RTCWeb Datagram Connection
draft-ietf-rtcweb-data-channel-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8831.
Expired & archived
|
|
|---|---|---|---|
| Authors | Randell Jesup , Salvatore Loreto , Michael Tüxen | ||
| Last updated | 2012-09-06 (Latest revision 2012-03-05) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | Became RFC 8831 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-rtcweb-data-channel-00
RTCWeb Working Group R. Jesup
Internet-Draft Mozilla
Intended status: Informational S. Loreto
Expires: September 7, 2012 Ericsson
M. Tuexen
Muenster Univ. of Appl. Sciences
March 6, 2012
RTCWeb Datagram Connection
draft-ietf-rtcweb-data-channel-00.txt
Abstract
The Web Real-Time Communication (WebRTC) working group is charged to
provide protocol support for direct interactive rich communication
using audio, video, and data between two peers' web-browsers. This
document describes the non-media data transport aspects of the WebRTC
framework. It provides an architectural overview of how the Stream
Control Transmission Protocol (SCTP) is used in the WebRTC context as
a generic transport service allowing Web Browser to exchange generic
data from peer to peer.
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."
This Internet-Draft will expire on September 7, 2012.
Copyright Notice
Copyright (c) 2012 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
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Use Cases for Unreliable Datagram Based Channels . . . . . 5
3.2. Use Cases for Reliable Channels (Datagram or Stream
Based) . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Datagrams over SCTP over DTLS over UDP . . . . . . . . . . . . 6
5. The Envisioned Usage of SCTP in the RTCWeb Context . . . . . . 8
5.1. Association Setup . . . . . . . . . . . . . . . . . . . . 8
5.2. SCTP Streams . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Channel Definition . . . . . . . . . . . . . . . . . . . . 9
5.4. Usage of Payload Protocol Identifier . . . . . . . . . . . 9
6. Minor Protocol and Message Format . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
10. Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The issue of how best to handle non-media data types in the context
of RTCWEB has reached a general consensus on the usage of SCTP
[RFC4960] encapsulated on DTLS [RFC6347]:
+----------+
| SCTP |
+----------+
| DTLS |
+----------+
| ICE/UDP |
+----------+
Figure 1: Basic stack diagram
The encapsulation of SCTP over DTLS over ICE/UDP provides a NAT
traversal solution together with confidentiality, source
authenticated, integrity protected transfers. This data transport
service operates in parallel to the media transports, and all of them
can eventually share a single transport-layer port number.
SCTP provides multiple streams natively with reliable, unreliable and
partially-reliable delivery modes.
The remainder of this document is organized as follows: Section 2 and
Section 3 provide requirements and use cases for both unreliable and
reliable peer to peer datagram base channel; Section 4 arguments SCTP
over DTLS over UDP; Section 5 provides an overview of how SCTP should
be used by the RTCWeb protocol framework for transporting non-media
data between browsers.
2. Requirements
This section lists the requirements for P2P data connections between
two browsers.
Req. 1 Multiple simultaneous datagram streams must be supported.
Note that there may 0 or more media streams in parallel with
the data streams, and the number and state (active/inactive)
of the media streams may change at any time.
Req. 2 Both reliable and unreliable datagram streams must be
supported.
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Req. 3 Data streams must be congestion controlled; either
individually, as a class, or in conjunction with the media
streams, to ensure that datagram exchanges don't cause
congestion problems for the media streams, and that the
rtcweb PeerConnection as a whole is fair with competing
streams such as TCP.
Req. 4 The application should be able to provide guidance as to the
relative priority of each datagram stream relative to each
other, and relative to the media streams. [ TBD: how this is
encoded and what the impact of this is. ] This will interact
with the congestion control algorithms.
Req. 5 Datagram streams must be encrypted; allowing for
confidentiality, integrity and source authentication. See
[I-D.ietf-rtcweb-security] and
[I-D.ietf-rtcweb-security-arch] for detailed info.
Req. 6 Consent and NAT traversal mechanism: These are handled by
the PeerConnection's ICE [RFC5245] connectivity checks and
optional TURN servers.
Req. 7 Data streams must provide message fragmentation support such
that IP-layer fragmentation does not occur no matter how
large a message the Javascript application passes to be
sent.
Req. 8 The data stream transport protocol must not encode local IP
addresses inside its protocol fields; doing so reveals
potentially private information, and leads to failure if the
address is depended upon.
Req. 9 The data stream protocol should support unbounded-length
"messages" (i.e., a virtual socket stream) at the
application layer, for such things as image-file-transfer;
or else it must support at least a maximum application-layer
message size of 4GB.
Req. 10 The data stream packet format/encoding must be such that it
is impossible for a malicious Javascript to generate an
application message crafted such that it could be
interpreted as a native protocol over UDP - such as UPnP,
RTP, SNMP, STUN, etc.
Req. 11 The data stream transport protocol must start with the
assumption of a PMTU of 1280 [ *** need justification ***]
bytes until measured otherwise.
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Req. 12 The data stream transport protocol must not rely on ICMP or
ICMPv6 being generated or being passed back, such as for
PMTU discovery.
Req. 13 It must be possible to implement the protocol stack in the
user application space.
3. Use Cases
3.1. Use Cases for Unreliable Datagram Based Channels
U-C 1 A real-time game where position and object state information
is sent via one or more unreliable data channels. Note that
at any time there may be no media channels, or all media
channels may be inactive, and that there may also be reliable
data channels in use.
U-C 2 Non-critical state updates about a user in a video chat or
conference, such as Mute state.
3.2. Use Cases for Reliable Channels (Datagram or Stream Based)
Note that either reliable datagrams or streams are possible; reliable
streams would be fairly simple to layer on top of SCTP reliable
datagrams with in-order delivery.
U-C 3 A real-time game where critical state information needs to be
transferred, such as control information. Typically this
would be datagrams. Such a game may have no media channels,
or they may be inactive at any given time, or may only be
added due to in-game actions.
U-C 4 Non-realtime file transfers between people chatting. This
could be datagrams or streaming. Note that this may involve a
large number of files to transfer sequentially or in parallel,
such as when sharing a folder of images or a directory of
files.
U-C 5 Realtime text chat while talking with an individual or with
multiple people in a conference. Typically this would be
datagrams.
U-C 6 Renegotiation of the set of media streams in the
PeerConnection. Typically this would be datagrams.
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U-C 7 Proxy browsing, where a browser uses data channels of a
PeerConnection to send and receive HTTP/HTTPS requests and
data, for example to avoid local internet filtering or
monitoring. Typically this would be streams.
4. Datagrams over SCTP over DTLS over UDP
The encapsulation of SCTP over DTLS as defined in
[I-D.tuexen-tsvwg-sctp-dtls-encaps] provides a NAT traversal solution
together with confidentiality, source authenticated, integrity
protected transfers. SCTP provides also natively several interesting
features for transporting non-media data between browsers:
o Support of multiple streams.
o Ordered and unordered delivery of user messages.
o Reliable and partial-reliable transport of user messages.
Each SCTP user message contains a so called Payload Protocol
Identifier (PPID) that is passed to SCTP by its upper layer and sent
to its peer. This value represents an application (or upper layer)
specified protocol identifier and be used to transport multiple
protocols over a single SCTP association. The sender provides for
each protocol a specific PPID and the receiver demultiplexes the
messages based on the received PPID.
The encapsulation of SCTP over DTLS, together with the SCTP features
listed above satisfies all the requirements listed in in Section 2.
The layering of protocols for WebRTC is shown in the following
Figure 2.
+------+
|WEBAPP|
+------+
| SCTP |
+--------------------+
| STUN | SRTP | DTLS |
+--------------------+
| ICE |
+--------------------+
| UDP1 | UDP2 | ... |
+--------------------+
Figure 2: WebRTC protocol layers
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This stack (especially in contrast to DTLS over SCTP [RFC6083]) has
been chosen because it
o supports the transmission of arbitrary large user messages.
o shares the DTLS connection with the media channels.
o provides privacy for the SCTP control information.
Considering the protocol stack of Figure 2 the usage of DTLS over UDP
is specified in [RFC6347], while the usage of SCTP on top of DTLS is
specified in [I-D.tuexen-tsvwg-sctp-dtls-encaps].
Since DTLS is typically implemented in user-land, an SCTP user-land
implementation must also be used.
When using DTLS as the lower layer, only single homed SCTP
associations can be used, since DTLS does not expose any address
management to its upper layer. The ICE/UDP layer can handle IP
address changes during a session without needing to notify the DTLS
and SCTP layers, though it would be advantageous to retest path MTU
on an IP address change.
DTLS implementations used for this stack must support controlling
fields of the IP layer like the Don't fragment (DF)-bit in case of
IPv4 and the Differentiated Services Code Point (DSCP) field. This
is required for performing path MTU discovery. The DTLS
implementation must also support sending user messages exceeding the
path MTU.
When supporting multiple SCTP associations over a single DTLS
connection, incoming ICMP or ICMPv6 messages can't be processed by
the SCTP layer, since there is no way to identify the corresponding
association. Therefore the number of SCTP associations should be
limited to one or ICMP and ICMPv6 messages should be ignored. In
general, the lower layer interface of an SCTP implementation has to
be adapted to address the differences between IPv4 or IPv6 (being
connection-less) or DTLS (being connection-oriented).
When protocol stack of Figure 2 is used, DTLS protects the complete
SCTP packet, so it provides confidentiality, integrity and source
authentication of the complete SCTP packet.
This protocol stack supports the usage of multiple SCTP streams. A
user message can be sent ordered or unordered and, if the SCTP
implementations support [RFC3758], with partial reliability. When
using partial reliability, it might make sense to use a policy
limiting the number of retransmissions. Limiting the number of
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retransmissions to zero provides a UDP like service where each user
messages is sent exactly once.
SCTP provides congestion control on a per-association base. This
means that all SCTP streams within a single SCTP association share
the same congestion window. Traffic not being sent over SCTP is not
covered by the SCTP congestion control. Due to the typical parallel
SRTP media streams, it will be advantageous to select a delay-
sensitive congestion control algorithm or to at least coordinate
congestion control between the data channels and the media streams to
avoid a data channel transfer ending up with most or all the channel
bandwidth.
5. The Envisioned Usage of SCTP in the RTCWeb Context
The appealing features of SCTP in the RTCWeb context are:
o TCP-friendly congestion control.
o The congestion control is modifiable for integration with media
stream congestion control.
o Support for multiple channels with different characteristics.
o Support for out-of-order delivery.
o Support for large datagrams and PMTU-discovery and fragmentation.
o Reliable or partial reliability support.
o Support of multiple streams.
Multihoming will not be used in this scenario. The SCTP layer would
simply act as if it were running on a single-homed host, since that
is the abstraction that the lower layer (a connection oriented,
unreliable datagram service) would expose.
5.1. Association Setup
The SCTP association would be set up when the two endpoints of the
WebRTC PeerConnection agree on opening it, as negotiated by JSEP
(typically an exchange of SDP) [I-D.ietf-rtcweb-jsep]. It would use
the DTLS connection created at the start of the PeerConnection
connection.
The application should indicate the number of simultaneous streams
required when opening the association, and if no value is supplied,
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the implementation should provide a default, with a suggested value
of 16. If more simultaneous streams are needed, [RFC6525] allows
adding additional (but not removing) streams to an existing
association. There can be up to 65536 SCTP streams per SCTP
association in each direction.
5.2. SCTP Streams
SCTP defines a stream as an unidirectional logical channel existing
within an SCTP association one to another SCTP endpoint. The streams
are used to provide the notion of in-sequence delivery. Each user
message is sent on a particular stream, either order or unordered.
Ordering is preserved only for all ordered messages sent on the same
stream.
5.3. Channel Definition
The W3C seems to have consensus on defining the application API for
WebRTC dataChannels to be bidirectional. They also consider the
notions of in-sequence, out-of-sequence, reliable and un-reliable as
properties of Channels.
A possible realization of a bidirectional Data Channel is a pair of
one incoming stream and one outcoming SCTP stream.
Closing of a Data Channel can be signalled resetting the
corresponding streams [RFC6525]. Resetting a stream set the Stream
Sequence Numbers (SSNs) of the stream back to 'zero' with a
corresponding notification to the application layer that the reset
has been performed. Closed streams are available to reuse.
[RFC6525] also guarantees that all the messages are delivered (or
expired) before resetting the stream.
It might be useful to use a specific pair of SCTP streams for
transporting control information.
5.4. Usage of Payload Protocol Identifier
The SCTP Payload Protocol Identifiers (PPIDs) can be used to signal
the interpretation of the "Payload data", like a string, ASCII or
binary data.
RFC 4960 [RFC4960] creates the registry from which these identifiers
have been assigned. Eventual PPIDs defined within the RTCWeb Context
have to be registered with IANA.
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6. Minor Protocol and Message Format
A separate draft (draft-jesup-rtcweb-data-protocol) is being
submitted to define the minor protocol to set up and manage the
bidirectional data channels needed to satisify the requirements in
this document for WebRTC.
Masking of the protocol is not needed if the lower layer always
encrypts with DTLS.
7. Security Considerations
To be done.
8. IANA Considerations
This document does not require any actions by the IANA.
9. Acknowledgments
Many thanks for comments, ideas, and text from Cullen Jennings, Eric
Rescorla, Randall Stewart, Justin Uberti, and Harald Alvestrand.
10. Informative References
[RFC3758] Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
Conrad, "Stream Control Transmission Protocol (SCTP)
Partial Reliability Extension", RFC 3758, May 2004.
[RFC4960] Stewart, R., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC6083] Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
Transport Layer Security (DTLS) for Stream Control
Transmission Protocol (SCTP)", RFC 6083, January 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
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[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, February 2012.
[I-D.ietf-rtcweb-security]
Rescorla, E., "Security Considerations for RTC-Web",
draft-ietf-rtcweb-security-01 (work in progress),
October 2011.
[I-D.ietf-rtcweb-security-arch]
Rescorla, E., "RTCWEB Security Architecture",
draft-ietf-rtcweb-security-arch-00 (work in progress),
January 2012.
[I-D.ietf-rtcweb-jsep]
Uberti, J. and C. Jennings, "Javascript Session
Establishment Protocol", draft-ietf-rtcweb-jsep-00 (work
in progress), March 2012.
[I-D.ietf-tsvwg-sctp-udp-encaps]
Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP
Packets", draft-ietf-tsvwg-sctp-udp-encaps-02 (work in
progress), December 2011.
[I-D.tuexen-tsvwg-sctp-dtls-encaps]
Jesup, R., Loreto, S., Stewart, R., and M. Tuexen, "DTLS
Encapsulation of SCTP Packets for RTCWEB",
draft-tuexen-tsvwg-sctp-dtls-encaps-00 (work in progress),
March 2012.
Authors' Addresses
Randell Jesup
Mozilla
USA
Email: randell-ietf@jesup.org
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: salvatore.loreto@ericsson.com
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Michael Tuexen
Muenster University of Applied Sciences
Stegerwaldstrasse 39
Steinfurt 48565
Germany
Email: tuexen@fh-muenster.de
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