Network Working Group J. Uberti
Internet-Draft Google
Intended status: Standards Track C. Jennings
Expires: April 20, 2016 Cisco
E. Rescorla, Ed.
Mozilla
October 18, 2015
Javascript Session Establishment Protocol
draft-ietf-rtcweb-jsep-12
Abstract
This document describes the mechanisms for allowing a Javascript
application to control the signaling plane of a multimedia session
via the interface specified in the W3C RTCPeerConnection API, and
discusses how this relates to existing signaling protocols.
Status of This Memo
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This Internet-Draft will expire on April 20, 2016.
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the Trust Legal Provisions and are provided without warranty as
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. General Design of JSEP . . . . . . . . . . . . . . . . . 3
1.2. Other Approaches Considered . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Semantics and Syntax . . . . . . . . . . . . . . . . . . . . 6
3.1. Signaling Model . . . . . . . . . . . . . . . . . . . . . 6
3.2. Session Descriptions and State Machine . . . . . . . . . 7
3.3. Session Description Format . . . . . . . . . . . . . . . 10
3.4. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.4.1. ICE Gathering Overview . . . . . . . . . . . . . . . 10
3.4.2. ICE Candidate Trickling . . . . . . . . . . . . . . . 11
3.4.2.1. ICE Candidate Format . . . . . . . . . . . . . . 11
3.4.3. ICE Candidate Policy . . . . . . . . . . . . . . . . 12
3.4.4. ICE Candidate Pool . . . . . . . . . . . . . . . . . 13
3.5. Video Size Negotiation . . . . . . . . . . . . . . . . . 13
3.5.1. Creating an imageattr Attribute . . . . . . . . . . . 13
3.5.2. Interpreting an imageattr Attribute . . . . . . . . . 14
3.6. Interactions With Forking . . . . . . . . . . . . . . . . 15
3.6.1. Sequential Forking . . . . . . . . . . . . . . . . . 15
3.6.2. Parallel Forking . . . . . . . . . . . . . . . . . . 16
4. Interface . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1. Methods . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1.1. Constructor . . . . . . . . . . . . . . . . . . . . . 17
4.1.2. createOffer . . . . . . . . . . . . . . . . . . . . . 19
4.1.3. createAnswer . . . . . . . . . . . . . . . . . . . . 20
4.1.4. SessionDescriptionType . . . . . . . . . . . . . . . 21
4.1.4.1. Use of Provisional Answers . . . . . . . . . . . 22
4.1.4.2. Rollback . . . . . . . . . . . . . . . . . . . . 22
4.1.5. setLocalDescription . . . . . . . . . . . . . . . . . 23
4.1.6. setRemoteDescription . . . . . . . . . . . . . . . . 24
4.1.7. localDescription . . . . . . . . . . . . . . . . . . 24
4.1.8. remoteDescription . . . . . . . . . . . . . . . . . . 24
4.1.9. canTrickleIceCandidates . . . . . . . . . . . . . . . 24
4.1.10. setConfiguration . . . . . . . . . . . . . . . . . . 25
4.1.11. addIceCandidate . . . . . . . . . . . . . . . . . . . 26
5. SDP Interaction Procedures . . . . . . . . . . . . . . . . . 26
5.1. Requirements Overview . . . . . . . . . . . . . . . . . . 26
5.1.1. Implementation Requirements . . . . . . . . . . . . . 26
5.1.2. Usage Requirements . . . . . . . . . . . . . . . . . 28
5.1.3. Profile Names and Interoperability . . . . . . . . . 28
5.2. Constructing an Offer . . . . . . . . . . . . . . . . . . 29
5.2.1. Initial Offers . . . . . . . . . . . . . . . . . . . 29
5.2.2. Subsequent Offers . . . . . . . . . . . . . . . . . . 34
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5.2.3. Options Handling . . . . . . . . . . . . . . . . . . 37
5.2.3.1. OfferToReceiveAudio . . . . . . . . . . . . . . . 37
5.2.3.2. OfferToReceiveVideo . . . . . . . . . . . . . . . 38
5.2.3.3. IceRestart . . . . . . . . . . . . . . . . . . . 38
5.2.3.4. VoiceActivityDetection . . . . . . . . . . . . . 38
5.3. Generating an Answer . . . . . . . . . . . . . . . . . . 39
5.3.1. Initial Answers . . . . . . . . . . . . . . . . . . . 39
5.3.2. Subsequent Answers . . . . . . . . . . . . . . . . . 43
5.3.3. Options Handling . . . . . . . . . . . . . . . . . . 44
5.3.3.1. VoiceActivityDetection . . . . . . . . . . . . . 45
5.4. Processing a Local Description . . . . . . . . . . . . . 45
5.5. Processing a Remote Description . . . . . . . . . . . . . 45
5.6. Parsing a Session Description . . . . . . . . . . . . . . 46
5.6.1. Session-Level Parsing . . . . . . . . . . . . . . . . 46
5.6.2. Media Section Parsing . . . . . . . . . . . . . . . . 48
5.6.3. Semantics Verification . . . . . . . . . . . . . . . 49
5.7. Applying a Local Description . . . . . . . . . . . . . . 50
5.8. Applying a Remote Description . . . . . . . . . . . . . . 51
5.9. Applying an Answer . . . . . . . . . . . . . . . . . . . 53
6. Configurable SDP Parameters . . . . . . . . . . . . . . . . . 54
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 55
7.1. Simple Example . . . . . . . . . . . . . . . . . . . . . 56
7.2. Normal Examples . . . . . . . . . . . . . . . . . . . . . 60
8. Security Considerations . . . . . . . . . . . . . . . . . . . 71
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 71
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 71
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 72
11.1. Normative References . . . . . . . . . . . . . . . . . . 72
11.2. Informative References . . . . . . . . . . . . . . . . . 75
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 76
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 79
1. Introduction
This document describes how the W3C WEBRTC RTCPeerConnection
interface[W3C.WD-webrtc-20140617] is used to control the setup,
management and teardown of a multimedia session.
1.1. General Design of JSEP
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
application as much as possible. The rationale is that different
applications may prefer to use different protocols, such as the
existing SIP or Jingle call signaling protocols, or something custom
to the particular application, perhaps for a novel use case. In this
approach, the key information that needs to be exchanged is the
multimedia session description, which specifies the necessary
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transport and media configuration information necessary to establish
the media plane.
With these considerations in mind, this document describes the
Javascript Session Establishment Protocol (JSEP) that allows for full
control of the signaling state machine from Javascript. JSEP removes
the browser almost entirely from the core signaling flow, which is
instead handled by the Javascript making use of two interfaces: (1)
passing in local and remote session descriptions and (2) interacting
with the ICE state machine.
In this document, the use of JSEP is described as if it always occurs
between two browsers. Note though in many cases it will actually be
between a browser and some kind of server, such as a gateway or MCU.
This distinction is invisible to the browser; it just follows the
instructions it is given via the API.
JSEP's handling of session descriptions is simple and
straightforward. Whenever an offer/answer exchange is needed, the
initiating side creates an offer by calling a createOffer() API. The
application optionally modifies that offer, and then uses it to set
up its local config via the setLocalDescription() API. The offer is
then sent off to the remote side over its preferred signaling
mechanism (e.g., WebSockets); upon receipt of that offer, the remote
party installs it using the setRemoteDescription() API.
To complete the offer/answer exchange, the remote party uses the
createAnswer() API to generate an appropriate answer, applies it
using the setLocalDescription() API, and sends the answer back to the
initiator over the signaling channel. When the initiator gets that
answer, it installs it using the setRemoteDescription() API, and
initial setup is complete. This process can be repeated for
additional offer/answer exchanges.
Regarding ICE [RFC5245], JSEP decouples the ICE state machine from
the overall signaling state machine, as the ICE state machine must
remain in the browser, because only the browser has the necessary
knowledge of candidates and other transport info. Performing this
separation also provides additional flexibility; in protocols that
decouple session descriptions from transport, such as Jingle, the
session description can be sent immediately and the transport
information can be sent when available. In protocols that don't,
such as SIP, the information can be used in the aggregated form.
Sending transport information separately can allow for faster ICE and
DTLS startup, since ICE checks can start as soon as any transport
information is available rather than waiting for all of it.
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Through its abstraction of signaling, the JSEP approach does require
the application to be aware of the signaling process. While the
application does not need to understand the contents of session
descriptions to set up a call, the application must call the right
APIs at the right times, convert the session descriptions and ICE
information into the defined messages of its chosen signaling
protocol, and perform the reverse conversion on the messages it
receives from the other side.
One way to mitigate this is to provide a Javascript library that
hides this complexity from the developer; said library would
implement a given signaling protocol along with its state machine and
serialization code, presenting a higher level call-oriented interface
to the application developer. For example, libraries exist to adapt
the JSEP API into an API suitable for a SIP or XMPP. Thus, JSEP
provides greater control for the experienced developer without
forcing any additional complexity on the novice developer.
1.2. Other Approaches Considered
One approach that was considered instead of JSEP was to include a
lightweight signaling protocol. Instead of providing session
descriptions to the API, the API would produce and consume messages
from this protocol. While providing a more high-level API, this put
more control of signaling within the browser, forcing the browser to
have to understand and handle concepts like signaling glare. In
addition, it prevented the application from driving the state machine
to a desired state, as is needed in the page reload case.
A second approach that was considered but not chosen was to decouple
the management of the media control objects from session
descriptions, instead offering APIs that would control each component
directly. This was rejected based on a feeling that requiring
exposure of this level of complexity to the application programmer
would not be beneficial; it would result in an API where even a
simple example would require a significant amount of code to
orchestrate all the needed interactions, as well as creating a large
API surface that needed to be agreed upon and documented. In
addition, these API points could be called in any order, resulting in
a more complex set of interactions with the media subsystem than the
JSEP approach, which specifies how session descriptions are to be
evaluated and applied.
One variation on JSEP that was considered was to keep the basic
session description-oriented API, but to move the mechanism for
generating offers and answers out of the browser. Instead of
providing createOffer/createAnswer methods within the browser, this
approach would instead expose a getCapabilities API which would
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provide the application with the information it needed in order to
generate its own session descriptions. This increases the amount of
work that the application needs to do; it needs to know how to
generate session descriptions from capabilities, and especially how
to generate the correct answer from an arbitrary offer and the
supported capabilities. While this could certainly be addressed by
using a library like the one mentioned above, it basically forces the
use of said library even for a simple example. Providing
createOffer/createAnswer avoids this problem, but still allows
applications to generate their own offers/answers (to a large extent)
if they choose, using the description generated by createOffer as an
indication of the browser's capabilities.
2. 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 [RFC2119].
3. Semantics and Syntax
3.1. Signaling Model
JSEP does not specify a particular signaling model or state machine,
other than the generic need to exchange session descriptions in the
fashion described by [RFC3264] (offer/answer) in order for both sides
of the session to know how to conduct the session. JSEP provides
mechanisms to create offers and answers, as well as to apply them to
a session. However, the browser is totally decoupled from the actual
mechanism by which these offers and answers are communicated to the
remote side, including addressing, retransmission, forking, and glare
handling. These issues are left entirely up to the application; the
application has complete control over which offers and answers get
handed to the browser, and when.
+-----------+ +-----------+
| Web App |<--- App-Specific Signaling -->| Web App |
+-----------+ +-----------+
^ ^
| SDP | SDP
V V
+-----------+ +-----------+
| Browser |<----------- Media ------------>| Browser |
+-----------+ +-----------+
Figure 1: JSEP Signaling Model
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3.2. Session Descriptions and State Machine
In order to establish the media plane, the user agent needs specific
parameters to indicate what to transmit to the remote side, as well
as how to handle the media that is received. These parameters are
determined by the exchange of session descriptions in offers and
answers, and there are certain details to this process that must be
handled in the JSEP APIs.
Whether a session description applies to the local side or the remote
side affects the meaning of that description. For example, the list
of codecs sent to a remote party indicates what the local side is
willing to receive, which, when intersected with the set of codecs
the remote side supports, specifies what the remote side should send.
However, not all parameters follow this rule; for example, the DTLS-
SRTP parameters [RFC5763] sent to a remote party indicate what
certificate the local side will use in DTLS setup, and thereby what
the remote party should expect to receive; the remote party will have
to accept these parameters, with no option to choose different
values.
In addition, various RFCs put different conditions on the format of
offers versus answers. For example, an offer may propose an
arbitrary number of media streams (i.e. m= sections), but an answer
must contain the exact same number as the offer.
Lastly, while the exact media parameters are only known only after an
offer and an answer have been exchanged, it is possible for the
offerer to receive media after they have sent an offer and before
they have received an answer. To properly process incoming media in
this case, the offerer's media handler must be aware of the details
of the offer before the answer arrives.
Therefore, in order to handle session descriptions properly, the user
agent needs:
1. To know if a session description pertains to the local or remote
side.
2. To know if a session description is an offer or an answer.
3. To allow the offer to be specified independently of the answer.
JSEP addresses this by adding both setLocalDescription and
setRemoteDescription methods and having session description objects
contain a type field indicating the type of session description being
supplied. This satisfies the requirements listed above for both the
offerer, who first calls setLocalDescription(sdp [offer]) and then
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later setRemoteDescription(sdp [answer]), as well as for the
answerer, who first calls setRemoteDescription(sdp [offer]) and then
later setLocalDescription(sdp [answer]).
JSEP also allows for an answer to be treated as provisional by the
application. Provisional answers provide a way for an answerer to
communicate initial session parameters back to the offerer, in order
to allow the session to begin, while allowing a final answer to be
specified later. This concept of a final answer is important to the
offer/answer model; when such an answer is received, any extra
resources allocated by the caller can be released, now that the exact
session configuration is known. These "resources" can include things
like extra ICE components, TURN candidates, or video decoders.
Provisional answers, on the other hand, do no such deallocation
results; as a result, multiple dissimilar provisional answers can be
received and applied during call setup.
In [RFC3264], the constraint at the signaling level is that only one
offer can be outstanding for a given session, but at 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 via the createOffer() method 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 sending a final answer. The state machine
for this is as follows:
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setRemote(OFFER) setLocal(PRANSWER)
/-----\ /-----\
| | | |
v | v |
+---------------+ | +---------------+ |
| |----/ | |----/
| | setLocal(PRANSWER) | |
| Remote-Offer |------------------- >| Local-Pranswer|
| | | |
| | | |
+---------------+ +---------------+
^ | |
| | setLocal(ANSWER) |
setRemote(OFFER) | |
| V setLocal(ANSWER) |
+---------------+ |
| | |
| |<---------------------------+
| Stable |
| |<---------------------------+
| | |
+---------------+ setRemote(ANSWER) |
^ | |
| | setLocal(OFFER) |
setRemote(ANSWER) | |
| V |
+---------------+ +---------------+
| | | |
| | setRemote(PRANSWER) | |
| Local-Offer |------------------- >|Remote-Pranswer|
| | | |
| |----\ | |----\
+---------------+ | +---------------+ |
^ | ^ |
| | | |
\-----/ \-----/
setLocal(OFFER) setRemote(PRANSWER)
Figure 2: JSEP State Machine
Aside from these state transitions there is no other difference
between the handling of provisional ("pranswer") and final ("answer")
answers.
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3.3. Session Description Format
In the WebRTC specification, session descriptions are formatted as
SDP messages. While this format is not optimal for manipulation from
Javascript, it is widely accepted, and frequently updated with new
features. Any alternate encoding of session descriptions would have
to keep pace with the changes to SDP, at least until the time that
this new encoding eclipsed SDP in popularity. As a result, JSEP
currently uses SDP as the internal representation for its session
descriptions.
However, to simplify Javascript processing, and provide for future
flexibility, the SDP syntax is encapsulated within a
SessionDescription object, which can be constructed from SDP, and be
serialized out to SDP. If future specifications agree on a JSON
format for session descriptions, we could easily enable this object
to generate and consume that JSON.
Other methods may be added to SessionDescription in the future to
simplify handling of SessionDescriptions from Javascript. In the
meantime, Javascript libraries can be used to perform these
manipulations.
Note that most applications should be able to treat the
SessionDescriptions produced and consumed by these various API calls
as opaque blobs; that is, the application will not need to read or
change them. The W3C WebRTC API specification will provide
appropriate APIs to allow the application to control various session
parameters, which will provide the necessary information to the
browser about what sort of SessionDescription to produce.
3.4. ICE
3.4.1. ICE Gathering Overview
JSEP gathers ICE candidates as needed by the application. Collection
of ICE candidates is referred to as a gathering phase, and this is
triggered either by the addition of a new or recycled m= line to the
local session description, or new ICE credentials in the description,
indicating an ICE restart. Use of new ICE credentials can be
triggered explicitly by the application, or implicitly by the browser
in response to changes in the ICE configuration.
When a new gathering phase starts, the ICE Agent will notify the
application that gathering is occurring through an event. Then, when
each new ICE candidate becomes available, the ICE Agent will supply
it to the application via an additional event; these candidates will
also automatically be added to the local session description.
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Finally, when all candidates have been gathered, an event will be
dispatched to signal that the gathering process is complete.
Note that gathering phases only gather the candidates needed by
new/recycled/restarting m= lines; other m= lines continue to use
their existing candidates.
3.4.2. ICE Candidate Trickling
Candidate trickling is a technique through which a caller may
incrementally provide candidates to the callee after the initial
offer has been dispatched; the semantics of "Trickle ICE" are defined
in [I-D.ietf-mmusic-trickle-ice]. This process allows the callee to
begin acting upon the call and setting up the ICE (and perhaps DTLS)
connections immediately, without having to wait for the caller to
gather all possible candidates. This results in faster media setup
in cases where gathering is not performed prior to initiating the
call.
JSEP supports optional candidate trickling by providing APIs, as
described above, that provide control and feedback on the ICE
candidate gathering process. Applications that support candidate
trickling can send the initial offer immediately and send individual
candidates when they get the notified of a new candidate;
applications that do not support this feature can simply wait for the
indication that gathering is complete, and then create and send their
offer, with all the candidates, at this time.
Upon receipt of trickled candidates, the receiving application will
supply them to its ICE Agent. This triggers the ICE Agent to start
using the new remote candidates for connectivity checks.
3.4.2.1. ICE Candidate Format
As with session descriptions, the syntax of the IceCandidate object
provides some abstraction, but can be easily converted to and from
the SDP candidate lines.
The candidate lines are the only SDP information that is contained
within IceCandidate, as they represent the only information needed
that is not present in the initial offer (i.e., for trickle
candidates). This information is carried with the same syntax as the
"candidate-attribute" field defined for ICE. For example:
candidate:1 1 UDP 1694498815 192.0.2.33 10000 typ host
The IceCandidate object also contains fields to indicate which m=
line it should be associated with. The m= line can be identified in
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one of two ways; either by a m= line index, or a MID. The m= line
index is a zero-based index, with index N referring to the N+1th m=
line in the SDP sent by the entity which sent the IceCandidate. The
MID uses the "media stream identification" attribute, as defined in
[RFC5888], Section 4, to identify the m= line. JSEP implementations
creating an ICE Candidate object MUST populate both of these fields.
Implementations receiving an ICE Candidate object MUST use the MID if
present, or the m= line index, if not (as it could have come from a
non-JSEP endpoint).
3.4.3. ICE Candidate Policy
Typically, when gathering ICE candidates, the browser will gather all
possible forms of initial candidates - host, server reflexive, and
relay. However, in certain cases, applications may want to have more
specific control over the gathering process, due to privacy or
related concerns. For example, one may want to suppress the use of
host candidates, to avoid exposing information about the local
network, or go as far as only using relay candidates, to leak as
little location information as possible (note that these choices come
with corresponding operational costs). To accomplish this, the
browser MUST allow the application to restrict which ICE candidates
are used in a session. In addition, administrators may also wish to
control the set of ICE candidates, and so the browser SHOULD also
allow control via local policy, with the most restrictive policy
prevailing.
There may also be cases where the application wants to change which
types of candidates are used while the session is active. A prime
example is where a callee may initially want to use only relay
candidates, to avoid leaking location information to an arbitrary
caller, but then change to use all candidates (for lower operational
cost) once the user has indicated they want to take the call. For
this scenario, the browser MUST allow the candidate policy to be
changed in mid-session, subject to the aforementioned interactions
with local policy.
To administer the ICE candidate policy, the browser will determine
the current setting at the start of each gathering phase. Then,
during the gathering phase, the browser MUST NOT expose candidates
disallowed by the current policy to the application, use them as the
source of connectivity checks, or indirectly expose them via other
fields, such as the raddr/rport attributes for other ICE candidates.
Later, if a different policy is specified by the application, the
application can apply it by kicking off a new gathering phase via an
ICE restart.
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3.4.4. ICE Candidate Pool
JSEP applications typically inform the browser to begin ICE gathering
via the information supplied to setLocalDescription, as this is where
the app specifies the number of media streams, and thereby ICE
components, for which to gather candidates. However, to accelerate
cases where the application knows the number of ICE components to use
ahead of time, it may ask the browser to gather a pool of potential
ICE candidates to help ensure rapid media setup.
When setLocalDescription is eventually called, and the browser goes
to gather the needed ICE candidates, it SHOULD start by checking if
any candidates are available in the pool. If there are candidates in
the pool, they SHOULD be handed to the application immediately via
the ICE candidate event. If the pool becomes depleted, either
because a larger-than-expected number of ICE components is used, or
because the pool has not had enough time to gather candidates, the
remaining candidates are gathered as usual.
One example of where this concept is useful is an application that
expects an incoming call at some point in the future, and wants to
minimize the time it takes to establish connectivity, to avoid
clipping of initial media. By pre-gathering candidates into the
pool, it can exchange and start sending connectivity checks from
these candidates almost immediately upon receipt of a call. Note
though that by holding on to these pre-gathered candidates, which
will be kept alive as long as they may be needed, the application
will consume resources on the STUN/TURN servers it is using.
3.5. Video Size Negotiation
Video size negotiation is the process through which a receiver can
use the "a=imageattr" SDP attribute [RFC6236] to indicate what video
frame sizes it is capable of receiving. A receiver may have hard
limits on what its video decoder can process, or it may wish to
constrain what it receives due to application preferences, e.g. a
specific size for the window in which the video will be displayed.
3.5.1. Creating an imageattr Attribute
In order to determine the limits on what video resolution a receiver
wants to receive, it will intersect its decoder hard limits with any
mandatory constraints that have been applied to the associated
MediaStreamTrack. If the decoder limits are unknown, e.g. when using
a software decoder, the mandatory constraints are used directly. For
the answerer, these mandatory constraints can be applied to the
remote MediaStreamTracks that are created by a setRemoteDescription
call, and will affect the output of the ensuing createAnswer call.
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Any constraints set after setLocalDescription is used to set the
answer will result in a new offer-answer exchange. For the offerer,
because it does not know about any remote MediaStreamTracks until it
receives the answer, the offer can only reflect decoder hard limits.
If the offerer wishes to set mandatory constraints on video
resolution, it must do so after receiving the answer, and the result
will be a new offer-answer to communicate them.
If there are no known decoder limits or mandatory constraints, the
"a=imageattr" attribute SHOULD be omitted.
Otherwise, an "a=imageattr" attribute is created with "recv"
direction, and the resulting resolution space formed by intersecting
the decoder limits and constraints is used to specify its minimum and
maximum x= and y= values. If the intersection is the null set, i.e.,
there are no resolutions that are permitted by both the decoder and
the mandatory constraints, this SHOULD be represented by x=0 and y=0
values.
The rules here express a single set of preferences, and therefore,
the "a=imageattr" q= value is not important. It SHOULD be set to
1.0.
The "a=imageattr" field is payload type specific. When all video
codecs supported have the same capabilities, use of a single
attribute, with the wildcard payload type (*), is RECOMMENDED.
However, when the supported video codecs have differing capabilities,
specific "a=imageattr" attributes MUST be inserted for each payload
type.
As an example, consider a system with a HD-capable, multiformat video
decoder, where the application has constrained the received track to
at most 360p. In this case, the implemention would generate this
attribute:
a=imageattr:* recv [x=[16:640],y=[16:360],q=1.0]
3.5.2. Interpreting an imageattr Attribute
[RFC6236] defines "a=imageattr" to be an advisory field. This means
that it does not absolutely constrain the video formats that the
sender can use, but gives an indication of the preferred values.
This specification prescribes more specific behavior. When a sender
of a given MediaStreamTrack, which is producing video of a certain
resolution, receives an "a=imageattr recv" attribute, it MUST first
check to see if the original resolution meets the criteria specified
in the attribute, and transmit it untouched if so. If the original
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resolution is too large for the attribute criteria, the sender SHOULD
apply downscaling to the output of the MediaStreamTrack in order to
satisfy the criteria.
If the receiver requires a minimum resolution which is greater than
the native resolution of the video, upscaling is needed, but this may
not be appropriate in all cases. To address this concern, the
application can set an upscaling policy for each sent track. For
this case, if upscaling is permitted by policy, the sender SHOULD
apply upscaling in order to provide the desired resolution.
Otherwise, the sender MUST NOT apply upscaling. The sender SHOULD
NOT upscale in other cases, even if the policy permits it.
If there is no appropriate and permitted scaling mechanism that
allows the received criteria to be satisfied, the sender MUST NOT
transmit the track.
In the special case of receiving a maximum resolution of [0, 0], as
described above, the sender MUST NOT transmit the track.
3.6. Interactions With 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
[RFC3261] 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 that is relevant. When forking happens 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, as well as 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:
o Start exchanging media with a given remote peer, but keep all the
resources reserved in the offer.
o Start exchanging media with a given remote peer, and free any
resources in the offer that are not being used.
3.6.1. 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.
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JSEP handles sequential forking well, allowing the application to
easily control the policy for selecting the desired remote endpoint.
When an answer arrives from one of the callees, the application can
choose to apply it either as a provisional answer, leaving open the
possibility of using a different answer in the future, or apply it as
a final answer, ending the setup flow.
In a "first-one-wins" situation, the first answer will be applied as
a final answer, and the application will reject any subsequent
answers. In SIP parlance, this would be ACK + BYE.
In a "last-one-wins" situation, all answers would be applied as
provisional answers, and any previous call leg will be terminated.
At some point, the application will end the setup process, perhaps
with a timer; at this point, the application could reapply the
existing remote description as a final answer.
3.6.2. Parallel Forking
Parallel forking involves a call being dispatched to multiple remote
callees, where each callee can accept the call, and multiple
simultaneous active signaling sessions can be established as a
result. If multiple callees send media at the same time, the
possibilities for handling this are described in Section 3.1 of
[RFC3960]. Most SIP devices today only support exchanging media with
a single device at a time, and do not try to mix multiple early media
audio sources, as that could result in a confusing situation. For
example, consider having a European ringback tone mixed together with
the North American ringback tone - the resulting sound would not be
like either tone, and would confuse the user. If the signaling
application wishes to only exchange media with one of the remote
endpoints at a time, then from a media engine point of view, this is
exactly like the sequential forking case.
In the parallel forking case where the Javascript application wishes
to simultaneously exchange media with multiple peers, the flow is
slightly more complex, but the Javascript application can follow the
strategy that [RFC3960] describes using UPDATE. The UPDATE approach
allows the signaling to set up a separate media flow for each peer
that it wishes to exchange media with. In JSEP, this offer used in
the UPDATE would be formed by simply creating a 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 [RFC3960].
As a result of sharing the media streams, the application will end up
with N parallel PeerConnection sessions, each with a local and remote
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description and their own local and remote addresses. The media flow
from these sessions can be managed by specifying SDP direction
attributes in the descriptions, or the application can choose to play
out the media from all sessions mixed together. Of course, if the
application wants to only keep a single session, it can simply
terminate the sessions that it no longer needs.
4. Interface
This section details the basic operations that must be present to
implement JSEP functionality. The actual API exposed in the W3C API
may have somewhat different syntax, but should map easily to these
concepts.
4.1. Methods
4.1.1. Constructor
The PeerConnection constructor allows the application to specify
global parameters for the media session, such as the STUN/TURN
servers and credentials to use when gathering candidates, as well as
the initial ICE candidate policy and pool size, and also the BUNDLE
policy to use.
If an ICE candidate policy is specified, it functions as described in
Section 3.4.3, causing the browser to only surface the permitted
candidates to the application, and only use those candidates for
connectivity checks. The set of available policies is as follows:
all: All candidates will be gathered and used.
public: Candidates with private IP addresses [RFC1918] will be
filtered out. This prevents exposure of internal network details,
at the cost of requiring relay usage even for intranet calls, if
the NAT does not allow hairpinning as described in [RFC4787],
section 6.
relay: All candidates except relay candidates will be filtered out.
This obfuscates the location information that might be ascertained
by the remote peer from the received candidates. Depending on how
the application deploys its relay servers, this could obfuscate
location to a metro or possibly even global level.
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Although it can be overridden by local policy, the default ICE
candidate policy MUST be set to allow all candidates, as this
minimizes use of application STUN/TURN server resources.
If a size is specified for the ICE candidate pool, this indicates the
number of ICE components to pre-gather candidates for. Because pre-
gathering results in utilizing STUN/TURN server resources for
potentially long periods of time, this must only occur upon
application request, and therefore the default candidate pool size
MUST be zero.
The application can specify its preferred policy regarding use of
BUNDLE, the multiplexing mechanism defined in
[I-D.ietf-mmusic-sdp-bundle-negotiation]. Regardless of policy, the
application will always try to negotiate BUNDLE onto a single
transport, and will offer a single BUNDLE group across all media
section; use of this single transport is contingent upon the answerer
accepting BUNDLE. However, by specifying a policy from the list
below, the application can control exactly how aggressively it will
try to BUNDLE media streams together, which affects how it will
interoperate with a non-BUNDLE-aware endpoint. When negotiating with
a non-BUNDLE-aware endpoint, only the streams not marked as bundle-
only streams will be established. The set of available policies is
as follows:
balanced: The first media section of each type (audio, video, or
application) will contain transport parameters, which will allow
an answerer to unbundle that section. The second and any
subsequent media section of each type will be marked bundle-only.
The result is that if there are N distinct media types, then
candidates will be gathered for for N media streams. This policy
balances desire to multiplex with the need to ensure basic audio
and video can still be negotiated in legacy cases.
max-compat: All media sections will contain transport parameters;
none will be marked as bundle-only. This policy will allow all
streams to be received by non-BUNDLE-aware endpoints, but require
separate candidates to be gathered for each media stream.
max-bundle: Only the first media section will contain transport
parameters; all streams other than the first will be marked as
bundle-only. This policy aims to minimize candidate gathering and
maximize multiplexing, at the cost of less compatibility with
legacy endpoints.
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As it provides the best tradeoff between performance and
compatibility with legacy endpoints, the default BUNDLE policy MUST
be set to "balanced".
The application can specify its preferred policy regarding use of
RTP/RTCP multiplexing [RFC5761] using one of the following policies:
negotiate: The browser will gather both RTP and RTCP candidates but
also will offer "a=rtcp-mux", thus allowing for compatibility with
either multiplexing or non-multiplexing endpoints.
require: The browser will only gather RTP candidates. This halves
the number of candidates that the offerer needs to gather. When
acting as answerer, the browser will reject any m= section that
does not provide an "a=rtcp-mux" attribute.
4.1.2. createOffer
The createOffer method generates a blob of SDP that contains a
[RFC3264] offer with the supported configurations for the session,
including descriptions of the local MediaStreams attached to this
PeerConnection, the codec/RTP/RTCP options supported by this
implementation, and any candidates that have been gathered by the ICE
Agent. An options parameter may be supplied to provide additional
control over the generated offer. This options parameter should
allow for the following manipulations to be performed:
o To indicate support for a media type even if no MediaStreamTracks
of that type have been added to the session (e.g., an audio call
that wants to receive video.)
o To trigger an ICE restart, for the purpose of reestablishing
connectivity.
In the initial offer, the generated SDP will contain all desired
functionality for the session (functionality that is supported but
not desired by default may be omitted); for each SDP line, the
generation of the SDP will follow the process defined for generating
an initial offer from the document that specifies the given SDP line.
The exact handling of initial offer generation is detailed in
Section 5.2.1 below.
In the event createOffer is called after the session is established,
createOffer will generate an offer to modify the current session
based on any changes that have been made to the session, e.g. adding
or removing MediaStreams, or requesting an ICE restart. For each
existing stream, the generation of each SDP line must follow the
process defined for generating an updated offer from the RFC that
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specifies the given SDP line. For each new stream, the generation of
the SDP must follow the process of generating an initial offer, as
mentioned above. If no changes have been made, or for SDP lines that
are unaffected by the requested changes, the offer will only contain
the parameters negotiated by the last offer-answer exchange. The
exact handling of subsequent offer generation is detailed in
Section 5.2.2. below.
Session descriptions generated by createOffer must be immediately
usable by setLocalDescription; if a system has limited resources
(e.g. a finite number of decoders), createOffer should return an
offer that reflects the current state of the system, so that
setLocalDescription will succeed when it attempts to acquire those
resources. Because this method may need to inspect the system state
to determine the currently available resources, it may be implemented
as an async operation.
Calling this method may do things such as generate new ICE
credentials, but does not result in candidate gathering, or cause
media to start or stop flowing.
4.1.3. createAnswer
The createAnswer method generates a blob of SDP that contains a
[RFC3264] SDP answer with the supported configuration for the session
that is compatible with the parameters supplied in the most recent
call to setRemoteDescription, which MUST have been called prior to
calling createAnswer. Like createOffer, the returned blob contains
descriptions of the local MediaStreams attached to this
PeerConnection, the codec/RTP/RTCP options negotiated for this
session, and any candidates that have been gathered by the ICE Agent.
An options parameter may be supplied to provide additional control
over the generated answer.
As an answer, the generated SDP will contain a specific configuration
that specifies how the media plane should be established; for each
SDP line, the generation of the SDP must follow the process defined
for generating an answer from the document that specifies the given
SDP line. The exact handling of answer generation is detailed in
Section 5.3. below.
Session descriptions generated by createAnswer must be immediately
usable by setLocalDescription; like createOffer, the returned
description should reflect the current state of the system. Because
this method may need to inspect the system state to determine the
currently available resources, it may need to be implemented as an
async operation.
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Calling this method may do things such as generate new ICE
credentials, but does not trigger candidate gathering or change media
state.
4.1.4. SessionDescriptionType
Session description objects (RTCSessionDescription) may be of type
"offer", "pranswer", "answer" or "rollback". These types provide
information as to how the description parameter should be parsed, and
how the media state should be changed.
"offer" indicates that a description should be parsed as an offer;
said description may include many possible media configurations. A
description used as an "offer" may be applied anytime the
PeerConnection is in a stable state, or as an update to a previously
supplied but unanswered "offer".
"pranswer" indicates that a description should be parsed as an
answer, but not a final answer, and so should not result in the
freeing of allocated resources. It may result in the start of media
transmission, if the answer does not specify an inactive media
direction. A description used as a "pranswer" may be applied as a
response to an "offer", or an update to a previously sent "pranswer".
"answer" indicates that a description should be parsed as an answer,
the offer-answer exchange should be considered complete, and any
resources (decoders, candidates) that are no longer needed can be
released. A description used as an "answer" may be applied as a
response to an "offer", or an update to a previously sent "pranswer".
The only difference between a provisional and final answer is that
the final answer results in the freeing of any unused resources that
were allocated as a result of the offer. As such, the application
can use some discretion on whether an answer should be applied as
provisional or final, and can change the type of the session
description as needed. For example, in a serial forking scenario, an
application may receive multiple "final" answers, one from each
remote endpoint. The application could choose to accept the initial
answers as provisional answers, and only apply an answer as final
when it receives one that meets its criteria (e.g. a live user
instead of voicemail).
"rollback" is a special session description type implying that the
state machine should be rolled back to the previous state, as
described in Section 4.1.4.2. The contents MUST be empty.
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4.1.4.1. Use of Provisional Answers
Most web applications will not need to create answers using the
"pranswer" type. While it is good practice to send an immediate
response to an "offer", in order to warm up the session transport and
prevent media clipping, the preferred handling for a web application
would be to create and send an "inactive" final answer immediately
after receiving the offer. Later, when the called user actually
accepts the call, the application can create a new "sendrecv" offer
to update the previous offer/answer pair and start the media flow.
While this could also be done with an inactive "pranswer", followed
by a sendrecv "answer", the initial "pranswer" leaves the offer-
answer exchange open, which means that neither side can send an
updated offer during this time.
As an example, 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 inactive or receive-only. It could then
ask the user to accept the call, 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 accepted the call and
triggered the new offer, it is likely that the ICE and DTLS
handshaking for all the channels will already have finished.
Of course, some applications may not be able to perform this double
offer-answer exchange, particularly ones that are attempting to
gateway to legacy signaling protocols. In these cases, "pranswer"
can still provide the application with a mechanism to warm up the
transport.
4.1.4.2. Rollback
In certain situations it may be desirable to "undo" a change made to
setLocalDescription or setRemoteDescription. Consider a case where a
call is ongoing, and one side wants to change some of the session
parameters; that side generates an updated offer and then calls
setLocalDescription. However, the remote side, either before or
after setRemoteDescription, decides it does not want to accept the
new parameters, and sends a reject message back to the offerer. Now,
the offerer, and possibly the answerer as well, need to return to a
stable state and the previous local/remote description. To support
this, we introduce the concept of "rollback".
A rollback discards any proposed changes to the session, returning
the state machine to the stable state, and setting the modified local
and/or remote description back to their previous values. Any
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resources or candidates that were allocated by the abandoned local
description are discarded; any media that is received will be
processed according to the previous local and remote descriptions.
Rollback can only be used to cancel proposed changes; there is no
support for rolling back from a stable state to a previous stable
state. Note that this implies that once the answerer has performed
setLocalDescription with his answer, this cannot be rolled back.
A rollback is performed by supplying a session description of type
"rollback" with empty contents to either setLocalDescription or
setRemoteDescription, depending on which was most recently used (i.e.
if the new offer was supplied to setLocalDescription, the rollback
should be done using setLocalDescription as well).
4.1.5. setLocalDescription
The setLocalDescription method instructs the PeerConnection to apply
the supplied session description as its local configuration. The
type field indicates whether the description should be processed as
an offer, provisional answer, or final answer; offers and answers are
checked differently, using the various rules that exist for each SDP
line.
This API changes the local media state; among other things, it sets
up local resources for receiving and decoding media. In order to
successfully handle scenarios where the application wants to offer to
change from one media format to a different, incompatible format, the
PeerConnection must be able to simultaneously support use of both the
old and new local descriptions (e.g. support codecs that exist in
both descriptions) until a final answer is received, at which point
the PeerConnection can fully adopt the new local description, or roll
back to the old description if the remote side denied the change.
This API indirectly controls the candidate gathering process. When a
local description is supplied, and the number of transports currently
in use does not match the number of transports needed by the local
description, the PeerConnection will create transports as needed and
begin gathering candidates for them.
If setRemoteDescription was previous called with an offer, and
setLocalDescription is called with an answer (provisional or final),
and the media directions are compatible, and media are available to
send, this will result in the starting of media transmission.
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4.1.6. setRemoteDescription
The setRemoteDescription method instructs the PeerConnection to apply
the supplied session description as the desired remote configuration.
As in setLocalDescription, the type field of the description
indicates how it should be processed.
This API changes the local media state; among other things, it sets
up local resources for sending and encoding media.
If setLocalDescription was previously called with an offer, and
setRemoteDescription is called with an answer (provisional or final),
and the media directions are compatible, and media are available to
send, this will result in the starting of media transmission.
4.1.7. localDescription
The localDescription method returns a copy of the current local
configuration, i.e. what was most recently passed to
setLocalDescription, plus any local candidates that have been
generated by the ICE Agent.
[[OPEN ISSUE: Do we need to expose accessors for both the current and
proposed local description? https://github.com/rtcweb-wg/jsep/
issues/16]]
A null object will be returned if the local description has not yet
been established.
4.1.8. remoteDescription
The remoteDescription method returns a copy of the current remote
configuration, i.e. what was most recently passed to
setRemoteDescription, plus any remote candidates that have been
supplied via processIceMessage.
[[OPEN ISSUE: Do we need to expose accessors for both the current and
proposed remote description? https://github.com/rtcweb-wg/jsep/
issues/16]]
A null object will be returned if the remote description has not yet
been established.
4.1.9. canTrickleIceCandidates
The canTrickleIceCandidates property indicates whether the remote
side supports receiving trickled candidates. There are three
potential values:
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null: No SDP has been received from the other side, so it is not
known if it can handle trickle. This is the initial value before
setRemoteDescription() is called.
true: SDP has been received from the other side indicating that it
can support trickle.
false: SDP has been received from the other side indicating that it
cannot support trickle.
As described in Section 3.4.2, JSEP implementations always provide
candidates to the application individually, consistent with what is
needed for Trickle ICE. However, applications can use the
canTrickleIceCandidates property to determine whether their peer can
actually do Trickle ICE, i.e., whether it is safe to send an initial
offer or answer followed later by candidates as they are gathered.
As "true" is the only value that definitively indicates remote
Trickle ICE support, an application which compares
canTrickleIceCandidates against "true" will by default attempt Half
Trickle on initial offers and Full Trickle on subsequent interactions
with a Trickle ICE-compatible agent.
4.1.10. setConfiguration
The setConfiguration method allows the global configuration of the
PeerConnection, which was initially set by constructor parameters, to
be changed during the session. The effects of this method call
depend on when it is invoked, and differ depending on which specific
parameters are changed:
o Any changes to the STUN/TURN servers to use affect the next
gathering phase. If gathering has already occurred, this will
cause the next call to createOffer to generate new ICE
credentials, for the purpose of forcing an ICE restart and kicking
off a new gathering phase, in which the new servers will be used.
If the ICE candidate pool has a nonzero size, any existing
candidates will be discarded, and new candidates will be gathered
from the new servers.
o Any changes to the ICE candidate policy also affect the next
gathering phase, in similar fashion to the server changes
described above. Note though that changes to the policy have no
effect on the candidate pool, because pooled candidates are not
surfaced to the application until a gathering phase occurs, and so
any necessary filtering can still be done on any pooled
candidates.
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o Any changes to the ICE candidate pool size take effect
immediately; if increased, additional candidates are pre-gathered;
if decreased, the now-superfluous candidates are discarded.
o The BUNDLE and RTCP-multiplexing policies MUST NOT be changed
after the construction of the PeerConnection.
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
being established.
4.1.11. addIceCandidate
The addIceCandidate method provides a remote candidate to the ICE
Agent, which, if parsed successfully, will be added to the remote
description according to the rules defined for Trickle ICE.
Connectivity checks will be sent to the new candidate.
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
being established.
5. SDP Interaction Procedures
This section describes the specific procedures to be followed when
creating and parsing SDP objects.
5.1. Requirements Overview
JSEP implementations must comply with the specifications listed below
that govern the creation and processing of offers and answers.
The first set of specifications is the "mandatory-to-implement" set.
All implementations must support these behaviors, but may not use all
of them if the remote side, which may not be a JSEP endpoint, does
not support them.
The second set of specifications is the "mandatory-to-use" set. The
local JSEP endpoint and any remote endpoint must indicate support for
these specifications in their session descriptions.
5.1.1. Implementation Requirements
This list of mandatory-to-implement specifications is derived from
the requirements outlined in [I-D.ietf-rtcweb-rtp-usage].
R-1 [RFC4566] is the base SDP specification and MUST be
implemented.
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R-2 [RFC5764] MUST be supported for signaling the UDP/TLS/RTP/SAVPF
[RFC5764] and TCP/DTLS/RTP/SAVPF
[I-D.nandakumar-mmusic-proto-iana-registration] RTP profiles.
R-3 [RFC5245] MUST be implemented for signaling the ICE credentials
and candidate lines corresponding to each media stream. The
ICE implementation MUST be a Full implementation, not a Lite
implementation.
R-4 [RFC5763] MUST be implemented to signal DTLS certificate
fingerprints.
R-5 [RFC4568] MUST NOT be implemented to signal SDES SRTP keying
information.
R-6 The [RFC5888] grouping framework MUST be implemented for
signaling grouping information, and MUST be used to identify m=
lines via the a=mid attribute.
R-7 [I-D.ietf-mmusic-msid] MUST be supported, in order to signal
associations between RTP objects and W3C MediaStreams and
MediaStreamTracks in a standard way.
R-8 The bundle mechanism in
[I-D.ietf-mmusic-sdp-bundle-negotiation] MUST be supported to
signal the ability to multiplex RTP streams on a single UDP
port, in order to avoid excessive use of port number resources.
R-9 The SDP attributes of "sendonly", "recvonly", "inactive", and
"sendrecv" from [RFC4566] MUST be implemented to signal
information about media direction.
R-10 [RFC5576] MUST be implemented to signal RTP SSRC values and
grouping semantics.
R-11 [RFC4585] MUST be implemented to signal RTCP based feedback.
R-12 [RFC5761] MUST be implemented to signal multiplexing of RTP and
RTCP.
R-13 [RFC5506] MUST be implemented to signal reduced-size RTCP
messages.
R-14 [RFC4588] MUST be implemented to signal RTX payload type
associations.
R-15 [RFC3556] with bandwidth modifiers MAY be supported for
specifying RTCP bandwidth as a fraction of the media bandwidth,
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RTCP fraction allocated to the senders and setting maximum
media bit-rate boundaries.
R-16 TODO: any others?
As required by [RFC4566], Section 5.13, JSEP implementations MUST
ignore unknown attribute (a=) lines.
5.1.2. Usage Requirements
All session descriptions handled by JSEP endpoints, both local and
remote, MUST indicate support for the following specifications. If
any of these are absent, this omission MUST be treated as an error.
R-1 ICE, as specified in [RFC5245], MUST be used. Note that the
remote endpoint may use a Lite implementation; implementations
MUST properly handle remote endpoints which do ICE-Lite.
R-2 DTLS [RFC6347] or DTLS-SRTP [RFC5763], MUST be used, as
appropriate for the media type, as specified in
[I-D.ietf-rtcweb-security-arch]
5.1.3. Profile Names and Interoperability
For media m= sections, JSEP endpoints MUST support both the "UDP/TLS/
RTP/SAVPF" and "TCP/DTLS/RTP/SAVPF" profiles and MUST indicate one of
these two profiles for each media m= line they produce in an offer.
For data m= sections, JSEP endpoints must support both the "UDP/DTLS/
SCTP" and "TCP/DTLS/SCTP" profiles and MUST indicate one of these two
profiles for each data m= line they produce in an offer. Because ICE
can select either TCP or UDP transport depending on network
conditions, both advertisements are consistent with ICE eventually
selecting either either UDP or TCP.
Unfortunately, in an attempt at compatibility, some endpoints
generate other profile strings even when they mean to support one of
these profiles. For instance, an endpoint might generate "RTP/AVP"
but supply "a=fingerprint" and "a=rtcp-fb" attributes, indicating its
willingness to support "(UDP,TCP)/TLS/RTP/SAVPF". In order to
simplify compatibility with such endpoints, JSEP endpoints MUST
follow the following rules when processing the media m= sections in
an offer:
o The profile in any "m=" line in any answer MUST exactly match the
profile provided in the offer.
o Any profile matching the following patterns MUST be accepted:
"RTP/[S]AVP[F]" and "(UDP/TCP)/TLS/RTP/SAVP[F]"
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o Because DTLS-SRTP is REQUIRED, the choice of SAVP or AVP has no
effect; support for DTLS-SRTP is determined by the presence of one
or more "a=fingerprint" attribute. Note that lack of an
"a=fingerprint" attribute will lead to negotiation failure.
o The use of AVPF or AVP simply controls the timing rules used for
RTCP feedback. If AVPF is provided, or an "a=rtcp-fb" attribute
is present, assume AVPF timing, i.e. a default value of "trr-
int=0". Otherwise, assume that AVPF is being used in an AVP
compatible mode and use AVP timing, i.e., "trr-int=4".
o For data m= sections, JSEP endpoints MUST support receiving the
"UDP/ DTLS/SCTP", "TCP/DTLS/SCTP", or "DTLS/SCTP" (for backwards
compatibility) profiles.
Note that re-offers by JSEP endpoints MUST use the correct profile
strings even if the initial offer/answer exchange used an (incorrect)
older profile string.
5.2. Constructing an Offer
When createOffer is called, a new SDP description must be created
that includes the functionality specified in
[I-D.ietf-rtcweb-rtp-usage]. The exact details of this process are
explained below.
5.2.1. Initial Offers
When createOffer is called for the first time, the result is known as
the initial offer.
The first step in generating an initial offer is to generate session-
level attributes, as specified in [RFC4566], Section 5.
Specifically:
o The first SDP line MUST be "v=0", as specified in [RFC4566],
Section 5.1
o The second SDP line MUST be an "o=" line, as specified in
[RFC4566], Section 5.2. The value of the <username> field SHOULD
be "-". The value of the <sess-id> field SHOULD be a
cryptographically random number. To ensure uniqueness, this
number SHOULD be at least 64 bits long. The value of the <sess-
version> field SHOULD be zero. The value of the <nettype>
<addrtype> <unicast-address> tuple SHOULD be set to a non-
meaningful address, such as IN IP4 0.0.0.0, to prevent leaking the
local address in this field. As mentioned in [RFC4566], the
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entire o= line needs to be unique, but selecting a random number
for <sess-id> is sufficient to accomplish this.
o The third SDP line MUST be a "s=" line, as specified in [RFC4566],
Section 5.3; to match the "o=" line, a single dash SHOULD be used
as the session name, e.g. "s=-". Note that this differs from the
advice in [RFC4566] which proposes a single space, but as both
"o=" and "s=" are meaningless, having the same meaningless value
seems clearer.
o Session Information ("i="), URI ("u="), Email Address ("e="),
Phone Number ("p="), Bandwidth ("b="), Repeat Times ("r="), and
Time Zones ("z=") lines are not useful in this context and SHOULD
NOT be included.
o Encryption Keys ("k=") lines do not provide sufficient security
and MUST NOT be included.
o A "t=" line MUST be added, as specified in [RFC4566], Section 5.9;
both <start-time> and <stop-time> SHOULD be set to zero, e.g. "t=0
0".
o An "a=msid-semantic:WMS" line MUST be added, as specified in
[I-D.ietf-mmusic-msid], Section 4.
The next step is to generate m= sections, as specified in [RFC4566]
Section 5.14, for each MediaStreamTrack that has been added to the
PeerConnection via the addStream method. (Note that this method
takes a MediaStream, which can contain multiple MediaStreamTracks,
and therefore multiple m= sections can be generated even if addStream
is only called once.) m=sections MUST be sorted first by the order in
which the MediaStreams were added to the PeerConnection, and then by
the alphabetical ordering of the media type for the MediaStreamTrack.
For example, if a MediaStream containing both an audio and a video
MediaStreamTrack is added to a PeerConnection, the resultant m=audio
section will precede the m=video section. If a second MediaStream
containing an audio MediaStreamTrack was added, it would follow the
m=video section.
Each m= section, provided it is not being bundled into another m=
section, MUST generate a unique set of ICE credentials and gather its
own unique set of ICE candidates. Otherwise, it MUST use the same
ICE credentials and candidates as the m= section into which it is
being bundled. Note that this means that for offers, any m= sections
which are not bundle-only MUST have unique ICE credentials and
candidates, since it is possible that the answerer will accept them
without bundling them.
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For DTLS, all m= sections MUST use the certificate for the identity
that has been specified for the PeerConnection; as a result, they
MUST all have the same [RFC4572] fingerprint value, or this value
MUST be a session-level attribute.
Each m= section should be generated as specified in [RFC4566],
Section 5.14. For the m= line itself, the following rules MUST be
followed:
o The port value is set to the port of the default ICE candidate for
this m= section, but given that no candidates have yet been
gathered, the "dummy" port value of 9 (Discard) MUST be used, as
indicated in [I-D.ietf-mmusic-trickle-ice], Section 5.1.
o To properly indicate use of DTLS, the <proto> field MUST be set to
"UDP/TLS/RTP/SAVPF", as specified in [RFC5764], Section 8, if the
default candidate uses UDP transport, or "TCP/DTLS/RTP/SAVPF", as
specified in[I-D.nandakumar-mmusic-proto-iana-registration] if the
default candidate uses TCP transport.
The m= line MUST be followed immediately by a "c=" line, as specified
in [RFC4566], Section 5.7. Again, as no candidates have yet been
gathered, the "c=" line must contain the "dummy" value "IN IP4
0.0.0.0", as defined in [I-D.ietf-mmusic-trickle-ice], Section 5.1.
[[NOTE: This has not yet changed in the trickle ICE draft.]]
Each m= section MUST include the following attribute lines:
o An "a=mid" line, as specified in [RFC5888], Section 4. When
generating mid values, it is RECOMMENDED that the values be 3
bytes or less, to allow them to efficiently fit into the RTP
header extension defined in
[I-D.ietf-mmusic-sdp-bundle-negotiation], Section 11.
o An "a=rtcp" line, as specified in [RFC3605], Section 2.1,
containing the dummy value "9 IN IP4 0.0.0.0", because no
candidates have yet been gathered.
o An "a=msid" line, as specified in [I-D.ietf-mmusic-msid],
Section 2.
o An "a=sendrecv" line, as specified in [RFC3264], Section 5.1.
o For each supported codec, "a=rtpmap" and "a=fmtp" lines, as
specified in [RFC4566], Section 6. The audio and video codecs
that MUST be supported are specified in [I-D.ietf-rtcweb-audio]
(see Section 3) and [I-D.ietf-rtcweb-video] (see Section 5).
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o If this m= section is for media with configurable frame sizes,
e.g. audio, an "a=maxptime" line, indicating the smallest of the
maximum supported frame sizes out of all codecs included above, as
specified in [RFC4566], Section 6.
o If this m= section is for video media, and there are known
limitations on the size of images which can be decoded, an
"a=imageattr" line, as specified in Section 3.5.
o For each primary codec where RTP retransmission should be used, a
corresponding "a=rtpmap" line indicating "rtx" with the clock rate
of the primary codec and an "a=fmtp" line that references the
payload type of the primary codec, as specified in [RFC4588],
Section 8.1.
o For each supported FEC mechanism, "a=rtpmap" and "a=fmtp" lines,
as specified in [RFC4566], Section 6. The FEC mechanisms that
MUST be supported are specified in [I-D.ietf-rtcweb-fec],
Section 6, and specific usage for each media type is outlined in
Sections 4 and 5.
o "a=ice-ufrag" and "a=ice-pwd" lines, as specified in [RFC5245],
Section 15.4.
o An "a=ice-options" line, with the "trickle" option, as specified
in [I-D.ietf-mmusic-trickle-ice], Section 4.
o An "a=fingerprint" line for each of the endpoint's certificates,
as specified in [RFC4572], Section 5; the digest algorithm used
for the fingerprint MUST match that used in the certificate
signature.
o An "a=setup" line, as specified in [RFC4145], Section 4, and
clarified for use in DTLS-SRTP scenarios in [RFC5763], Section 5.
The role value in the offer MUST be "actpass".
o An "a=rtcp-mux" line, as specified in [RFC5761], Section 5.1.1.
o An "a=rtcp-rsize" line, as specified in [RFC5506], Section 5.
o For each supported RTP header extension, an "a=extmap" line, as
specified in [RFC5285], Section 5. The list of header extensions
that SHOULD/MUST be supported is specified in
[I-D.ietf-rtcweb-rtp-usage], Section 5.2. Any header extensions
that require encryption MUST be specified as indicated in
[RFC6904], Section 4.
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o For each supported RTCP feedback mechanism, an "a=rtcp-fb"
mechanism, as specified in [RFC4585], Section 4.2. The list of
RTCP feedback mechanisms that SHOULD/MUST be supported is
specified in [I-D.ietf-rtcweb-rtp-usage], Section 5.1.
o An "a=ssrc" line, as specified in [RFC5576], Section 4.1,
indicating the SSRC to be used for sending media, along with the
mandatory "cname" source attribute, as specified in Section 6.1,
indicating the CNAME for the source. The CNAME MUST be generated
in accordance with Section 4.9 of [I-D.ietf-rtcweb-rtp-usage].
o If RTX is supported for this media type, another "a=ssrc" line
with the RTX SSRC, and an "a=ssrc-group" line, as specified in
[RFC5576], section 4.2, with semantics set to "FID" and including
the primary and RTX SSRCs.
o If FEC is supported for this media type, another "a=ssrc" line
with the FEC SSRC, and an "a=ssrc-group" line with semantics set
to "FEC-FR" and including the primary and FEC SSRCs, as specified
in [RFC5956], section 4.3. For simplicity, if both RTX and FEC
are supported, the FEC SSRC MUST be the same as the RTX SSRC.
o If the BUNDLE policy for this PeerConnection is set to "max-
bundle", and this is not the first m= section, or the BUNDLE
policy is set to "balanced", and this is not the first m= section
for this media type, an "a=bundle-only" line.
Lastly, if a data channel has been created, a m= section MUST be
generated for data. The <media> field MUST be set to "application"
and the <proto> field MUST be set to "UDP/DTLS/SCTP" if the default
candidate uses UDP transport, or "TCP/DTLS/SCTP" if the default
candidate uses TCP transport [I-D.ietf-mmusic-sctp-sdp]. The "fmt"
value MUST be set to "webrtc-datachannel" as specified in
[I-D.ietf-mmusic-sctp-sdp], Section 4.1.
Within the data m= section, the "a=mid", "a=ice-ufrag", "a=ice-
passwd", "a=ice-options", "a=candidate", "a=fingerprint", and
"a=setup" lines MUST be included as mentioned above, along with an
"a=fmtp:webrtc-datachannel" line and an "a=sctp-port" line
referencing the SCTP port number as defined in
[I-D.ietf-mmusic-sctp-sdp], Section 4.1.
Once all m= sections have been generated, a session-level "a=group"
attribute MUST be added as specified in [RFC5888]. This attribute
MUST have semantics "BUNDLE", and MUST include the mid identifiers of
each m= section. The effect of this is that the browser offers all
m= sections as one BUNDLE group. However, whether the m= sections
are bundle-only or not depends on the BUNDLE policy.
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The next step is to generate session-level lip sync groups as defined
in [RFC5888], Section 7. For each MediaStream with more than one
MediaStreamTrack, a group of type "LS" MUST be added that contains
the mid values for each MediaStreamTrack in that MediaStream.
Attributes which SDP permits to either be at the session level or the
media level SHOULD generally be at the media level even if they are
identical. This promotes readability, especially if one of a set of
initially identical attributes is subsequently changed.
Attributes other than the ones specified above MAY be included,
except for the following attributes which are specifically
incompatible with the requirements of [I-D.ietf-rtcweb-rtp-usage],
and MUST NOT be included:
o "a=crypto"
o "a=key-mgmt"
o "a=ice-lite"
Note that when BUNDLE is used, any additional attributes that are
added MUST follow the advice in [I-D.ietf-mmusic-sdp-mux-attributes]
on how those attributes interact with BUNDLE.
Note that these requirements are in some cases stricter than those of
SDP. Implementations MUST be prepared to accept compliant SDP even
if it would not conform to the requirements for generating SDP in
this specification.
5.2.2. Subsequent Offers
When createOffer is called a second (or later) time, or is called
after a local description has already been installed, the processing
is somewhat different than for an initial offer.
If the initial offer was not applied using setLocalDescription,
meaning the PeerConnection is still in the "stable" state, the steps
for generating an initial offer should be followed, subject to the
following restriction:
o The fields of the "o=" line MUST stay the same except for the
<session-version> field, which MUST increment if the session
description changes in any way, including the addition of ICE
candidates.
If the initial offer was applied using setLocalDescription, but an
answer from the remote side has not yet been applied, meaning the
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PeerConnection is still in the "local-offer" state, an offer is
generated by following the steps in the "stable" state above, along
with these exceptions:
o The "s=" and "t=" lines MUST stay the same.
o Each "m=" and c=" line MUST be filled in with the port, protocol,
and address of the default candidate for the m= section, as
described in [RFC5245], Section 4.3. If ICE checking has already
completed for one or more candidate pairs and a candidate pair is
in active use, then that pair MUST be used, even if ICE has not
yet completed. Note that this differs from the guidance in
[RFC5245], Section 9.1.2.2, which only refers to offers created
when ICE has completed. Each "a=rtcp" attribute line MUST also be
filled in with the port and address of the appropriate default
candidate, either the default RTP or RTCP candidate, depending on
whether RTCP multiplexing is currently active or not. Note that
if RTCP multiplexing is being offered, but not yet active, the
default RTCP candidate MUST be used, as indicated in [RFC5761],
section 5.1.3. In each case, if no candidates of the desired type
have yet been gathered, dummy values MUST be used, as described
above.
o Each "a=mid" line MUST stay the same.
o Each "a=ice-ufrag" and "a=ice-pwd" line MUST stay the same, unless
the ICE configuration has changed (either changes to the supported
STUN/TURN servers, or the ICE candidate policy), or the
"IceRestart" option (Section 5.2.3.3 was specified.
o Within each m= section, for each candidate that has been gathered
during the most recent gathering phase (see Section 3.4.1), an
"a=candidate" line MUST be added, as specified in [RFC5245],
Section 4.3., paragraph 3. If candidate gathering for the section
has completed, an "a=end-of-candidates" attribute MUST be added,
as described in [I-D.ietf-mmusic-trickle-ice], Section 9.3.
o For MediaStreamTracks that are still present, the "a=msid",
"a=ssrc", and "a=ssrc-group" lines MUST stay the same.
o If any MediaStreamTracks have been removed, either through the
removeStream method or by removing them from an added MediaStream,
their m= sections MUST be marked as recvonly by changing the value
of the [RFC3264] directional attribute to "a=recvonly". The
"a=msid", "a=ssrc", and "a=ssrc-group" lines MUST be removed from
the associated m= sections.
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o If any MediaStreamTracks have been added, and there exist m=
sections of the appropriate media type with no associated
MediaStreamTracks (i.e. as described in the preceding paragraph),
those m= sections MUST be recycled by adding the new
MediaStreamTrack to the m= section. This is done by adding the
necessary "a=msid", "a=ssrc", and "a=ssrc-group" lines to the
recycled m= section, and removing the "a=recvonly" attribute.
If the initial offer was applied using setLocalDescription, and an
answer from the remote side has been applied using
setRemoteDescription, meaning the PeerConnection is in the "remote-
pranswer" or "stable" states, an offer is generated based on the
negotiated session descriptions by following the steps mentioned for
the "local-offer" state above, along with these exceptions: [OPEN
ISSUE: should this be permitted in the remote-pranswer state?
https://github.com/rtcweb-wg/jsep/issues/143]
o If a m= section exists in the current local description, but does
not have an associated local MediaStreamTrack (possibly because
said MediaStreamTrack was removed since the last exchange), a m=
section MUST still be generated in the new offer, as indicated in
[RFC3264], Section 8. The disposition of this section will depend
on the state of the remote MediaStreamTrack associated with this
m= section. If one exists, and it is still in the "live" state,
the new m= section MUST be marked as "a=recvonly", with no
"a=msid" or related attributes present. If no remote
MediaStreamTrack exists, or it is in the "ended" state, the m=
section MUST be marked as rejected, by setting the port to zero,
as indicated in [RFC3264], Section 8.2.
o If any MediaStreamTracks have been added, and there exist recvonly
m= sections of the appropriate media type with no associated
MediaStreamTracks, or rejected m= sections of any media type,
those m= sections MUST be recycled, and a local MediaStreamTrack
associated with these recycled m= sections until all such existing
m= sections have been used. This includes any recvonly or
rejected m= sections created by the preceding paragraph.
In addition, for each non-recycled, non-rejected m= section in the
new offer, the following adjustments are made based on the contents
of the corresponding m= section in the current remote description:
o The m= line and corresponding "a=rtpmap" and "a=fmtp" lines MUST
only include codecs present in the remote description.
o The RTP header extensions MUST only include those that are present
in the remote description.
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o The RTCP feedback extensions MUST only include those that are
present in the remote description.
o The "a=rtcp-mux" line MUST only be added if present in the remote
description.
o The "a=rtcp-rsize" line MUST only be added if present in the
remote description.
The "a=group:BUNDLE" attribute MUST include the mid identifiers
specified in the BUNDLE group in the most recent answer, minus any m=
sections that have been marked as rejected, plus any newly added or
re-enabled m= sections. In other words, the BUNDLE attribute must
contain all m= sections that were previously bundled, as long as they
are still alive, as well as any new m= sections.
The "LS" groups are generated in the same way as with initial offers.
5.2.3. Options Handling
The createOffer method takes as a parameter an RTCOfferOptions
object. Special processing is performed when generating a SDP
description if the following options are present.
5.2.3.1. OfferToReceiveAudio
If the "OfferToReceiveAudio" option is specified, with an integer
value of N, and M audio MediaStreamTracks have been added to the
PeerConnection, the offer MUST include N non-rejected m= sections
with media type "audio", even if N is greater than M. This allows
the offerer to receive audio, including multiple independent streams,
even when not sending it; accordingly, the directional attribute on
the N-M audio m= sections without associated MediaStreamTracks MUST
be set to recvonly.
If N is set to a value less than M, the offer MUST mark the m=
sections associated with the M-N most recently added (since the last
setLocalDescription) MediaStreamTracks as sendonly. This allows the
offerer to indicate that it does not want to receive audio on some or
all of its newly created streams. For m= sections that have
previously been negotiated, this setting has no effect. [TODO: refer
to RTCRtpSender in the future]
For backwards compatibility with pre-standard versions of this
specification, a value of "true" is interpreted as equivalent to N=1,
and "false" as N=0.
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5.2.3.2. OfferToReceiveVideo
If the "OfferToReceiveVideo" option is specified, with an integer
value of N, and M video MediaStreamTracks have been added to the
PeerConnection, the offer MUST include N non-rejected m= sections
with media type "video", even if N is greater than M. This allows
the offerer to receive video, including multiple independent streams,
even when not sending it; accordingly, the directional attribute on
the N-M video m= sections without associated MediaStreamTracks MUST
be set to recvonly.
If N is set to a value less than M, the offer MUST mark the m=
sections associated with the M-N most recently added (since the last
setLocalDescription) MediaStreamTracks as sendonly. This allows the
offerer to indicate that it does not want to receive video on some or
all of its newly created streams. For m= sections that have
previously been negotiated, this setting has no effect. [TODO: refer
to RTCRtpSender in the future]
For backwards compatibility with pre-standard versions of this
specification, a value of "true" is interpreted as equivalent to N=1,
and "false" as N=0.
5.2.3.3. IceRestart
If the "IceRestart" option is specified, with a value of "true", the
offer MUST indicate an ICE restart by generating new ICE ufrag and
pwd attributes, as specified in [RFC5245], Section 9.1.1.1. If this
option is specified on an initial offer, it has no effect (since a
new ICE ufrag and pwd are already generated). Similarly, if the ICE
configuration has changed, this option has no effect, since new ufrag
and pwd attributes will be generated automatically. This option is
primarily useful for reestablishing connectivity in cases where
failures are detected by the application.
5.2.3.4. VoiceActivityDetection
If the "VoiceActivityDetection" option is specified, with a value of
"true", the offer MUST indicate support for silence suppression in
the audio it receives by including comfort noise ("CN") codecs for
each offered audio codec, as specified in [RFC3389], Section 5.1,
except for codecs that have their own internal silence suppression
support. For codecs that have their own internal silence suppression
support, the appropriate fmtp parameters for that codec MUST be
specified to indicate that silence suppression for received audio is
desired. For example, when using the Opus codec, the "usedtx=1"
parameter would be specified in the offer. This option allows the
endpoint to significantly reduce the amount of audio bandwidth it
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receives, at the cost of some fidelity, depending on the quality of
the remote VAD algorithm.
5.3. Generating an Answer
When createAnswer is called, a new SDP description must be created
that is compatible with the supplied remote description as well as
the requirements specified in [I-D.ietf-rtcweb-rtp-usage]. The exact
details of this process are explained below.
5.3.1. Initial Answers
When createAnswer is called for the first time after a remote
description has been provided, the result is known as the initial
answer. If no remote description has been installed, an answer
cannot be generated, and an error MUST be returned.
Note that the remote description SDP may not have been created by a
JSEP endpoint and may not conform to all the requirements listed in
Section 5.2. For many cases, this is not a problem. However, if any
mandatory SDP attributes are missing, or functionality listed as
mandatory-to-use above is not present, this MUST be treated as an
error, and MUST cause the affected m= sections to be marked as
rejected.
The first step in generating an initial answer is to generate
session-level attributes. The process here is identical to that
indicated in the Initial Offers section above.
The next step is to generate lip sync groups as defined in [RFC5888],
Section 7. For each MediaStream with more than one MediaStreamTrack,
a group of type "LS" MUST be added that contains the mid values for
each MediaStreamTrack in that MediaStream. In some cases this may
result in adding a mid to a given LS group that was not in that LS
group in the associated offer. Although this is not allowed by
[RFC5888], it is allowed when implementing this specification.
[[OPEN ISSUE: This is still under discussion. See:
https://github.com/rtcweb-wg/jsep/issues/162.]]
The next step is to generate m= sections for each m= section that is
present in the remote offer, as specified in [RFC3264], Section 6.
For the purposes of this discussion, any session-level attributes in
the offer that are also valid as media-level attributes SHALL be
considered to be present in each m= section.
The next step is to go through each offered m= section. If there is
a local MediaStreamTrack of the same type which has been added to the
PeerConnection via addStream and not yet associated with a m=
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section, and the specific m= section is either sendrecv or recvonly,
the MediaStreamTrack will be associated with the m= section at this
time. MediaStreamTracks are assigned to m= sections using the
canonical order described in Section 5.2.1. If there are more m=
sections of a certain type than MediaStreamTracks, some m= sections
will not have an associated MediaStreamTrack. If there are more
MediaStreamTracks of a certain type than compatible m= sections, only
the first N MediaStreamTracks will be able to be associated in the
constructed answer. The remainder will need to be associated in a
subsequent offer.
For each offered m= section, if the associated remote
MediaStreamTrack has been stopped, and is therefore in state "ended",
and no local MediaStreamTrack has been associated, the corresponding
m= section in the answer MUST be marked as rejected by setting the
port in the m= line to zero, as indicated in [RFC3264], Section 6.,
and further processing for this m= section can be skipped.
Provided that is not the case, each m= section in the answer should
then be generated as specified in [RFC3264], Section 6.1. For the m=
line itself, the following rules must be followed:
o The port value would normally be set to the port of the default
ICE candidate for this m= section, but given that no candidates
have yet been gathered, the "dummy" port value of 9 (Discard) MUST
be used, as indicated in [I-D.ietf-mmusic-trickle-ice],
Section 5.1.
o The <proto> field MUST be set to exactly match the <proto> field
for the corresponding m= line in the offer.
The m= line MUST be followed immediately by a "c=" line, as specified
in [RFC4566], Section 5.7. Again, as no candidates have yet been
gathered, the "c=" line must contain the "dummy" value "IN IP4
0.0.0.0", as defined in [I-D.ietf-mmusic-trickle-ice], Section 5.1.
If the offer supports BUNDLE, all m= sections to be BUNDLEd must use
the same ICE credentials and candidates; all m= sections not being
BUNDLEd must use unique ICE credentials and candidates. Each m=
section MUST include the following:
o If present in the offer, an "a=mid" line, as specified in
[RFC5888], Section 9.1. The "mid" value MUST match that specified
in the offer.
o An "a=rtcp" line, as specified in [RFC3605], Section 2.1,
containing the dummy value "9 IN IP4 0.0.0.0", because no
candidates have yet been gathered.
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o If a local MediaStreamTrack has been associated, an "a=msid" line,
as specified in [I-D.ietf-mmusic-msid], Section 2.
o Depending on the directionality of the offer, the disposition of
any associated remote MediaStreamTrack, and the presence of an
associated local MediaStreamTrack, the appropriate directionality
attribute, as specified in [RFC3264], Section 6.1. If the offer
was sendrecv, and the remote MediaStreamTrack is still "live", and
there is a local MediaStreamTrack that has been associated, the
directionality MUST be set as sendrecv. If the offer was
sendonly, and the remote MediaStreamTrack is still "live", the
directionality MUST be set as recvonly. If the offer was
recvonly, and a local MediaStreamTrack has been associated, the
directionality MUST be set as sendonly. If the offer was
inactive, the directionality MUST be set as inactive.
o For each supported codec that is present in the offer, "a=rtpmap"
and "a=fmtp" lines, as specified in [RFC4566], Section 6, and
[RFC3264], Section 6.1. The audio and video codecs that MUST be
supported are specified in [I-D.ietf-rtcweb-audio] (see Section 3)
and [I-D.ietf-rtcweb-video] (see Section 5). Note that for
simplicity, the answerer MAY use different payload types for
codecs than the offerer, as it is not prohibited by Section 6.1.
o If this m= section is for media with configurable frame sizes,
e.g. audio, an "a=maxptime" line, indicating the smallest of the
maximum supported frame sizes out of all codecs included above, as
specified in [RFC4566], Section 6.
o If this m= section is for video media, and there are known
limitations on the size of images which can be decoded, an
"a=imageattr" line, as specified in Section 3.5.
o If "rtx" is present in the offer, for each primary codec where RTP
retransmission should be used, a corresponding "a=rtpmap" line
indicating "rtx" with the clock rate of the primary codec and an
"a=fmtp" line that references the payload type of the primary
codec, as specified in [RFC4588], Section 8.1.
o For each supported FEC mechanism, "a=rtpmap" and "a=fmtp" lines,
as specified in [RFC4566], Section 6. The FEC mechanisms that
MUST be supported are specified in [I-D.ietf-rtcweb-fec],
Section 6, and specific usage for each media type is outlined in
Sections 4 and 5.
o "a=ice-ufrag" and "a=ice-passwd" lines, as specified in [RFC5245],
Section 15.4.
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o If the "trickle" ICE option is present in the offer, an "a=ice-
options" line, with the "trickle" option, as specified in
[I-D.ietf-mmusic-trickle-ice], Section 4.
o An "a=fingerprint" line for each of the endpoint's certificates,
as specified in [RFC4572], Section 5; the digest algorithm used
for the fingerprint MUST match that used in the certificate
signature.
o An "a=setup" line, as specified in [RFC4145], Section 4, and
clarified for use in DTLS-SRTP scenarios in [RFC5763], Section 5.
The role value in the answer MUST be "active" or "passive"; the
"active" role is RECOMMENDED.
o If present in the offer, an "a=rtcp-mux" line, as specified in
[RFC5761], Section 5.1.1. If the "require" RTCP multiplexing
policy is set and no "a=rtcp-mux" line is present in the offer,
then the m=line MUST be marked as rejected by setting the port in
the m= line to zero, as indicated in [RFC3264], Section 6.
o If present in the offer, an "a=rtcp-rsize" line, as specified in
[RFC5506], Section 5.
o For each supported RTP header extension that is present in the
offer, an "a=extmap" line, as specified in [RFC5285], Section 5.
The list of header extensions that SHOULD/MUST be supported is
specified in [I-D.ietf-rtcweb-rtp-usage], Section 5.2. Any header
extensions that require encryption MUST be specified as indicated
in [RFC6904], Section 4.
o For each supported RTCP feedback mechanism that is present in the
offer, an "a=rtcp-fb" mechanism, as specified in [RFC4585],
Section 4.2. The list of RTCP feedback mechanisms that SHOULD/
MUST be supported is specified in [I-D.ietf-rtcweb-rtp-usage],
Section 5.1.
o If a local MediaStreamTrack has been associated, an "a=ssrc" line,
as specified in [RFC5576], Section 4.1, indicating the SSRC to be
used for sending media, along with the mandatory "cname" source
attribute, as specified in Section 6.1, indicating the CNAME for
the source. The CNAME MUST be generated in accordance with
Section 4.9 of [I-D.ietf-rtcweb-rtp-usage].
o If a local MediaStreamTrack has been associated, and RTX has been
negotiated for this m= section, another "a=ssrc" line with the RTX
SSRC, and an "a=ssrc-group" line, as specified in [RFC5576],
section 4.2, with semantics set to "FID" and including the primary
and RTX SSRCs.
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o If a local MediaStreamTrack has been associated, and FEC has been
negotiated for this m= section, another "a=ssrc" line with the FEC
SSRC, and an "a=ssrc-group" line with semantics set to "FEC-FR"
and including the primary and FEC SSRCs, as specified in
[RFC5956], section 4.3. For simplicity, if both RTX and FEC are
supported, the FEC SSRC MUST be the same as the RTX SSRC.
If a data channel m= section has been offered, a m= section MUST also
be generated for data. The <media> field MUST be set to
"application" and the <proto> and "fmt" fields MUST be set to exactly
match the fields in the offer.
Within the data m= section, the "a=mid", "a=ice-ufrag", "a=ice-
passwd", "a=ice-options", "a=candidate", "a=fingerprint", and
"a=setup" lines MUST be included as mentioned above, along with an
"a=fmtp:webrtc-datachannel" line and an "a=sctp-port" line
referencing the SCTP port number as defined in
[I-D.ietf-mmusic-sctp-sdp], Section 4.1.
If "a=group" attributes with semantics of "BUNDLE" are offered,
corresponding session-level "a=group" attributes MUST be added as
specified in [RFC5888]. These attributes MUST have semantics
"BUNDLE", and MUST include the all mid identifiers from the offered
BUNDLE groups that have not been rejected. Note that regardless of
the presence of "a=bundle-only" in the offer, no m= sections in the
answer should have an "a=bundle-only" line.
Attributes that are common between all m= sections MAY be moved to
session-level, if explicitly defined to be valid at session-level.
The attributes prohibited in the creation of offers are also
prohibited in the creation of answers.
5.3.2. Subsequent Answers
When createAnswer is called a second (or later) time, or is called
after a local description has already been installed, the processing
is somewhat different than for an initial answer.
If the initial answer was not applied using setLocalDescription,
meaning the PeerConnection is still in the "have-remote-offer" state,
the steps for generating an initial answer should be followed,
subject to the following restriction:
o The fields of the "o=" line MUST stay the same except for the
<session-version> field, which MUST increment if the session
description changes in any way from the previously generated
answer.
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If any session description was previously supplied to
setLocalDescription, an answer is generated by following the steps in
the "have-remote-offer" state above, along with these exceptions:
o The "s=" and "t=" lines MUST stay the same.
o Each "m=" and c=" line MUST be filled in with the port and address
of the default candidate for the m= section, as described in
[RFC5245], Section 4.3. Note, however, that the m= line protocol
need not match the default candidate, because this protocol value
must instead match what was supplied in the offer, as described
above. Each "a=rtcp" attribute line MUST also be filled in with
the port and address of the appropriate default candidate, either
the default RTP or RTCP candidate, depending on whether RTCP
multiplexing is enabled in the answer. In each case, if no
candidates of the desired type have yet been gathered, dummy
values MUST be used, as described in the initial answer section
above.
o Each "a=ice-ufrag" and "a=ice-pwd" line MUST stay the same.
o Within each m= section, for each candidate that has been gathered
during the most recent gathering phase (see Section 3.4.1), an
"a=candidate" line MUST be added, as specified in [RFC5245],
Section 4.3., paragraph 3. If candidate gathering for the section
has completed, an "a=end-of-candidates" attribute MUST be added,
as described in [I-D.ietf-mmusic-trickle-ice], Section 9.3.
o For MediaStreamTracks that are still present, the "a=msid",
"a=ssrc", and "a=ssrc-group" lines MUST stay the same.
5.3.3. Options Handling
The createAnswer method takes as a parameter an RTCAnswerOptions
object. The set of parameters for RTCAnswerOptions is different than
those supported in RTCOfferOptions; the OfferToReceiveAudio,
OfferToReceiveVideo, and IceRestart options mentioned in
Section 5.2.3 are meaningless in the context of generating an answer,
as there is no need to generate extra m= lines in an answer, and ICE
credentials will automatically be changed for all m= lines where the
offerer chose to perform ICE restart.
The following options are supported in RTCAnswerOptions.
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5.3.3.1. VoiceActivityDetection
Silence suppression in the answer is handled as described in
Section 5.2.3.4.
5.4. Processing a Local Description
When a SessionDescription is supplied to setLocalDescription, the
following steps MUST be performed:
o First, the type of the SessionDescription is checked against the
current state of the PeerConnection:
* If the type is "offer", the PeerConnection state MUST be either
"stable" or "have-local-offer".
* If the type is "pranswer" or "answer", the PeerConnection state
MUST be either "have-remote-offer" or "have-local-pranswer".
o If the type is not correct for the current state, processing MUST
stop and an error MUST be returned.
o Next, the SessionDescription is parsed into a data structure, as
described in the Section 5.6 section below. If parsing fails for
any reason, processing MUST stop and an error MUST be returned.
o Finally, the parsed SessionDescription is applied as described in
the Section 5.7 section below.
5.5. Processing a Remote Description
When a SessionDescription is supplied to setRemoteDescription, the
following steps MUST be performed:
o First, the type of the SessionDescription is checked against the
current state of the PeerConnection:
* If the type is "offer", the PeerConnection state MUST be either
"stable" or "have-remote-offer".
* If the type is "pranswer" or "answer", the PeerConnection state
MUST be either "have-local-offer" or "have-remote-pranswer".
o If the type is not correct for the current state, processing MUST
stop and an error MUST be returned.
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o Next, the SessionDescription is parsed into a data structure, as
described in the Section 5.6 section below. If parsing fails for
any reason, processing MUST stop and an error MUST be returned.
o Finally, the parsed SessionDescription is applied as described in
the Section 5.8 section below.
5.6. Parsing a Session Description
[The behavior described herein is a draft version, and needs more
discussion to resolve various open issues.]
When a SessionDescription of any type is supplied to setLocal/
RemoteDescription, the implementation must parse it and reject it if
it is invalid. The exact details of this process are explained
below.
The SDP contained in the session description object consists of a
sequence of text lines, each containing a key-value expression, as
described in [RFC4566], Section 5. The SDP is read, line-by-line,
and converted to a data structure that contains the deserialized
information. However, SDP allows many types of lines, not all of
which are relevant to JSEP applications. For each line, the
implementation will first ensure it is syntactically correct
according its defining ABNF [TODO: reference], check that it conforms
to [RFC4566] and [RFC3264] semantics, and then either parse and store
or discard the provided value, as described below. [TODO: ensure
that every line is listed below.] If the line is not well-formed, or
cannot be parsed as described, the parser MUST stop with an error and
reject the session description. This ensures that implementations do
not accidentally misinterpret ambiguous SDP.
5.6.1. Session-Level Parsing
First, the session-level lines are checked and parsed. These lines
MUST occur in a specific order, and with a specific syntax, as
defined in [RFC4566], Section 5. Note that while the specific line
types (e.g. "v=", "c=") MUST occur in the defined order, lines of the
same type (typically "a=") can occur in any order, and their ordering
is not meaningful.
For non-attribute (non-"a=") lines, their sequencing, syntax, and
semantics, are checked, as mentioned above. The following lines are
not meaningful in the JSEP context and MAY be discarded once they
have been checked.
The "c=" line MUST be checked for syntax but its value is not
used. This supersedes the guidance in [RFC5245], Section 6.1, to
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use "ice-mismatch" to indicate mismatches between "c=" and the
candidate lines; because JSEP always uses ICE, "ice-mismatch" is
not useful in this context.
TODO
The remaining lines are processed as follows:
The "b=" line, if present, MUST be parsed as specified in
[RFC4566], Section 5.8, and the bwtype and bandwidth values
stored.
[OPEN ISSUE: is this WG consensus? Are there other non-a= lines
that we need to do more than just syntactical validation, e.g.
v=?]
Specific processing MUST be applied for the following session-level
attribute ("a=") lines:
o Any "a=group" lines are parsed as specified in [RFC5888],
Section 5, and the group's semantics and mids are stored.
o If present, a single "a=ice-lite" line is parsed as specified in
[RFC5245], Section 15.3, and a value indicating the presence of
ice-lite is stored.
o If present, a single "a=ice-ufrag" line is parsed as specified in
[RFC5245], Section 15.4, and the ufrag value is stored.
o If present, a single "a=ice-pwd" line is parsed as specified in
[RFC5245], Section 15.4, and the password value is stored.
o If present, a single "a=ice-options" line is parsed as specified
in [RFC5245], Section 15.5, and the set of specified options is
stored.
o Any "a=fingerprint" lines are parsed as specified in [RFC4572],
Section 5, and the set of fingerprint and algorithm values is
stored.
o If present, a single "a=setup" line is parsed as specified in
[RFC4145], Section 4, and the setup value is stored.
o Any "a=extmap" lines are parsed as specified in [RFC5285],
Section 5, and their values are stored.
o TODO: msid-semantic, identity, rtcp-rsize, rtcp-mux, and any other
attribs valid at session level.
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Once all the session-level lines have been parsed, processing
continues with the lines in media sections.
5.6.2. Media Section Parsing
Like the session-level lines, the media session lines MUST occur in
the specific order and with the specific syntax defined in [RFC4566],
Section 5.
The "m=" line itself MUST be parsed as described in [RFC4566],
Section 5.14, and the media, port, proto, and fmt values stored.
Following the "m=" line, specific processing MUST be applied for the
following non-attribute lines:
o As with the "c=" line at the session level, the "c=" line MUST be
parsed according to [RFC4566], Section 5.7, but its value is not
used.
o The "b=" line, if present, MUST be parsed as specified in
[RFC4566], Section 5.8, and the bwtype and bandwidth values
stored.
Specific processing MUST also be applied for the following attribute
lines:
o If present, a single "a=ice-ufrag" line is parsed as specified in
[RFC5245], Section 15.4, and the ufrag value is stored.
o If present, a single "a=ice-pwd" line is parsed as specified in
[RFC5245], Section 15.4, and the password value is stored.
o If present, a single "a=ice-options" line is parsed as specified
in [RFC5245], Section 15.5, and the set of specified options is
stored.
o Any "a=fingerprint" lines are parsed as specified in [RFC4572],
Section 5, and the set of fingerprint and algorithm values is
stored.
o If present, a single "a=setup" line is parsed as specified in
[RFC4145], Section 4, and the setup value is stored.
If the "m=" proto value indicates use of RTP, as decribed in the
Section 5.1.3 section above, the following attribute lines MUST be
processed:
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o The "m=" fmt value MUST be parsed as specified in [RFC4566],
Section 5.14, and the individual values stored.
o Any "a=rtpmap" or "a=fmtp" lines MUST be parsed as specified in
[RFC4566], Section 6, and their values stored.
o If present, a single "a=ptime" line MUST be parsed as described in
[RFC4566], Section 6, and its value stored.
o If present, a single direction attribute line (e.g. "a=sendrecv")
MUST be parsed as described in [RFC4566], Section 6, and its value
stored.
o Any "a=ssrc" or "a=ssrc-group" attributes MUST be parsed as
specified in [RFC5576], Sections 4.1-4.2, and their values stored.
o Any "a=extmap" attributes MUST be parsed as specified in
[RFC5285], Section 5, and their values stored.
o Any "a=rtcp-fb" attributes MUST be parsed as specified in
[RFC4585], Section 4.2., and their values stored.
o If present, a single "a=rtcp-mux" line MUST be parsed as specified
in [RFC5761], Section 5.1.1, and its presence or absence flagged
and stored.
o TODO: a=rtcp-rsize, a=rtcp, a=msid, a=candidate, a=end-of-
candidates
Otherwise, if the "m=" proto value indicats use of SCTP, the
following attribute lines MUST be processed:
o The "m=" fmt value MUST be parsed as specified in
[I-D.ietf-mmusic-sctp-sdp], Section 4.3, and the application
protocol value stored.
o An "a=sctp-port" attribute MUST be present, and it MUST be parsed
as specified in [I-D.ietf-mmusic-sctp-sdp], Section 5.2, and the
value stored.
o TODO: max message size
5.6.3. Semantics Verification
Assuming parsing completes successfully, the parsed description is
then evaluated to ensure internal consistency as well as proper
support for mandatory features. Specifically, the following checks
are performed:
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o For each m= section, valid values for each of the mandatory-to-use
features enumerated in Section 5.1.2 MUST be present. These
values MAY either be present at the media level, or inherited from
the session level.
* ICE ufrag and password values, which MUST comply with the size
limits specified in [RFC5245], Section 15.4.
* DTLS setup value, which MUST be set according to the rules
specified in [RFC5763], Section 5, and MUST be consistent with
the selected role of the current DTLS connection, if one
exists.[TODO: may need revision, i.e., use of actpass
* DTLS fingerprint values, where at least one fingerprint MUST be
present.
o Each m= section is also checked to ensure prohibited features are
not used. If this is a local description, the "ice-lite"
attribute MUST NOT be specified.
If this session description is of type "pranswer" or "answer", the
following additional checks are applied:
o The session description must follow the rules defined in
[RFC3264], Section 6, including the requirement that the number of
m= sections MUST exactly match the number of m= sections in the
associated offer.
o For each m= section, the media type and protocol values MUST
exactly match the media type and protocol values in the
corresponding m= section in the associated offer.
5.7. Applying a Local Description
The following steps are performed at the media engine level to apply
a local description.
First, the parsed parameters are checked to ensure that any
modifications performed fall within those explicitly permitted by
Section 6; otherwise, processing MUST stop and an error MUST be
returned.
Next, media sections are processed. For each media section, the
following steps MUST be performed; if any parameters are out of
bounds, or cannot be applied, processing MUST stop and an error MUST
be returned.
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o If this media section is new, begin gathering candidates for it,
as defined in [RFC5245], Section 4.1.1, unless it has been marked
as bundle-only.
o Or, if the ICE ufrag and password values have changed, trigger the
ICE Agent to start an ICE restart and begin gathering new
candidates for the media section, as defined in [RFC5245],
Section 9.1.1.1, unless it has been marked as bundle-only.
o If the media section proto value indicates use of RTP:
* If RTCP mux is indicated, prepare to demux RTP and RTCP from
the RTP ICE component, as specified in [RFC5761],
Section 5.1.1. If RTCP mux is not indicated, but was indicated
in a previous description, this MUST result in an error.
* For each specified RTP header extension, establish a mapping
between the extension ID and URI. If any indicated RTP header
extension is unknown, this MUST result in an error. [TODO:
ref]
* If the MID header extension is supported, prepare to demux RTP
data intended for this media section based on the MID header
extension. [TODO: ref]
* For each specified payload type, establish a mapping between
the payload type ID and the actual media format. [TODO: ref]
If any indicated payload type is unknown, this MUST result in
an error.
* For each specified "rtx" media format, establish a mapping
between the RTX payload type and its associated primary payload
type. [TODO: ref] If any referenced primary payload types are
not present, this MUST result in an error.
* If the directional attribute is of type "sendrecv" or
"recvonly", enable receipt and decoding of media.
Finally, if this description is of type "pranswer" or "answer",
follow the processing defined in the Section 5.9 section below.
5.8. Applying a Remote Description
The following steps are performed at the media engine level to apply
a remote description.
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For each media section, the following steps MUST be performed; if any
parameters are out of bounds, or cannot be applied, processing MUST
stop and an error MUST be returned.
o If the description is of type "offer", and the ICE ufrag or
password changed from the previous remote description, [TODO:
ref], mark that an ICE restart is needed.
o Configure the ICE components associated with this media section to
use the supplied ICE remote ufrag and password for their
connectivity checks.
o Pair any supplied ICE candidates with any gathered local
candidates, as described in [TODO: ref] and start connectivity
checks with the appropriate credentials.
o If the media section proto value indicates use of RTP:
* [TODO: header extensions]
* For each specified payload type that is also supported by the
local implementation, establish a mapping between the payload
type ID and the actual media format. [TODO: ref] If any
indicated payload type is unknown, it MUST be ignored. [TODO:
should fail on answers]
* For each specified "rtx" media format, establish a mapping
between the RTX payload type and its associated primary payload
type. [TODO: ref] If any referenced primary payload types are
not present, this MUST result in an error.
* For each specified fmtp parameter that is supported by the
local implementation, enable them on the associated payload
types.
* For each specified RTCP feedback mechanism that is supported by
the local implementation, enable them on the associated payload
types.
* For any specified "TIAS" bandwidth value, set this value as the
maximum RTP bitrate to be used when sending media. If a "TIAS"
value is not present, but an "AS" value is, generate a TIAS a
value using this formula: [TODO: convert AS to TIAS].
* [TODO: handling of CN, telephone-event, "red"]
* If the media section if of type audio:
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+ For any specified "ptime" value, configure the available
payload types to use the specified packet size. If the
specified size is not supported for a payload type, use the
next closest value instead.
Finally, if this description is of type "pranswer" or "answer",
follow the processing defined in the Section 5.9 section below.
5.9. Applying an Answer
In addition to the steps mentioned above for processing a local or
remote description, the following steps are performed when processing
a description of type "pranswer" or "answer".
For each media section, the following steps MUST be performed:
o If the media section has been rejected (i.e. port is set to zero
in the answer), stop any reception or transmission of media for
this section, and discard any associated ICE components. [TODO:
ref]
o If the remote DTLS fingerprint has been changed, tear down the
existing DTLS connection.
o If no valid DTLS connection exists, prepare to start a DTLS
connection, using the specified roles and fingerprints, on any
underlying ICE components, once they are active.
o If the media section proto value indicates use of RTP:
* If the media section has RTCP mux enabled, discard any RTCP
component, and begin or continue muxing RTCP over the RTP
component, as specified in [RFC5761], Section 5.1.3.
Otherwise, transmit RTCP over the RTCP component; if no RTCP
component exists, because RTCP mux was previously enabled, this
MUST result in an error.
* If the media section has reduced-size RTCP enabled, configure
the RTCP transmission for this media section to use reduced-
size RTCP, as specified in [TODO: ref]
* If the directional attribute in the answer is of type
"sendrecv" or "sendonly", prepare to start transmitting media
using the specified primary SSRC and one of the selected
payload types, once the underlying transport layers have been
established. Otherwise, stop transmitting RTP media, although
RTCP should still be sent. [TODO: ref]
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o If the media section proto value indicates use of SCTP:
* If no SCTP association yet exists, prepare to initiate a SCTP
association over the associated ICE component and DTLS
connection, using the local SCTP port value from the local
description, and the remote SCTP port value from the remote
description. [TODO: ref]
If the answer contains valid BUNDLE groups, discard any ICE
components for the m= sections that will be bundled onto the primary
ICE components in each BUNDLE, and begin muxing these m= sections
accordingly. [TODO: ref]
If the answer contains any "a=ice-options" attributes where "trickle"
is listed as an attribute, update the PeerConnection canTrickle
property to be true. Otherwise, set this property to false.
6. Configurable SDP Parameters
It is possible to change elements in the SDP returned from
createOffer before passing it to setLocalDescription. When an
implementation receives modified SDP it MUST either:
o Accept the changes and adjust its behavior to match the SDP.
o Reject the changes and return an error via the error callback.
Changes MUST NOT be silently ignored.
The following elements of the session description MUST NOT be changed
between the createOffer and the setLocalDescription (or between the
createAnswer and the setLocalDescription), since they reflect
transport attributes that are solely under browser control, and the
browser MUST NOT honor an attempt to change them:
o The number, type and port number of m= lines.
o The generated ICE credentials (a=ice-ufrag and a=ice-pwd).
o The set of ICE candidates and their parameters (a=candidate).
o The DTLS fingerprint(s) (a=fingerprint).
o The contents of BUNDLE groups, bundle-only parameters, or "a=rtcp-
mux" parameters.
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The following modifications, if done by the browser to a description
between createOffer/createAnswer and the setLocalDescription, MUST be
honored by the browser:
o Remove or reorder codecs (m=)
The following parameters may be controlled by options passed into
createOffer/createAnswer. As an open issue, these changes may also
be be performed by manipulating the SDP returned from createOffer/
createAnswer, as indicated above, as long as the capabilities of the
endpoint are not exceeded (e.g. asking for a resolution greater than
what the endpoint can encode):
o [[OPEN ISSUE: This is a placeholder for other modifications, which
we may continue adding as use cases appear.]]
Implementations MAY choose to either honor or reject any elements not
listed in the above two categories, but must do so explicitly as
described at the beginning of this section. Note that future
standards may add new SDP elements to the list of elements which must
be accepted or rejected, but due to version skew, applications must
be prepared for implementations to accept changes which must be
rejected and vice versa.
The application can also modify the SDP to reduce the capabilities in
the offer it sends to the far side or the offer that it installs from
the far side in any way the application sees fit, as long as it is a
valid SDP offer and specifies a subset of what was in the original
offer. This is safe because the answer is not permitted to expand
capabilities and therefore will just respond to what is actually in
the offer.
As always, the application is solely responsible for what it sends to
the other party, and all incoming SDP will be processed by the
browser to the extent of its capabilities. It is an error to assume
that all SDP is well-formed; however, one should be able to assume
that any implementation of this specification will be able to
process, as a remote offer or answer, unmodified SDP coming from any
other implementation of this specification.
7. Examples
Note that this example section shows several SDP fragments. To
format in 72 columns, some of the lines in SDP have been split into
multiple lines, where leading whitespace indicates that a line is a
continuation of the previous line. In addition, some blank lines
have been added to improve readability but are not valid in SDP.
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More examples of SDP for WebRTC call flows can be found in
[I-D.nandakumar-rtcweb-sdp].
7.1. Simple Example
This section shows a very simple example that sets up a minimal audio
/ video call between two browsers and does not use trickle ICE. The
example in the following section provides a more realistic example of
what would happen in a normal browser to browser connection.
The flow shows Alice's browser initiating the session to Bob's
browser. The messages from Alice's JS to Bob's JS are assumed to
flow over some signaling protocol via a web server. The JS on both
Alice's side and Bob's side waits for all candidates before sending
the offer or answer, so the offers and answers are complete. Trickle
ICE is not used. Both Alice and Bob are using the default policy of
balanced.
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// set up local media state
AliceJS->AliceUA: create new PeerConnection
AliceJS->AliceUA: addStream with stream containing audio and video
AliceJS->AliceUA: createOffer to get offer
AliceJS->AliceUA: setLocalDescription with offer
AliceUA->AliceJS: multiple onicecandidate events with candidates
// wait for ICE gathering to complete
AliceUA->AliceJS: onicecandidate event with null candidate
AliceJS->AliceUA: get |offer-A1| from value of localDescription
// |offer-A1| is sent over signaling protocol to Bob
AliceJS->WebServer: signaling with |offer-A1|
WebServer->BobJS: signaling with |offer-A1|
// |offer-A1| arrives at Bob
BobJS->BobUA: create a PeerConnection
BobJS->BobUA: setRemoteDescription with |offer-A1|
BobUA->BobJS: onaddstream event with remoteStream
// Bob accepts call
BobJS->BobUA: addStream with local media
BobJS->BobUA: createAnswer
BobJS->BobUA: setLocalDescription with answer
BobUA->BobJS: multiple onicecandidate events with candidates
// wait for ICE gathering to complete
BobUA->BobJS: onicecandidate event with null candidate
BobJS->BobUA: get |answer-A1| from value of localDescription
// |answer-A1| is sent over signaling protocol to Alice
BobJS->WebServer: signaling with |answer-A1|
WebServer->AliceJS: signaling with |answer-A1|
// |answer-A1| arrives at Alice
AliceJS->AliceUA: setRemoteDescription with |answer-A1|
AliceUA->AliceJS: onaddstream event with remoteStream
// media flows
BobUA->AliceUA: media sent from Bob to Alice
AliceUA->BobUA: media sent from Alice to Bob
The SDP for |offer-A1| looks like:
v=0
o=- 4962303333179871722 1 IN IP4 0.0.0.0
s=-
t=0 0
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a=msid-semantic:WMS
a=group:BUNDLE a1 v1
m=audio 56500 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 192.0.2.1
a=mid:a1
a=rtcp:56501 IN IP4 192.0.2.1
a=msid:47017fee-b6c1-4162-929c-a25110252400
f83006c5-a0ff-4e0a-9ed9-d3e6747be7d9
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=maxptime:120
a=ice-ufrag:ETEn1v9DoTMB9J4r
a=ice-pwd:OtSK0WpNtpUjkY4+86js7ZQl
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=rtcp-mux
a=rtcp-rsize
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=ssrc:1732846380 cname:EocUG1f0fcg/yvY7
a=candidate:3348148302 1 udp 2113937151 192.0.2.1 56500
typ host
a=candidate:3348148302 2 udp 2113937151 192.0.2.1 56501
typ host
a=end-of-candidates
m=video 56502 UDP/TLS/RTP/SAVPF 100 101
c=IN IP4 192.0.2.1
a=rtcp:56503 IN IP4 192.0.2.1
a=mid:v1
a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae
f30bdb4a-5db8-49b5-bcdc-e0c9a23172e0
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=ice-ufrag:BGKkWnG5GmiUpdIV
a=ice-pwd:mqyWsAjvtKwTGnvhPztQ9mIf
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
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:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=rtcp-mux
a=rtcp-rsize
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:mid
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=ssrc:1366781083 cname:EocUG1f0fcg/yvY7
a=ssrc:1366781084 cname:EocUG1f0fcg/yvY7
a=ssrc-group:FID 1366781083 1366781084
a=candidate:3348148302 1 udp 2113937151 192.0.2.1 56502
typ host
a=candidate:3348148302 2 udp 2113937151 192.0.2.1 56503
typ host
a=end-of-candidates
The SDP for |answer-A1| looks like:
v=0
o=- 6729291447651054566 1 IN IP4 0.0.0.0
s=-
t=0 0
a=msid-semantic:WMS
m=audio 20000 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 192.0.2.2
a=mid:a1
a=rtcp:20000 IN IP4 192.0.2.2
a=msid:PI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1
PI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1a0
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=maxptime:120
a=ice-ufrag:6sFvz2gdLkEwjZEr
a=ice-pwd:cOTZKZNVlO9RSGsEGM63JXT2
a=fingerprint:sha-256 6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35
:DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:active
a=rtcp-mux
a=rtcp-rsize
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=ssrc:3429951804 cname:Q/NWs1ao1HmN4Xa5
a=candidate:2299743422 1 udp 2113937151 192.0.2.2 20000
typ host
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a=end-of-candidates
m=video 20001 UDP/TLS/RTP/SAVPF 100 101
c=IN IP4 192.0.2.2
a=rtcp 20001 IN IP4 192.0.2.2
a=mid:v1
a=msid:PI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1
PI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1v0
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=ice-ufrag:6sFvz2gdLkEwjZEr
a=ice-pwd:cOTZKZNVlO9RSGsEGM63JXT2
a=fingerprint:sha-256 6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35
:DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:active
a=rtcp-mux
a=rtcp-rsize
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=ssrc:3229706345 cname:Q/NWs1ao1HmN4Xa5
a=ssrc:3229706346 cname:Q/NWs1ao1HmN4Xa5
a=ssrc-group:FID 3229706345 3229706346
a=candidate:2299743422 1 udp 2113937151 192.0.2.2 20001
typ host
a=end-of-candidates
7.2. Normal Examples
This section shows a typical example of a session between two
browsers setting up an audio channel and a data channel. Trickle ICE
is used in full trickle mode with a bundle policy of max-bundle, an
RTCP mux policy of require, and a single TURN server. Later, two
video flows, one for the presenter and one for screen sharing, are
added to the session. This example shows Alice's browser initiating
the session to Bob's browser. The messages from Alice's JS to Bob's
JS are assumed to flow over some signaling protocol via a web server.
// set up local media state
AliceJS->AliceUA: create new PeerConnection
AliceJS->AliceUA: addStream that contains audio track
AliceJS->AliceUA: createDataChannel to get data channel
AliceJS->AliceUA: createOffer to get |offer-B1|
AliceJS->AliceUA: setLocalDescription with |offer-B1|
// |offer-B1| is sent over signaling protocol to Bob
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AliceJS->WebServer: signaling with |offer-B1|
WebServer->BobJS: signaling with |offer-B1|
// |offer-B1| arrives at Bob
BobJS->BobUA: create a PeerConnection
BobJS->BobUA: setRemoteDescription with |offer-B1|
BobUA->BobJS: onaddstream with audio track from Alice
// candidates are sent to Bob
AliceUA->AliceJS: onicecandidate event with |candidate-B1| (host)
AliceJS->WebServer: signaling with |candidate-B1|
AliceUA->AliceJS: onicecandidate event with |candidate-B2| (srflx)
AliceJS->WebServer: signaling with |candidate-B2|
AliceUA->AliceJS: onicecandidate event with |candidate-B3| (relay)
AliceJS->WebServer: signaling with |candidate-B3|
WebServer->BobJS: signaling with |candidate-B1|
BobJS->BobUA: addIceCandidate with |candidate-B1|
WebServer->BobJS: signaling with |candidate-B2|
BobJS->BobUA: addIceCandidate with |candidate-B2|
WebServer->BobJS: signaling with |candidate-B3|
BobJS->BobUA: addIceCandidate with |candidate-B3|
// Bob accepts call
BobJS->BobUA: addStream with local audio stream
BobJS->BobUA: createDataChannel to get data channel
BobJS->BobUA: createAnswer to get |answer-B1|
BobJS->BobUA: setLocalDescription with |answer-B1|
// |answer-B1| is sent to Alice
BobJS->WebServer: signaling with |answer-B1|
WebServer->AliceJS: signaling with |answer-B1|
AliceJS->AliceUA: setRemoteDescription with |answer-B1|
AliceUA->AliceJS: onaddstream event with audio track from Bob
// candidates are sent to Alice
BobUA->BobJS: onicecandidate event with |candidate-B4| (host)
BobJS->WebServer: signaling with |candidate-B4|
BobUA->BobJS: onicecandidate event with |candidate-B5| (srflx)
BobJS->WebServer: signaling with |candidate-B5|
BobUA->BobJS: onicecandidate event with |candidate-B6| (relay)
BobJS->WebServer: signaling with |candidate-B6|
WebServer->AliceJS: signaling with |candidate-B4|
AliceJS->AliceUA: addIceCandidate with |candidate-B4|
WebServer->AliceJS: signaling with |candidate-B5|
AliceJS->AliceUA: addIceCandidate with |candidate-B5|
WebServer->AliceJS: signaling with |candidate-B6|
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AliceJS->AliceUA: addIceCandidate with |candidate-B6|
// data channel opens
BobUA->BobJS: ondatachannel event
AliceUA->AliceJS: ondatachannel event
BobUA->BobJS: onopen
AliceUA->AliceJS: onopen
// media is flowing between browsers
BobUA->AliceUA: audio+data sent from Bob to Alice
AliceUA->BobUA: audio+data sent from Alice to Bob
// some time later Bob adds two video streams
// note, no candidates exchanged, because of BUNDLE
BobJS->BobUA: addStream with first video stream
BobJS->BobUA: addStream with second video stream
BobJS->BobUA: createOffer to get |offer-B2|
BobJS->BobUA: setLocalDescription with |offer-B2|
// |offer-B2| is sent to Alice
BobJS->WebServer: signaling with |offer-B2|
WebServer->AliceJS: signaling with |offer-B2|
AliceJS->AliceUA: setRemoteDescription with |offer-B2|
AliceUA->AliceJS: onaddstream event with first video stream
AliceUA->AliceJS: onaddstream event with second video stream
AliceJS->AliceUA: createAnswer to get |answer-B2|
AliceJS->AliceUA: setLocalDescription with |answer-B2|
// |answer-B2| is sent over signaling protocol to Bob
AliceJS->WebServer: signaling with |answer-B2|
WebServer->BobJS: signaling with |answer-B2|
BobJS->BobUA: setRemoteDescription with |answer-B2|
// media is flowing between browsers
BobUA->AliceUA: audio+video+data sent from Bob to Alice
AliceUA->BobUA: audio+video+data sent from Alice to Bob
The SDP for |offer-B1| looks like:
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v=0
o=- 4962303333179871723 1 IN IP4 0.0.0.0
s=-
t=0 0
a=msid-semantic:WMS
a=group:BUNDLE a1 d1
m=audio 9 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
a=mid:a1
a=msid:57017fee-b6c1-4162-929c-a25110252400
e83006c5-a0ff-4e0a-9ed9-d3e6747be7d9
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=maxptime:120
a=ice-ufrag:ATEn1v9DoTMB9J4r
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=rtcp-mux
a=rtcp-rsize
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=ssrc:1732846380 cname:FocUG1f0fcg/yvY7
m=application 9 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=mid:d1
a=fmtp:webrtc-datachannel max-message-size=65536
a=sctp-port 5000
a=ice-ufrag:ATEn1v9DoTMB9J4r
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=ice-options:trickle
a=fingerprint:sha-256 19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
The SDP for |candidate-B1| looks like:
candidate:109270923 1 udp 2122194687 192.168.1.2 51556 typ host
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The SDP for |candidate-B2| looks like:
candidate:4036177503 1 udp 1685987071 11.22.33.44 52546 typ srflx
raddr 192.168.1.2 rport 51556
The SDP for |candidate-B3| looks like:
candidate:3671762466 1 udp 41819903 22.33.44.55 61405 typ relay
raddr 11.22.33.44 rport 52546
The SDP for |answer-B1| looks like:
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v=0
o=- 7729291447651054566 1 IN IP4 0.0.0.0
s=-
t=0 0
a=msid-semantic:WMS
a=group:BUNDLE a1 d1
m=audio 9 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
a=mid:a1
a=msid:QI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1
QI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1a0
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=maxptime:120
a=ice-ufrag:7sFvz2gdLkEwjZEr
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=ice-options:trickle
a=fingerprint:sha-256 6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35
:DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:active
a=rtcp-mux
a=rtcp-rsize
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=ssrc:4429951804 cname:Q/NWs1ao1HmN4Xa5
m=application 9 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 0.0.0.0
a=mid:d1
a=fmtp:webrtc-datachannel max-message-size=65536
a=sctp-port 5000
a=ice-ufrag:7sFvz2gdLkEwjZEr
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=ice-options:trickle
a=fingerprint:sha-256 6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35
:DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:active
The SDP for |candidate-B4| looks like:
candidate:109270924 1 udp 2122194687 192.168.2.3 61665 typ host
The SDP for |candidate-B5| looks like:
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candidate:4036177504 1 udp 1685987071 55.66.77.88 64532 typ srflx
raddr 192.168.2.3 rport 61665
The SDP for |candidate-B6| looks like:
candidate:3671762467 1 udp 41819903 66.77.88.99 50416 typ relay
raddr 55.66.77.88 rport 64532
The SDP for |offer-B2| looks like: (note the increment of the version
number in the o= line, and the c= and a=rtcp lines, which indicate
the local candidate that was selected)
v=0
o=- 7729291447651054566 2 IN IP4 0.0.0.0
s=-
t=0 0
a=msid-semantic:WMS
a=group:BUNDLE a1 d1 v1 v2
m=audio 64532 UDP/TLS/RTP/SAVPF 96 0 8 97 98
c=IN IP4 55.66.77.88
a=rtcp:64532 IN IP4 55.66.77.88
a=mid:a1
a=msid:QI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1
QI39StLS8W7ZbQl1sJsWUXkr3Zf12fJUvzQ1a0
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=maxptime:120
a=ice-ufrag:7sFvz2gdLkEwjZEr
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=ice-options:trickle
a=fingerprint:sha-256 6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35
:DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:actpass
a=rtcp-mux
a=rtcp-rsize
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=ssrc:4429951804 cname:Q/NWs1ao1HmN4Xa5
a=candidate:109270924 1 udp 2122194687 192.168.2.3 61665 typ host
a=candidate:4036177504 1 udp 1685987071 55.66.77.88 64532 typ srflx
raddr 192.168.2.3 rport 61665
a=candidate:3671762467 1 udp 41819903 66.77.88.99 50416 typ relay
raddr 55.66.77.88 rport 64532
a=end-of-candidates
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m=application 64532 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 55.66.77.88
a=mid:d1
a=fmtp:webrtc-datachannel max-message-size=65536
a=sctp-port 5000
a=ice-ufrag:7sFvz2gdLkEwjZEr
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=ice-options:trickle
a=fingerprint:sha-256 6B:8B:F0:65:5F:78:E2:51:3B:AC:6F:F3:3F:46:1B:35
:DC:B8:5F:64:1A:24:C2:43:F0:A1:58:D0:A1:2C:19:08
a=setup:actpass
a=candidate:109270924 1 udp 2122194687 192.168.2.3 61665 typ host
a=candidate:4036177504 1 udp 1685987071 55.66.77.88 64532 typ srflx
raddr 192.168.2.3 rport 61665
a=candidate:3671762467 1 udp 41819903 66.77.88.99 50416 typ relay
raddr 55.66.77.88 rport 64532
a=end-of-candidates
m=video 64532 UDP/TLS/RTP/SAVPF 100 101
c=IN IP4 55.66.77.88
a=rtcp:64532 IN IP4 55.66.77.88
a=mid:v1
a=msid:61317484-2ed4-49d7-9eb7-1414322a7aae
f30bdb4a-5db8-49b5-bcdc-e0c9a23172e0
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=ice-ufrag:7sFvz2gdLkEwjZEr
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=rtcp-mux
a=rtcp-rsize
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=ssrc:1366781083 cname:Q/NWs1ao1HmN4Xa5
a=ssrc:1366781084 cname:Q/NWs1ao1HmN4Xa5
a=ssrc-group:FID 1366781083 1366781084
a=candidate:109270924 1 udp 2122194687 192.168.2.3 61665 typ host
a=candidate:4036177504 1 udp 1685987071 55.66.77.88 64532 typ srflx
raddr 192.168.2.3 rport 61665
a=candidate:3671762467 1 udp 41819903 66.77.88.99 50416 typ relay
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raddr 55.66.77.88 rport 64532
a=end-of-candidates
m=video 64532 UDP/TLS/RTP/SAVPF 100 101
c=IN IP4 55.66.77.88
a=rtcp:64532 IN IP4 55.66.77.88
a=mid:v1
a=msid:71317484-2ed4-49d7-9eb7-1414322a7aae
f30bdb4a-5db8-49b5-bcdc-e0c9a23172e0
a=sendrecv
a=rtpmap:100 VP8/90000
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=ice-ufrag:7sFvz2gdLkEwjZEr
a=ice-pwd:dOTZKZNVlO9RSGsEGM63JXT2
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:actpass
a=rtcp-mux
a=rtcp-rsize
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=ssrc:2366781083 cname:Q/NWs1ao1HmN4Xa5
a=ssrc:2366781084 cname:Q/NWs1ao1HmN4Xa5
a=ssrc-group:FID 2366781083 2366781084
a=candidate:109270924 1 udp 2122194687 192.168.2.3 61665 typ host
a=candidate:4036177504 1 udp 1685987071 55.66.77.88 64532 typ srflx
raddr 192.168.2.3 rport 61665
a=candidate:3671762467 1 udp 41819903 66.77.88.99 50416 typ relay
raddr 55.66.77.88 rport 64532
a=end-of-candidates
The SDP for |answer-B2| looks like: (note the use of setup:passive to
maintain the existing DTLS roles, and the use of a=recvonly to
indicate that the video streams are one-way)
v=0
o=- 4962303333179871723 2 IN IP4 0.0.0.0
s=-
t=0 0
a=msid-semantic:WMS
a=group:BUNDLE a1 d1 v1 v2
m=audio 52546 UDP/TLS/RTP/SAVPF 96 0 8 97 98
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c=IN IP4 11.22.33.44
a=rtcp:52546 IN IP4 11.22.33.44
a=mid:a1
a=msid:57017fee-b6c1-4162-929c-a25110252400
e83006c5-a0ff-4e0a-9ed9-d3e6747be7d9
a=sendrecv
a=rtpmap:96 opus/48000/2
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:97 telephone-event/8000
a=rtpmap:98 telephone-event/48000
a=maxptime:120
a=ice-ufrag:ATEn1v9DoTMB9J4r
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:passive
a=rtcp-mux
a=rtcp-rsize
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=ssrc:1732846380 cname:FocUG1f0fcg/yvY7
a=candidate:109270923 1 udp 2122194687 192.168.1.2 51556 typ host
a=candidate:4036177503 1 udp 1685987071 11.22.33.44 52546 typ srflx
raddr 192.168.1.2 rport 51556
a=candidate:3671762466 1 udp 41819903 22.33.44.55 61405 typ relay
raddr 11.22.33.44 rport 52546
a=end-of-candidates
m=application 52546 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 11.22.33.44
a=mid:d1
a=fmtp:webrtc-datachannel max-message-size=65536
a=sctp-port 5000
a=ice-ufrag:ATEn1v9DoTMB9J4r
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=ice-options:trickle
a=fingerprint:sha-256 19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:passive
a=candidate:109270923 1 udp 2122194687 192.168.1.2 51556 typ host
a=candidate:4036177503 1 udp 1685987071 11.22.33.44 52546 typ srflx
raddr 192.168.1.2 rport 51556
a=candidate:3671762466 1 udp 41819903 22.33.44.55 61405 typ relay
raddr 11.22.33.44 rport 52546
a=end-of-candidates
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m=video 52546 UDP/TLS/RTP/SAVPF 100 101
c=IN IP4 11.22.33.44
a=rtcp:52546 IN IP4 11.22.33.44
a=mid:v1
a=recvonly
a=rtpmap:100 VP8/90000
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=ice-ufrag:ATEn1v9DoTMB9J4r
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:passive
a=rtcp-mux
a=rtcp-rsize
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=candidate:109270923 1 udp 2122194687 192.168.1.2 51556 typ host
a=candidate:4036177503 1 udp 1685987071 11.22.33.44 52546 typ srflx
raddr 192.168.1.2 rport 51556
a=candidate:3671762466 1 udp 41819903 22.33.44.55 61405 typ relay
raddr 11.22.33.44 rport 52546
a=end-of-candidates
m=video 52546 UDP/TLS/RTP/SAVPF 100 101
c=IN IP4 11.22.33.44
a=rtcp:52546 IN IP4 11.22.33.44
a=mid:v2
a=recvonly
a=rtpmap:100 VP8/90000
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=ice-ufrag:ATEn1v9DoTMB9J4r
a=ice-pwd:AtSK0WpNtpUjkY4+86js7ZQl
a=ice-options:trickle
a=fingerprint:sha-256
19:E2:1C:3B:4B:9F:81:E6:B8:5C:F4:A5:A8:D8:73:04
:BB:05:2F:70:9F:04:A9:0E:05:E9:26:33:E8:70:88:A2
a=setup:passive
a=rtcp-mux
a=rtcp-rsize
a=extmap:2 urn:ietf:params:rtp-hdrext:sdes:mid
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
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a=rtcp-fb:100 nack pli
a=candidate:109270923 1 udp 2122194687 192.168.1.2 51556 typ host
a=candidate:4036177503 1 udp 1685987071 11.22.33.44 52546 typ srflx
raddr 192.168.1.2 rport 51556
a=candidate:3671762466 1 udp 41819903 22.33.44.55 61405 typ relay
raddr 11.22.33.44 rport 52546
a=end-of-candidates
8. Security Considerations
The IETF has published separate documents
[I-D.ietf-rtcweb-security-arch] [I-D.ietf-rtcweb-security] describing
the security architecture for WebRTC as a whole. The remainder of
this section describes security considerations for this document.
While formally the JSEP interface is an API, it is better to think of
it is an Internet protocol, with the JS being untrustworthy from the
perspective of the browser. Thus, the threat model of [RFC3552]
applies. In particular, JS can call the API in any order and with
any inputs, including malicious ones. This is particularly relevant
when we consider the SDP which is passed to setLocalDescription().
While correct API usage requires that the application pass in SDP
which was derived from createOffer() or createAnswer() (perhaps
suitably modified as described in Section 6, there is no guarantee
that applications do so. The browser MUST be prepared for the JS to
pass in bogus data instead.
Conversely, the application programmer MUST recognize that the JS
does not have complete control of browser behavior. One case that
bears particular mention is that editing ICE candidates out of the
SDP or suppressing trickled candidates does not have the expected
behavior: implementations will still perform checks from those
candidates even if they are not sent to the other side. Thus, for
instance, it is not possible to prevent the remote peer from learning
your public IP address by removing server reflexive candidates.
Applications which wish to conceal their public IP address should
instead configure the ICE agent to use only relay candidates.
9. IANA Considerations
This document requires no actions from IANA.
10. Acknowledgements
Significant text incorporated in the draft as well and review was
provided by Harald Alvestrand and Suhas Nandakumar. Dan Burnett,
Neil Stratford, Eric Rescorla, Anant Narayanan, Andrew Hutton,
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Richard Ejzak, Adam Bergkvist and Matthew Kaufman all provided
valuable feedback on this proposal.
11. References
11.1. Normative References
[I-D.ietf-mmusic-msid]
Alvestrand, H., "Cross Session Stream Identification in
the Session Description Protocol", draft-ietf-mmusic-
msid-01 (work in progress), August 2013.
[I-D.ietf-mmusic-sctp-sdp]
Loreto, S. and G. Camarillo, "Stream Control Transmission
Protocol (SCTP)-Based Media Transport in the Session
Description Protocol (SDP)", draft-ietf-mmusic-sctp-sdp-04
(work in progress), June 2013.
[I-D.ietf-mmusic-sdp-bundle-negotiation]
Holmberg, C., Alvestrand, H., and C. Jennings,
"Multiplexing Negotiation Using Session Description
Protocol (SDP) Port Numbers", draft-ietf-mmusic-sdp-
bundle-negotiation-04 (work in progress), June 2013.
[I-D.ietf-mmusic-sdp-mux-attributes]
Nandakumar, S., "A Framework for SDP Attributes when
Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-01
(work in progress), February 2014.
[I-D.ietf-mmusic-trickle-ice]
Ivov, E., Rescorla, E., and J. Uberti, "Trickle ICE:
Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol", draft-ietf-
mmusic-trickle-ice-00 (work in progress), March 2013.
[I-D.ietf-rtcweb-audio]
Valin, J. and C. Bran, "WebRTC Audio Codec and Processing
Requirements", draft-ietf-rtcweb-audio-02 (work in
progress), August 2013.
[I-D.ietf-rtcweb-data-protocol]
Jesup, R., Loreto, S., and M. Tuexen, "WebRTC Data Channel
Protocol", draft-ietf-rtcweb-data-protocol-04 (work in
progress), February 2013.
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[I-D.ietf-rtcweb-fec]
Uberti, J., "WebRTC Forward Error Correction
Requirements", draft-ietf-rtcweb-fec-00 (work in
progress), February 2015.
[I-D.ietf-rtcweb-rtp-usage]
Perkins, C., Westerlund, M., and J. Ott, "Web Real-Time
Communication (WebRTC): Media Transport and Use of RTP",
draft-ietf-rtcweb-rtp-usage-09 (work in progress),
September 2013.
[I-D.ietf-rtcweb-security]
Rescorla, E., "Security Considerations for WebRTC", draft-
ietf-rtcweb-security-06 (work in progress), January 2014.
[I-D.ietf-rtcweb-security-arch]
Rescorla, E., "WebRTC Security Architecture", draft-ietf-
rtcweb-security-arch-09 (work in progress), February 2014.
[I-D.ietf-rtcweb-video]
Roach, A., "WebRTC Video Processing and Codec
Requirements", draft-ietf-rtcweb-video-00 (work in
progress), July 2014.
[I-D.nandakumar-mmusic-proto-iana-registration]
Nandakumar, S., "IANA registration of SDP 'proto'
attribute for transporting RTP Media over TCP under
various RTP profiles.", September 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552, July
2003.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605, October
2003.
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[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
September 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[RFC4585] Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey,
"Extended RTP Profile for Real-time Transport Control
Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July
2006.
[RFC5124] Ott, J. and E. Carrara, "Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback
(RTP/SAVPF)", RFC 5124, February 2008.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5285] Singer, D. and H. Desineni, "A General Mechanism for RTP
Header Extensions", RFC 5285, July 2008.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[RFC5888] Camarillo, G. and H. Schulzrinne, "The Session Description
Protocol (SDP) Grouping Framework", RFC 5888, June 2010.
[RFC6236] Johansson, I. and K. Jung, "Negotiation of Generic Image
Attributes in the Session Description Protocol (SDP)", RFC
6236, May 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6904] Lennox, J., "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904, April
2013.
[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, September 2013.
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11.2. Informative References
[I-D.nandakumar-rtcweb-sdp]
Nandakumar, S. and C. Jennings, "SDP for the WebRTC",
draft-nandakumar-rtcweb-sdp-02 (work in progress), July
2013.
[RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC
3556, July 2003.
[RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
Tone Generation in the Session Initiation Protocol (SIP)",
RFC 3960, December 2004.
[RFC4568] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
July 2006.
[RFC5506] Johansson, I. and M. Westerlund, "Support for Reduced-Size
Real-Time Transport Control Protocol (RTCP): Opportunities
and Consequences", RFC 5506, April 2009.
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.
[RFC5763] Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
for Establishing a Secure Real-time Transport Protocol
(SRTP) Security Context Using Datagram Transport Layer
Security (DTLS)", RFC 5763, May 2010.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in
the Session Description Protocol", RFC 5956, September
2010.
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[W3C.WD-webrtc-20140617]
Bergkvist, A., Burnett, D., Narayanan, A., and C.
Jennings, "WebRTC 1.0: Real-time Communication Between
Browsers", World Wide Web Consortium WD WD-webrtc-
20140617, June 2014,
<http://www.w3.org/TR/2011/WD-webrtc-20140617>.
Appendix A. Change log
Note: This section will be removed by RFC Editor before publication.
Changes in draft-12:
o Filled in sections on applying local and remote descriptions.
o Discussed downscaling and upscaling to fulfill imageattr
requirements.
o Updated what SDP can be modified by the application.
o Updated to latest datachannel SDP.
o Allowed multiple fingerprint lines.
o Switched back to IPv4 for dummy candidates.
o Added additional clarity on ICE default candidates.
Changes in draft-11:
o Clarified handling of RTP CNAMEs.
o Updated what SDP lines should be processed or ignored.
o Specified how a=imageattr should be used.
Changes in draft-10:
o TODO
Changes in draft-09:
o Don't return null for {local,remote}Description after close().
o Changed TCP/TLS to UDP/DTLS in RTP profile names.
o Separate out bundle and mux policy.
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o Added specific references to FEC mechanisms.
o Added canTrickle mechanism.
o Added section on subsequent answers and, answer options.
o Added text defining set{Local,Remote}Description behavior.
Changes in draft-08:
o Added new example section and removed old examples in appendix.
o Fixed <proto> field handling.
o Added text describing a=rtcp attribute.
o Reworked handling of OfferToReceiveAudio and OfferToReceiveVideo
per discussion at IETF 90.
o Reworked trickle ICE handling and its impact on m= and c= lines
per discussion at interim.
o Added max-bundle-and-rtcp-mux policy.
o Added description of maxptime handling.
o Updated ICE candidate pool default to 0.
o Resolved open issues around AppID/receiver-ID.
o Reworked and expanded how changes to the ICE configuration are
handled.
o Some reference updates.
o Editorial clarification.
Changes in draft-07:
o Expanded discussion of VAD and Opus DTX.
o Added a security considerations section.
o Rewrote the section on modifying SDP to require implementations to
clearly indicate whether any given modification is allowed.
o Clarified impact of IceRestart on CreateOffer in local-offer
state.
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o Guidance on whether attributes should be defined at the media
level or the session level.
o Renamed "default" bundle policy to "balanced".
o Removed default ICE candidate pool size and clarify how it works.
o Defined a canonical order for assignment of MSTs to m= lines.
o Removed discussion of rehydration.
o Added Eric Rescorla as a draft editor.
o Cleaned up references.
o Editorial cleanup
Changes in draft-06:
o Reworked handling of m= line recycling.
o Added handling of BUNDLE and bundle-only.
o Clarified handling of rollback.
o Added text describing the ICE Candidate Pool and its behavior.
o Allowed OfferToReceiveX to create multiple recvonly m= sections.
Changes in draft-05:
o Fixed several issues identified in the createOffer/Answer sections
during document review.
o Updated references.
Changes in draft-04:
o Filled in sections on createOffer and createAnswer.
o Added SDP examples.
o Fixed references.
Changes in draft-03:
o Added text describing relationship to W3C specification
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Changes in draft-02:
o Converted from nroff
o Removed comparisons to old approaches abandoned by the working
group
o Removed stuff that has moved to W3C specification
o Align SDP handling with W3C draft
o Clarified section on forking.
Changes in draft-01:
o Added diagrams for architecture and state machine.
o Added sections on forking and rehydration.
o Clarified meaning of "pranswer" and "answer".
o Reworked how ICE restarts and media directions are controlled.
o Added list of parameters that can be changed in a description.
o Updated suggested API and examples to match latest thinking.
o Suggested API and examples have been moved to an appendix.
Changes in draft -00:
o Migrated from draft-uberti-rtcweb-jsep-02.
Authors' Addresses
Justin Uberti
Google
747 6th Ave S
Kirkland, WA 98033
USA
Email: justin@uberti.name
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Cullen Jennings
Cisco
170 West Tasman Drive
San Jose, CA 95134
USA
Email: fluffy@iii.ca
Eric Rescorla (editor)
Mozilla
331 Evelyn Ave
Mountain View, CA 94041
USA
Email: ekr@rtfm.com
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