Internet Engineering Task Force (IETF) R. Jesup

Internet Engineering Task Force (IETF) R. Jesup
Request for Comments: 8832 Mozilla
Category: Standards Track S. Loreto
ISSN: 2070-1721 Ericsson
M. Tüxen
Münster Univ. of Appl. Sciences
January 2021

WebRTC Data Channel Establishment Protocol

Abstract

The WebRTC framework specifies protocol support for direct
interactive rich communication using audio, video, and data between
two peers' web browsers. This document specifies a simple protocol
for establishing symmetric data channels between the peers. It uses
a two-way handshake and allows sending of user data without waiting
for the handshake to complete.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8832.

Copyright Notice

Copyright (c) 2021 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these 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.

Table of Contents

1. Introduction
2. Conventions
3. Terminology
4. Protocol Overview
5. Message Formats
5.1. DATA_CHANNEL_OPEN Message
5.2. DATA_CHANNEL_ACK Message
6. Procedures
7. Security Considerations
8. IANA Considerations
8.1. SCTP Payload Protocol Identifier
8.2. New Standalone Registry for DCEP
8.2.1. New Message Type Registry
8.2.2. New Channel Type Registry
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses

1. Introduction

The Data Channel Establishment Protocol (DCEP) is designed to
provide, in the WebRTC data channel context [RFC8831], a simple in-
band method for opening symmetric data channels. As discussed in
[RFC8831], the protocol uses the Stream Control Transmission Protocol
(SCTP) [RFC4960] encapsulated in Datagram Transport Layer Security
(DTLS) (described in [RFC8261]). This allows DCEP to benefit from
the already standardized transport and security features of SCTP and
DTLS. DTLS 1.0 is defined in [RFC4347]; the present latest version,
DTLS 1.2, is defined in [RFC6347]; and an upcoming version, DTLS 1.3,
is defined in [TLS-DTLS13].

2. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP?14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

3. Terminology

This document uses the following terms:

Association: An SCTP association.

Stream: A unidirectional stream of an SCTP association. It is
uniquely identified by an SCTP stream identifier (0-65534). Note:
The SCTP stream identifier 65535 is reserved due to SCTP INIT and
INIT-ACK chunks only allowing a maximum of 65535 streams to be
negotiated (0-65534).

Stream Identifier: The SCTP stream identifier uniquely identifying a
stream.

Data Channel: Two streams with the same stream identifier, one in
each direction, which are managed together.

4. Protocol Overview

The Data Channel Establishment Protocol is a simple, low-overhead way
to establish bidirectional data channels over an SCTP association
with a consistent set of properties.

The set of consistent properties includes:

* reliable or unreliable message transmission. In case of
unreliable transmissions, the same level of unreliability is used.

* in-order or out-of-order message delivery.

* the priority of the data channel.

* an optional label for the data channel.

* an optional protocol for the data channel.

* the streams.

This protocol uses a two-way handshake to open a data channel. The
handshake pairs one incoming and one outgoing stream, both having the
same stream identifier, into a single bidirectional data channel.
The peer that initiates opening a data channel selects a stream
identifier for which the corresponding incoming and outgoing streams
are unused and sends a DATA_CHANNEL_OPEN message on the outgoing
stream. The peer responds with a DATA_CHANNEL_ACK message on its
corresponding outgoing stream. Then the data channel is open. DCEP
messages are sent on the same stream as the user messages belonging
to the data channel. The demultiplexing is based on the SCTP Payload
Protocol Identifier (PPID), since DCEP uses a specific PPID.

| Note: The opening side MAY send user messages before the
| DATA_CHANNEL_ACK is received.

To avoid collisions where both sides try to open a data channel with
the same stream identifiers, each side MUST use streams with either
even or odd stream identifiers when sending a DATA_CHANNEL_OPEN
message. When using SCTP over DTLS [RFC8261], the method used to
determine which side uses odd or even is based on the underlying DTLS
connection role: the side acting as the DTLS client MUST use streams
with even stream identifiers; the side acting as the DTLS server MUST
use streams with odd stream identifiers.

| Note: There is no attempt to ensure uniqueness for the label;
| if both sides open a data channel labeled "x" at the same time,
| there will be two data channels labeled "x" -- one on an even
| stream pair, one on an odd pair.

The purpose of the protocol field is to ease cross-application
interoperation ("federation") by identifying the user data being
passed by means of an IANA-registered string from the "WebSocket
Subprotocol Name Registry" defined in [RFC6455]. The field may be
useful for homogeneous applications that may create more than one
type of data channel. Note that there is no attempt to ensure
uniqueness for the protocol field.

5. Message Formats

Every DCEP message starts with a one-byte field called "Message Type"
that indicates the type of the message. The corresponding values are
managed by IANA (see Section 8.2.1).

5.1. DATA_CHANNEL_OPEN Message

This message is initially sent using the data channel on the stream
used for user messages.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | Channel Type | Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reliability Parameter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Length | Protocol Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ /
| Label |
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ /
| Protocol |
/ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Message Type: 1 byte (unsigned integer)
This field holds the IANA-defined message type for the
DATA_CHANNEL_OPEN message. The value of this field is 0x03, as
specified in Section 8.2.1.

Channel Type: 1 byte (unsigned integer)
This field specifies the type of data channel to be opened. The
values are managed by IANA (see Section 8.2.2):

DATA_CHANNEL_RELIABLE (0x00): The data channel provides a
reliable in-order bidirectional communication.

DATA_CHANNEL_RELIABLE_UNORDERED (0x80): The data channel provides
a reliable unordered bidirectional communication.

DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT (0x01): The data channel
provides a partially reliable in-order bidirectional
communication. User messages will not be retransmitted more
times than specified in the Reliability Parameter.

DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED (0x81): The data
channel provides a partially reliable unordered bidirectional
communication. User messages will not be retransmitted more
times than specified in the Reliability Parameter.

DATA_CHANNEL_PARTIAL_RELIABLE_TIMED (0x02): The data channel
provides a partially reliable in-order bidirectional
communication. User messages might not be transmitted or
retransmitted after a specified lifetime given in milliseconds
in the Reliability Parameter. This lifetime starts when
providing the user message to the protocol stack.

DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED (0x82): The data
channel provides a partially reliable unordered bidirectional
communication. User messages might not be transmitted or
retransmitted after a specified lifetime given in milliseconds
in the Reliability Parameter. This lifetime starts when
providing the user message to the protocol stack.

Priority: 2 bytes (unsigned integer)
The priority of the data channel, as described in [RFC8831].

Reliability Parameter: 4 bytes (unsigned integer)
For reliable data channels, this field MUST be set to 0 on the
sending side and MUST be ignored on the receiving side. If a
partially reliable data channel with a limited number of
retransmissions is used, this field specifies the number of
retransmissions. If a partially reliable data channel with a
limited lifetime is used, this field specifies the maximum
lifetime in milliseconds. The following table summarizes this:

+================================================+=============+
| Channel Type | Reliability |
| | Parameter |
+================================================+=============+
| DATA_CHANNEL_RELIABLE | Ignored |
+------------------------------------------------+-------------+
| DATA_CHANNEL_RELIABLE_UNORDERED | Ignored |
+------------------------------------------------+-------------+
| DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT | Number of |
| | RTX |
+------------------------------------------------+-------------+
| DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED | Number of |
| | RTX |
+------------------------------------------------+-------------+
| DATA_CHANNEL_PARTIAL_RELIABLE_TIMED | Lifetime in |
| | ms |
+------------------------------------------------+-------------+
| DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED | Lifetime in |
| | ms |
+------------------------------------------------+-------------+

Table 1

Label Length: 2 bytes (unsigned integer)
The length of the label field in bytes.

Protocol Length: 2 bytes (unsigned integer)
The length of the protocol field in bytes.

Label: Variable Length (sequence of characters)
The name of the data channel as a UTF-8-encoded string, as
specified in [RFC3629]. This may be an empty string.

Protocol: Variable Length (sequence of characters)
If this is an empty string, the protocol is unspecified. If it is
a non-empty string, it specifies a protocol registered in the
"WebSocket Subprotocol Name Registry" created in [RFC6455]. This
string is UTF-8 encoded, as specified in [RFC3629].

5.2. DATA_CHANNEL_ACK Message

This message is sent in response to a DATA_CHANNEL_OPEN_RESPONSE
message. It is sent on the stream used for user messages using the
data channel. Reception of this message tells the opener that the
data channel setup handshake is complete.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |
+-+-+-+-+-+-+-+-+

Message Type: 1 byte (unsigned integer)
This field holds the IANA-defined message type for the
DATA_CHANNEL_ACK message. The value of this field is 0x02, as
specified in Section 8.2.1.

6. Procedures

All DCEP messages MUST be sent using ordered delivery and reliable
transmission. They MUST be sent on the same outgoing stream as the
user messages belonging to the corresponding data channel.
Multiplexing and demultiplexing is done by using the SCTP PPID.
Therefore, a DCEP message MUST be sent with the assigned PPID for the
Data Channel Establishment Protocol (see Section 8.1). Other
messages MUST NOT be sent using this PPID.

The peer that initiates opening a data channel selects a stream
identifier for which the corresponding incoming and outgoing streams
are unused. If the side is acting as the DTLS client, it MUST choose
an even stream identifier; if the side is acting as the DTLS server,
it MUST choose an odd one. The initiating peer fills in the
parameters of the DATA_CHANNEL_OPEN message and sends it on the
chosen stream.

If a DATA_CHANNEL_OPEN message is received on an unused stream, the
stream identifier corresponds to the role of the peer, and all
parameters in the DATA_CHANNEL_OPEN message are valid, then a
corresponding DATA_CHANNEL_ACK message is sent on the stream with the
same stream identifier as the one the DATA_CHANNEL_OPEN message was
received on.

If the DATA_CHANNEL_OPEN message doesn't satisfy the conditions
above, the receiver MUST close the corresponding data channel using
the procedure described in [RFC8831] and MUST NOT send a
DATA_CHANNEL_ACK message in response to the received message. This
might occur if, for example, a DATA_CHANNEL_OPEN message is received
on an already used stream, there are problems with parameters within
the DATA_CHANNEL_OPEN message, the odd/even rule is violated, or the
DATA_CHANNEL_OPEN message itself is not well formed. Therefore,
receiving an SCTP stream-reset request for a stream on which no
DATA_CHANNEL_ACK message has been received indicates to the sender of
the corresponding DATA_CHANNEL_OPEN message the failure of the data
channel setup procedure. After also successfully resetting the
corresponding outgoing stream, which concludes the data channel
closing initiated by the peer, a new DATA_CHANNEL_OPEN message can be
sent on the stream.

After the DATA_CHANNEL_OPEN message has been sent, the sender of that
message MAY start sending messages containing user data without
waiting for the reception of the corresponding DATA_CHANNEL_ACK
message. However, before the DATA_CHANNEL_ACK message or any other
message has been received on a data channel, all other messages
containing user data and belonging to this data channel MUST be sent
ordered, no matter whether the data channel is ordered or not. After
the DATA_CHANNEL_ACK or any other message has been received on the
data channel, messages containing user data MUST be sent ordered on
ordered data channels and MUST be sent unordered on unordered data
channels. Therefore, receiving a message containing user data on an
unused stream indicates an error. In that case, the corresponding
data channel MUST be closed, as described in [RFC8831].

7. Security Considerations

The DATA_CHANNEL_OPEN message contains two variable-length fields:
the protocol and the label. A receiver must be prepared to receive
DATA_CHANNEL_OPEN messages where these fields have the maximum length
of 65535 bytes. Error cases such as using inconsistent lengths of
fields, using unknown parameter values, or violating the odd/even
rule must also be handled by closing the corresponding data channel.
An end point must also be prepared for the peer to open the maximum
number of data channels.

This protocol does not provide privacy, integrity, or authentication.
It needs to be used as part of a protocol suite that contains all
these things. Such a protocol suite is specified in [RFC8261].

For general considerations, see [RFC8826] and [RFC8827].

8. IANA Considerations

IANA has updated the reference of an already existing SCTP PPID
assignment (Section 8.1) and created a new standalone registry with
its own URL for DCEP (Section 8.2) containing two new registration
tables (Sections 8.2.1 and 8.2.2).

8.1. SCTP Payload Protocol Identifier

This document uses an SCTP Payload Protocol Identifier (PPID)
previously registered as "WebRTC Control". [RFC4960] created the
"SCTP Payload Protocol Identifiers" registry, in which this
identifier was assigned. IANA has updated the PPID name from "WebRTC
Control" to "WebRTC DCEP" and has updated the reference to point to
this document. The corresponding date has been kept.

Therefore, this assignment now appears as follows:

+=============+===========+===========+============+
| Value | SCTP PPID | Reference | Date |
+=============+===========+===========+============+
| WebRTC DCEP | 50 | RFC 8832 | 2013-09-20 |
+-------------+-----------+-----------+------------+

Table 2

8.2. New Standalone Registry for DCEP

IANA has created the "Data Channel Establishment Protocol (DCEP)
Parameters" registry. It contains the two tables provided in
Sections 8.2.1 and 8.2.2.

8.2.1. New Message Type Registry

IANA has created the "Message Types" registry for DCEP to manage the
one-byte "Message Type" field in DCEP messages (see Section 5). This
registration table is a subregistry of the registry described in
Section 8.2.

The assignment of new message types is done through an RFC Required
action, as defined in [RFC8126]. Documentation of new message types
MUST contain the following information:

1. A name for the new message type.

2. A detailed procedural description of how each message type is
used with within DCEP.

The following are the initial registrations:

+===================+===========+===========+
| Name | Type | Reference |
+===================+===========+===========+
| Reserved | 0x00 | RFC 8832 |
+-------------------+-----------+-----------+
| Reserved | 0x01 | RFC 8832 |
+-------------------+-----------+-----------+
| DATA_CHANNEL_ACK | 0x02 | RFC 8832 |
+-------------------+-----------+-----------+
| DATA_CHANNEL_OPEN | 0x03 | RFC 8832 |
+-------------------+-----------+-----------+
| Unassigned | 0x04-0xfe | |
+-------------------+-----------+-----------+
| Reserved | 0xff | RFC 8832 |
+-------------------+-----------+-----------+

Table 3

Note that values 0x00 and 0x01 are reserved to avoid interoperability
problems, since they have been used in draft versions of the
document. The value 0xff has been reserved for future extensibility.
The range of possible values is from 0x00 to 0xff.

8.2.2. New Channel Type Registry

IANA has created the "Channel Types" registry for DCEP to manage the
one-byte "Channel Type" field in DATA_CHANNEL_OPEN messages (see
Section 5.1). This registration table is a subregistry within the
registry described in Section 8.2.

The assignment of new message types is done through an RFC Required
action, as defined in [RFC8126]. Documentation of new Channel Types
MUST contain the following information:

1. A name for the new Channel Type.

2. A detailed procedural description of the user message handling
for data channels using this new Channel Type.

If new Channel Types support ordered and unordered message delivery,
the high-order bit MUST be used to indicate whether or not the
message delivery is unordered.

The following are the initial registrations:

+================================================+======+===========+
| Name | Type | Reference |
+================================================+======+===========+
| DATA_CHANNEL_RELIABLE | 0x00 | RFC 8832 |
+------------------------------------------------+------+-----------+
| DATA_CHANNEL_RELIABLE_UNORDERED | 0x80 | RFC 8832 |
+------------------------------------------------+------+-----------+
| DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT | 0x01 | RFC 8832 |
+------------------------------------------------+------+-----------+
| DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED | 0x81 | RFC 8832 |
+------------------------------------------------+------+-----------+
| DATA_CHANNEL_PARTIAL_RELIABLE_TIMED | 0x02 | RFC 8832 |
+------------------------------------------------+------+-----------+
| DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED | 0x82 | RFC 8832 |
+------------------------------------------------+------+-----------+
| Reserved | 0x7f | RFC 8832 |
+------------------------------------------------+------+-----------+
| Reserved | 0xff | RFC 8832 |
+------------------------------------------------+------+-----------+
| Unassigned | rest | |
+------------------------------------------------+------+-----------+

Table 4

Values 0x7f and 0xff have been reserved for future extensibility.
The range of possible values is from 0x00 to 0xff.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.

[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8261] Tuexen, M., Stewart, R., Jesup, R., and S. Loreto,
"Datagram Transport Layer Security (DTLS) Encapsulation of
SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, November
2017, <https://www.rfc-editor.org/info/rfc8261>.

[RFC8831] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
Channels", RFC 8831, DOI 10.17487/RFC8831, January 2021,
<https://www.rfc-editor.org/info/rfc8831>.

9.2. Informative References

[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, DOI 10.17487/RFC4347, April 2006,
<https://www.rfc-editor.org/info/rfc4347>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, DOI 10.17487/RFC6455, December 2011,
<https://www.rfc-editor.org/info/rfc6455>.

[RFC8826] Rescorla, E., "Security Considerations for WebRTC",
RFC 8826, DOI 10.17487/RFC8826, January 2021,
<https://www.rfc-editor.org/info/rfc8826>.

[RFC8827] Rescorla, E., "WebRTC Security Architecture", RFC 8827,
DOI 10.17487/RFC8827, January 2021,
<https://www.rfc-editor.org/info/rfc8827>.

[TLS-DTLS13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
dtls13-39, 2 November 2020,
<https://tools.ietf.org/html/draft-ietf-tls-dtls13-39>.

Acknowledgements

The authors wish to thank Harald Alvestrand, Richard Barnes, Adam
Bergkvist, Spencer Dawkins, Barry Dingle, Stefan Håkansson, Cullen
Jennings, Paul Kyzivat, Doug Leonard, Alexey Melnikov, Pete Resnick,
Irene Rüngeler, Randall Stewart, Peter Thatcher, Martin Thomson,
Justin Uberti, and many others for their invaluable comments.

Authors' Addresses

Randell Jesup
Mozilla
United States of America

Email: [email protected]

Salvatore Loreto
Ericsson
Hirsalantie 11
FI-02420 Jorvas
Finland

Email: [email protected]

Michael Tüxen
Münster University of Applied Sciences
Stegerwaldstrasse 39
48565 Steinfurt
Germany

Email: [email protected]