RTCWEB M. Thomson
Internet-Draft Microsoft
Intended status: Standards Track February 18, 2013
Expires: August 22, 2013
Data Channels for RTCWEB
draft-thomson-rtcweb-data-00
Abstract
RTCWEB have selected SCTP over DTLS over UDP with ICE for peer-to-
peer data channels. There is some debate over the best way to
negotiate channels. This proposal is a nose-to-tail description of
an alternative to existing proposals.
Status of this Memo
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This Internet-Draft will expire on August 22, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . . 3
3. (In)consistent Properties . . . . . . . . . . . . . . . . . . . 5
3.1. Negotiation . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Dealing with Mismatched Properties . . . . . . . . . . . . 5
4. Available Data Channel Properties . . . . . . . . . . . . . . . 6
5. SDP Format . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Payload Protocol Identifiers . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
RTCWEB [I-D.ietf-rtcweb-overview] has defined the use of the Stream
Control Transmission Protocol (SCTP) [RFC4960] over Datagram
Transport Layer Security (DTLS) [RFC6347] over UDP with Interactive
Connectivity Establishment (ICE) [RFC5245] for peer-to-peer data
channels.
This document describes a proposal for how this protocol stack is
used. The proposal attempts to reconcile the following basic
requirements:
o the ability to have data channels used interchangeably with
websockets, after establishment
o the ability to use as many SCTP features as possible
Like other proposals, this proposal uses an API that is largely
interchangeable with the WebSockets API [REF:TBD]. Of course, that
alone is insufficient because the way that data channels are created
is completely different to websockets [RFC6455]. Only the general
usage of the API follows the WebSockets API, channel establishment
requires a very different process.
Furthermore, not every application will care for compatibility with
the WebSockets API. For those applications, additional properties
are exposed to enable valuable SCTP features.
In these aspects, all data channel proposals are the same. The
details differ. For example, this one doesn't need an in-band
protocol. It even avoids the need for negotiation, except where it
is needed. If not for the fact that the WebSockets API designers -
in their infinite wisdom - decided to distinguish text from binary,
it wouldn't even need to use a PPID to identify textual messages.
1.1. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [RFC2119] and indicate requirement
levels for compliant implementations.
2. Overview of Operation
A data channel is a bidirectional communication medium between WebRTC
peers.
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Every data channel is bound to a specific SCTP stream number. The
same SCTP stream identifier is used for both directions of the data
channel. Though SCTP streams are unidirectional, and this concept
doesn't hold any particular meaning for SCTP, this simplification
ensures that channels can be created with minimal overhead.
Each data channel has a set of properties that governs how it sends
messages. Unlike other SCTP APIs where properties like reliability
settings are set on a per-message basis, this API places these
properties on the data channel. This allows the API to behave
exactly like the WebSockets API when sending messages. Details of
the available data channel properties are included in Section 4.
There are three ways that a data channel can be created. All three
result in an object representing the data channel being provided to
the application. Each differs in the manner of delivery and how
properties are selected for the data channel.
1. The application can request the creation of a new data channel
directly. The browser selects appropriate properties for the
channel, using any values provided by the application and
providing defaults for others.
This triggers a notification to the application that indicates
that it needs to renegotiate the session.
2. Offer/answer negotiation can trigger the creation of a new data
channel. In this case, the session description provided in an
offer or answer describes the properties of the channel.
3. Messages can arrive on an SCTP stream that does not have a data
channel allocated. If messages arrive on a stream, the browser
provides default values for all stream properties.
Channel creation can fail if there are an insufficient number of
available SCTP streams. This is based on either a local
unwillingness to receive more streams, or based on knowledge of the
unwillingness of the peer to receive more streams.
Creation can also fail if the application specifies a stream ID that
is already in use. These should trigger the appropriate error
mechanisms (exceptions or something).
Channels are closed by sending a RE-CONFIG chunk that includes
Incoming and Outgoing SSN Reset Request parameters, as defined in
[RFC6525]. Closing a channel doesn't permit the sending of a code
and message as exposed in the WebSockets API, any values that are
provided by the application are discarded.
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3. (In)consistent Properties
The creation of the first data channel will require offer/answer
negotiation. This is necessary to ensure that the SCTP association
is created, including ICE, the DTLS handshake, plus any
authentication that might be required.
Once an SCTP association is live, data channels can be used to
exchange messages immediately after they are created. The drawback
is that messages arrive at the peer without any information about
what properties the sender attaches to the corresponding data
channel.
How much property consistency matters to the application will depend
on the application. If the application is performing SS7 signaling
using M3UA [RFC4666], this is unlikely to matter, but some
applications could rely on having consistent data channel properties.
3.1. Negotiation
The safest (and slowest) way to establish new channels with
consistent properties is to negotiate them. This is performed using
an offer/answer exchange. The application is able to choose where
and when this negotiation occurs. If there is an existing data
channel, then this provides a low latency path for performing this
negotiation.
The negotiation includes a description for every SCTP stream that a
peer is sending that includes all of the data channel properties (see
Section 4), so that the receiver can create a data channel with the
same properties. The browser creates a data channel with the
described properties and provides that to the application. If the
data channel description appears in an offer, the answer describes
the data channel that is used on the same stream number.
An offer or answer that includes a description for a data channel
that already exists, then the properties of that data channel are not
modified. An answer MUST include the properties of the existent data
channel, not the channel that the offer describes.
3.2. Dealing with Mismatched Properties
An application that chooses to send on data channels prior to
negotiation will cause the receiving peer to create a data channel
with a default configuration. Applications can handle this in a
number of ways:
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o The application on the receiving peer can create data channels in
the same order as the sender to ensure consistent properties.
This is possible because stream identifiers are assigned in the
same way by both peers.
o The application on the receiving peer might apply application-
specific default values for all non-negotiated channels.
o The application on the receiving peer might not care about having
consistent data channel properties. Note that data channel
properties only apply to the sending of messages.
Note: It is possible to provide an application with information about
the values that are in use by a peer. This would in most cases be
possible after negotiation, though some properties are revealed when
new messages arrive. Of course, this is a lot of effort after the
application has already effectively declared that it doesn't care.
Given that the application could exchange this information using the
data channel(s) it has convenient, adding new APIs seem of very low
value.
[[Irrelevant API Note: Adding a data channel triggers a notification
to the application that it should renegotiate the session. Normally,
the 'negotiation needed' state is cleared when the negotiation
commences (or completes?). If the application decides to send
packets, then the damage is done and there is no point negotiating.
That being the case, a data channel could removed from the set of
unnegotiated things upon sending a packet. Negotiation from this
point isn't going to change anything.]]
4. Available Data Channel Properties
The following properties are exposed to the application. All of
these properties can be set during the creation of a data channel.
Once the channel object is created, these properties are mutable,
with the exception of "streamId".
streamId The SCTP stream ID to use for the channel. If not provided
by the application, the lowest valued stream ID that is not
already in use by a data channel is selected. If the channel is
created as a result of negotiation or packet arrival, the stream
ID has already been chosen.
binaryPPID The SCTP payload protocol identifier (PPID) that is used
for binary messages. Textual messages are always sent using a
PPID that indicates textual content, so this value only determines
the PPID that is attached to binary messages. This field is a 32-
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bit number.
Unless otherwise specified, channels use the PPID for WebRTC
binary data channels. Channels created in response to the receipt
of a message use the PPID from the received message, unless the
message uses the PPID for WebRTC textual data channels, which
causes the binary PPID to be selected instead. Details on the
newly defined PPIDs are included in Section 6.
reliabilityTime The amount of time (in milliseconds) that the
browser will attempt to retransmit messages for reliable delivery.
Together with reliabilityRetransmissions, this enables unreliable
or partially reliable transmission of data. The default value for
this property is the largest number available (e.g.,
Number.POSITIVE_INFINITY).
reliabilityRetransmissions The numer of retransmissions that the
browser will make for any packet reliable delivery. Together with
reliabilityTime, this enables unreliable or partially reliable
transmission of data. The default value for this property is the
largest number available (e.g., Number.POSITIVE_INFINITY).
label The label to assign to the data channel. A default value is
selected by the receiving browser.
protocol A protocol label that identifies the protocol used on the
data channel. This property is undefined unless set by the
application.
binaryType The BinaryType defined in The WebSockets API determines
whether binary data is provided to the application as Blob or
ArrayBuffer objects. The default value is "blob". Note that
changing this might not have an immediate effect if messages have
started to arrive prior to the change.
All these properties are specific to each message that is sent.
Changes to mutable properties take effect for the next message that
is sent on the channel.
This isn't a W3C WebRTC document, so specifics of the API aren't
really relevant, but it is imagined that these properties could be
passed in a dictionary to the method that creates a new data channel
and exposed as attributes on the data channel object.
5. SDP Format
The properties described above appear in SDP. All properties are
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declarative. Though the specifics of the syntax doesn't matter much,
the following example could indicate something that would work:
m=application 12345 SCTP/DTLS 0 1 2 5
c=IN IP6 ::1
a=fmtp:0 binaryPPID=177;label=control
a=fmtp:1 label=chat
a=fmtp:2 label=characters;reliabilityTime=2000;protocol=lrudfb
a=fmtp:5 label=bullets;reliabilityTime=5000
[[Formal SDP grammar TBD]]
This assumes that the SCTP port number (inside the DTLS
encapsulation) is fixed, so that this doesn't need to be indicated
anywhere. I'm not sure whether asking IANA for a port allocation
makes sense though.
6. Payload Protocol Identifiers
Two SCTP payload protocol identifiers are defined for WebRTC data
channels.
The WebRTC textual data channel PPID (number TBD) is used for all
messages that are identified as being textual. The payload of
messages marked with this PPID MUST be UTF-8 encoded text.
The WebRTC binary data channel PPID (number TBD) is used as the
default PPID for new data channels.
7. IANA Considerations
This document probably should register the PPIDs, but I don't really
have time to do that right now.
8. Security Considerations
I thought about this, and I can't think of any specific security
considerations. I could blather on about willingness to accept
streams and large volumes of data, but that's pretty lame.
I'm sure something will turn up eventually.
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9. Acknowledgments
This document is a rush job. If it survives for any longer than a
couple of weeks, no doubt this section will come in handy.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, February 2012.
10.2. Informative References
[I-D.ietf-rtcweb-overview]
Alvestrand, H., "Overview: Real Time Protocols for Brower-
based Applications", draft-ietf-rtcweb-overview-05 (work
in progress), December 2012.
[RFC4666] Morneault, K. and J. Pastor-Balbas, "Signaling System 7
(SS7) Message Transfer Part 3 (MTP3) - User Adaptation
Layer (M3UA)", RFC 4666, September 2006.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, December 2011.
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Author's Address
Martin Thomson
Microsoft
3210 Porter Drive
Palo Alto, CA 94304
US
Phone: +1 650-353-1925
Email: martin.thomson@skype.net
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