Network Working Group H. Alvestrand
Internet-Draft Google
Intended status: Standards Track June 5, 2011
Expires: December 7, 2011
Overview: Real Time Protocols for Brower-based Applications
draft-alvestrand-rtcweb-overview-00
Abstract
This document gives an overview and context of a protocol suite
intended for use with real-time applications that can be deployed in
browsers - "real time communication on the Web".
It intends to serve as a starting and coordination point to make sure
all the parts that are needed to achieve this goal are findable, and
that the parts that belong in the Internet protocol suite are fully
specified and on the right publication track.
This work is an attempt to synthesize the input of many people, but
makes no claims to fully represent the views of any of them. All
parts of the document should be regarded as open for discussion,
unless the RTCWEB chairs have declared consensus on an item.
This document is a work item of the RTCWEB working group.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on December 7, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Principles and Terminology . . . . . . . . . . . . . . . . . . 5
2.1. Goals of this overview . . . . . . . . . . . . . . . . . . 5
2.2. Relationship between API and protocol . . . . . . . . . . 5
2.3. On interoperability and innovation . . . . . . . . . . . . 6
2.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
3. Functionality groups . . . . . . . . . . . . . . . . . . . . . 7
4. Data transport . . . . . . . . . . . . . . . . . . . . . . . . 8
5. Data framing and securing . . . . . . . . . . . . . . . . . . 9
6. Data formats . . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Connection management . . . . . . . . . . . . . . . . . . . . 10
8. Presentation and control . . . . . . . . . . . . . . . . . . . 11
9. Local system support functions . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Security Considerations . . . . . . . . . . . . . . . . . . . 12
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
13.1. Normative References . . . . . . . . . . . . . . . . . . . 13
13.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
The Internet was, from very early in its lifetime, considered a
possible veichle for the deployment of real-time, interactive
applications - with the most easily imaginable being audio
conversations (aka "Internet telephony") and videoconferencing.
The first attempts to build this were dependent on special networks,
special hardware and custom-built software, often at very high prices
or at low quality, placing great demands on the infrastructure.
As the available bandwidth has increased, and as processors and other
hardware has become ever faster, the barriers to participation have
decreased, and it is possible to deliver a satisfactory experience on
commonly available computing hardware.
Still, there are a number of barriers to the ability to communicate
universally - one of these is that there are, as of yet, no single
set of communication protocols that all agree should be made
available for communication; another is the sheer lack of universal
identification systems (such as is served by telephone numbers or
email addresses in other communications systems).
Development of The Universal Solution has proved hard, however, for
all the usual reasons. This memo aims to take a more building-block-
oriented approach, and try to find consensus on a set of substrate
components that we think will be useful in any real-time
communications systems.
The last few years have also seen a new platform rise for deployment
of services: The browser-embedded application, or "Web application".
It turns out that as long as the browser platform has the necessary
interfaces, it is possible to deliver almost any kind of service on
it.
Traditionally, these interfaces have been delivered by plugins, which
had to be downloaded and installed separately from the browser; in
the development of HTML5, much promise is seen by the possiblitiy of
making those interfaces available in a standardized way within the
browser.
Other efforts, for instance the W3C WebRTC, Web Applications and
Device API working groups, focus on making standardized APIs and
interfaces available, within or alongside the HTML5 effort, for those
functions; this memo concentrates on specifying the protocols and
subprotocols that are needed to specify the interactions that happen
across the network.
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2. Principles and Terminology
2.1. Goals of this overview
The goal of the RTCWEB protocol specification is to specify a set of
protocols that, if all are implemented, will allow the implementation
to communicate with another implementation using audio, video and
auxillary data sent along the most direct possible path between the
participants.
This document is intended to serve as the roadmap to the RTCWEB
specifications. It defines terms used by other pieces of
specification, lists references to other specifications that don't
need further elaboration in the RTCWEB context, and gives pointers to
other documents that form part of the RTCWEB suite.
By reading this document and the documents it refers to, it should be
possible to have all information needed to implement an RTCWEB
compatible implementation.
2.2. Relationship between API and protocol
The total RTCWEB/WEBRTC effort consists of two pieces:
o A protocol specification, done in the IETF
o A Javascript API specification, done in the W3C
Together, these two specifications aim to provide an environment
where Javascript embedded in any page, viewed in any compatible
browser, when suitably authorized by its user, is able to set up
communication using audio, video and auxillary data, where the
browser environment does not constrain the types of application in
which this functionality can be used.
The protocol specification does not assume that all implementations
implement this API; it is not intended to be possible by observing
the bits on the wire whether they come from a browser or from another
device implementing this specification.
The goal of cooperation between the protocol specification and the
API specification is that for all options and features of the
protocol specification, it should be clear which API calls to make to
exercise that option or feature; similarly, for any sequence of API
calls, it should be clear which protocol options and features will be
invoked. Both subject to constraints of the implementation, of
course.
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2.3. On interoperability and innovation
The "Mission statement of the IETF" [RFC3935] states that "The
benefit of a standard to the Internet is in interoperability - that
multiple products implementing a standard are able to work together
in order to deliver valuable functions to the Internet's users."
Communication on the Internet frequently occurs in two phases:
o Two parties communicate, through some mechanism, what
functionality they both are able to support
o They use that shared communicative functionality to communicate,
or, failing to find anything in common, give up on communication.
There are often many choices that can be made for communicative
functionality; the history of the Internet is rife with the proposal,
standardization, implementation, and success or failure of many types
of options, in all sorts of protocols.
The goal of having a mandatory to implement function set is to
prevent negotiation failure, not to preempt or prevent negotiation.
The presence of a mandatory to implement function set serves as a
strong changer of the marketplace of deployment - in that it gives a
guarantee that, as long as you conform to a specification, and the
other party is willing to accept communication at the base level of
that specification, you can communicate successfully.
The alternative - that of having no mandatory to implement - does not
mean that you cannot communicate, it merely means that in order to be
part of the communications partnership, you have to implement the
standard "and then some" - that "and then some" usually being called
a profile of some sort; in the version most antithetical to the
Internet ethos, that "and then some" consists of having to use a
specific vendor's product only.
2.4. Terminology
The following terms are used in this document, and as far as possible
across the documents specifying the RTCWEB suite, in the specific
meanings given here. Other terms are used in their commonly used
meaning.
The list is in alphabetical order.
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API Application Programming Interface - a specification of a set of
calls and events, usually tied to a programming language or an
abstract formal specification such as WebIDL, with its defined
semantics.
Interactive Communication between multiple parties, where the
expectation is that an action from one party can cause a reaction
by another party, and the reaction can be observed by the first
party, with the total time required for the action/reaction/
observation is on the order of no more than hundreds of
milliseconds.
Media Audio and video content. Not to be confused with
"transmission media" such as wires.
Protocol A specification of a set of data units, their
representation, and rules for their transmission, with their
defined semantics. A protocol is usually thought of as going
between systems.
Real-time media Media where generation of content and display of
content are intended to occur closely together in time (on the
order of no more than hundreds of milliseconds).
NOTE: Where common definitions exist for these terms, those
definitions should be used to the greatest extent possible.
TODO: Extend this list with other terms that might prove slippery.
3. Functionality groups
The functionallity groups that are needed can be specified, more or
less from the bottom up, as:
o Data transport: TCP, UDP and the means to securely set up
connections between entities, as well as the functions for
deciding when to send data: Congestion management, bandwith
estimation and so on.
o Data framing: RTP and other data formats that serve as containers,
and their functions for data confidentiality and integrity.
o Data formats: Codec specifications, format specifications and
functionality specifications for the data passed between systems.
Audio and video codecs, as well as formats for data and document
sharing, belong in this category. In order to make use of data
formats, a way to describe them, a session description, is needed.
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o Connection management: Setting up connections, agreeing on data
formats, changing data formats during the duration of a call; SIP
and Jingle/XMPP belong in this category.
o Presentation and control: What needs to happen in order to ensure
that interactions behave in a non-surprising manner. This can
include floor control, screen layout, voice activated image
switching and other such functions - where part of the system
require the cooperation between parties. Cisco/Tandberg's TIP was
one attempt at specifying this functionality.
o Local system support functions: These are things that need not be
specified uniformly, because each participant may choose to do
these in a way of the participant's choosing, without affecting
the bits on the wire in a way that others have to be cognizant of.
Examples in this category include echo cancellation (some forms of
it), local authentication and authorization mechanisms, OS access
control and the ability to do local recording of conversations.
Within each functionality group, it is important to preserve both
freedom to innovate and the ability for global communication.
Freedom to innovate is helped by doing the specification in terms of
interfaces, not implementation; any implementation able to
communicate according to the interfaces is a valid implementation.
Ability to communicate globally is helped both by having core
specifications be unencumbered by IPR issues and by having the
formats and protocols be fully enough specified to allow for
independent implementation.
One can think of the three first groups as forming a "media transport
infrastructure", and of the three last groups as forming a "media
service". In many contexts, it makes sense to use a common
specification for the media transport infrastructure, which can be
embedded in browsers and accessed using standard interfaces, and "let
a thousand flowers bloom" in the "media service" layer; to achieve
interoperable services, however, at least the first five of the six
groups need to be specified.
4. Data transport
Datagram transport is the subject of a separate draft, "A Datagram
Transport for the RTC-Web
profile".[I-D.alvestrand-dispatch-rtcweb-datagram] The basic approach
is to use ICE as a setup mechanism, and to specify mechanisms to use
ICE over connections that utilize UDP and TCP if needed to support a
basic datagram-passing function with adequate security. In order to
deal with complex NAT/firewall situations, relaying using TURN MUST
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be supported.
For octet-stream transport, TCP is used. (QUESTION: Do we need a TCP
relay specification? The use of TURN over TCP and TLS is specified
in the TURN RFC - is it suitable?)
(The role of Web Sockets [I-D.ietf-hybi-thewebsocketprotocol] needs
to be clarified.)
The data transport MUST behave reasonably in the presence of
congested networks; this is usually interpreted as reducing the send
rate when congestion is encountered. TCP, when correctly
implemented, does this automatically; this is not the case with UDP,
and the RTP framing specification does not contain a congestion
control component.
Determining an useful congestion handling mechanism is a high
priority for work with this specification suite.
Usually when designing data transport for media, one separates out
the functions of bandwith estimation (which is a determinant for
which codec and which codec parameters to use) and congestion
management (reacting to events that change the available bandwidth,
such as congestion or media change, in an appropriate manner). The
totality of these features MUST ensure that an implementation of the
RTCWEB suite is able to coexist on a network with other users,
including TCP-based data transfers, without starving them of
resources, and without letting itself be starved.
5. Data framing and securing
RTP [RFC3550]and SRTP [RFC3711]. The RTP/SAVP profile, defined as
part of SRTP, is supported, and "extended RTCP", RTP/SAVPF [RFC4585],
with its secured version RTP/SAVPF [RFC5124]is used in order to
support codec functionality that depends on this RTP profile, such as
The implementation of SRTP used MUST support encryption using AES-CM
with MIC, on both RTP and RTCP channels. <TODO: Add pointer to
appropriate profile here> (Note that like for all mandatory-to-
implement, there is no requirement that these protocols be used, just
that it is possible to negotiate them.)
[OPEN ISSUE; We need to specify a securable format of passing data
that is not RTP. One proposal has been to use DTLS over DCCCP,
although specifying a "data codec" and using SRTP has been proposed
too.]
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6. Data formats
The intent of this specification is to allow each communications
event to use the data formats that are best suited for that
particular instance, where a format is supported by both sides of the
connection. However, a minimum standard is greatly helpful in order
to ensure that communication can be achieved. This document
specifies a minimum baseline that will be supported by all
implementations of this specification, and leaves further codecs to
be included at the will of the implementor.
NOTE IN DRAFT: The particular codecs named are NOT A DECISION. They
are included to illustrate possible choices, and to check with the
group that the references given are necessary and sufficient for the
purpose of specifying an interoperable codec suite.
In audio, the OPUS codec[I-D.ietf-codec-opus] MUST be supported. For
ease of interoperability with gateways to older equipment, G.711
U-law, audio/PCMU, defined in RFC 1890 [RFC1890] section 4.4.12, is
also mandatory to implement. There is no third mandatory to
implement.
In video, the VP8 codec [I-D.westin-payload-vp8] MUST be supported.
The Theora codec is also freely available. H.264/AVC and H.264/SVC
[I-D.ietf-avt-rtp-svc] are widely enough used that it gives a wider
range of communications partners if they are supported.
The overall set of data formats and parameters, and the identifiers
that allow the partners to bind data streams to application-level
entities, form a session description. It is vital that the
communicating parties have the same session description, and that the
session description can be updated while the connection is in
progress.
7. Connection management
This specification is silent on the definition of connection
management protocols. It envisions that implementors will make a
choice on whether to implement connection management protocols as a
downloadable component, as a browser plug-in, or as a frontend/
backend split, where a part of the protocol machinery is downloaded
into the browser and uses some mechanism (for instance WebSockets) to
communicate back to a backend implementing the rest of the connection
management protocol.
XMPP, and its Jingle component, has proved a versatile tool in
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building interoperable communities, and so has SIP. This suite
requires that the browser support establishing and describing
connections using a data format for session description capable of
representing the information needed by these two protocols, such as
one that can be one-to-one transformed into SDP. The exact
specification of this API is done elsewhere <insert reference when
available>; this API is powerful enough that all interesting
parameters of the transport mechanisms specified above are settable,
and clear enough that how to connect the API to the protocols is
obvious.
8. Presentation and control
The most important part of control is the user's control over the
browser's interaction with input/output devices and communications
channels. It is important that the user have some way of figuring
out where his audio, video or texting is being sent, for what
purported reason, and what guarantees are made by the parties that
form part of this control channel. This is largely a local function
between the browser, the underlying operating system and the user
interface; this is being worked on as part of the W3C API effort.
9. Local system support functions
These are characterized by the fact that the quality of these
functions strongly influences the user experience, but the exact
algorithm does not need coordination. In some cases (for instance
echo cancellation, as described below), the overall system definition
may need to specify that the overall system needs to have some
characteristics for which these facilities are useful, without
requiring them to be implemented a certain way.
Local functions include echo cancellation, volume control, camera
management including focus, zoom, pan/tilt controls (if available),
and more.
Certain parts of the system SHOULD conform to certain properties, for
instance:
o Echo cancellation should be good enough that feedback (defined as
a rising volume of sound with no local sound input) does not
occur.
o Privacy concerns must be satisfied; for instance, if remote
control of camera is offered, the APIs should be available to let
the local participant to figure out who's controlling the camera,
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and possibly decide to revoke the permission for camera usage.
o Automatic gain control, if present, should normalize a speaking
voice into <whatever dB metrics makes sense here - most important
that we have one only>
10. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
11. Security Considerations
Security of the web-enabled real time communications comes in several
pieces:
o Security of the components: The browsers, and other servers
involved. The most target-rich environment here is probably the
browser; the aim here should be that the introduction of these
components introduces no additional vulnerability.
o Security of the communication channels: It should be easy for a
participant to reassure himself of the security of his
communication - by verifying the crypto parameters of the links he
himself participates in, and to get reassurances from the other
parties to the communication that they promise that appropriate
measures are taken.
o Security of the partners' identity: verifying that the
participants are who they say they are (when positivie
identification is appropriate), or that their identity cannot be
uncovered (when anonymity is a goal of the application).
This specification addresses some, but not all, of these concerns,
and makes some assumptions about the security considerations of other
parts of the environment; it is up to the implementor to see that
these security assumptions are warranted. In particular:
o We assume that the ICE security mechanism is a necessary and
sufficient criterion for accepting that a connection attempt is
from a communications partner. This means that we trust the
randomness of ICE "usernames" and the security of ICE "passwords".
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o We assume that the SRTP key exchange mechanisms and security
profiles specified provide an adequate level of protection for
audio and video media.
(there needs to be more text here)
12. Acknowledgements
13. References
13.1. Normative References
[I-D.alvestrand-dispatch-rtcweb-datagram]
Alvestrand, H., "A Datagram Transport for the RTC-Web
profile", draft-alvestrand-dispatch-rtcweb-datagram-01
(work in progress), February 2011.
[I-D.ietf-codec-opus]
Valin, J. and K. Vos, "Definition of the Opus Audio
Codec", draft-ietf-codec-opus-05 (work in progress),
March 2011.
[I-D.ietf-hybi-thewebsocketprotocol]
Fette, I., "The WebSocket protocol",
draft-ietf-hybi-thewebsocketprotocol-07 (work in
progress), April 2011.
[I-D.westin-payload-vp8]
Westin, P. and H. Lundin, "Proposal for the IETF on "RTP
Payload Format for VP8 Video"",
draft-westin-payload-vp8-02 (work in progress),
March 2011.
[RFC1890] Schulzrinne, H., "RTP Profile for Audio and Video
Conferences with Minimal Control", RFC 1890, January 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
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[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.
13.2. Informative References
[I-D.ietf-avt-rtp-svc]
Wenger, S., Wang, Y., Schierl, T., and A. Eleftheriadis,
"RTP Payload Format for Scalable Video Coding",
draft-ietf-avt-rtp-svc-27 (work in progress),
February 2011.
[RFC3935] Alvestrand, H., "A Mission Statement for the IETF",
BCP 95, RFC 3935, October 2004.
Author's Address
Harald T. Alvestrand
Google
Kungsbron 2
Stockholm, 11122
Sweden
Email: harald@alvestrand.no
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