draft-ietf-rtcweb-jsep-01.txt   draft-ietf-rtcweb-jsep-02.txt 
Network Working Group J. Uberti Network Working Group J. Uberti
Internet-Draft Google Internet-Draft Google
Intended status: Standards Track C. Jennings Intended status: Standards Track C. Jennings
Expires: December 6, 2012 Cisco Systems, Inc. Expires: April 25, 2013 Cisco
June 4, 2012 October 22, 2012
Javascript Session Establishment Protocol Javascript Session Establishment Protocol
draft-ietf-rtcweb-jsep-01 draft-ietf-rtcweb-jsep-02
Abstract Abstract
This document proposes a mechanism for allowing a Javascript This document proposes a mechanism for allowing a Javascript
application to fully control the signaling plane of a multimedia application to fully control the signaling plane of a multimedia
session, and discusses how this would work with existing signaling session, and discusses how this would work with existing signaling
protocols. 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 Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 26, 2012. This Internet-Draft will expire on April 25, 2013.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Other Approaches Considered . . . . . . . . . . . . . . . . . 5
2. JSEP Approach . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Other Approaches Considered . . . . . . . . . . . . . . . . . . 6 4. Semantics and Syntax . . . . . . . . . . . . . . . . . . . . . 7
4. Semantics and Syntax . . . . . . . . . . . . . . . . . . . . . 7 4.1. Signaling Model . . . . . . . . . . . . . . . . . . . . . 7
4.1. Signaling Model . . . . . . . . . . . . . . . . . . . . . . 7 4.2. Session Descriptions and State Machine . . . . . . . . . . 7
4.2. Session Descriptions and State Machine . . . . . . . . . . 7 4.3. Session Description Format . . . . . . . . . . . . . . . . 9
4.3. Session Description Format . . . . . . . . . . . . . . . . 9 4.4. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Separation of Signaling and ICE State Machines . . . . . . 10 4.4.1. ICE Candidate Trickling . . . . . . . . . . . . . . . 10
4.5. ICE Candidate Trickling . . . . . . . . . . . . . . . . . . 10 4.4.1.1. ICE Candidate Format . . . . . . . . . . . . . . . 10
4.6. ICE Candidate Format . . . . . . . . . . . . . . . . . . . 11 4.5. Interactions With Forking . . . . . . . . . . . . . . . . 11
4.7. Interactions With Forking . . . . . . . . . . . . . . . . . 11 4.5.1. Sequential Forking . . . . . . . . . . . . . . . . . . 11
4.7.1. Serial Forking . . . . . . . . . . . . . . . . . . . . 11 4.5.2. Parallel Forking . . . . . . . . . . . . . . . . . . . 12
4.7.2. Parallel Forking . . . . . . . . . . . . . . . . . . . 12 4.6. Session Rehydration . . . . . . . . . . . . . . . . . . . 13
4.8. Session Rehydration . . . . . . . . . . . . . . . . . . . . 12 5. Interface . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.1. SDP Requirements . . . . . . . . . . . . . . . . . . . . . 14
5.1. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2. Methods . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1.1. createOffer . . . . . . . . . . . . . . . . . . . . . . 13 5.2.1. createOffer . . . . . . . . . . . . . . . . . . . . . 15
5.1.2. createAnswer . . . . . . . . . . . . . . . . . . . . . 14 5.2.2. createAnswer . . . . . . . . . . . . . . . . . . . . . 15
5.1.3. SessionDescriptionType . . . . . . . . . . . . . . . . 14 5.2.3. SessionDescriptionType . . . . . . . . . . . . . . . . 16
5.1.4. setLocalDescription . . . . . . . . . . . . . . . . . . 15 5.2.3.1. Creating Answers . . . . . . . . . . . . . . . . . 17
5.1.5. setRemoteDescription . . . . . . . . . . . . . . . . . 15 5.2.4. setLocalDescription . . . . . . . . . . . . . . . . . 17
5.1.6. localDescription . . . . . . . . . . . . . . . . . . . 16 5.2.5. setRemoteDescription . . . . . . . . . . . . . . . . . 18
5.1.7. remoteDescription . . . . . . . . . . . . . . . . . . . 16 5.2.6. localDescription . . . . . . . . . . . . . . . . . . . 18
5.1.8. updateIce . . . . . . . . . . . . . . . . . . . . . . . 16 5.2.7. remoteDescription . . . . . . . . . . . . . . . . . . 18
5.1.9. addIceCandidate . . . . . . . . . . . . . . . . . . . . 17 5.2.8. updateIce . . . . . . . . . . . . . . . . . . . . . . 18
5.2. Configurable SDP Parameters . . . . . . . . . . . . . . . . 17 5.2.9. addIceCandidate . . . . . . . . . . . . . . . . . . . 19
6. Media Setup Overview . . . . . . . . . . . . . . . . . . . . . 17 6. Configurable SDP Parameters . . . . . . . . . . . . . . . . . 20
6.1. Initiating the Session . . . . . . . . . . . . . . . . . . 18 7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
6.1.1. Generating An Offer . . . . . . . . . . . . . . . . . . 18 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
6.1.2. Applying the Offer . . . . . . . . . . . . . . . . . . 18 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.3. Handling ICE Callbacks . . . . . . . . . . . . . . . . 18 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1.4. Serializing the Offer and Candidates . . . . . . . . . 19 10.1. Normative References . . . . . . . . . . . . . . . . . . . 24
6.2. Receiving the Session . . . . . . . . . . . . . . . . . . . 19 10.2. Informative References . . . . . . . . . . . . . . . . . . 24
6.2.1. Receiving the Offer . . . . . . . . . . . . . . . . . . 19 Appendix A. JSEP Implementation Examples . . . . . . . . . . . . 26
6.2.2. Handling ICE Messages . . . . . . . . . . . . . . . . . 19 A.1. Example API Flows . . . . . . . . . . . . . . . . . . . . 26
6.2.3. Generating the Answer . . . . . . . . . . . . . . . . . 20 A.1.1. Call using ROAP . . . . . . . . . . . . . . . . . . . 26
6.2.4. Applying the Answer . . . . . . . . . . . . . . . . . . 20 A.1.2. Call using XMPP . . . . . . . . . . . . . . . . . . . 27
6.2.5. Serializing the Answer . . . . . . . . . . . . . . . . 20 A.1.3. Adding video to a call, using XMPP . . . . . . . . . . 28
6.3. Completing the Session . . . . . . . . . . . . . . . . . . 20 A.1.4. Simultaneous add of video streams, using XMPP . . . . 28
6.3.1. Receiving the Answer . . . . . . . . . . . . . . . . . 20 A.1.5. Call using SIP . . . . . . . . . . . . . . . . . . . . 29
A.1.6. Handling early media (e.g. 1-800-GO FEDEX), using
6.4. Updates to the Session . . . . . . . . . . . . . . . . . . 20 SIP . . . . . . . . . . . . . . . . . . . . . . . . . 30
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 21 Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 32
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. JSEP Implementation Examples . . . . . . . . . . . . . 22
A.1. Example API . . . . . . . . . . . . . . . . . . . . . . . . 22
A.2. Example API Flows . . . . . . . . . . . . . . . . . . . . . 23
A.2.1. Call using ROAP . . . . . . . . . . . . . . . . . . . . 23
A.2.2 Call using XMPP . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
1. Introduction 1. Introduction
The thinking behind WebRTC call setup has been to fully specify and The thinking behind WebRTC call setup has been to fully specify and
control the media plane, but to leave the signaling plane up to the control the media plane, but to leave the signaling plane up to the
application as much as possible. The rationale is that different application as much as possible. The rationale is that different
applications may prefer to use different protocols, such as the applications may prefer to use different protocols, such as the
existing SIP or Jingle call signaling protocols, or something custom existing SIP or Jingle call signaling protocols, or something custom
to the particular application, perhaps for a novel use case. In this to the particular application, perhaps for a novel use case. In this
approach, the key information that needs to be exchanged is the approach, the key information that needs to be exchanged is the
multimedia session description, which specifies the necessary multimedia session description, which specifies the necessary
transport and media configuration information necessary to establish transport and media configuration information necessary to establish
the media plane. 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 [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
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 The browser environment also has its own challenges that cause
problems for an embedded signaling state machine. One of these is 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, 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 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 push the current state back down to the page and resume the call
where it left off. where it left off.
If instead the state machine is run at the browser end, and is This document describes the Javascript Session Establishment Protocol
instantiated within, for example, the PeerConnection object, that (JSEP) that pulls the signaling state machine out of the browser and
state machine will be reinitialized when the page is reloaded and the into Javascript. This mechanism effectively removes the browser
JavaScript re-executed. This actually complicates the design of any almost completely from the core signaling flow; the only interface
interoperability service, as all cases where an offer or answer has needed is a way for the application to pass in the local and remote
already been generated but is now "forgotten" must now be handled by session descriptions negotiated by whatever signaling mechanism is
trying to move the client state machine forward to the same state it used, and a way to interact with the ICE state machine.
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 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 JSEP's handling of session descriptions is simple and
straightforward. Whenever an offer/answer exchange is needed, the straightforward. Whenever an offer/answer exchange is needed, the
initiating side creates an offer by calling a createOffer() API. The initiating side creates an offer by calling a createOffer() API. The
application can do massaging of that offer, if it wants to, and then application optionally modifies that offer, and then uses it to set
uses it to set up its local config via a setLocalDescription() API. up its local config via the setLocalDescription() API. The offer is
The offer is then sent off to the remote side over its preferred then sent off to the remote side over its preferred signaling
signaling mechanism (e.g. WebSockets); upon receipt of that offer, mechanism (e.g., WebSockets); upon receipt of that offer, the remote
the remote party installs it using a setRemoteDescription() API. party installs it using the setRemoteDescription() API.
When the call is accepted, the callee uses a createAnswer() API to When the call is accepted, the callee uses the createAnswer() API to
generate an appropriate answer, applies it using generate an appropriate answer, applies it using
setLocalDescription(), and sends the answer back to the initiator setLocalDescription(), and sends the answer back to the initiator
over the signaling channel. When the offerer gets that answer, it over the signaling channel. When the offerer gets that answer, it
installs it using setRemoteDescription(), and initial setup is installs it using setRemoteDescription(), and initial setup is
complete. This process can be repeated for additional offer/answer complete. This process can be repeated for additional offer/answer
exchanges. exchanges.
Regarding ICE, JSEP decouples the ICE state machine from the overall Regarding ICE, JSEP decouples the ICE state machine from the overall
signaling state machine, as the ICE state machine must remain in the signaling state machine, as the ICE state machine must remain in the
browser, since only the browser has the necessary knowledge of browser, because only the browser has the necessary knowledge of
candidates and other transport info. Performing this separation it candidates and other transport info. Performing this separation also
provides additional flexibility; in protocols that decouple session provides additional flexibility; in protocols that decouple session
descriptions from transport, such as Jingle, the transport descriptions from transport, such as Jingle, the transport
information can be sent separately; in protocols that don't, such as information can be sent separately; in protocols that don't, such as
SIP, the information can be easily aggregated and recombined. Sending SIP, the information can be used in the aggregated form. Sending
transport information separately can allow for faster ICE and DTLS transport information separately can allow for faster ICE and DTLS
startup, since the necessary roundtrips can occur while waiting for startup, since the necessary roundtrips can occur while waiting for
the remote side to accept the session. the remote side to accept the session.
The JSEP approach does come with a minor downside. As the application The JSEP approach does come with a minor downside. As the
now is responsible for driving the signaling state machine, slightly application now is responsible for driving the signaling state
more application code is necessary to perform call setup; the machine, slightly more application code is necessary to perform call
application must call the right APIs at the right times, and convert setup; the application must call the right APIs at the right times,
the session descriptions and ICE information into the defined and convert the session descriptions and ICE information into the
messages of its chosen signaling protocol, instead of simply defined messages of its chosen signaling protocol, instead of simply
forwarding the messages emitted from the browser. forwarding the messages emitted from the browser.
One way to mitigate this is to provide a Javascript library that One way to mitigate this is to provide a Javascript library that
hides this complexity from the developer, which would implement the hides this complexity from the developer, which would implement the
state machine and serialization of the desired signaling protocol. state machine and serialization of the desired signaling protocol.
For example, this library could convert easily adapt the JSEP API For example, this library could convert easily adapt the JSEP API
into the exact ROAP API, thereby implementing the ROAP signaling into the exact ROAP API [I-D.jennings-rtcweb-signaling], thereby
protocol. Such a library could of course also implement other popular implementing the ROAP signaling protocol. Such a library could of
signaling protocols, including SIP or Jingle. In this fashion we can course also implement other popular signaling protocols, including
enable greater control for the experienced developer without forcing SIP or Jingle. In this fashion we can enable greater control for the
any additional complexity on the novice developer. experienced developer without forcing any additional complexity on
the novice developer.
3. Other Approaches Considered 2. Other Approaches Considered
Another approach that was considered for JSEP was to move the Another approach that was considered for JSEP was to move the
mechanism for generating offers and answers out of the browser as mechanism for generating offers and answers out of the browser as
well. Instead of providing createOffer/createAnswer methods within well. Instead of providing createOffer/createAnswer methods within
the browser, this approach would instead expose a getCapabilities API the browser, this approach would instead expose a getCapabilities API
which would provide the application with the information it needed in which would provide the application with the information it needed in
order to generate its own session descriptions. This increases the order to generate its own session descriptions. This increases the
amount of work that the application needs to do; it needs to know how amount of work that the application needs to do; it needs to know how
to generate session descriptions from capabilities, and especially to generate session descriptions from capabilities, and especially
how to generate the correct answer from an arbitrary offer and the how to generate the correct answer from an arbitrary offer and the
supported capabilities. While this could certainly be addressed by supported capabilities. While this could certainly be addressed by
using a library like the one mentioned above, it basically forces the using a library like the one mentioned above, it basically forces the
use of said library even for a simple example. Exposing use of said library even for a simple example. Exposing createOffer/
createOffer/createAnswer avoids that problem, but still allows createAnswer avoids that problem, but still allows applications to
applications to generate their own offers/answers if they choose, generate their own offers/answers if they choose, using the
using the description generated by createOffer as an indication of description generated by createOffer as an indication of the
the browser's capabilities. browser's capabilities.
Note also that while JSEP transfers more control to Javascript, it is 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 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 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 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, that is hard to specify and test. In the approach proposed here,
control is performed via session descriptions; this requires only a control is performed via session descriptions; this requires only a
few APIs to handle these descriptions, and they are evaluated in a few APIs to handle these descriptions, and they are evaluated in a
specific fashion, which reduces the number of possible states and specific fashion, which reduces the number of possible states and
interactions. interactions.
4. Semantics and Syntax 3. Terminology
4.1. Signaling Model 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].
4. Semantics and Syntax
4.1. Signaling Model
JSEP does not specify a particular signaling model or state machine, JSEP does not specify a particular signaling model or state machine,
other than the generic need to exchange RFC 3264 offers and answers 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 in order for both sides of the session to know how to conduct the
session. JSEP provides mechanisms to create offers and answers, as session. JSEP provides mechanisms to create offers and answers, as
well as to apply them to a session. However, the actual mechanism by well as to apply them to a session. However, the actual mechanism by
which these offers and answers are communicated to the remote side, which these offers and answers are communicated to the remote side,
including addressing, retransmission, forking, and glare handling, is including addressing, retransmission, forking, and glare handling, is
left entirely up to the application. left entirely up to the application.
+-----------+ +-----------+ +-----------+ +-----------+
| Web App |<--- App-Specific Signaling --->| Web App | | Web App |<--- App-Specific Signaling -->| Web App |
+-----------+ +-----------+ +-----------+ +-----------+
| | ^ ^
| SDP | SDP | SDP | SDP
V V V V
+-----------+ +-----------+ +-----------+ +-----------+
| Browser |<----------- Media ------------>| Browser | | Browser |<----------- Media ------------>| Browser |
+-----------+ +-----------+ +-----------+ +-----------+
Figure 1: JSEP Signaling Model Figure 1: JSEP Signaling Model
4.2. Session Descriptions and State Machine 4.2. Session Descriptions and State Machine
In order to establish the media plane, the user agent needs specific In order to establish the media plane, the user agent needs specific
parameters to indicate what to transmit to the remote side, as well parameters to indicate what to transmit to the remote side, as well
as how to handle the media that is received. These parameters are as how to handle the media that is received. These parameters are
determined by the exchange of session descriptions in offers and determined by the exchange of session descriptions in offers and
answers, and there are certain details to this process that must be answers, and there are certain details to this process that must be
handled in the JSEP APIs. handled in the JSEP APIs.
Whether a session description was sent or received affects the Whether a session description was sent or received affects the
meaning of that description. For example, the list of codecs sent to meaning of that description. For example, the list of codecs sent to
a remote party indicates what the local side is willing to decode, a remote party indicates what the local side is willing to decode,
and what the remote party should send. Not all parameters follow this and what the remote party should send. Not all parameters follow
rule; for example, the SRTP parameters [RFC4568] sent to a remote this rule; for example, the SRTP parameters [RFC4568] sent to a
party indicate what the local side will use to encrypt, and thereby remote party indicate what the local side will use to encrypt, and
how the remote party should expect to receive. thereby how the remote party should expect to receive.
In addition, various RFCs put different conditions on the format of In addition, various RFCs put different conditions on the format of
offers versus answers. For example, a offer may propose multiple SRTP offers versus answers. For example, a offer may propose multiple
configurations, but an answer may only contain a single SRTP SRTP configurations, but an answer may only contain a single SRTP
configuration. configuration.
Lastly, while the exact media parameters are only known only after a Lastly, while the exact media parameters are only known only after a
offer and an answer have been exchanged, it is possible for the offer and an answer have been exchanged, it is possible for the
offerer to receive media after they have sent an offer and before offerer to receive media after they have sent an offer and before
they have received an answer. To properly process incoming media in they have received an answer. To properly process incoming media in
this case, the offerer's media handler must be aware of the details this case, the offerer's media handler must be aware of the details
of the offerer before the answer arrives. of the offerer before the answer arrives.
Therefore, in order to handle session descriptions properly, the user Therefore, in order to handle session descriptions properly, the user
agent needs: agent needs:
1. To know if a session description pertains to the local or 1. To know if a session description pertains to the local or remote
remote side. side.
2. To know if a session description is an offer or an answer. 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. 3. To allow the offer to be specified independently of the answer.
JSEP addresses this by adding both a setLocalDescription and a JSEP addresses this by adding both a setLocalDescription and a
setRemoteDescription method, and both these methods take a parameter setRemoteDescription method and having session description objects
to indicate the type of session description being supplied. This contain a type field indicating the type of session description being
satisfies the requirements listed above for both the offerer, who supplied. This satisfies the requirements listed above for both the
first calls setLocalDescription("offer", sdp) and then later offerer, who first calls setLocalDescription(sdp [offer]) and then
setRemoteDescription("answer", sdp), as well as for the answerer, who later setRemoteDescription(sdp [answer]), as well as for the
first calls setRemoteDescription("offer", sdp) and then later answerer, who first calls setRemoteDescription(sdp [offer]) and then
setLocalDescription("answer", sdp). While it could be possible to later setLocalDescription(sdp [answer]). While it could be possible
implicitly determine the value of the offer/answer argument, to implicitly determine the value of the offer/answer argument,
requiring it to be specified explicitly is more robust, allowing requiring it to be specified explicitly is more robust, allowing
invalid combinations (i.e. an answer before an offer) to generate an invalid combinations (i.e. an answer before an offer) to generate an
appropriate error. appropriate error.
It also allows for an answer to be treated as provisional by the JSEP also allows for an answer to be treated as provisional by the
application. Provisional answers provide a way for an answerer to application. Provisional answers provide a way for an answerer to
communicate session parameters back to the offerer, in order for the communicate initial session parameters back to the offerer, in order
session to begin, while allowing a final answer to be specified to allow the session to begin, while allowing a final answer to be
later. This concept of a final answer is important to the specified later. This concept of a final answer is important to the
offer/answer model; when such an answer is received, any extra offer/answer model; when such an answer is received, any extra
resources allocated by the caller can be released, now that the exact resources allocated by the caller can be released, now that the exact
session configuration is known. These "resources" can include things session configuration is known. These "resources" can include things
like extra ICE components, TURN candidates, or video decoders. like extra ICE components, TURN candidates, or video decoders.
Provisional answers, on the other hand, do no such deallocation; as a Provisional answers, on the other hand, do no such deallocation
result, multiple dissimilar provisional answers can be received and results; as a result, multiple dissimilar provisional answers can be
applied during call setup. received and applied during call setup.
As in [RFC3264], an offerer can send an offer, and update it as long In [RFC3264], the constraints at the signaling level is that only one
as it has not been answered. The answerer can send back zero or more offer can be outstanding for a given session but from the media stack
level, a new offer can be generated at any point. For example, when
using SIP for signaling, if one offer is sent, then cancelled using a
SIP CANCEL, another offer can be generated even though no answer was
received for the first offer. To support this, the JSEP media layer
can provide an offer whenever the Javascript application needs one
for the signaling. The answerer can send back zero or more
provisional answers, and finally end the offer-answer exchange by provisional answers, and finally end the offer-answer exchange by
sending a final answer. The state machine for this is as follows: sending a final answer. The state machine for this is as follows:
+-----------+ +-----------+
| | | |
| | | |
| Stable |<---------------\ | Stable |<---------------\
| | | | | |
| | | | | |
+-----------+ | +-----------+ |
^ | | ^ | |
| | OFFER | | | OFFER |
ANSWER | | | ANSWER ANSWER | | | ANSWER
| V | | V |
+-----------+ +-----------+ +-----------+ +-----------+
| | | | | | | |
| | PRANSWER | | | | PRANSWER | |
| Offer |--------->| Pranswer | | Offer |-------- >| Pranswer |
| | | | | | | |
| |----\ | |----\ | |----\ | |----\
+-----------+ | +-----------+ | +-----------+ | +-----------+ |
^ | ^ | ^ | ^ |
| | | | | | | |
\-----/ \-----/ \-----/ \-----/
OFFER PRANSWER OFFER PRANSWER
Figure 2: JSEP State Machine Figure 2: JSEP State Machine
Aside from these state transitions, there is no other difference Aside from these state transitions, there is no other difference
between the handling of provisional ("pranswer") and final ("answer") between the handling of provisional ("pranswer") and final ("answer")
answers. answers.
4.3. Session Description Format 4.3. Session Description Format
In the current WebRTC specification, session descriptions are
formatted as SDP messages. While this format is not optimal for In the WebRTC specification, session descriptions are formatted as
manipulation from Javascript, it is widely accepted, and frequently SDP messages. While this format is not optimal for manipulation from
updated with new features. Any alternate encoding of session Javascript, it is widely accepted, and frequently updated with new
descriptions would have to keep pace with the changes to SDP, at features. Any alternate encoding of session descriptions would have
least until the time that this new encoding eclipsed SDP in to keep pace with the changes to SDP, at least until the time that
popularity. As a result, JSEP continues to use SDP as the internal this new encoding eclipsed SDP in popularity. As a result, JSEP
representation for its session descriptions. continues to use SDP as the internal representation for its session
descriptions.
However, to simplify Javascript processing, and provide for future However, to simplify Javascript processing, and provide for future
flexibility, the SDP syntax is encapsulated within a flexibility, the SDP syntax is encapsulated within a
SessionDescription object, which can be constructed from SDP, and be 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 serialized out to SDP. If future specifications agree on a JSON
session descriptions, we could easily enable this object to format for session descriptions, we could easily enable this object
generate/expect JSON. to generate and consume that JSON.
Other methods may be added to SessionDescription in the future to Other methods may be added to SessionDescription in the future to
simplify handling of SessionDescriptions from Javascript. simplify handling of SessionDescriptions from Javascript. Though it
is unclear exactly what manipulations developer will commonly want to
4.4. Separation of Signaling and ICE State Machines do to SDP, it would be simple to write a Javascript library to
perform these manipulations.
JSEP does away with the SDP Agent within the browser, and this 4.4. ICE
functionality is now controlled directly by the application, which
uses the setLocalDescription and setRemoteDescription APIs to tell
the browser what SDP has been negotiated. The ICE Agent remains in
the browser, as it still needs to drive the process of gathering
candidates, connectivity checks, and related ICE functionality.
When a new ICE candidate is available, the ICE Agent will notify the When a new ICE candidate is available, the ICE Agent will notify the
application via a callback; these candidates will automatically be application via a callback; these candidates will automatically be
added to the local session description. When all candidates have been added to the local session description. When all candidates have
gathered, the callback will also be invoked to signal that the been gathered, the callback will also be invoked to signal that the
gathering process is complete. gathering process is complete.
4.5. ICE Candidate Trickling 4.4.1. ICE Candidate Trickling
Candidate trickling is a technique through which a caller may Candidate trickling is a technique through which a caller may
incrementally provide candidates to the callee after the initial incrementally provide candidates to the callee after the initial
offer has been dispatched. This allows the callee to begin acting offer has been dispatched; the semantics of "Trickle ICE" are defined
upon the call and setting up the ICE (and perhaps DTLS) connections in [I-D.rescorla-mmusic-ice-trickle]. This process allows the callee
immediately, without having to wait for the caller to allocate all to begin acting upon the call and setting up the ICE (and perhaps
possible candidates, resulting in faster call startup in many cases. DTLS) connections immediately, without having to wait for the caller
to gather all possible candidates. This results in faster call
startup in cases where gathering is not performed prior to initating
the call.
JSEP supports optional candidate trickling by providing APIs that JSEP supports optional candidate trickling by providing APIs that
provide control and feedback on the ICE candidate gathering process. provide control and feedback on the ICE candidate gathering process.
Applications that support candidate trickling can send the initial Applications that support candidate trickling can send the initial
offer immediately and send individual candidates when they get the offer immediately and send individual candidates when they get the
onicecandidate callback with a new candidate; applications that do notified of a new candidate; applications that do not support this
not support this feature can simply wait for the final onicecandidate feature can simply wait for the indication that gathering is
callback that indicates gathering is complete, and create and send complete, and then create and send their offer, with all the
their offer, with all the candidates, at this time. candidates, at this time.
Upon receipt of trickled candidates, the receiving application can Upon receipt of trickled candidates, the receiving application will
supply them to its ICE Agent by calling an addIceCandidate method. supply them to its ICE Agent. This triggers the ICE Agent to start
This triggers the ICE Agent to start using this remote candidate for using the new remote candidates for connectivity checks.
connectivity checks. Applications that do not make use of candidate
tricking can ignore addIceCandidate entirely, and use the
onicecandidate callback solely to indicate when candidate gathering
is complete.
4.6. ICE Candidate Format 4.4.1.1. ICE Candidate Format
As with session descriptions, we choose to provide an IceCandidate As with session descriptions, the syntax of the IceCandidate object
object that provides some abstraction, but can be easily converted provides some abstraction, but can be easily converted to and from
to/from SDP a=candidate lines. the SDP a=candidate lines.
The IceCandidate object has fields to indicate which m= line it The a=candidate lines are the only SDP information that is contained
should be associated with, and a method to convert to a SDP within IceCandidate, as they represent the only information needed
representation, ex: that is not present in the initial offer (i.e. for trickle
candidates). This information is carried with the same syntax as the
"a=candidate" line in SDP. For example:
a=candidate:1 1 UDP 1694498815 66.77.88.99 10000 typ host a=candidate:1 1 UDP 1694498815 192.0.2.33 10000 typ host
Currently, a=candidate lines are the only SDP information that is The IceCandidate object also contains fields to indicate which m=
contained within IceCandidate, as they represent the only information line it should be associated with. The m line can be identified in
needed that is not present in the initial offer (i.e. for trickle one of two ways; either by a m-line index, or a MID. The m-line
candidates). index is a zero-based index, referring to the Nth m-line in the SDP.
The MID uses the "media stream identification", as defined in [RFC
3388], to identify the m-line. WebRTC implementations creating an
ICE Candidate object MUST populate both of these fields.
Implementations receiving an ICE Candidate object SHOULD use the MID
if they implement that functionality, or the m-line index, if not.
4.7. Interactions With Forking 4.5. Interactions With Forking
4.7.1. Serial Forking Some call signaling systems allow various types of forking where an
SDP Offer may be provided to more than one device. For example, SIP
RFC 3261 defines both a "Parallel Search" and "Sequential Search".
Although these are primarily signaling level issues that are outside
the scope of JSEP, they do have some impact on the configuration of
the media plane, which is relevant. When forking is happening at the
signaling layer, the Javascript application responsible for the
signaling needs to make the decisions about what media should be sent
or received at any point of time and which remote endpoint it should
communicate with. JSEP is used to make sure the media engine can
make the RTP and media perform as required by the application. The
basic operations that the applications can have the media engine do
are:
Serial forking involves a call being dispatched to multiple remote Start exchanging media to a given remote peer but keep all the
callees, where each callee can accept the call, but only one active resources reserved in the offer.
session ever exists at a time; no mixing of received media is
Start exchanging media with a given remote peer and free any
resources in the offer that are not being used.
4.5.1. Sequential Forking
Sequential forking involves a call being dispatched to multiple
remote callees, where each callee can accept the call, but only one
active session ever exists at a time; no mixing of received media is
performed. performed.
JSEP handles serial forking well, allowing the application to easily JSEP handles serial forking well, allowing the application to easily
control the policy for selecting the desired remote endpoint. When an control the policy for selecting the desired remote endpoint. When
answer arrives from one of the callees, the application can choose to an answer arrives from one of the callees, the application can choose
apply it either as a provisional answer, leaving open the possibility to apply it either as a provisional answer, leaving open the
of using a different answer in the future, or apply it as a final possibility of using a different answer in the future, or apply it as
answer, ending the setup flow. a final answer, ending the setup flow.
In a "first-one-wins" situation, the first answer will be applied as In a "first-one-wins" situation, the first answer will be applied as
a final answer, and the application will send a terminate message to a final answer, and the application will reject any subsequent
any subsequent answers. In SIP parlance, this would be ACK + BYE. answers. In SIP parlance, this would be ACK + BYE.
In a "last-one-wins" situation, all answers would be applied as In a "last-one-wins" situation, all answers would be applied as
provisional answers, and any previous call leg will be terminated. At provisional answers, and any previous call leg will be terminated.
some point, the application will end the setup process, perhaps with At some point, the application will end the setup process, perhaps
a timer; At this point, the application could reapply the existing with a timer; at this point, the application could reapply the
remote description as a final answer. existing remote description as a final answer.
4.7.2. Parallel Forking 4.5.2. Parallel Forking
Parallel forking involves a call being dispatched to multiple remote Parallel forking involves a call being dispatched to multiple remote
callees, where each callee can accept the call, and multiple callees, where each callee can accept the call, and multiple
simultaneous active sessions can be established as a result. If simultaneous active signaling sessions can be established as a
multiple callees send media, this media is mixed and played out at result. If multiple callees send media at the same time, the
the caller side. possibilities for handling this are described in Section 3.1 of RFC
3960. Most SIP devices today only support exchanging media with a
JSEP can handle parallel forking by "cloning" the session when needed single device at a time, and do not try to mix multiple early media
to create multiple parallel sessions. When the first answer is audio sources, as that could result in a confusing situation. For
received, the caller can clone the existing session, and then apply example. consider having a European ringback tone mixed together with
the answer as a final answer to the original session. Upon receiving the North American ringback tone - the resulting sound would not be
the next answer, the cloned session is cloned again, and the received like either tone, and would confuse the user. If the signaling
answer is applied as a final answer to the first clone. This process application wishes to only exchange media with one of the remote
repeats until the caller decides to end the setup flow, and closes endpoints at a time, then from a media engine point of view, this is
the final cloned session. exactly like the sequential forking case.
Cloned sessions inherit the local session description and candidates In the parallel forking case where the Javascript application wishes
from their parent, and an empty remote description; only sessions to simultaneously exchange media with multiple peers, the flow is
that have not yet applied an answer can be cloned. Each cloned slightly more complex, but the Javascript application can follow the
session may discover new peer-reflexive candidates; these candidates strategy that RFC 3960 describes using UPDATE. (It is worth noting
will be supplied via the onicecandidate callback to that specific that use cases where this is the desired behavior are very unusual.)
session. Since the clone uses the same local description as its The UPDATE approach allows the signaling to set up a separate media
parent, creating a clone will fail if it is not possible to reserve flow for each peer that it wishes to exchange media with. In JSEP,
the same resources for the clone as have already been reserved by the this offer used in the UPDATE would be formed by simply creating a
parent. new PeerConnection and making sure that the same local media streams
have been added into this new PeerConnection. Then the new
PeerConnection object would produce a SDP offer that could be used by
the signaling to perform the UPDATE strategy discussed in RFC 3690.
As a result of this cloning, the application will end up with N As a result of sharing the media streams, the application will end up
parallel sessions, each with a local and remote description and their with N parallel PeerConnection sessions, each with a local and remote
own local and remote addresses. The media flow from these sessions description and their own local and remote addresses. The media flow
can be managed by specifying SDP direction attributes in the from these sessions can be managed by specifying SDP direction
descriptions, or the application can choose to play out the media attributes in the descriptions, or the application can choose to play
from all sessions mixed together. Of course, if the application wants out the media from all sessions mixed together. Of course, if the
to only keep a single session, it can simply terminate the sessions application wants to only keep a single session, it can simply
that it no longer needs. terminate the sessions that it no longer needs.
4.8. Session Rehydration 4.6. Session Rehydration
In the event that the local application state is reinitialized, In the event that the local application state is reinitialized,
either due to a user reload of the page, or a decision within the either due to a user reload of the page, or a decision within the
application to reload itself (perhaps to update to a new version), it application to reload itself (perhaps to update to a new version), it
is possible to keep an existing session alive via a process called is possible to keep an existing session alive via a process called
"rehydration". "rehydration".
With rehydration, the current local session description is persisted With rehydration, the current signaling state is persisted somewhere
somewhere outside of the page, perhaps on the application server, or outside of the page, perhaps on the application server, or in browser
in browser local storage. The page is then reloaded, and a new local storage. The page is then reloaded, and a new session object
session object is created in Javascript. The saved local session is is created in Javascript. The saved signaling state is now
now retrieved, but the previous ICE candidates will no longer be retrieved, and a new PeerConnection object is created for the
valid in this case, so we will need to perform an ICE restart; to do session. At this point a new offer can be generated by the new
so, we simply generate a new ICE ufrag/pwd combo for the local PeerConnection, with new ICE and SDES credentials. This can then be
description. used to re-initiate the session with the existing remote endpoint,
who simply sees the new offer as an in-call renegotiation, and will
reply with an answer that can be supplied to setRemoteDescription.
ICE processing proceeds as usual, and as soon as connectivity is
established, the session will be back up and running again.
The modified local description is then installed via Open Issue: EKR proposed an alternative rehydration approach where
setLocalDescription, and sent off as an offer to the remote side, who the actual internal PeerConnection object in the browser was kept
will reply with an answer that can be supplied to alive for some time after the web page was killed and provided some
setRemoteDescription. ICE processing proceeds as usual, and as soon way for a new page to acquire the old PeerConnection object.
as connectivity is established, the session will be back up and
running again.
5. Interface 5. Interface
This section details the basic operations that must be present to This section details the basic operations that must be present to
implement JSEP functionality. The actual API exposed in the W3C API implement JSEP functionality. The actual API exposed in the W3C API
may have somewhat different syntax, but should map easily to these may have somewhat different syntax, but should map easily to these
concepts. concepts.
5.1. Methods 5.1. SDP Requirements
5.1.1. createOffer Note: The text in this section may not represent working group
consensus and is put here so that the working group can discuss it
and find out how to change it such that it does have consensus.
When generating SDP blobs, either for offers or answers, the
generated SDP needs to conform to the following specifications.
Similarly, in order to properly process received SDP blobs,
implementations need to implement the functionality described in the
following specifications. This list is derived from
[I-D.ietf-rtcweb-rtp-usage].
RFC4566 is the base SDP specification and MUST be implemented.
RFC5124 MUST be supported for signaling RTP/SAVPF RTP profile.
RFC5104 MUST be implemented to signal RTCP based feedback.
RFC5761 MUST be implemented to signal multiplexing of RTP and
RTCP.
RFC5245 MUST be implemented for signaling the ICE candidate lines
corresponding to each media stream.
RFC3264 MUST be implemented to signal information about media
direction.
The RFC5888 grouping framework MUST be implemented for signaling
the grouping information.
RFC5506 MAY be implemented to signal Reduced-Size RTCP messages.
RFC5576 MAY be implemented to signal RTP SSRC values.
RFC3556 with bandwidth modifiers MAY be supported for specifying
RTCP bandwidth as a fraction of the media bandwidth, RTCP fraction
allocated to the senders and setting maximum media bit-rate
boundaries.
As required by RFC 4566 Section 5.13 JSEP implementations MUST ignore
unknown attributes (a=) lines.
Example SDP for RTCWeb call flows can be found in
[I-D.nandakumar-rtcweb-sdp].
5.2. Methods
5.2.1. createOffer
The createOffer method generates a blob of SDP that contains a RFC The createOffer method generates a blob of SDP that contains a RFC
3264 offer with the supported configurations for the session, 3264 offer with the supported configurations for the session,
including descriptions of the local MediaStreams attached to this including descriptions of the local MediaStreams attached to this
PeerConnection, the codec/RTP/RTCP options supported by this PeerConnection, the codec/RTP/RTCP options supported by this
implementation, and any candidates that have been gathered by the ICE implementation, and any candidates that have been gathered by the ICE
Agent. A constraints parameters may be supplied to provide additional Agent. A constraints parameters may be supplied to provide
control over the generated offer, e.g. to get a full set of session additional control over the generated offer, e.g. to get a full set
capabilities, or to request a new set of ICE credentials. of session capabilities, or to request a new set of ICE credentials.
In the initial offer, the generated SDP will contain all desired In the initial offer, the generated SDP will contain all desired
functionality for the session (certain parts that are supported but functionality for the session (certain parts that are supported but
not desired by default may be omitted); for each SDP line, the not desired by default may be omitted); for each SDP line, the
generation of the SDP must follow the appropriate process for generation of the SDP must follow the appropriate process for
generating an offer. In the event createOffer is called after the generating an offer. In the event createOffer is called after the
session is established, createOffer will generate an offer that is session is established, createOffer will generate an offer that is
compatible with the current session, incorporating any changes that compatible with the current session, incorporating any changes that
have been made to the session since the last complete offer-answer have been made to the session since the last complete offer-answer
exchange, such as addition or removal of streams. If no changes have exchange, such as addition or removal of streams. If no changes have
been made, the offer will be identical to the current local been made, the offer will be identical to the current local
description. description.
Session descriptions generated by createOffer must be immediately Session descriptions generated by createOffer must be immediately
usable by setLocalDescription; if a system has limited resources usable by setLocalDescription; if a system has limited resources
(e.g. a finite number of decoders), createOffer should return an (e.g. a finite number of decoders), createOffer should return an
offer that reflects the current state of the system, so that offer that reflects the current state of the system, so that
setLocalDescription will succeed when it attempts to acquire those setLocalDescription will succeed when it attempts to acquire those
resources. Because this method may need to inspect the system state resources. Because this method may need to inspect the system state
to determine the currently available resources, it may be implemented to determine the currently available resources, it may be implemented
as an async operation. as an async operation.
Calling this method does not change state; its use is not required. Calling this method may do things such as generate new ICE
credentials, but does not change media state.
5.1.2. createAnswer 5.2.2. createAnswer
The createAnswer method generates a blob of SDP that contains a RFC The createAnswer method generates a blob of SDP that contains a RFC
3264 SDP answer with the supported configuration for the session that 3264 SDP answer with the supported configuration for the session that
is compatible with the parameters supplied in |offer|. Like is compatible with the parameters supplied in the offer. Like
createOffer, the returned blob contains descriptions of the local createOffer, the returned blob contains descriptions of the local
MediaStreams attached to this PeerConnection, the codec/RTP/RTCP MediaStreams attached to this PeerConnection, the codec/RTP/RTCP
options negotiated for this session, and any candidates that have options negotiated for this session, and any candidates that have
been gathered by the ICE Agent. A constraints parameter may be been gathered by the ICE Agent. A constraints parameter may be
supplied to provide additional control over the generated answer. supplied to provide additional control over the generated answer.
As an answer, the generated SDP will contain a specific configuration As an answer, the generated SDP will contain a specific configuration
that specifies how the media plane should be established. For each that specifies how the media plane should be established.
SDP line, the generation of the SDP must follow the appropriate
process for generating an answer.
Session descriptions generated by createAnswer must be immediately Session descriptions generated by createAnswer must be immediately
usable by setLocalDescription; like createOffer, the returned usable by setLocalDescription; like createOffer, the returned
description should reflect the current state of the system. Because description should reflect the current state of the system. Because
this method may need to inspect the system state to determine the this method may need to inspect the system state to determine the
currently available resources, it may need to be implemented as an currently available resources, it may need to be implemented as an
async operation. async operation.
Calling this method does not change state; its use is not required. Calling this method may do things such as generate new ICE
credentials, but does not change media state.
5.1.3. SessionDescriptionType 5.2.3. SessionDescriptionType
The strings "offer", "pranswer", and "answer" serve as type arguments Session description objects (RTCSessionDescription) may be of type
to setLocalDescription and setRemoteDescription. They provide "offer", "pranswer", and "answer". These types provide information
information as to how the description parameter should be parsed, and as to how the description parameter should be parsed, and how the
how the media state should be changed. media state should be changed.
"offer" indicates that a description should be parsed as an offer; "offer" indicates that a description should be parsed as an offer;
said description may include many possible media configurations. A said description may include many possible media configurations. A
description used as an "offer" may be applied anytime the description used as an "offer" may be applied anytime the
PeerConnection is in a stable state, or as an update to a previously PeerConnection is in a stable state, or as an update to a previously
sent but unanswered "offer". sent but unanswered "offer".
"pranswer" indicates that a description should be parsed as an "pranswer" indicates that a description should be parsed as an
answer, but not a final answer, and so should not result in the 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 freeing of allocated resources. It may result in the start of media
transmission, if the answer does not specify an inactive media transmission, if the answer does not specify an inactive media
direction. A description used as a "pranswer" may be applied as a direction. A description used as a "pranswer" may be applied as a
response to an "offer", or an update to a previously sent "answer". response to an "offer", or an update to a previously sent "answer".
"answer" indicates that a description should be parsed as an answer, "answer" indicates that a description should be parsed as an answer,
the offer-answer exchange should be considered complete, and any the offer-answer exchange should be considered complete, and any
resources (decoders, candidates) that are no longer needed can be resources (decoders, candidates) that are no longer needed can be
released. A description used as an "answer" may be applied as a released. A description used as an "answer" may be applied as a
response to a "offer", or an update to a previously sent "pranswer". response to a "offer", or an update to a previously sent "pranswer".
The application can use some discretion on whether an answer should The application can use some discretion on whether an answer should
be applied as provisional or final. For example, in a serial forking be applied as provisional or final. For example, in a serial forking
scenario, an application may receive multiple "final" answers, one scenario, an application may receive multiple "final" answers, one
from each remote endpoint. The application could accept the initial from each remote endpoint. The application could accept the initial
answers as provisional answers, and only apply an answer as final answers as provisional answers, and only apply an answer as final
when it receives one that meets its criteria (e.g. a live user when it receives one that meets its criteria (e.g. a live user
instead of voicemail). instead of voicemail).
5.1.4. setLocalDescription 5.2.3.1. Creating Answers
Most web applications will not need to create answers using the
"pranswer" type. The general recommendation for a web application
would be to create an answer more or less immediately after receiving
the offer, instead of waiting for a human user to provide input.
Later when the human input is received, the applications can create a
new offer to update the previous offer/answer pair. Some
applications may not be able to do this, particularly ones that Some
application may not be able to do this, particular ones that are
attempting to gateway to other signaling protocols.
Consider a typical web application that will set up a data channel,
an audio channel, and a video channel. When an endpoint receives an
offer with these channels, it could send an answer accepting the data
channel for two-way data, and accepting the audio and video tracks as
receive-only. It could then ask the user if they wanted to transmit
audio and video to the far end, acquire the local media streams, and
send a new offer to the remote side moving the audio and video to be
two-way media. By the time the human has authorized sending media,
it is likely that the ICE and DTLS handshaking with the remote side
will already be set up.
5.2.4. setLocalDescription
The setLocalDescription method instructs the PeerConnection to apply The setLocalDescription method instructs the PeerConnection to apply
the supplied SDP blob as its local configuration. The type parameter the supplied SDP blob as its local configuration. The type field
indicates whether the blob should be processed as an offer, indicates whether the blob should be processed as an offer,
provisional answer, or final answer; offers and answers are checked provisional answer, or final answer; offers and answers are checked
differently, using the various rules that exist for each SDP line. differently, using the various rules that exist for each SDP line.
This API changes the local media state; among other things, it sets This API changes the local media state; among other things, it sets
up local resources for receiving and decoding media. In order to up local resources for receiving and decoding media. In order to
successfully handle scenarios where the application wants to offer to successfully handle scenarios where the application wants to offer to
change from one media format to a different, incompatible format, the change from one media format to a different, incompatible format, the
PeerConnection must be able to simultaneously support use of both the PeerConnection must be able to simultaneously support use of both the
old and new local descriptions (e.g. support codecs that exist in old and new local descriptions (e.g. support codecs that exist in
both descriptions) until a final answer is received, at which point both descriptions) until a final answer is received, at which point
the PeerConnection can fully adopt the new local description, or roll the PeerConnection can fully adopt the new local description, or roll
back to the old description if the remote side denied the change. back to the old description if the remote side denied the change.
If setRemoteDescription was previous called with an offer, and If setRemoteDescription was previous called with an offer, and
setLocalDescription is called with an answer (provisional or final), setLocalDescription is called with an answer (provisional or final),
and the media directions are compatible, this will result in the and the media directions are compatible, this will result in the
starting of media transmission. starting of media transmission.
5.1.5. setRemoteDescription 5.2.5. setRemoteDescription
The setRemoteDescription method instructs the PeerConnection to apply The setRemoteDescription method instructs the PeerConnection to apply
the supplied SDP blob as the desired remote configuration. As in the supplied SDP blob as the desired remote configuration. As in
setLocalDescription, the |type| parameter indicates how the blob setLocalDescription, the type field of the indicates how the blob
should be processed. should be processed.
This API changes the local media state; among other things, it sets This API changes the local media state; among other things, it sets
up local resources for sending and encoding media. up local resources for sending and encoding media.
If setRemoteDescription was previous called with an offer, and If setRemoteDescription was previous called with an offer, and
setLocalDescription is called with an answer (provisional or final), setLocalDescription is called with an answer (provisional or final),
and the media directions are compatible, this will result in the and the media directions are compatible, this will result in the
starting of media transmission. starting of media transmission.
5.1.6. localDescription 5.2.6. localDescription
The localDescription method returns a copy of the current local The localDescription method returns a copy of the current local
configuration, i.e. what was most recently passed to configuration, i.e. what was most recently passed to
setLocalDescription, plus any local candidates that have been setLocalDescription, plus any local candidates that have been
generated by the ICE Agent. generated by the ICE Agent.
A null object will be returned if the local description has not yet A null object will be returned if the local description has not yet
been established. been established.
5.1.7. remoteDescription 5.2.7. remoteDescription
The remoteDescription method returns a copy of the current remote The remoteDescription method returns a copy of the current remote
configuration, i.e. what was most recently passed to configuration, i.e. what was most recently passed to
setRemoteDescription, plus any remote candidates that have been setRemoteDescription, plus any remote candidates that have been
supplied via processIceMessage. supplied via processIceMessage.
A null object will be returned if the remote description has not yet A null object will be returned if the remote description has not yet
been established. been established.
5.1.8. updateIce 5.2.8. updateIce
The updateIce method allows the configuration of the ICE Agent to be The updateIce method allows the configuration of the ICE Agent to be
changed during the session, primarily for changing which types of changed during the session, primarily for changing which types of
local candidates are provided to the application and used for local candidates are provided to the application and used for
connectivity checks. A callee may initially configure the ICE Agent connectivity checks. A callee may initially configure the ICE Agent
to use only relay candidates, to avoid leaking location information, to use only relay candidates, to avoid leaking location information,
but update this configuration to use all candidates once the call is but update this configuration to use all candidates once the call is
accepted. accepted.
Regardless of the configuration, the gathering process collects all Regardless of the configuration, the gathering process collects all
available candidates, but excluded candidates will not be surfaced in available candidates, but excluded candidates will not be surfaced in
onicecallback or used for connectivity checks. onicecallback or used for connectivity checks.
This call may result in a change to the state of the ICE Agent, and This call may result in a change to the state of the ICE Agent, and
may result in a change to media state if it results in connectivity may result in a change to media state if it results in connectivity
being established. being established.
5.1.9. addIceCandidate 5.2.9. addIceCandidate
The addIceCandidate method provides a remote candidate to the ICE The addIceCandidate method provides a remote candidate to the ICE
Agent, which will be added to the remote description. Connectivity Agent, which will be added to the remote description. Connectivity
checks will be sent to the new candidate. checks will be sent to the new candidate.
This call will result in a change to the state of the ICE Agent, and This call will result in a change to the state of the ICE Agent, and
may result in a change to media state if it results in connectivity may result in a change to media state if it results in connectivity
being established. being established.
5.2. Configurable SDP Parameters 6. Configurable SDP Parameters
Note: This section is still very early and is likely to
significantly change as we get a better understanding of the a) the
use cases for this b) the implications at the protocol level c)
feedback from implementors on what they can do.
The following is a partial list of SDP parameters that an application The following is a partial list of SDP parameters that an application
may want to control, in either local or remote descriptions, using may want to control, in either local or remote descriptions, using
this API. this API.
- remove or reorder codecs (m=) o 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 typical call setup using the JSEP model,
indicating the functions that are called and the state changes that
occur. We assume the following architecture in this example, where UA
is synonymous with "browser", and JS is synonymous with "web
application":
OffererUA <-> OffererJS <-> WebServer <-> AnswererJS <-> AnswererUA
6.1. Initiating the Session
The initiator creates a PeerConnection, hooks up to its ICE callback,
and adds the desired MediaStreams (presumably obtained via
getUserMedia). The ICE gathering process begins to gather candidates
for a default number of streams, as the exact number will not be
known until the local description is 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 description to offer to the
callee. This description includes the codecs and other necessary
session parameters, as well as information about each of the streams
that has been added (e.g. SSRC, CNAME, etc.) The created description
includes all parameters that the offerer's UA supports; if the
initiator wants to influence the created offer, they can pass in a
MediaConstraints object to createOffer that allows for customization
(e.g. if the initiator wants to receive but not send video). The
initiator can also directly manipulate the created session
description as well, perhaps if it wants to change the priority of
the offered codecs.
OffererJS->OffererUA: var offer = pc.createOffer(null);
6.1.2. Applying the Offer
The initiator then instructs the PeerConnection to use this offer as
the 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 the UA handle the description
properly, the initiator marks it as an offer when calling
setLocalDescription; this indicates to the UA that multiple
capabilities have been 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 Callbacks
The initiator starts to receive callbacks on its onicecandidate
handler. Candidates are provided to the IceCallback as they are
allocated; when the last allocation completes or times out, this
callback will be invoked with a null argument.
OffererUA->OffererJS: onIceCandidate(candidate);
6.1.4. Serializing the Offer and Candidates
At this point, the offerer is ready to send its offer to the callee
using its preferred signaling protocol. Depending on the protocol, it
can either send the initial session description first, and then
"trickle" the ICE candidates as they are given to the application, or
it can wait for all the ICE candidates to be collected, and then send
the offer and list of candidates all at once.
6.2. Receiving the Session
Through the chosen signaling protocol, the recipient is notified of
an incoming session request. It creates a PeerConnection, and sets up
its own ICE callback. The ICE gathering process begins to 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 Offer
The recipient converts the received offer from its signaling protocol
into SDP format, and supplies it to its PeerConnection, again marking
it as an offer. As a remote description, the offer indicates what
codecs the remote side wants to use for receiving, as well as what
SRTP keys it will use for sending. The setting of the remote
description causes callbacks to be issued, informing the application
of what kinds of streams are present in the offer.
This step will also cause encoder resources to be 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 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.addIceCandidate(candidate);
6.2.3. Generating the Answer
Once the recipient has decided to accept the session, it generates an
answer session description. This process performs the appropriate
intersection of codecs and other parameters to generate the correct
answer. As with the offer, MediaConstraints can be provided to
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 o change codec attributes (a=fmtp; ptime)
The recipient then instructs the PeerConnection to use the answer as o enable/disable BUNDLE (a=group)
its local description 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 the
PeerConnection to move to the ACTIVE state, and transmission of media
by the answerer to start (assuming both sides have indicated this in
their descriptions).
AnswererJS->AnswererUA: pc.setLocalDescription("answer", answer); o enable/disable RTCP mux (a=rtcp-mux)
AnswererUA->OffererUA: <media>
6.2.5. Serializing the Answer o change send resolution or framerate (TBD)
As with the offer, the answer (with or without candidates) is now o change desired recv resolution or framerate (TBD)
converted to the desired signaling format and sent to the initiator.
6.3. Completing the Session o change total bandwidth (b=)
6.3.1. Receiving the Answer o remove desired AVPF mechanisms (a=rtcp-fb)
The initiator converts the answer from the signaling protocol and o remove RTP header extensions (a=rtphdr-ext)
applies it as the remote description, marking it as an answer. This
causes the PeerConnection to move to the ACTIVE state, and
transmission of media by the offerer to start (assuming both sides
have indicated this in their descriptions).
OffererJS->OffererUA: pc.setRemoteDescription("answer", answer); o add/change SSRC grouping (e.g. FID, RTX, etc) (a=ssrc-group)
OffererUA->AnswererUA: <media>
6.4. Updates to the Session o add SSRC attributes (a=ssrc)
Updates to the session are handled with a new offer/answer exchange. o change media send/recv state (a=sendonly/recvonly/inactive)
However, since media will already be flowing at this point, the new
offerer needs to support both its old session description as well as
the new one it has offered, until the change is accepted by the
remote side.
Note also that in an update scenario, the roles may be reversed, i.e. For example, an application could implement call hold by adding an
the update offerer can be different than the original offerer. a=inactive attribute to its local description, and then applying and
signaling that description.
7. Security Considerations 7. Security Considerations
TODO TODO
8. IANA Considerations 8. IANA Considerations
This document requires no actions from IANA. This document requires no actions from IANA.
9. Acknowledgements 9. Acknowledgements
Harald Alvestrand, Dan Burnett, Neil Stratford, Eric Rescorla, Anant Harald Alvestrand, Dan Burnett, Neil Stratford, Eric Rescorla, Anant
Narayanan, and Adam Bergkvist all provided valuable feedback on this Narayanan, and Adam Bergkvist all provided valuable feedback on this
proposal. Matthew Kaufman provided the observation that keeping state proposal. Suhas Nandakumar provided text and input for SDP
out of the browser allows a call to continue even if the page is requirements. Matthew Kaufman provided the observation that keeping
reloaded. Richard Ejzak provided the specifics on session cloning. state out of the browser allows a call to continue even if the page
is reloaded.
10. References 10. References
10.1. Normative References 10.1. Normative References
[I-D.rescorla-mmusic-ice-trickle]
Rescorla, E., Uberti, J., and E. Ivov, "Trickle ICE:
Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol",
draft-rescorla-mmusic-ice-trickle-00 (work in progress),
October 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June 2002. with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006. 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 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", May 2011.
Available at http://dev.w3.org/2012/webrtc/editor/webrtc.html
Appendix A. JSEP Implementation Examples
A.1. Example API
The interface below shows a basic Javascript API that could be used
to expose the functionality discussed in this document. This API is
used for the examples in the following parts of this Appendix.
// actions, for setLocalDescription/setRemoteDescription 10.2. Informative References
enum SessionDescriptionType { "offer", "pranswer", "answer" }
// constraints that can be supplied to the ctor or createXXXX [I-D.ietf-rtcweb-rtp-usage]
enum MediaConstraints { Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
"offerConfig", // controls the kind of offer created; Communication (WebRTC): Media Transport and Use of RTP",
// "default" (normal offer) draft-ietf-rtcweb-rtp-usage-04 (work in progress),
// "caps" (all capabilities) July 2012.
// "new" (brand new description)
// "iceRestart" (new ICE creds)
"iceTransports", // controls ICE candidates; can be [I-D.jennings-rtcweb-signaling]
// "none" (no candidates) Jennings, C., Rosenberg, J., and R. Jesup, "RTCWeb Offer/
// "relay" (only relay candidates) Answer Protocol (ROAP)",
// "all" (all available candidates) draft-jennings-rtcweb-signaling-01 (work in progress),
} October 2011.
[Constructor (int index, DOMString id, in DOMString candidateLine)] [I-D.nandakumar-rtcweb-sdp]
interface IceCandidate { Nandakumar, S. and C. Jennings, "SDP for the WebRTC",
// the m= line index for this candidate draft-nandakumar-rtcweb-sdp-00 (work in progress),
readonly attribute int mLineIndex October 2012.
// the mid for the m= line for this candidate
readonly attribute DOMString mLineId;
// creates a SDP-ized form of this candidate
stringifier DOMString ();
};
[Constructor (DOMString sdp)] [RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
interface SessionDescription { Description Protocol (SDP) Security Descriptions for Media
// adds the specified candidate to the description Streams", RFC 4568, July 2006.
void addCandidate(IceCandidate candidate);
// serializes the description to SDP
stringifier DOMString ();
};
[Constructor (DOMString configuration, [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
optional MediaConstraints constraints)] (ICE): A Protocol for Network Address Translator (NAT)
interface PeerConnection { Traversal for Offer/Answer Protocols", RFC 5245,
// creates a blob of SDP to be provided as an offer. April 2010.
SessionDescription createOffer (
SessionDescriptionCallback successCb,
optional ErrorCallback errorCb,
optional MediaContraints constraints);
// creates a blob of SDP to be provided as an answer.
SessionDescription createAnswer (
SessionDescription offer,
SessionDescriptionCallback successCb,
optional ErrorCallback errorCb,
optional MediaContraints constraints);
// sets the local session description [W3C.WD-webrtc-20111027]
void setLocalDescription ( Bergkvist, A., Burnett, D., Narayanan, A., and C.
SessionDescriptionType action, Jennings, "WebRTC 1.0: Real-time Communication Between
SessionDescription desc); Browsers", World Wide Web Consortium WD WD-webrtc-
// sets the remote session description 20111027, October 2011,
void setRemoteDescription ( <http://www.w3.org/TR/2011/WD-webrtc-20111027>.
SessionDescriptionType action,
SessionDescription desc)
// returns the current local session description
readonly attribute SessionDescription localDescription;
// returns the current remote session description
readonly attribute SessionDescription remoteDescription;
// updates the constraints for ICE processing Appendix A. JSEP Implementation Examples
void updateIce (
optional DOMString configuration,
optional MediaConstraints constraints);
// starts using a received remote ICE candidate
void addIceCandidate (
IceCandidate candidate);
// notifies the application of a new local ICE candidate
attribute Function? onicecandidate;
};
A.2. Example API Flows A.1. Example API Flows
Below are several sample flows for the new PeerConnection and library Below are several sample flows for the new PeerConnection and library
APIs, demonstrating when the various APIs are called in different APIs, demonstrating when the various APIs are called in different
situations and with various transport protocols. For clarity and situations and with various transport protocols. For clarity and
simplicity, the createOffer/createAnswer calls are assumed to be simplicity, the createOffer/createAnswer calls are assumed to be
synchronous in these examples, whereas the actual APIs are async. synchronous in these examples, whereas the actual APIs are async.
A.2.1. Call using ROAP A.1.1. Call using ROAP
This example demonstrates a ROAP call, without the use of trickle This example demonstrates a ROAP call, without the use of trickle
candidates. candidates.
// Call is initiated toward Answerer // Call is initiated toward Answerer
OffererJS->OffererUA: pc = new PeerConnection(); OffererJS->OffererUA: pc = new PeerConnection();
OffererJS->OffererUA: pc.addStream(localStream, null); OffererJS->OffererUA: pc.addStream(localStream, null);
OffererUA->OffererJS: iceCallback(candidate); OffererUA->OffererJS: iceCallback(candidate);
OffererJS->OffererUA: offer = pc.createOffer(null); OffererJS->OffererUA: offer = pc.createOffer(null);
OffererJS->OffererUA: pc.setLocalDescription("offer", offer); OffererJS->OffererUA: pc.setLocalDescription("offer", offer);
OffererJS->AnswererJS: {"type":"OFFER", "sdp":offer } OffererJS->AnswererJS: {"type":"OFFER", "sdp":offer }
skipping to change at page 24, line 40 skipping to change at page 27, line 5
// ICE Completes (at Answerer) // ICE Completes (at Answerer)
AnswererUA->AnswererJS: onopen(); AnswererUA->AnswererJS: onopen();
AnswererUA->OffererUA: Media AnswererUA->OffererUA: Media
// ICE Completes (at Offerer) // ICE Completes (at Offerer)
OffererUA->OffererJS: onopen(); OffererUA->OffererJS: onopen();
OffererJS->AnswererJS: {"type":"OK" } OffererJS->AnswererJS: {"type":"OK" }
OffererUA->AnswererUA: Media OffererUA->AnswererUA: Media
A.2.2 Call using XMPP A.1.2. Call using XMPP
This example demonstrates an XMPP call, making use of trickle This example demonstrates an XMPP call, making use of trickle
candidates. candidates.
// Call is initiated toward Answerer // Call is initiated toward Answerer
OffererJS->OffererUA: pc = new PeerConnection(); OffererJS->OffererUA: pc = new PeerConnection();
OffererJS->OffererUA: pc.addStream(localStream, null); OffererJS->OffererUA: pc.addStream(localStream, null);
OffererJS->OffererUA: offer = pc.createOffer(null); OffererJS->OffererUA: offer = pc.createOffer(null);
OffererJS->OffererUA: pc.setLocalDescription("offer", offer); OffererJS->OffererUA: pc.setLocalDescription("offer", offer);
OffererJS: xmpp = createSessionInitiate(offer); OffererJS: xmpp = createSessionInitiate(offer);
skipping to change at page 25, line 46 skipping to change at page 28, line 11
OffererUA->OffererJS: onaddstream(remoteStream); OffererUA->OffererJS: onaddstream(remoteStream);
// ICE Completes (at Answerer) // ICE Completes (at Answerer)
AnswererUA->AnswererJS: onopen(); AnswererUA->AnswererJS: onopen();
AnswererUA->OffererUA: Media AnswererUA->OffererUA: Media
// ICE Completes (at Offerer) // ICE Completes (at Offerer)
OffererUA->OffererJS: onopen(); OffererUA->OffererJS: onopen();
OffererUA->AnswererUA: Media OffererUA->AnswererUA: Media
A.2.3. Adding video to a call, using XMPP A.1.3. Adding video to a call, using XMPP
This example demonstrates an XMPP call, where the XMPP content-add This example demonstrates an XMPP call, where the XMPP content-add
mechanism is used to add video media to an existing session. For mechanism is used to add video media to an existing session. For
simplicity, candidate exchange is not shown. simplicity, candidate exchange is not shown.
Note that the offerer for the change to the session may be different Note that the offerer for the change to the session may be different
than the original call offerer. than the original call offerer.
// Offerer adds video stream // Offerer adds video stream
OffererJS->OffererUA: pc.addStream(videoStream) OffererJS->OffererUA: pc.addStream(videoStream)
OffererJS->OffererUA: offer = pc.createOffer(null); OffererJS->OffererUA: offer = pc.createOffer(null);
OffererJS: xmpp = createContentAdd(offer); OffererJS: xmpp = createContentAdd(offer);
OffererJS->OffererUA: pc.setLocalDescription("offer", offer); OffererJS->OffererUA: pc.setLocalDescription("offer", offer);
skipping to change at page 26, line 27 skipping to change at page 28, line 39
AnswererJS->AnswererUA: pc.setRemoteDescription("offer", offer); AnswererJS->AnswererUA: pc.setRemoteDescription("offer", offer);
AnswererJS->AnswererUA: answer = pc.createAnswer(offer, null); AnswererJS->AnswererUA: answer = pc.createAnswer(offer, null);
AnswererJS->AnswererUA: pc.setLocalDescription("answer", answer); AnswererJS->AnswererUA: pc.setLocalDescription("answer", answer);
AnswererJS: xmpp = createContentAccept(answer); AnswererJS: xmpp = createContentAccept(answer);
AnswererJS->OffererJS: <jingle action="content-accept"/> AnswererJS->OffererJS: <jingle action="content-accept"/>
// content-accept arrives at Offerer // content-accept arrives at Offerer
OffererJS: answer = parseContentAccept(xmpp); OffererJS: answer = parseContentAccept(xmpp);
OffererJS->OffererUA: pc.setRemoteDescription("answer", answer); OffererJS->OffererUA: pc.setRemoteDescription("answer", answer);
A.2.4. Simultaneous add of video streams, using XMPP A.1.4. Simultaneous add of video streams, using XMPP
This example demonstrates an XMPP call, where new video sources are 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 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 these sources only affects one side of the call, there is no
conflict. The XMPP description-info mechanism is used to indicate the conflict. The XMPP description-info mechanism is used to indicate
new sources to the remote side. the new sources to the remote side.
// Offerer and "Answerer" add video streams at the same time // Offerer and "Answerer" add video streams at the same time
OffererJS->OffererUA: pc.addStream(offererVideoStream2) OffererJS->OffererUA: pc.addStream(offererVideoStream2)
OffererJS->OffererUA: offer = pc.createOffer(null); OffererJS->OffererUA: offer = pc.createOffer(null);
OffererJS: xmpp = createDescriptionInfo(offer); OffererJS: xmpp = createDescriptionInfo(offer);
OffererJS->OffererUA: pc.setLocalDescription("offer", offer); OffererJS->OffererUA: pc.setLocalDescription("offer", offer);
OffererJS->AnswererJS: <jingle action="description-info"/> OffererJS->AnswererJS: <jingle action="description-info"/>
AnswererJS->AnswererUA: pc.addStream(answererVideoStream2) AnswererJS->AnswererUA: pc.addStream(answererVideoStream2)
AnswererJS->AnswererUA: offer = pc.createOffer(null); AnswererJS->AnswererUA: offer = pc.createOffer(null);
AnswererJS: xmpp = createDescriptionInfo(offer); AnswererJS: xmpp = createDescriptionInfo(offer);
AnswererJS->AnswererUA: pc.setLocalDescription("offer", offer); AnswererJS->AnswererUA: pc.setLocalDescription("offer", offer);
AnswererJS->OffererJS: <jingle action="description-info"/> AnswererJS->OffererJS: <jingle action="description-info"/>
// description-info arrives at "Answerer", and is acked // description-info arrives at "Answerer", and is acked
AnswererJS: offer = parseDescriptionInfo(xmpp); AnswererJS: offer = parseDescriptionInfo(xmpp);
AnswererJS->OffererJS: <iq type="result/> // ack AnswererJS->OffererJS: <iq type="result"/> // ack
// description-info arrives at Offerer, and is acked // description-info arrives at Offerer, and is acked
OffererJS: offer = parseDescriptionInfo(xmpp); OffererJS: offer = parseDescriptionInfo(xmpp);
OffererJS->AnswererJS: <iq type="result/> // ack OffererJS->AnswererJS: <iq type="result"/> // ack
// ack arrives at Offerer; remote offer is used as an answer // ack arrives at Offerer; remote offer is used as an answer
OffererJS->OffererUA: pc.setRemoteDescription("answer", offer); OffererJS->OffererUA: pc.setRemoteDescription("answer", offer);
// ack arrives at "Answerer"; remote offer is used as an answer // ack arrives at "Answerer"; remote offer is used as an answer
AnswererJS->AnswererUA: pc.setRemoteDescription("answer", offer); AnswererJS->AnswererUA: pc.setRemoteDescription("answer", offer);
A.2.5. Call using SIP A.1.5. Call using SIP
This example demonstrates a simple SIP call (e.g. where the client This example demonstrates a simple SIP call (e.g. where the client
talks to a SIP proxy over WebSockets). talks to a SIP proxy over WebSockets).
// Call is initiated toward Answerer // Call is initiated toward Answerer
OffererJS->OffererUA: pc = new PeerConnection(); OffererJS->OffererUA: pc = new PeerConnection();
OffererJS->OffererUA: pc.addStream(localStream, null); OffererJS->OffererUA: pc.addStream(localStream, null);
OffererUA->OffererJS: onicecandidate(candidate); OffererUA->OffererJS: onicecandidate(candidate);
OffererJS->OffererUA: offer = pc.createOffer(null); OffererJS->OffererUA: offer = pc.createOffer(null);
OffererJS->OffererUA: pc.setLocalDescription("offer", offer); OffererJS->OffererUA: pc.setLocalDescription("offer", offer);
skipping to change at page 28, line 4 skipping to change at page 30, line 37
// 200 OK arrives at Offerer // 200 OK arrives at Offerer
OffererJS: answer = parseResponse(sip); OffererJS: answer = parseResponse(sip);
OffererJS->OffererUA: peer.setRemoteDescription("answer", answer); OffererJS->OffererUA: peer.setRemoteDescription("answer", answer);
OffererUA->OffererJS: onaddstream(remoteStream); OffererUA->OffererJS: onaddstream(remoteStream);
OffererJS->AnswererJS: ACK OffererJS->AnswererJS: ACK
// ICE Completes (at Answerer) // ICE Completes (at Answerer)
AnswererUA->AnswererJS: onopen(); AnswererUA->AnswererJS: onopen();
AnswererUA->OffererUA: Media AnswererUA->OffererUA: Media
// ICE Completes (at Offerer) // ICE Completes (at Offerer)
OffererUA->OffererJS: onopen(); OffererUA->OffererJS: onopen();
OffererUA->AnswererUA: Media OffererUA->AnswererUA: Media
A.2.6. Handling early media (e.g. 1-800-FEDEX), using SIP A.1.6. Handling early media (e.g. 1-800-GO FEDEX), using SIP
This example demonstrates how early media could be handled; for This example demonstrates how early media could be handled; for
simplicity, only the offerer side of the call is shown. simplicity, only the offerer side of the call is shown.
// Call is initiated toward Answerer // Call is initiated toward Answerer
OffererJS->OffererUA: pc = new PeerConnection(); OffererJS->OffererUA: pc = new PeerConnection();
OffererJS->OffererUA: pc.addStream(localStream, null); OffererJS->OffererUA: pc.addStream(localStream, null);
OffererUA->OffererJS: onicecandidate(candidate); OffererUA->OffererJS: onicecandidate(candidate);
OffererJS->OffererUA: offer = pc.createOffer(null); OffererJS->OffererUA: offer = pc.createOffer(null);
OffererJS->OffererUA: pc.setLocalDescription("offer", offer); OffererJS->OffererUA: pc.setLocalDescription("offer", offer);
skipping to change at page 28, line 36 skipping to change at page 32, line 5
// ICE Completes (at Offerer) // ICE Completes (at Offerer)
OffererUA->OffererJS: onopen(); OffererUA->OffererJS: onopen();
OffererUA->AnswererUA: Media OffererUA->AnswererUA: Media
// 200 OK arrives at Offerer // 200 OK arrives at Offerer
OffererJS: answer = parseResponse(sip); OffererJS: answer = parseResponse(sip);
OffererJS->OffererUA: pc.setRemoteDescription("answer", answer); OffererJS->OffererUA: pc.setRemoteDescription("answer", answer);
OffererJS->AnswererJS: ACK OffererJS->AnswererJS: ACK
A.3. Full Example Application Appendix B. Change log
The following example demonstrates a simple video calling Changes in draft -02:
application, using both trickle candidates and provisional answers to
speed up call setup.
// Usage: o Converted from nroff
// Caller calls start(true)
// Callee calls start(false) to prepare the call/start connecting,
// and then accept() to start transmitting.
var signalingChannel = createSignalingChannel(); o Removed comparisons to old approaches abandoned by the working
var pc = null; group
var localStream = null;
signalingChannel.onmessage = handleMessage;
// Set up the call, get access to local media,
// and establish connectivity.
function start(isCaller) {
// Create a PeerConnection and hook up the IceCallback.
pc = new webkitPeerConnection(null, null);
pc.onicecandidate = function(evt) {
sendMessage("candidate", evt.candidate);
};
// Get the local stream and show it in the local video element; o Removed stuff that has moved to W3C specificaiton
// if we're the caller, ship off an offer once we get the stream.
navigator.webkitGetUserMedia(
{"audio": true, "video": true}, function (stream) {
selfView.src = webkitURL.createObjectURL(stream);
localStream = stream;
if (isCaller) {
pc.addStream(stream);
pc.createOffer(function(sdp) {
setLocalAndSendMessage("offer", sdp);
});
});
// When the remote stream arrives, show it in the remote o Align SDP handling with W3C draft
// video element.
pc.onaddstream = function(evt) {
remoteView.src = webkitURL.createObjectURL(evt.stream);
};
}
// The callee has accepted the call, attach their media o Clarified section on forking.
// and send a final answer.
function accept() {
// 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 the stream already
pc.createAnswer(msg.sdp, function(sdp) {
setLocalAndSendMessage("answer", sdp);
});
}
// -- internal methods -- Changes in draft -01:
// Apply SDP locally and send it to the remote side. o Added diagrams for architecture and state machine.
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": type, "sdp": obj }));
}
// Handle incoming signaling messages. o Added sections on forking and rehydration.
function handleMessage(str) {
var msg = JSON.parse(str);
switch (msg.type) {
case "offer":
// create the PeerConnection
start(false);
// feed the received offer into the PeerConnection
pc.setRemoteDescription(msg.type, msg.sdp);
// create provisional answer to 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;
}
}
Appendix B. Change log o Clarified meaning of "pranswer" and "answer".
01: Added diagrams for architecture and state machine. o Reworked how ICE restarts and media directions are controlled.
Added sections on forking and rehydration.
Clarified meaning of "pranswer" and "answer". o Added list of parameters that can be changed in a description.
Reworked how ICE restarts and media directions are controlled.
Added list of parameters that can be changed in a description. o Updated suggested API and examples to match latest thinking.
Updated suggested API and examples to match latest thinking.
Suggested API and examples have been moved to an appendix. o Suggested API and examples have been moved to an appendix.
00: Migrated from draft-uberti-rtcweb-jsep-02.
Changes in draft -00:
o Migrated from draft-uberti-rtcweb-jsep-02.
Authors' Addresses Authors' Addresses
Justin Uberti Justin Uberti
Google Google
5 Cambridge Center 747 6th Ave S
Cambridge, MA 02142 Kirkland, WA 98033
Email: justin@uberti.name USA
Email: justin@uberti.name
Cullen Jennings Cullen Jennings
Cisco Cisco
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
USA USA
Email: fluffy@cisco.com Email: fluffy@iii.ca
 End of changes. 167 change blocks. 
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