|RFC 8832||WebRTC Data Channel Establishment Protoc||January 2021|
|Jesup, et al.||Standards Track||[Page]|
- Internet Engineering Task Force (IETF)
- Standards Track
WebRTC Data Channel Establishment Protocol
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.¶
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.¶
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 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.¶
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].¶
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.¶
This document uses the following terms:¶
- An SCTP association.¶
- 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.¶
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.¶
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.¶
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.¶
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)
- Channel Type: 1 byte (unsigned integer)
- 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)
- 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_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|
- 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)
- 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].¶
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 | +-+-+-+-+-+-+-+-+¶
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].¶
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.¶
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).¶
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:¶
|WebRTC DCEP||50||RFC 8832||2013-09-20|
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.¶
- A name for the new message type.¶
- A detailed procedural description of how each message type is used with within DCEP.¶
The following are the initial registrations:¶
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.¶
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.¶
- A name for the new Channel Type.¶
- 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:¶
Values 0x7f and 0xff have been reserved for future extensibility. The range of possible values is from 0x00 to 0xff.¶
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
- Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, , <https://www.rfc-editor.org/info/rfc3629>.
- Stewart, R., Ed., "Stream Control Transmission Protocol", RFC 4960, DOI 10.17487/RFC4960, , <https://www.rfc-editor.org/info/rfc4960>.
- Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <https://www.rfc-editor.org/info/rfc8126>.
- Tuexen, M., Stewart, R., Jesup, R., and S. Loreto, "Datagram Transport Layer Security (DTLS) Encapsulation of SCTP Packets", RFC 8261, DOI 10.17487/RFC8261, , <https://www.rfc-editor.org/info/rfc8261>.
- Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data Channels", RFC 8831, DOI 10.17487/RFC8831, , <https://www.rfc-editor.org/info/rfc8831>.
- Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security", RFC 4347, DOI 10.17487/RFC4347, , <https://www.rfc-editor.org/info/rfc4347>.
- Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, , <https://www.rfc-editor.org/info/rfc6347>.
- Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC 6455, DOI 10.17487/RFC6455, , <https://www.rfc-editor.org/info/rfc6455>.
- Rescorla, E., "Security Considerations for WebRTC", RFC 8826, DOI 10.17487/RFC8826, , <https://www.rfc-editor.org/info/rfc8826>.
- Rescorla, E., "WebRTC Security Architecture", RFC 8827, DOI 10.17487/RFC8827, , <https://www.rfc-editor.org/info/rfc8827>.
- 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, , <https://tools.ietf.org/html/draft-ietf-tls-dtls13-39>.
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.¶