaling protocols, or something custom
   to the particular application, perhaps for a novel use case. In this
   approach, the key information that needs to be exchanged is the
   multimedia session description, which specifies the necessary
   transport and media configuration information necessary to establish
   the media plane.

   The original spec for WebRTC attempted to implement this protocol-
   agnostic signaling by providing a mechanism to exchange session
   descriptions in the form of SDP blobs. Upon starting a session, the
   browser would generate a SDP blob, which would be passed to the
   application for transport over its preferred signaling protocol. On
   the remote side, this blob would be passed into the browser from the
   application, and the browser would then generate a blob of its own in
   response. Upon transmission back to the initiator, this blob would be
   plugged into their browser, and the handshake would be complete.

   Experimentation with this mechanism turned up several shortcomings,
   which generally stemmed from there being insufficient context at the
   browser to fully determine the meaning of a SDP blob. For example,
   determining whether a blob is an offer or an answer, or
   differentiating a new offer from a retransmit.

   The ROAP proposal, specified in http://tools.ietf.org/html/draft-
   jennings-rtcweb-signaling-01, [I-D.draft-jennings-rtcweb-signaling-
   01], attempted to resolve these issues by providing additional
   structure in the messaging - in essence, to create a generic
   signaling protocol that specifies how the browser signaling state
   machine should operate. However, even though the protocol is
   abstracted, the state machine forces a least-common-
   denominator least-common-denominator
   approach on the signaling interactions. For example, in Jingle, the
   call initiator can provide additional ICE candidates even after the
   initial offer has been sent, which allows the offer to be sent
   immediately for quicker call startup. However, in the browser state
   machine, there is no notion of sending an updated offer before the
   initial offer has been responded to, rendering this functionality
   impossible.

   While specific concerns like this could be addressed by modifying the
   generic protocol, others would likely be discovered later. The main
   reason this mechanism is inflexible is because it embeds a signaling
   state machine within the browser. Since the browser generates the
   session descriptions on its own, and fully controls the possible
   states and advancement of the signaling state machine, modification
   of the session descriptions or use of alternate state machines
   becomes difficult or impossible.

   The browser environment also has its own challenges that cause
   problems for an embedded signaling state machine. One of these is
   that the user may reload the web page at any time. If this happens,
   and the state machine is being run at a server, the server can simply
   push the current state back down to the page and resume the call
   where it left off.

   If instead the state machine is run at the browser end, and is
   instantiated within, for example, the PeerConnection object, that
   state machine will be reinitialized when the page is reloaded and the
   JavaScript re-executed. This actually complicates the design of any
   interoperability service, as all cases where an offer or answer has
   already been generated but is now "forgotten" must now be handled by
   trying to move the client state machine forward to the same state it
   had been in previously in order to match what has already been
   delivered to and/or answered by the far side, or handled by ensuring
   that aborts are cleanly handled from every state and the negotiation
   rapidly restarted.

1.1. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2. JSEP Approach

   To resolve these issues, the issues mentioned above, this document proposes the
   Javascript Session Establishment Protocol (JSEP) that pulls the
   signaling state machine out of the browser and into Javascript. This
   mechanism effectively removes the browser almost completely from the
   core signaling flow; the only interface needed is a way for the
   application to pass in the local and remote session descriptions
   negotiated by whatever signaling mechanism is used, and a way to
   interact with the ICE state machine.

   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 on
   PeerConnection. API. The
   application can do massaging of that offer, if it wants to, and then
   uses it to set up its local config via a setLocalDescription() API.
   The offer is then sent off to the remote side over its preferred
   signaling mechanism (e.g. WebSockets); upon receipt of that offer,
   the remote party installs it using a setRemoteDescription() API.

   When the call is accepted, the callee uses a 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. This process can be repeated for additional offer/answer
   exchanges.

   Regarding ICE, in this approach we decouple JSEP decouples the ICE state machine from the overall
   signaling state machine; machine, as the ICE state machine must remain in the
   browser, given that since only the browser has the necessary knowledge of
   candidates and other transport info. While transport has
   typically been lumped in with session descriptions, performing Performing this separation it
   provides additional flexibility. In flexibility; in protocols that decouple session
   descriptions from transport, such as Jingle, the transport
   information can be sent separately; in protocols that don't, such as
   SIP, the information can be easily aggregated and recombined. Sending
   transport information separately can allow for faster ICE and DTLS
   startup, since the necessary roundtrips can occur while waiting for
   the remote side to accept the session.

   The JSEP approach does come with a minor downside. As the application
   now is responsible for driving the signaling state machine, slightly
   more application code is necessary to perform call setup; the
   application must call the right APIs at the right times, and convert
   the session desciptions descriptions and ICE information into the defined
   messages of its chosen signaling protocol, instead of simply
   forwarding the messages emitted from the browser.

   One way to mitigate this is to provide a Javascript library that
   hides this complexity from the developer, which would implement the
   state machine and serialization of the desired signaling protocol.
   For example, this library could convert easily adapt the JSEP API
   into the exact ROAP API, thereby implementing the ROAP signaling
   protocol. Such a library could of course also implement other popular
   signaling protocols, including SIP or Jingle. In this fashion we can
   enable greater control for the experienced developer without forcing
   any additional complexity on the novice developer.

3. Other Approaches Considered

   Another approach that was considered for JSEP was to move the
   mechanism for generating offers and answers out of the browser as
   well. This Instead of providing createOffer/createAnswer methods within
   the browser, this approach would add instead expose a getCapabilities API
   which would provide the application with the information it needed in
   order to generate its own session descriptions. This increases the
   amount of work that the application needs to do; it needs to know how
   to generate session descriptions from capabilities, and especially
   how to generate the correct answer from an arbitrary offer and available the
   supported capabilities. While this could certainly be addressed by
   using a library like the one mentioned above, some experimentation also indicates that coming
   up with a sufficiently complete getCapabilities API is a nontrivial
   undertaking. Nevertheless, if we wanted to go down this road, JSEP
   makes it significantly easier; if a getCapabilities API is added in
   the future, basically forces the application can generate session descriptions
   accordingly and pass those
   use of said library even for a simple example. Exposing
   createOffer/createAnswer avoids that problem, but still allows
   applications to generate their own offers/answers if they choose,
   using the
   setLocalDescription/setRemoteDescription APIs added description generated by JSEP. (Even
   with JSEP, an application could still perform its own browser
   fingerprinting and generate approximate session descriptions createOffer as a
   result.) an indication of
   the browser's capabilities.

   Note also that while JSEP transfers more control to Javascript, it is
   not intended to be an example of a "low-level" API. The general
   argument against a low-level API is that there are too many necessary
   API points, and they can be called in any order, leading to something
   that is hard to specify and test. In the approach proposed here,
   control is performed via session descriptions; this requires only a
   few APIs to handle these descriptions, and they are evaluated in a
   specific fashion, which reduces the number of possible states and
   interactions.

4. Semantics and Syntax

4.1. Signaling Model

   JSEP does not specify a particular signaling model or state machine,
   other than the generic need to exchange RFC 3264 offers and answers
   in order for both sides of the session to know how to conduct the
   session. JSEP provides mechanisms to create offers and answers, as
   well as to apply them to a PeerConnection. session. However, the actual mechanism by
   which these offers and answers are communicated to the remote side,
   including addressing, retransmission, forking, and glare handling, is
   left entirely up to the application.

       +-----------+                                +-----------+
       |  Web App  |<--- App-Specific Signaling --->|  Web App  |
       +-----------+                                +-----------+
             |                                            |
             |  SDP                                       |  SDP
             V                                            V
       +-----------+                                +-----------+
       |  Browser  |<----------- Media ------------>|  Browser  |
       +-----------+                                +-----------+

                     Figure 1: JSEP Signaling Model

4.2. Session Descriptions and State Machine

   In order to establish the media plane, PeerConnection the user agent needs specific
   parameters to indicate what to transmit to the remote side, as well
   as how to handle the media that is received. These parameters are
   determined by the exchange of session descriptions in offers and
   answers, and there are certain details to this process that must be
   handled in the JSEP APIs.

   Whether a session description was sent or received affects the
   meaning of that description. For example, the list of codecs sent to
   a remote party indicates what the local side is willing to decode,
   and what the remote party should send. Not all parameters follow this
   rule; for example, the SRTP parameters [RFC4568] sent to a remote
   party indicate what the local side will use to encrypt, and thereby
   how the remote party should expect to receive.

   In addition, various RFCs put different conditions on the format of
   offers versus answers. For example, a offer may propose multiple SRTP
   configurations, but an answer may only contain a single SRTP
   configuration.

   Lastly, while the exact media parameters are only known only after a
   offer and an answer have been exchanged, it is possible for the
   offerer to receive media after they have sent an offer and before
   they have received an answer. To properly process incoming media in
   this case, the offerer's media handler must be aware of the details
   of the offerer before the answer arrives.

   Therefore, in order to handle session descriptions properly,
   PeerConnection the user
   agent needs:

      1. To know if a session description pertains to the local or
      remote side.

      2. To know if a session description is an offer or an answer.

      3. To allow the offer to be specified independently of the answer.

   JSEP addresses this by adding both a setLocalDescription and a
   setRemoteDescription method, and both these methods take as a first parameter either
   to indicate the value SDP_OFFER, SDP_PRANSWER (for a non-final
   answer) or SDP_ANSWER (for a final answer). type of session description being supplied. This
   satisfies the requirements listed above for both the offerer, who
   first calls
   setLocalDescription(SDP_OFFER, setLocalDescription("offer", sdp) and then later
   setRemoteDescription(SDP_ANSWER,
   setRemoteDescription("answer", sdp), as well as for the answerer, who
   first calls setRemoteDescription(SDP_OFFER, setRemoteDescription("offer", sdp) and then later
   setLocalDescription(SDP_ANSWER,
   setLocalDescription("answer", sdp). While it could be possible to
   implicitly determine the value of the offer/answer argument inside of PeerConnection, argument,
   requiring it to be specified explicitly seems substantially is more robust, allowing
   invalid combinations (i.e. an answer before an offer) to generate an
   appropriate error.

   It also allows for an answer to be treated as provisional by the
   application. Provisional answers provide a way for an answerer to
   communicate session parameters back to the offerer, in order for the
   session to begin, while allowing a final answer to be specified
   later. This concept of a final answer is important to the
   offer/answer model; when such an answer is received, any extra
   resources allocated by the caller can be released, now that the exact
   session configuration is known. These "resources" can include things
   like extra ICE components, TURN candidates, or video decoders.
   Provisional answers, on the other hand, do no such deallocation; as a
   result, multiple dissimilar provisional answers can be received and
   applied during call setup.

   As in [RFC3264], an offerer can send an offer, and update it as long
   as it has not been answered. The answerer can send back zero or more
   provisional answers, and finally end the offer-answer exchange by
   sending a final answer. The state machine for this is as follows:

         +-----------+
         |           |
         |           |
         |  Stable   |<---------------\
         |           |                |
         |           |                |
         +-----------+                |
             ^   |                    |
             |   | OFFER              |
      ANSWER |   |                    | ANSWER
             |   V                    |
         +-----------+          +-----------+
         |           |          |           |
         |           | PRANSWER |           |
         |   Offer   |--------->| Pranswer  |
         |           |          |           |
         |           |----\     |           |----\
         +-----------+    |     +-----------+    |
                    ^     |                ^     |
                    |     |                |     |
                    \-----/                \-----/
                     OFFER                 PRANSWER

                   Figure 2: JSEP State Machine

   Aside from these state transitions, there is no other difference
   between the handling of provisional ("pranswer") and final ("answer")
   answers.

4.3. Session Description Format
   In the current WebRTC specification, session descriptions are
   formatted as SDP messages. While this format is not optimal for
   manipulation from Javascript, it is widely accepted, and frequently
   updated with new features. Any alternate encoding of session
   descriptions would have to keep pace with the changes to SDP, at
   least until the time that this new encoding eclipsed SDP in
   popularity. As a result, JSEP continues to use SDP as the internal
   representation for its session descriptions.

   However, to simplify Javascript processing, and provide for future
   flexibility, the SDP syntax is encapsulated within a
   SessionDescription object, which can be constructed from SDP, and be
   serialized out to SDP. If we were able to agree on a JSON format for
   session descriptions, we could easily enable this object to
   generate/expect JSON.

   Other methods may be added to SessionDescription in the future to
   simplify handling of SessionDescriptions from Javascript.

4.4. Separation of Signaling and ICE State Machines

   Previously, PeerConnection operated two state machines, referred to
   in the spec as an "ICE Agent", which handles the establishment of
   peer-to-peer connectivity, and an "SDP Agent", which handles the
   state of the offer-answer signaling. The states of these state
   machines were exposed through the iceState and sdpState attributes on
   PeerConnection, with an additional readyState attribute that
   reflected the high-level state of the PeerConnection.

   JSEP does away with the SDP Agent within the browser; browser, and this
   functionality is now controlled directly by the application, which
   uses the setLocalDescription and setRemoteDescription APIs to tell
   PeerConnection
   the browser what SDP has been negotiated. The ICE Agent remains in
   the browser, as it still needs to perform gathering drive the process of gathering
   candidates, connectivity checking, checks, and related ICE functionality.

   The net effect of this

   When a new ICE candidate is that sdpState goes away, and
   processSignalingMessage becomes processIceMessage, which now
   specifically handles incoming available, the ICE candidates. To allow Agent will notify the
   application via a callback; these candidates will automatically be
   added to control exactly when it wants to start ICE negotiation
   (e.g. either on receipt of the call, or only after accepting the
   call), a startIce method has local session description. When all candidates have been added.
   gathered, the callback will also be invoked to signal that the
   gathering process is complete.

4.5. ICE Candidate Trickling

   Candidate trickling is a technique through which a caller may
   incrementally provide candidates to the callee after the initial
   offer has been dispatched. This allows the callee to begin acting
   upon the call and setting up the ICE (and perhaps DTLS) connections
   immediately, without having to wait for the caller to allocate all
   possible candidates, resulting in faster call startup in many cases.

   JSEP supports optional candidate trickling by providing APIs that
   provide control and feedback on the ICE candidate gathering process.
   Applications that support candidate trickling can send the initial
   offer immediately and send individual candidates when they get a the
   onicecandidate callback with a new candidate; applications that do
   not support this feature can simply wait for the final onicecandidate
   callback that indicates gathering is complete, and simply create and send
   their offer, with all the candidates, at this time.

   To be clear, aplications

   Upon receipt of trickled candidates, the receiving application can
   supply them to its ICE Agent by calling an addIceCandidate method.
   This triggers the ICE Agent to start using this remote candidate for
   connectivity checks. Applications that do not make use of candidate
   tricking can ignore processIceMessage addIceCandidate entirely, and use IceCallback the
   onicecandidate callback solely to indicate when candidate gathering
   is complete.

4.6. ICE Candidate Format

   As with session descriptions, we choose to provide an IceCandidate
   object that provides some abstraction, but can be easily converted
   to/from SDP a=candidate lines.

   The IceCandidate object has a field fields to indicate which m= line it
   should be associated with, and a method to convert to a SDP
   representation, ex:

      a=candidate:1 1 UDP 1694498815 66.77.88.99 10000 typ host

   Currently, a=candidate lines are the only thing SDP information that are is
   contained within IceCandidate, as this is they represent the only information that is
   needed that is not present in the initial offer (i.e. for trickle
   candidates).

5. Media Setup Overview

   The example here shows

4.7. Interactions With Forking

4.7.1. Serial Forking

   Serial forking involves a typical call setup using the JSEP model. We
   assume the following architecture in this example, being dispatched to multiple remote
   callees, where UA is
   synonymous with "browser", and JS is synonymous with "web
   application":

   OffererUA <-> OffererJS <-> WebServer <-> AnswererJS <-> AnswererUA

5.1. Initiating each callee can accept the Session

   The initiator creates call, but only one active
   session ever exists at a PeerConnection, installs its IceCallback, and
   adds the desired MediaStreams (presumably obtained via getUserMedia).
   The PeerConnection time; no mixing of received media is in
   performed.

   JSEP handles serial forking well, allowing the NEW state.

   OffererJS->OffererUA: var pc = new PeerConnection(config, iceCb);
   OffererJS->OffererUA: pc.addStream(stream);

5.1.1. Generating An Offer

   The initiator then creates a session description to offer application to easily
   control the
   callee. This description includes policy for selecting the codecs and other necessary
   session parameters, as well as information about each desired remote endpoint. When an
   answer arrives from one of the streams
   that has been added (e.g. SSRC, CNAME, etc.) The created description
   includes all parameters that callees, the offerer's UA supports; if the
   initiator wants to influence the created offer, they application can pass in a
   MediaHints object to createOffer that allows for customization (e.g.
   if the initiator wants choose to receive but not send video). The initiator
   can also directly manipulate the created session description as well,
   perhaps if
   apply it wants to change either as a provisional answer, leaving open the priority possibility
   of using a different answer in the offered codecs.

   OffererJS->OffererUA: var offer = pc.createOffer(null);

5.1.2. Applying future, or apply it as a final
   answer, ending the Offer

   The initiator then instructs setup flow.

   In a "first-one-wins" situation, the PeerConnection to use this offer first answer will be applied as
   a final answer, and the local description for application will send a terminate message to
   any subsequent answers. In SIP parlance, this session, i.e. what codecs it would be ACK + BYE.

   In a "last-one-wins" situation, all answers would be applied as
   provisional answers, and any previous call leg will use
   for received media, what SRTP keys it be terminated. At
   some point, the application will use for sending media (if
   using SDES), etc. In order that end the UA handle setup process, perhaps with
   a timer; At this point, the description
   properly, application could reapply the initiator marks it existing
   remote description as an offer when calling
   setLocalDescription; this indicates a final answer.

4.7.2. Parallel Forking

   Parallel forking involves a call being dispatched to multiple remote
   callees, where each callee can accept the UA that call, and multiple
   capabilities have been offered, but this set may
   simultaneous active sessions can be pared back later,
   when established as a result. If
   multiple callees send media, this media is mixed and played out at
   the answer arrives.

   Since caller side.

   JSEP can handle parallel forking by "cloning" the local user agent must be prepared to receive media upon
   applying the offer, this operation will cause local decoder resources session when needed
   to be allocated, based on create multiple parallel sessions. When the codecs indicated in first answer is
   received, the offer.

   OffererJS->OffererUA: pc.setLocalDescription(SDP_OFFER, offer);

5.1.3. Initiating ICE

   The initiator caller can now start clone the ICE process of candidate generation existing session, and connectivity checking. This results in callbacks to the
   application's IceCallback. Candidates are provided to then apply
   the IceCallback answer as they are allocated, with the |moreToFollow| argument set a final answer to true
   if there are still allocations pending; when the last allocation
   completes or times out, this callback will be invoked with
   |moreToFollow| set to false.

   OffererJS->OffererUA: pc.startIce();
   OffererUA->OffererJS: iceCallback(candidate, ...);

5.1.4. Serializing original session. Upon receiving
   the Offer next answer, the cloned session is cloned again, and Candidates

   At this point, the offerer received
   answer is ready applied as a final answer to send its offer the first clone. This process
   repeats until the caller decides to end the callee
   using its preferred signaling protocol. Depending on setup flow, and closes
   the protocol, it
   can either send final cloned session.

   Cloned sessions inherit the initial local session description first, and then
   "trickle" the ICE candidates as they are given to the application, or
   it
   from their parent, and an empty remote description; only sessions
   that have not yet applied an answer can wait for all the ICE candidates to be collected, and then send
   the offer and list of cloned. Each cloned
   session may discover new peer-reflexive candidates; these candidates all at once.

5.2. Receiving
   will be supplied via the Session

   Through onicecandidate callback to that specific
   session. Since the chosen signaling protocol, clone uses the recipient same local description as its
   parent, creating a clone will fail if it is notified not possible to reserve
   the same resources for the clone as have already been reserved by the
   parent.

   As a result of
   an incoming session request. It creates this cloning, the application will end up with N
   parallel sessions, each with a PeerConnection, local and
   installs its remote description and their
   own IceCallback.

   AnswererJS->AnswererUA: var pc = new PeerConnection(config, iceCb);

5.2.1. Receiving the Offer local and remote addresses. The recipient converts the received offer media flow from its signaling protocol
   into these sessions
   can be managed by specifying SDP format, and supplies it direction attributes in the
   descriptions, or the application can choose to its PeerConnection, again marking
   it as an offer. As a remote description, play out the offer indicates what
   codecs media
   from all sessions mixed together. Of course, if the remote side application wants
   to use for receiving, as well as what
   SRTP keys only keep a single session, it will use for sending. The setting of can simply terminate the remote
   description causes callbacks to be issued, informing sessions
   that it no longer needs.

4.8. Session Rehydration

   In the event that the local application state is reinitialized,
   either due to a user reload of what kinds of streams are present in the offer.

   This step will also cause encoder resources page, or a decision within the
   application to be allocated, based reload itself (perhaps to update to a new version), it
   is possible to keep an existing session alive via a process called
   "rehydration".

   With rehydration, the current local session description is persisted
   somewhere outside of the page, perhaps on the codecs specified application server, or
   in |offer|.

   AnswererJS->AnswererUA: pc.setRemoteDescription(SDP_OFFER, offer);
   AnswererUA->AnswererJS: onAddStream(stream);

5.2.2. Initiating ICE browser local storage. The recipient page is then starts its own reloaded, and a new
   session object is created in Javascript. The saved local session is
   now retrieved, but the previous ICE state machine, to allow
   connectivity to candidates will no longer be established as quickly as possible.

   AnswererJS->AnswererUA: pc.startIce();
   AnswererUA->AnswererJS: iceCallback(candidate, ...);

5.2.3. Handling
   valid in this case, so we will need to perform an ICE Messages

   If restart; to do
   so, we simply generate a new ICE candidates from ufrag/pwd combo for the remote site were included in the offer,
   the ICE Agent will automatically start trying to use them. Otherwise,
   if ICE candidates are sent separately, they are passed into the
   PeerConnection when they arrive.

   AnswererJS->AnswererUA: pc.processIceMessage(candidate);

5.2.4. Generating the Answer
   Once the recipient has decided to accept the session, it generates an
   answer session local
   description. This process performs the appropriate
   intersection of codecs

   The modified local description is then installed via
   setLocalDescription, and other parameters sent off as an offer to generate the correct
   answer. As remote side, who
   will reply with the offer, MediaHints an answer that can be provided supplied to influence
   the answer that
   setRemoteDescription. ICE processing proceeds as usual, and as soon
   as connectivity is generated, and/or the application can post-process established, the answer manually.

   AnswererJS->AnswererUA: pc.createAnswer(offer, null);

5.2.5. Applying session will be back up and
   running again.

5. Interface

   This section details the Answer basic operations that must be present to
   implement JSEP functionality. The recipient then instructs actual API exposed in the PeerConnection W3C API
   may have somewhat different syntax, but should map easily to use these
   concepts.

5.1. Methods

5.1.1. createOffer

   The createOffer method generates a blob of SDP that contains a RFC
   3264 offer with the answer as
   its local description supported configurations for this session, i.e. what codecs it will use
   to receive media, etc. It also marks the description as an answer,
   which tells the UA that these parameters are final. This causes session,
   including descriptions of the
   PeerConnection to move local MediaStreams attached to this
   PeerConnection, the ACTIVE state, codec/RTP/RTCP options supported by this
   implementation, and transmission of media any candidates that have been gathered by the answerer ICE
   Agent. A constraints parameters may be supplied to start.

   AnswererJS->AnswererUA: pc.setLocalDescription(SDP_ANSWER, answer);
   AnswererUA->OffererUA:  <media>

5.2.6. Serializing the Answer

   As with provide additional
   control over the generated offer, the answer (with e.g. to get a full set of session
   capabilities, or without candidates) is now
   converted to request a new set of ICE credentials.

   In the initial offer, the generated SDP will contain all desired signaling format and sent to
   functionality for the initiator.

5.3. Completing session (certain parts that are supported but
   not desired by default may be omitted); for each SDP line, the Session

5.3.1. Receiving
   generation of the Answer

   The initiator converts SDP must follow the answer from appropriate process for
   generating an offer. In the signaling protocol and
   applies it as event createOffer is called after the remote description, marking it as
   session is established, createOffer will generate an answer. This
   causes offer that is
   compatible with the PeerConnection to move current session, incorporating any changes that
   have been made to the ACTIVE state, and
   transmission session since the last complete offer-answer
   exchange, such as addition or removal of media by streams. If no changes have
   been made, the offerer to start.

   OffererJS->OffererUA:  pc.setRemoteDescription(SDP_ANSWER, answer);
   OffererUA->AnswererUA: <media>

5.4. Updates offer will be identical to the current local
   description.

   Session

   Updates to the session are handled with descriptions generated by createOffer must be immediately
   usable by setLocalDescription; if a new offer/answer exchange.
   However, since media system has limited resources
   (e.g. a finite number of decoders), createOffer should return an
   offer that reflects the current state of the system, so that
   setLocalDescription will already be flowing at succeed when it attempts to acquire those
   resources. Because this point, method may need to inspect the new
   offerer needs system state
   to support both its old session description as well as determine the new one currently available resources, it has offered, until the may be implemented
   as an async operation.

   Calling this method does not change state; its use is accepted by the
   remote side.

   Note also not required.

5.1.2. createAnswer

   The createAnswer method generates a blob of SDP that in an update scenario, contains a RFC
   3264 SDP answer with the roles may be reversed, i.e. supported configuration for the update offerer can be different than session that
   is compatible with the original offerer.

6. Proposed WebRTC API changes

6.1. PeerConnection API

   The text below indicates parameters supplied in |offer|. Like
   createOffer, the recommended changes to returned blob contains descriptions of the
   PeerConnection API local
   MediaStreams attached to implement this PeerConnection, the JSEP functionality. Methods
   marked with a [+] are new/proposed; methods marked with a [-] codec/RTP/RTCP
   options negotiated for this session, and any candidates that have
   been removed in this proposal.

   [Constructor (in DOMString configuration, in IceCallback iceCb)]
   interface PeerConnection {
       // creates a blob of SDP to gathered by the ICE Agent. A constraints parameter may be provided as
   supplied to provide additional control over the generated answer.

   As an offer.
   [+] SessionDescription createOffer (MediaHints hints);
       // creates answer, the generated SDP will contain a blob specific configuration
   that specifies how the media plane should be established. For each
   SDP line, the generation of the SDP to be provided as must follow the appropriate
   process for generating an answer.
   [+] SessionDescription

   Session descriptions generated by createAnswer (DOMString offer,
                                        MediaHints hints);
       // actions, for setLocalDescription/setRemoteDescription
   [+] const unsigned short SDP_OFFER = 0x100;
   [+] const unsigned short SDP_PRANSWER = 0x200;
   [+] const unsigned short SDP_ANSWER = 0x300;
       // sets the local session description
   [+] void setLocalDescription (unsigned short action,
                                 SessionDescription desc);
       // sets must be immediately
   usable by setLocalDescription; like createOffer, the remote session returned
   description
   [+] void setRemoteDescription (unsigned short action,
                                  SessionDescription desc);
       // returns should reflect the current local session description
   [+] readonly SessionDescription localDescription;
       // returns state of the current remote session description
   [+] readonly SessionDescription remoteDescription;
   [-] void processSignalingMessage (DOMString message);
       const unsigned short NEW = 0;     // initial system. Because
   this method may need to inspect the system state
   [+] const unsigned short OPENING = 1; // to determine the
   currently available resources, it may need to be implemented as an
   async operation.

   Calling this method does not change state; its use is not required.

5.1.3. SessionDescriptionType

   The strings "offer", "pranswer", and "answer" serve as type arguments
   to setLocalDescription and setRemoteDescription. They provide
   information as to how the description parameter should be parsed, and
   how the media state should be changed.

   "offer" indicates that a description should be parsed as an offer;
   said description may include many possible media configurations. A
   description used as an "offer" may be applied anytime the
   PeerConnection is in a stable state, or as an update to a previously
   sent but unanswered "offer".

   "pranswer" indicates that a description should be parsed as an
   answer, but not a final answer, and so should not result in the
   freeing of allocated resources. It may result in the start of media
   transmission, if the answer does not specify an inactive media
   direction. A description used as a "pranswer" may be applied as a
   response to an "offer", or an update to a previously sent "answer".

   "answer" indicates that a description should be parsed as an answer,
   the offer-answer exchange should be considered complete, and any
   resources (decoders, candidates) that are no longer needed can be
   released. A description used as an "answer" may be applied as a
   response to a "offer", or an update to a previously sent "pranswer".

   The application can use some discretion on whether an answer should
   be applied as provisional or final. For example, in a serial forking
   scenario, an application may receive multiple "final" answers, one
   from each remote endpoint. The application could 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).

5.1.4. setLocalDescription

   The setLocalDescription method instructs the PeerConnection to apply
   the supplied SDP blob as its local configuration. The type parameter
   indicates whether the blob should be processed as an offer,
   provisional answer, or final answer; offers and answers are checked
   differently, using the various rules that exist for each SDP line.

   This API changes the local media state; among other things, it sets
   up local resources for receiving and decoding media. In order to
   successfully handle scenarios where the application wants to offer to
   change from one media format to a different, incompatible format, the
   PeerConnection must be able to simultaneously support use of both the
   old and new local descriptions (e.g. support codecs that exist in
   both descriptions) until a final answer is received, at which point
   the PeerConnection can fully adopt the new local description, or roll
   back to the old description if the remote side denied the change.

   If setRemoteDescription was previous called with an offer, and
   setLocalDescription is called with an answer (provisional or final),
   and the media directions are compatible, this will result in the
   starting of media transmission.

5.1.5. setRemoteDescription
   The setRemoteDescription method instructs the PeerConnection to apply
   the supplied SDP blob as the desired remote configuration. As in
   setLocalDescription, the |type| parameter indicates how the blob
   should be processed.

   This API changes the local media state; among other things, it sets
   up local resources for sending and encoding media.

   If setRemoteDescription was previous called with an offer, and
   setLocalDescription is called with an answer (provisional or remote desc set
       const unsigned short ACTIVE = 2;  // local final),
   and remote desc set
       const unsigned short CLOSED = 3;  // ended state
       readonly attribute unsigned short readyState;
       // starts ICE connection/handshaking
   [+] void startIce (optional IceOptions options);
       // processes received ICE information
   [+] void processIceMessage (IceCandidate candidate);
       const unsigned short ICE_GATHERING = 0x100;
       const unsigned short ICE_WAITING = 0x200;
       const unsigned short ICE_CHECKING = 0x300;
       const unsigned short ICE_CONNECTED = 0x400;
       const unsigned short ICE_COMPLETED = 0x500;
       const unsigned short ICE_FAILED = 0x600;
       const unsigned short ICE_CLOSED = 0x700;
       readonly attribute unsigned short iceState;

   [-] const unsigned short SDP_IDLE = 0x1000;
   [-] const unsigned short SDP_WAITING = 0x2000;
   [-] const unsigned short SDP_GLARE = 0x3000;
   [-] readonly attribute unsigned short sdpState;
       void addStream (MediaStream stream, MediaStreamHints hints);
       void removeStream (MediaStream stream);
       readonly attribute MediaStream[]  localStreams;
       readonly attribute MediaStream[]  remoteStreams;
       void close ();
       [ rest the media directions are compatible, this will result in the
   starting of media transmission.

5.1.6. localDescription

   The localDescription method returns a copy of interface omitted ]
   };

   [Constructor (in DOMString sdp)]
   interface SessionDescription {
     // adds the specified candidate current local
   configuration, i.e. what was most recently passed to
   setLocalDescription, plus any local candidates that have been
   generated by the ICE Agent.

   A null object will be returned if the local description
     void addCandidate(IceCandidate candidate);
     // serializes has not yet
   been established.

5.1.7. remoteDescription

   The remoteDescription method returns a copy of the current remote
   configuration, i.e. what was most recently passed to
   setRemoteDescription, plus any remote candidates that have been
   supplied via processIceMessage.

   A null object will be returned if the remote description has not yet
   been established.

5.1.8. updateIce

   The updateIce method allows the configuration of the ICE Agent to SDP
     DOMString toSdp();
   };

   [Constructor (in DOMString label, in DOMString candidateLine)]
   interface IceCandidate {
     // be
   changed during the m= line this candidate is associated with
     readonly DOMString label;
     // creates a SDP-ized form session, primarily for changing which types of
   local candidates are provided to the application and used for
   connectivity checks. A callee may initially configure the ICE Agent
   to use only relay candidates, to avoid leaking location information,
   but update this candidate
     DOMString toSdp();
   };

6.1.1 MediaHints

   MediaHints configuration to use all candidates once the call is an object that can
   accepted.

   Regardless of the configuration, the gathering process collects all
   available candidates, but excluded candidates will not be passed into createOffer surfaced in
   onicecallback or
   createAnswer used for connectivity checks.

   This call may result in a change to affect the type state of offer/answer that is generated.

   The following properties can be set on MediaHints:

      has_audio: boolean

      Indicates whether we want to receive audio; defaults the ICE Agent, and
   may result in a change to true media state if we
      have audio streams, else false

      has_video: boolean

      Indicates whether we want it results in connectivity
   being established.

5.1.9. addIceCandidate

   The addIceCandidate method provides a remote candidate to receive video; defaults the ICE
   Agent, which will be added to true if we
      have video streams, else false

   As an example, MediaHints could the remote description. Connectivity
   checks will be used sent to create the new candidate.

   This call will result in a session that
   transmits only audio, but is able change to receive video from the remote
   side, by forcing the inclusion state of the ICE Agent, and
   may result in a m=video line even when no video
   sources are provided.

6.1.2 createOffer change to media state if it results in connectivity
   being established.

5.2. Configurable SDP Parameters

   The createOffer method generates following is a blob partial list of SDP parameters that contains an application
   may want to control, in either local or remote descriptions, using
   this API.

    - remove or reorder codecs (m=)
    - change codec attributes (a=fmtp; ptime)
    - enable/disable BUNDLE (a=group)
    - enable/disable RTCP mux (a=rtcp-mux)
    - remove or reorder SRTP crypto-suites (a=crypto)
    - change SRTP parameters or keys (a=crypto)
    - change send resolution or framerate (TBD)
    - change desired recv resolution or framerate (TBD)
    - change total bandwidth (b=)
    - remove desired AVPF mechanisms (a=rtcp-fb)
    - remove RTP header extensions (a=rtphdr-ext)
    - add/change SSRC grouping (e.g. FID, RTX, etc) (a=ssrc-group)
    - add SSRC attributes (a=ssrc)
    - change ICE ufrag/password (a=ice-ufrag/pwd)
    - change media send/recv state (a=sendonly/recvonly/inactive)

   For example, an application could implement call hold by adding an
   a=inactive attribute to its local description, and then applying and
   signaling that description.

6. Media Setup Overview

   The example here shows a RFC
   3264 offer with the supported configurations for typical call setup using the session,
   including descriptions of JSEP model,
   indicating the local MediaStreams attached to this
   PeerConnection, functions that are called and the codec/RTP/RTCP options supported by state changes that
   occur. We assume the following architecture in this
   implementation, example, where UA
   is synonymous with "browser", and any candidates that have been gathered by JS is synonymous with "web
   application":

   OffererUA <-> OffererJS <-> WebServer <-> AnswererJS <-> AnswererUA

6.1. Initiating the ICE
   Agent. Session

   The |hints| parameter may be supplied initiator creates a PeerConnection, hooks up to provide additional
   control over the generated offer.

   As an offer, the generated SDP will contain the full set of
   capabilities supported by its ICE callback,
   and adds the session (as opposed to an answer, which
   will include only a specific negotiated subset desired MediaStreams (presumably obtained via
   getUserMedia). The ICE gathering process begins to use); gather candidates
   for each SDP
   line, the generation a default number of streams, as the SDP must follow the appropriate process
   for generating an offer. In exact number will not be
   known until the event createOffer local description is called after applied. The PeerConnection is
   in the NEW state.

   OffererJS->OffererUA: var pc = new PeerConnection(config, null);
   OffererJS->OffererUA: pc.onicecandidate = onIceCandidate;
   OffererJS->OffererUA: pc.addStream(stream);

6.1.1. Generating An Offer

   The initiator then creates a session is established, createOffer will generate an description to offer that is
   compatible with the current session, incorporating any changes that
   have been made to the session since
   callee. This description includes the last complete offer-answer
   exchange, such codecs and other necessary
   session parameters, as addition or removal well as information about each of streams. If no changes have
   been made, the offer will be identical to streams
   that has been added (e.g. SSRC, CNAME, etc.) The created description
   includes all parameters that the current local
   description.

   Session descriptions generated by createOffer must be immediately
   usable by setLocalDescription; offerer's UA supports; if the
   initiator wants to influence the created offer, they can pass in a system has limited resources
   (e.g. a finite number of decoders),
   MediaConstraints object to createOffer should return an
   offer that reflects allows for customization
   (e.g. if the current state of initiator wants to receive but not send video). The
   initiator can also directly manipulate the system, so that
   setLocalDescription will succeed when created session
   description as well, perhaps if it attempts wants to acquire those
   resources.

   Calling this method does not change the state priority of
   the PeerConnection;
   its use is not required.

   A TBD exception is thrown if offered codecs.

   OffererJS->OffererUA: var offer = pc.createOffer(null);

6.1.2. Applying the |hints| parameter is malformed.

6.1.3 createAnswer Offer

   The createAnswer method generates a blob of SDP that contains a RFC
   3264 SDP answer with initiator then instructs the supported configuration for PeerConnection to use this offer as
   the session local description for this session, i.e. what codecs it will use
   for received media, what SRTP keys it will use for sending media (if
   using SDES), etc. In order that
   is compatible with the parameters supplied in |offer|. Like
   createOffer, UA handle the returned blob contains descriptions of description
   properly, the local
   MediaStreams attached to initiator marks it as an offer when calling
   setLocalDescription; this PeerConnection, indicates to the codec/RTP/RTCP
   options negotiated for this session, and any candidates UA that multiple
   capabilities have been gathered by offered, but this set may be pared back later,
   when the answer arrives.

   Since the local user agent must be prepared to receive media upon
   applying the offer, this operation will cause local decoder resources
   to be allocated, based on the codecs indicated in the offer.

   OffererJS->OffererUA: pc.setLocalDescription("offer", offer);

6.1.3. Handling ICE Agent. Callbacks
   The |hints| parameter may be supplied initiator starts to receive callbacks on its onicecandidate
   handler. Candidates are provided to provide additional control over the generated answer.

   As an answer, IceCallback as they are
   allocated; when the generated SDP last allocation completes or times out, this
   callback will contain be invoked with a specific configuration
   that specifies how null argument.

   OffererUA->OffererJS: onIceCandidate(candidate);

6.1.4. Serializing the media plane should be established. For each
   SDP line, Offer and Candidates

   At this point, the generation of offerer is ready to send its offer to the SDP must follow callee
   using its preferred signaling protocol. Depending on the appropriate
   process for generating an answer.

   Session descriptions generated by createAnswer must be immediately
   usable by setLocalDescription; like createOffer, protocol, it
   can either send the returned initial session description should reflect first, and then
   "trickle" the current state of ICE candidates as they are given to the system.

   Calling this method does not change application, or
   it can wait for all the state ICE candidates to be collected, and then send
   the offer and list of candidates all at once.

6.2. Receiving the PeerConnection;
   its use is not required.

   A TBD exception is thrown if Session

   Through the |hints| parameter is malformed, or chosen signaling protocol, the |offer| parameter recipient is missing or malformed.

6.1.4 SDP_OFFER, SDP_PRANSWER, notified of
   an incoming session request. It creates a PeerConnection, and SDP_ANSWER sets up
   its own ICE callback. The SDP_XXXX enums serve as arguments to setLocalDescription and
   setRemoteDescription. They provide information as ICE gathering process begins to how gather
   candidates for a default number of streams.

   AnswererJS->AnswererUA: var pc = new PeerConnection(config, null);
   AnswererJS->AnswererUA: pc.onicecandidate = onIceCandidate;

6.2.1. Receiving the
   |description| parameter should be parsed, Offer

   The recipient converts the received offer from its signaling protocol
   into SDP format, and how supplies it to its PeerConnection, again marking
   it as an offer. As a remote description, the media state
   should be changed.

   SDP_OFFER offer indicates that a description should be parsed what
   codecs the remote side wants to use for receiving, as an offer;
   said description may include many possible media configurations. A
   description used well as a SDP_OFFER may what
   SRTP keys it will use for sending. The setting of the remote
   description causes callbacks to be applied anytime issued, informing the
   PeerConnection is application
   of what kinds of streams are present in a stable state, or as an update the offer.

   This step will also cause encoder resources to a previously
   sent but unanswered SDP_OFFER.

   SDP_PRANSWER indicates that a description should be parsed as an
   answer, but not a final answer, and so should not result allocated, based on
   the codecs specified in |offer|.

   AnswererJS->AnswererUA: pc.setRemoteDescription("offer", offer);
   AnswererUA->AnswererJS: onAddStream(stream);

6.2.2. Handling ICE Messages

   If ICE candidates from the
   starting of media transmission. A description used as a SDP_PRANSWER
   may be applied as a response to a SDP_OFFER, or an update remote site were included in the offer,
   the ICE Agent will automatically start trying to a
   previously use them. Otherwise,
   if ICE candidates are sent SDP_PRANSWER.

   SDP_ANSWER indicates that a description should be parsed as separately, they are passed into the
   PeerConnection when they arrive.

   AnswererJS->AnswererUA: pc.addIceCandidate(candidate);

6.2.3. Generating the Answer

   Once the recipient has decided to accept the session, it generates an
   answer,
   answer session description. This process performs the appropriate
   intersection of codecs and other parameters to generate the offer-answer exchange should be considered complete.
   A description used as a SDP_ANSWER may correct
   answer. As with the offer, MediaConstraints can be applied as a response to a
   SDP_OFFER, or an update provided to a previously send SDP_PRANSWER.

6.1.5 setLocalDescription
   influence the answer that is generated, and/or the application can
   post-process the answer manually.

   AnswererJS->AnswererUA: pc.createAnswer(offer, null);

6.2.4. Applying the Answer

   The setLocalDescription method recipient then instructs the PeerConnection to apply use the supplied SDP blob answer as
   its local configuration. The |type|
   parameter indicates whether description for this session, i.e. what codecs it will use
   to receive media, etc. It also marks the blob should be processed description as an offer
   (SDP_OFFER), provisional answer (SDP_PRANSWER), or final answer
   (SDP_ANSWER); offers and answers are checked differently, using answer,
   which tells the
   various rules UA that exist for each SDP line. these parameters are final. This API changes the local media state; among other things, it sets
   up local resources for receiving and decoding media. In order to
   successfully handle scenarios where causes the application wants
   PeerConnection to offer move to
   change from one the ACTIVE state, and transmission of media format to a different, incompatible format,
   by the
   PeerConnection must be able answerer to simultaneously support use of start (assuming both the
   old and new local descriptions (e.g. support codecs that exist sides have indicated this in
   both descriptions) until a final answer is received, at which point
   their descriptions).

   AnswererJS->AnswererUA: pc.setLocalDescription("answer", answer);
   AnswererUA->OffererUA:  <media>

6.2.5. Serializing the PeerConnection can fully adopt Answer

   As with the new local description, offer, the answer (with or roll
   back without candidates) is now
   converted to the old description if desired signaling format and sent to the remote side denied initiator.

6.3. Completing the change.

   Changes to Session

6.3.1. Receiving the state of media transmission will only occur when a
   final answer is successfully applied.

   A TBD exception is thrown if |description| is invalid. A TBD
   exception is thrown if there are insufficient local resources to
   apply |description|.

6.1.6 setRemoteDescription Answer

   The setRemoteDescription method instructs initiator converts the PeerConnection to apply answer from the supplied SDP blob signaling protocol and
   applies it as the desired remote configuration. As in
   setLocalDescription, the |type| parameter indicates how the blob
   should be processed. description, marking it as an answer. This API changes
   causes the local media state; among other things, it sets
   up local resources for sending and encoding media.

   Changes PeerConnection to move to the state of media ACTIVE state, and
   transmission will only occur when a
   final answer is successfully applied.

   A TBD exception is thrown if |description| is invalid. A TBD
   exception is thrown if there are insufficient local resources to
   apply |description|.

6.1.7 localDescription

   The localDescription method returns a copy of media by the current local
   configuration, i.e. what was most recently passed offerer to
   setLocalDescription, plus any local candidates that start (assuming both sides
   have been
   generated by indicated this in their descriptions).

   OffererJS->OffererUA:  pc.setRemoteDescription("answer", answer);
   OffererUA->AnswererUA: <media>

6.4. Updates to the ICE Agent.

   A null object Session

   Updates to the session are handled with a new offer/answer exchange.
   However, since media will already be returned if flowing at this point, the local new
   offerer needs to support both its old session description as well as
   the new one it has not yet
   been established.

6.1.8 remoteDescription

   The remoteDescription method returns a copy of offered, until the change is accepted by the current remote
   configuration, i.e. what was most recently passed to
   setRemoteDescription, plus any
   remote candidates side.

   Note also that have been
   supplied via processIceMessage.

   A null object will in an update scenario, the roles may be returned if reversed, i.e.
   the update offerer can be different than the original offerer.

7. Security Considerations

   TODO

8. IANA Considerations

   This document requires no actions from IANA.

9. Acknowledgements

   Harald Alvestrand, Dan Burnett, Neil Stratford, Eric Rescorla, Anant
   Narayanan, and Adam Bergkvist all provided valuable feedback on this
   proposal. Matthew Kaufman provided the remote description has not yet
   been established.

6.1.9 IceOptions

   IceOptions is an object observation that can be passed into startIce keeping state
   out of the browser allows a call to restrict continue even if the candidates that are page is
   reloaded. Richard Ejzak provided to the application and used specifics on session cloning.

10. References

10.1. Normative References

   [RFC2119]  Bradner, S., "Key words for
   connectivity checks. This can be useful if the application wants to
   only use TURN candidates in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
   with Session Description Protocol (SDP)", RFC 3264, June 2002.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
   Description Protocol", RFC 4566, July 2006.

10.2. Informative References

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
   Description Protocol (SDP) Security Descriptions for privacy reasons, or only local + STUN
   candidates Media Streams",
   RFC 4568, July 2006.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
   (ICE): A Protocol for cost reasons. Network Address Translator (NAT) Traversal for
   Offer/Answer Protocols", RFC 5245, April 2010.

   [webrtc-api] Bergkvist, Burnett, Jennings, Narayanan, "WebRTC 1.0:

   Real-time Communication Between Browsers", May 2011.

   Available at http://dev.w3.org/2012/webrtc/editor/webrtc.html

Appendix A. JSEP Implementation Examples

A.1. Example API

   The following properties can interface below shows a basic Javascript API that could be set on IceOptions:

      use_candidates: "all", "no_relay", "only_relay"

      Indicates what types used
   to expose the functionality discussed in this document. This API is
   used for the examples in the following parts of local candidates should this Appendix.

   // actions, for setLocalDescription/setRemoteDescription
   enum SessionDescriptionType { "offer", "pranswer", "answer" }

   // constraints that can be used; defaults supplied to "all"

6.1.10 startIce

   The startIce method starts the ctor or updates createXXXX
   enum MediaConstraints {
       "offerConfig",    // controls the ICE Agent process kind of
   gathering local candidates and pinging remote candidates. The
   |options| argument offer created;
                         //   "default"    (normal offer)
                         //   "caps"       (all capabilities)
                         //   "new"        (brand new description)
                         //   "iceRestart" (new ICE creds)

       "iceTransports",  // controls ICE candidates; can be used to restrict which types of local
   candidates are provided to
                         //   "none"  (no candidates)
                         //   "relay" (only relay candidates)
                         //   "all"   (all available candidates)
   }

   [Constructor (int index, DOMString id, in DOMString candidateLine)]
   interface IceCandidate {
       // the application and used m= line index for pinging; this
   can be used to limit candidate
       readonly attribute int mLineIndex
       // the use of TURN candidates by a callee to avoid
   leaking location information prior to mid for the call being accepted.

   This call may result in m= line for this candidate
       readonly attribute DOMString mLineId;
       // creates a change SDP-ized form of this candidate
       stringifier DOMString ();
   };

   [Constructor (DOMString sdp)]
   interface SessionDescription {
       // adds the specified candidate to the state of description
       void addCandidate(IceCandidate candidate);
       // serializes the ICE Agent, and
   may result in description to SDP
       stringifier DOMString ();
   };

   [Constructor (DOMString configuration,
                 optional MediaConstraints constraints)]
   interface PeerConnection {
       // creates a change blob of SDP to media state if it results in connectivity
   being established.

   A TBD exception will be thrown if |options| is malformed.

6.1.11 processIceMessage

   The processIceMessage method provides provided as an offer.
       SessionDescription createOffer (
           SessionDescriptionCallback successCb,
           optional ErrorCallback errorCb,
           optional MediaContraints constraints);
       // creates a remote candidate blob of SDP to the ICE
   Agent, which will be added to provided as an answer.
       SessionDescription createAnswer (
           SessionDescription offer,
           SessionDescriptionCallback successCb,
           optional ErrorCallback errorCb,
           optional MediaContraints constraints);

       // sets the local session description
       void setLocalDescription (
           SessionDescriptionType action,
           SessionDescription desc);
       // sets the remote description. If startIce has
   been called, connectivity checks will be sent to session description
       void setRemoteDescription (
           SessionDescriptionType action,
           SessionDescription desc)
       // returns the new candidates.

   This call will result in current local session description
       readonly attribute SessionDescription localDescription;
       // returns the current remote session description
       readonly attribute SessionDescription remoteDescription;

       // updates the constraints for ICE processing
       void updateIce (
         optional DOMString configuration,
         optional MediaConstraints constraints);
       // starts using a change to received remote ICE candidate
       void addIceCandidate (
         IceCandidate candidate);
       // notifies the state application of the ICE Agent, and
   may result in a change to media state if it results in connectivity
   being established.

   A TBD exception will be thrown if |candidate| is missing or
   malformed.

7. new local ICE candidate
       attribute Function?          onicecandidate;
   };

A.2. Example API Flows

   Below are several sample flows for the new PeerConnection and library
   APIs, demonstrating when the various APIs are called in different
   situations and with various transport protocols.

7.1. For clarity and
   simplicity, the createOffer/createAnswer calls are assumed to be
   synchronous in these examples, whereas the actual APIs are async.

A.2.1. Call using ROAP
   This example demonstrates a ROAP call, without the use of trickle
   candidates.

   // Call is initiated toward Answerer
   OffererJS->OffererUA:   pc = new PeerConnection();
   OffererJS->OffererUA:   pc.addStream(localStream, null);
   OffererJS->OffererUA:   pc.startIce();
   OffererUA->OffererJS:   iceCallback(candidate, false);   iceCallback(candidate);
   OffererJS->OffererUA:   offer = pc.createOffer(null);
   OffererJS->OffererUA:   pc.setLocalDescription(SDP_OFFER, offer.toSdp());   pc.setLocalDescription("offer", offer);
   OffererJS->AnswererJS:  {"type":"OFFER", "sdp":"<offer>"} "sdp":offer }

   // OFFER arrives at Answerer
   AnswererJS->AnswererUA: pc = new PeerConnection();
   AnswererJS->AnswererUA: pc.setRemoteDescription(SDP_OFFER, pc.setRemoteDescription("offer", msg.sdp);
   AnswererUA->AnswererJS: onaddstream(remoteStream);
   AnswererJS->AnswererUA: pc.startIce();
   AnswererUA->OffererUA:  iceCallback(candidate, false);  iceCallback(candidate);

   // Answerer accepts call
   AnswererJS->AnswererUA: peer.addStream(localStream, null);
   AnswererJS->AnswererUA: answer = peer.createAnswer(msg.offer, peer.createAnswer(msg.sdp, null);
   AnswererJS->AnswererUA: peer.setLocalDescription(SDP_ANSWER, peer.setLocalDescription("answer", answer);
   AnswererJS->OffererJS:  {"type":"ANSWER","sdp":"<answer>"}  {"type":"ANSWER","sdp":answer }

   // ANSWER arrives at Offerer
   OffererJS->OffererUA:   peer.setRemoteDescription(ANSWER,   peer.setRemoteDescription("answer", answer);
   OffererUA->OffererJS:   onaddstream(remoteStream);

   // ICE Completes (at Answerer)
   AnswererUA->AnswererJS: onopen();
   AnswererUA->OffererUA:  Media

   // ICE Completes (at Offerer)
   OffererUA->OffererJS:   onopen();
   OffererJS->AnswererJS:  {"type":"OK" }
   OffererUA->AnswererUA:  Media

7.2.

A.2.2 Call using XMPP

   This example demonstrates an XMPP call, making use of trickle
   candidates.

   // Call is initiated toward Answerer
   OffererJS->OffererUA:   pc = new PeerConnection();
   OffererJS->OffererUA:   pc.addStream(localStream, null);
   OffererJS->OffererUA:   offer = pc.createOffer(null);
   OffererJS->OffererUA:   pc.setLocalDescription(SDP_OFFER,   pc.setLocalDescription("offer", offer);
   OffererJS:              xmpp = createSessionInitiate(offer);
   OffererJS->AnswererJS:  <jingle action="session-initiate"/>
   OffererJS->OffererUA:   pc.startIce();
   OffererUA->OffererJS:   iceCallback(cand);   onicecandidate(cand);
   OffererJS:              createTransportInfo(cand, ...);              createTransportInfo(cand);
   OffererJS->AnswererJS:  <jingle action="transport-info"/>

   // session-initiate arrives at Answerer
   AnswererJS->AnswererUA: pc = new PeerConnection();
   AnswererJS:             offer = parseSessionInitiate(xmpp);
   AnswererJS->AnswererUA: pc.setRemoteDescription(SDP_OFFER, pc.setRemoteDescription("offer", offer);
   AnswererUA->AnswererJS: onaddstream(remoteStream);

   // transport-infos arrive at Answerer
   AnswererJS->AnswererUA: candidates candidate = parseTransportInfo(xmpp);
   AnswererJS->AnswererUA: pc.processIceMessage(candidates);
   AnswererJS->AnswererUA: pc.startIce(); pc.addIceCandidate(candidate);
   AnswererUA->AnswererJS: iceCallback(cand, ...) onicecandidate(cand)
   AnswererJS:             createTransportInfo(cand);
   AnswererJS->OffererJS:  <jingle action="transport-info"/>

   // transport-infos arrive at Offerer
   OffererJS->OffererUA:   candidates = parseTransportInfo(xmpp);
   OffererJS->OffererUA:  pc.processIceMessage(candidates);   pc.addIceCandidate(candidates);

   // Answerer accepts call
   AnswererJS->AnswererUA: peer.addStream(localStream, null);
   AnswererJS->AnswererUA: answer = peer.createAnswer(offer, null);
   AnswererJS:             xmpp = createSessionAccept(answer);
   AnswererJS->AnswererUA: pc.setLocalDescription(SDP_ANSWER, pc.setLocalDescription("answer", answer);
   AnswererJS->OffererJS:  <jingle action="session-accept"/>

   // session-accept arrives at Offerer
   OffererJS:              answer = parseSessionAccept(xmpp);
   OffererJS->OffererUA:   peer.setRemoteDescription(ANSWER,   peer.setRemoteDescription("answer", answer);
   OffererUA->OffererJS:   onaddstream(remoteStream);

   // ICE Completes (at Answerer)
   AnswererUA->AnswererJS: onopen();
   AnswererUA->OffererUA:  Media

   // ICE Completes (at Offerer)
   OffererUA->OffererJS:   onopen();
   OffererUA->AnswererUA:  Media

7.3.

A.2.3. Adding video to a call, using XMPP

   This example demonstrates an XMPP call, where the XMPP content-add
   mechanism is used to add video media to an existing session. For
   simplicity, candidate exchange is not shown.

   Note that the offerer for the change to the session may be different
   than the original call offerer.

   // Offerer adds video stream
   OffererJS->OffererUA:   pc.addStream(videoStream)
   OffererJS->OffererUA:   offer = pc.createOffer(null);
   OffererJS:              xmpp = createContentAdd(offer);
   OffererJS->OffererUA:   pc.setLocalDescription(SDP_OFFER,   pc.setLocalDescription("offer", offer);
   OffererJS->AnswererJS:  <jingle action="content-add"/>

   // content-add arrives at Answerer
   AnswererJS:             offer = parseContentAdd(xmpp);
   AnswererJS->AnswererUA: pc.setRemoteDescription(SDP_OFFER, pc.setRemoteDescription("offer", offer);
   AnswererJS->AnswererUA: answer = pc.createAnswer(offer, null);
   AnswererJS->AnswererUA: pc.setLocalDescription(SDP_ANSWER, pc.setLocalDescription("answer", answer);
   AnswererJS:             xmpp = createContentAccept(answer);
   AnswererJS->OffererJS:  <jingle action="content-accept"/>

   // content-accept arrives at Offerer
   OffererJS:              answer = parseContentAccept(xmpp);
   OffererJS->OffererUA:   pc.setRemoteDescription(SDP_ANSWER,   pc.setRemoteDescription("answer", answer);

7.4.

A.2.4. Simultaneous add of video streams, using XMPP

   This example demonstrates an XMPP call, where new video sources are
   added at the same time to a call that already has video; since adding
   these sources only affects one side of the call, there is no
   conflict. The XMPP description-info mechanism is used to indicate the
   new sources to the remote side.

   // Offerer and "Answerer" add video streams at the same time
   OffererJS->OffererUA:   pc.addStream(offererVideoStream2)
   OffererJS->OffererUA:   offer = pc.createOffer(null);
   OffererJS:              xmpp = createDescriptionInfo(offer);
   OffererJS->OffererUA:   pc.setLocalDescription(SDP_OFFER,   pc.setLocalDescription("offer", offer);
   OffererJS->AnswererJS:  <jingle action="description-info"/>

   AnswererJS->AnswererUA: pc.addStream(answererVideoStream2)
   AnswererJS->AnswererUA: offer = pc.createOffer(null);
   AnswererJS:             xmpp = createDescriptionInfo(offer);
   AnswererJS->AnswererUA: pc.setLocalDescription(SDP_OFFER, pc.setLocalDescription("offer", offer);
   AnswererJS->OffererJS:  <jingle action="description-info"/>

   // description-info arrives at "Answerer", and is acked
   AnswererJS:             offer = parseDescriptionInfo(xmpp);
   AnswererJS->OffererJS:  <iq type="result/>  // ack
   // description-info arrives at Offerer, and is acked
   OffererJS:              offer = parseDescriptionInfo(xmpp);
   OffererJS->AnswererJS:  <iq type="result/>  // ack

   // ack arrives at Offerer; remote offer is used as an answer
   OffererJS->OffererUA:   pc.setRemoteDescription(SDP_ANSWER,   pc.setRemoteDescription("answer", offer);

   // ack arrives at "Answerer"; remote offer is used as an answer
   AnswererJS->AnswererUA: pc.setRemoteDescription(SDP_ANSWER, pc.setRemoteDescription("answer", offer);

7.5.

A.2.5. Call using SIP

   This example demonstrates a simple SIP call (e.g. where the client
   talks to a SIP proxy over WebSockets).

   // Call is initiated toward Answerer
   OffererJS->OffererUA:   pc = new PeerConnection();
   OffererJS->OffererUA:   pc.addStream(localStream, null);
   OffererJS->OffererUA:   pc.startIce();
   OffererUA->OffererJS:   iceCallback(candidate, false);   onicecandidate(candidate);
   OffererJS->OffererUA:   offer = pc.createOffer(null);
   OffererJS->OffererUA:   pc.setLocalDescription(SDP_OFFER,   pc.setLocalDescription("offer", offer);
   OffererJS:              sip = createInvite(offer);- createInvite(offer);
   OffererJS->AnswererJS:  SIP INVITE w/ SDP

   // INVITE arrives at Answerer
   AnswererJS->AnswererUA: pc = new PeerConnection();
   AnswererJS:             offer = parseInvite(sip);
   AnswererJS->AnswererUA: pc.setRemoteDescription(SDP_OFFER, pc.setRemoteDescription("offer", offer);
   AnswererUA->AnswererJS: onaddstream(remoteStream);
   AnswererJS->AnswererUA: pc.startIce();
   AnswererUA->OffererUA:  iceCallback(candidate, false);  onicecandidate(candidate);

   // Answerer accepts call
   AnswererJS->AnswererUA: peer.addStream(localStream, null);
   AnswererJS->AnswererUA: answer = peer.createAnswer(offer, null);
   AnswererJS:             sip = createResponse(200, answer);
   AnswererJS->AnswererUA: peer.setLocalDescription(SDP_ANSWER, peer.setLocalDescription("answer", answer);
   AnswererJS->OffererJS:  200 OK w/ SDP

   // 200 OK arrives at Offerer
   OffererJS:              answer = parseResponse(sip);
   OffererJS->OffererUA:   peer.setRemoteDescription(ANSWER,   peer.setRemoteDescription("answer", answer);
   OffererUA->OffererJS:   onaddstream(remoteStream);
   OffererJS->AnswererJS:  ACK

   // ICE Completes (at Answerer)
   AnswererUA->AnswererJS: onopen();
   AnswererUA->OffererUA:  Media
   // ICE Completes (at Offerer)
   OffererUA->OffererJS:   onopen();
   OffererUA->AnswererUA:  Media

7.6.

A.2.6. Handling early media (e.g. 1-800-FEDEX), using SIP

   This example demonstrates how early media could be handled; for
   simplicity, only the offerer side of the call is shown.

   // Call is initiated toward Answerer
   OffererJS->OffererUA:   pc = new PeerConnection();
   OffererJS->OffererUA:   pc.addStream(localStream, null);
   OffererJS->OffererUA:   pc.startIce();
   OffererUA->OffererJS:   iceCallback(candidate, false);   onicecandidate(candidate);
   OffererJS->OffererUA:   offer = pc.createOffer(null);
   OffererJS->OffererUA:   pc.setLocalDescription(SDP_OFFER,   pc.setLocalDescription("offer", offer);
   OffererJS:              sip = createInvite(offer);
   OffererJS->AnswererJS:  SIP INVITE w/ SDP

   // 180 Ringing is received by offerer, w/ SDP
   OffererJS:              answer = parseResponse(sip);
   OffererJS->OffererUA:   pc.setRemoteDescription(SDP_PRANSWER,   pc.setRemoteDescription("pranswer", answer);
   OffererUA->OffererJS:   onaddstream(remoteStream);

   // ICE Completes (at Offerer)
   OffererUA->OffererJS:   onopen();
   OffererUA->AnswererUA:  Media

   // 200 OK arrives at Offerer
   OffererJS:              answer = parseResponse(sip);
   OffererJS->OffererUA:   pc.setRemoteDescription(SDP_ANSWER,   pc.setRemoteDescription("answer", answer);
   OffererJS->AnswererJS:  ACK

8.

A.3. Full Example Application

   The following example demonstrates a simple video calling
   application, roughly corresponding using both trickle candidates and provisional answers to
   speed up call setup.

   // Usage:
   // Caller calls start(true)
   // Callee calls start(false) to prepare the flow in Example 7.1. call/start connecting,
   // and then accept() to start transmitting.

   var signalingChannel = createSignalingChannel();
   var pc = null;
   var hasCandidates localStream = false; null;
   signalingChannel.onmessage = handleMessage;
   // Set up the call, get access to local media,
   // and establish connectivity.
   function start(isCaller) {
     // create Create a PeerConnection and hook up the IceCallback IceCallback.
     pc = new webkitPeerConnection(
            "", function (candidate, moreToFollow) {
       if (!moreToFollow) {
         hasCandidates webkitPeerConnection(null, null);
     pc.onicecandidate = true;
         maybeSignal(isCaller);
       }
     }); function(evt) {
       sendMessage("candidate", evt.candidate);
     };

     // get Get the local stream and show it in the local video element element;
     // if we're the caller, ship off an offer once we get the stream.
     navigator.webkitGetUserMedia(
         {"audio": true, "video": true}, function (localStream) (stream) {
       selfView.src = webkitURL.createObjectURL(localStream);
       pc.addStream(localStream);
       maybeSignal(isCaller);
     } webkitURL.createObjectURL(stream);
       localStream = stream;
       if (isCaller) {
         pc.addStream(stream);
         pc.createOffer(function(sdp) {
             setLocalAndSendMessage("offer", sdp);
         });
     });

     // once When the remote stream arrives, show it in the remote
     // video element element.
     pc.onaddstream = function(evt) {
       remoteView.src = webkitURL.createObjectURL(evt.stream);
     };
   }

   // if we're The callee has accepted the caller, create and install our offer, call, attach their media
   // and start candidate generation
     if (isCaller) {
       offer = pc.createOffer(null);
       pc.setLocalDescription(SDP_OFFER, offer);
       pc.startIce();
     }
   } send a final answer.
   function maybeSignal(isCaller) accept() {
     // only signal once The addStream could also be done for the pranswer,
     // although that would delay the pranswer
     // (due to the need for user consent)
     pc.addStream(localStream);  // assumes we have a local the stream and local candidates
     if (localStreams.size() == 0 || !hasCandidates) return;
     if (isCaller) already
     pc.createAnswer(msg.sdp, function(sdp) {
       offer = pc.localDescription;
       signalingChannel.send(
           JSON.stringify({ "type": "offer", "sdp": offer }));
       setLocalAndSendMessage("answer", sdp);
     });
   } else {

   // if we're the callee, generate, apply, -- internal methods --

   // Apply SDP locally and send it to the answer
       answer = pc.createAnswer(pc.remoteDescription, null);
       pc.setLocalDescription(SDP_ANSWER, answer); remote side.
   function setLocalAndSendMessage(type, sdp) {
     pc.setLocalDescription(type, sdp);
     sendMessage(type, sdp);
   }
   // Send a signaling message to the remote side.
   function sendMessage(type, obj) {
     signalingChannel.send(
          JSON.stringify({ "type": "answer", type, "sdp": answer obj }));
   }
   }

   signalingChannel.onmessage = function(evt)

   // Handle incoming signaling messages.
   function handleMessage(str) {
     var msg = JSON.parse(evt.data);
     if (msg.type == "offer") JSON.parse(str);
     switch (msg.type) {
       case "offer":
         // create the PeerConnection
         start(false);
         // feed the received offer into the PeerConnection and
       // start candidate generation
       pc.setRemoteDescription(PeerConnection.SDP_OFFER,
         pc.setRemoteDescription(msg.type, msg.sdp);
       pc.startIce();
     } else if (msg.type == "answer") {
         // feed the create provisional answer into the PeerConnection to complete setup
       pc.setRemoteDescription(PeerConnection.SDP_ANSWER, allow ICE/DTLS to start
         pc.createAnswer(msg.sdp, function(sdp) {
           setDirection(sdp, "recvonly");
           setLocalAndSendMessage("pranswer", sdp);
         });
         break;
       case "pranswer":
       case "answer":
         pc.setRemoteDescription(msg.type, msg.sdp);
         break;
       case "candidate":
         pc.addIceCandidate(msg.sdp);
         break;
     }

9. Security Considerations

   TODO

10. IANA Considerations

   This document requires no actions from IANA.

11. Acknowledgements

   Harald Alvestrand, Dan Burnett, Neil Stratford, Eric Rescorla,
   }

Appendix B. Change log

   01: Added diagrams for architecture and
   Anant Narayanan all provided valuable feedback on this proposal.
   Matthew Kaufman provided the observation that keeping state out of
   the browser allows a call to continue even if the page is reloaded.
   Adam Bergvist provided a code example that served as the basis for
   the example in Section 8.

12. References

12.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. machine.
       Added sections on forking and H. Schulzrinne, "An Offer/Answer Model
   with Session Description Protocol (SDP)", RFC 3264, June 2002.

   [RFC4566]  Handley, M., Jacobson, V., rehydration.
       Clarified meaning of "pranswer" and C. Perkins, "SDP: Session
   Description Protocol", RFC 4566, July 2006.

12.2. Informative References

   [RFC4568]  Andreasen, F., Baugher, M., "answer".
       Reworked how ICE restarts and D. Wing, "Session
   Description Protocol (SDP) Security Descriptions for Media Streams",
   RFC 4568, July 2006.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
   (ICE): A Protocol for Network Address Translator (NAT) Traversal for
   Offer/Answer Protocols", RFC 5245, April 2010.

   [webrtc-api] Bergkvist, Burnett, Jennings, Narayanan, "WebRTC 1.0:
   Real-time Communication Between Browsers", October 2011.

   Available at http://dev.w3.org/2011/webrtc/editor/webrtc.html

Appendix A. Open Issues

   - Determine media directions are controlled.
       Added list of exceptions parameters that can be thrown by each method.
   Leaning toward something like a PCException, changed in a la
   https://developer.mozilla.org/en/IndexedDB/IDBDatabaseException

   - Need callback description.
       Updated suggested API and examples to indicate that the transport is down, e.g.
   ICE_DISCONNECTED or ondisconnected().

Appendix B. Change log match latest thinking.
       Suggested API and examples have been moved to an appendix.
   00: Migrated from draft-uberti-rtcweb-jsep-02.

Authors' Addresses

   Justin Uberti
   Google
   5 Cambridge Center
   Cambridge, MA 02142
   Email: justin@uberti.name

   Cullen Jennings
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email:  fluffy@cisco.com