Internet Engineering Task Force (IETF) K. Drage

Internet Engineering Task Force (IETF) K. Drage
Request for Comments: 8864 M. Makaraju
Category: Standards Track R. Ejzak
ISSN: 2070-1721 J. Marcon
Unaffiliated
R. Even, Ed.
January 2021

Negotiation Data Channels Using the Session Description Protocol (SDP)

Abstract

Data channel setup can be done using either the in-band Data Channel
Establishment Protocol (DCEP) or some out-of-band non-DCEP protocol.
This document specifies how the SDP (Session Description Protocol)
offer/answer exchange can be used to achieve an out-of-band non-DCEP
negotiation for establishing a data channel.

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/rfc8864.

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
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
2. Conventions
3. Terminology
4. Applicability Statement
5. SDP Data Channel Attributes
5.1. SDP DCMAP Attribute
5.1.1. DCMAP Attribute Syntax
5.1.2. 'dcmap-stream-id' Parameter
5.1.3. 'label' Parameter
5.1.4. 'subprotocol' Parameter
5.1.5. 'max-retr' Parameter
5.1.6. 'max-time' Parameter
5.1.7. 'ordered' Parameter
5.1.8. 'priority' Parameter
5.1.9. DCMAP Multiplexing Category
5.2. SDP DCSA Attribute
5.2.1. DCSA Attribute Syntax
5.2.2. DCSA Multiplexing Category
6. SDP Offer/Answer Procedures
6.1. Managing Stream Identifiers
6.2. Negotiating Data Channel Parameters
6.3. Generating the Initial Offer for a Data Channel
6.4. Generating the SDP Answer
6.5. Offerer Processing of the SDP Answer
6.6. Modifying the Session
6.6.1. Closing a Data Channel
6.7. Various SDP Offer/Answer Considerations
7. Examples
8. Security Considerations
9. IANA Considerations
9.1. Subprotocol Identifiers
9.2. New SDP Attributes
9.2.1. dcmap
9.2.2. dcsa
9.3. Registering Attributes for Use with Data Channels
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Generic Data Channel Negotiation Aspects when Not
Using DCEP
A.1. Stream Identifier Numbering
A.2. Generic Data Channel Negotiation Not Using DCEP
A.2.1. Overview
A.2.2. Opening a Data Channel
A.2.3. Closing a Data Channel
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

The concept of establishing a bidirectional data channel running on
top of the Stream Control Transmission Protocol (SCTP) is discussed
in [RFC8831], allowing applications to use data channels. An in-band
Data Channel Establishment Protocol (DCEP) is described in [RFC8832];
however, other in-band or out-of-band protocols may be used for
establishing data channels. Each data channel consists of paired
SCTP streams sharing the same SCTP Stream Identifier. Data channels
are created by endpoint applications using (1) the WebRTC API
(Application Programming Interface) [WebRtcAPI] or (2) other
protocols (e.g., Controlling Multiple Streams for Telepresence (CLUE)
[RFC8850]). The protocols can be signaled by the data channel
'subprotocol' parameter, conceptually similar to a WebSocket
subprotocol as described in [RFC6455]. However, apart from the
"subprotocol" value transmitted to the peer, an endpoint application
can agree on how to instantiate a given subprotocol on a data
channel, and whether it is signaled in-band using DCEP or out-of-band
using a non-DCEP protocol (or both).

This document defines Session Description Protocol (SDP) offer/answer
procedures [RFC3264] that enable out-of-band negotiation for
establishing data channels for transport of well-defined
subprotocols. These procedures are based on generic SDP offer/answer
negotiation rules for SCTP-based media transport as specified in
[RFC8841] for the SDP "m=" line proto values UDP/DTLS/SCTP and
TCP/DTLS/SCTP.

This document uses MSRP (the Message Session Relay Protocol)
[RFC4975] and BFCP (the Binary Floor Control Protocol) [RFC8855] in
several examples. It does not provide a complete specification of
how to negotiate the use of a data channel to transport MSRP.
Procedures specific to each subprotocol would have to be documented
elsewhere. For MSRP, they are documented in [RFC8873]. The use of
MSRP in some examples is only to show how the generic procedures
described herein might apply to a specific subprotocol.

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:

Data channel: A WebRTC data channel as specified in [RFC8831].

Data channel stack: An entity that, upon application request, runs
the data channel protocol to keep track of states as well as the
sending and receiving of data. If the application is a browser-
based JavaScript application, then this stack resides in the
browser. If the application is a native application, then this
stack resides in the application and is accessible via some sort
of API or APIs.

Data channel properties: Fixed properties assigned to a data channel
at the time of its creation. Some of these properties determine
the way the data channel stack transmits data on this channel
(e.g., stream identifier, reliability, order of delivery).

Data channel subprotocol: The application protocol that is
transported over a single data channel. Data channel subprotocol
messages are sent as data channel payload over an established data
channel. An SDP offer/answer exchange can be used as specified in
this document to negotiate the establishment of data channels,
corresponding data channel properties, associated data channel
subprotocols, and data channel subprotocol properties. In this
case, the data channel subprotocols may be identified by the
values of the 'subprotocol' parameters of the SDP "a=dcmap:"
attribute as described in Section 5.1.4. Within this document,
the term "data channel subprotocol" is often abbreviated as just
"subprotocol".

DCEP: Data Channel Establishment Protocol, as defined in [RFC8832].

In-band: Transmission through the peer-to-peer SCTP association.

Out-of-band: Transmission through the application signaling path.

Peer: From the perspective of one of the agents in a session, its
peer is the other agent. Specifically, from the perspective of
the SDP offerer, the peer is the SDP answerer. From the
perspective of the SDP answerer, the peer is the SDP offerer.

SCTP Stream Sequence Number (SSN): The SCTP Stream Sequence Number,
as specified in [RFC4960].

Stream identifier: The identifier of the outbound and inbound SCTP
streams composing a data channel.

4. Applicability Statement

The mechanism described in this document only applies to SDP
[RFC8866] when used together with the SDP offer/answer mechanism
[RFC3264]. Declarative usage of SDP is out of scope for this
document and is thus undefined.

5. SDP Data Channel Attributes

This section defines two new SDP media-level attributes that can be
used together with the SDP Offer/Answer mechanism to negotiate data-
channel-specific and subprotocol-specific parameters without the
usage of DCEP [RFC8832]. The first attribute (Section 5.1) provides
for negotiation of channel-specific parameters. The second attribute
(Section 5.2) provides for negotiation of subprotocol-specific
parameters.

| Note: Appendix A provides information regarding how data
| channels work in general. In particular, it summarizes some
| key aspects that should be considered for the negotiation of
| data channels if DCEP is not used.

5.1. SDP DCMAP Attribute

This section defines a new media-level attribute, "a=dcmap:", that
defines the data channel parameters for each data channel to be
negotiated.

This attribute is used to create bidirectional SCTP data channels
having the same set of attributes. The data channel properties
(reliable / partially reliable, ordered/unordered) need to be
suitable per the subprotocol transport requirements.

5.1.1. DCMAP Attribute Syntax

"a=dcmap:" is a media-level attribute having the following definition
and ABNF (Augmented Backus-Naur Form) syntax [RFC5234].

+=================================+
| "a=dcmap:" Attribute |
+===================+=============+
| Name | dcmap |
+-------------------+-------------+
| Value | dcmap-value |
+-------------------+-------------+
| Usage Level | media |
+-------------------+-------------+
| Charset Dependent | No |
+-------------------+-------------+

Table 1: "a=dcmap:" Attribute
Definition

Formal syntax:

dcmap-value = dcmap-stream-id
[ SP dcmap-opt *(";" dcmap-opt) ]
dcmap-opt = ordering-opt / subprotocol-opt / label-opt
/ maxretr-opt / maxtime-opt / priority-opt
; maxretr-opt and maxtime-opt are
; mutually exclusive

dcmap-stream-id = 1*5DIGIT
ordering-opt = "ordered=" ordering-value
ordering-value = "true" / "false"
subprotocol-opt = "subprotocol=" quoted-string
label-opt = "label=" quoted-string
maxretr-opt = "max-retr=" maxretr-value
maxretr-value = "0" / integer
; number of retransmissions,
; less than 2^32,
; derived from 'Reliability Parameter' [RFC8832]
maxtime-opt = "max-time=" maxtime-value
maxtime-value = "0" / integer
; milliseconds,
; less than 2^32,
; derived from 'Reliability Parameter' [RFC8832]
priority-opt = "priority=" priority-value
priority-value = "0" / integer
; unsigned integer value indicating the priority of
; the data channel,
; less than 2^16,
; derived from 'Priority' [RFC8832]

quoted-string = DQUOTE *(quoted-char / escaped-char) DQUOTE
quoted-char = SP / quoted-visible
quoted-visible = %x21 / %x23-24 / %x26-7E ; VCHAR without " or %
escaped-char = "%" HEXDIG HEXDIG
DQUOTE = <from RFC 5234>
integer = <from RFC 8866>

Examples:

a=dcmap:0
a=dcmap:1 subprotocol="bfcp";max-time=60000;priority=512
a=dcmap:2 subprotocol="msrp";ordered=true;label="msrp"
a=dcmap:3 label="Label 1";ordered=false;max-retr=5;priority=128
a=dcmap:4 label="foo%09bar";ordered=true;max-time=15000

| Note: The last example (a=dcmap:4) shows a 'label' parameter
| value that contains one nonprintable 'escaped-char' character
| (the tabulator character).

Within an "a=dcmap:" attribute line's 'dcmap-opt' value, only one
'maxretr-opt' parameter or one 'maxtime-opt' parameter may be
present. Both parameters MUST NOT be present.

5.1.2. 'dcmap-stream-id' Parameter

The 'dcmap-stream-id' parameter indicates the SCTP stream identifier
within the SCTP association used to form the data channel.

5.1.3. 'label' Parameter

The 'label' parameter indicates the name of the channel. It
represents a label that can be used to distinguish, in the context of
the WebRTC API [WebRtcAPI], an RTCDataChannel object from other
RTCDataChannel objects. This parameter maps to the 'Label' parameter
defined in [RFC8832]. The 'label' parameter is optional. If it is
not present, then its value defaults to the empty string.

In order to communicate with the WebRTC API, the 'label' parameter
should

* Serialize the WebRTC label as a UTF-8 string [RFC3629].

* Treat the UTF-8 serialization as a series of bytes.

* For each byte in the serialization,

- If the byte can be expressed as a 'quoted-char', do so.

- Otherwise, express the byte as an 'escaped-char'.

| Note: The empty string can also be explicitly used as a 'label'
| value, such that 'label=""' is equivalent to the 'label'
| parameter not being present at all. [RFC8832] allows the
| DATA_CHANNEL_OPEN message's 'Label' value to be an empty
| string.

5.1.4. 'subprotocol' Parameter

The 'subprotocol' parameter indicates which protocol the client
expects to exchange via the channel. This parameter maps to the
'Protocol' parameter defined in [RFC8832]. Section 9.1 specifies how
values for new subprotocol parameters are registered. 'subprotocol'
is an optional parameter. If the 'subprotocol' parameter is not
present, then its value defaults to an empty string.

| Note: The empty string can also be explicitly used as a
| 'subprotocol' value, such that 'subprotocol=""' is equivalent
| to the 'subprotocol' parameter not being present at all.
| [RFC8832] allows the DATA_CHANNEL_OPEN message's 'Protocol'
| value to be an empty string.

5.1.5. 'max-retr' Parameter

This parameter indicates that the data channel is partially reliable.
The 'max-retr' parameter indicates the maximal number of times a user
message will be retransmitted. The 'max-retr' parameter is optional.
If the 'max-retr' parameter and the 'max-time' parameter are not
present, then reliable transmission is performed as specified in
[RFC4960]. This parameter maps to the 'Number of RTX' parameter
defined in [RFC8832].

5.1.6. 'max-time' Parameter

This parameter indicates that the data channel is partially reliable.
A user message will no longer be transmitted or retransmitted after a
specified lifetime, given in milliseconds, in the 'max-time'
parameter. The lifetime starts when providing the user message to
the protocol stack. The 'max-time' parameter is optional. If the
'max-retr' parameter and the 'max-time' parameter are not present,
then reliable transmission is performed as specified in [RFC4960].
This parameter maps to the 'Lifetime in ms' parameter defined in
[RFC8832].

5.1.7. 'ordered' Parameter

The 'ordered' parameter with value "true" indicates that the receiver
will dispatch DATA chunks in the data channel to the upper layer
while preserving the order. The 'ordered' parameter is optional and
takes two values -- "true" for ordered delivery and "false" for
unordered delivery -- with "true" as the default value. Any other
value is ignored, and the default "ordered=true" is assumed. In the
absence of this parameter, "ordered=true" is assumed. This parameter
maps to the ordered or unordered data channel types as defined in
[RFC8832].

5.1.8. 'priority' Parameter

The 'priority' parameter indicates the data channel's priority
relative to the priorities of other data channels, which may
additionally exist over the same SCTP association. The 'priority'
parameter maps to the 'Priority' parameter defined in [RFC8832]. The
'priority' parameter is optional. In the absence of this parameter,
"priority=256" is assumed.

5.1.9. DCMAP Multiplexing Category

The multiplexing category [RFC8859] of the "a=dcmap:" attribute is
SPECIAL.

As the usage of multiple SCTP associations on top of a single DTLS
association is outside the scope of [RFC8841], no "a=dcmap:"
attribute multiplexing rules are specified for the UDP/DTLS/SCTP and
TCP/DTLS/SCTP proto values. If future extensions of [RFC8841] define
how to negotiate multiplexing of multiple SCTP associations on top of
a single DTLS association or how to add multiple SCTP associations to
one BUNDLE group, then multiplexing rules for the "a=dcmap:"
attribute need to be defined as well -- for instance, in an extension
of this specification.

5.2. SDP DCSA Attribute

In the SDP media description, each data channel declaration MAY also
be followed by other SDP attributes, which apply to the corresponding
data channel and its subprotocol. Each of these attributes is
represented by one new "a=dcsa:" attribute line that references
another SDP attribute defined for use with this data channel's
subprotocol. Instructions for registering attributes for use with a
data channel are given in Section 9.3.

Each SDP attribute that is related to the subprotocol and that would
normally be used to negotiate the subprotocol using the SDP offer/
answer mechanism is replaced with an attribute of the form
"a=dcsa:stream-id original-attribute", where "dcsa" stands for "data
channel subprotocol attribute", "stream-id" is the SCTP stream
identifier assigned to this subprotocol instance, and "original-
attribute" represents the contents of the subprotocol-related
attribute to be included.

The same syntax applies to any other SDP attribute required for
negotiation of this instance of the subprotocol.

The detailed offer/answer procedures for the dcsa attribute are
dependent on the associated subprotocol. If no offer/answer
procedures exist for the subprotocol when used outside of the dcsa
attribute, no specification is needed for use with dcsa. The IANA
(Internet Assigned Numbers Authority) registration procedures for the
"WebSocket Subprotocol Name Registry" (Section 9.1) do not strictly
require a specification of the offer/answer procedures for the
subprotocol when used with dcsa. If the subprotocol has defined
offer/answer procedures when used outside of dcsa, such a
specification is encouraged to ensure interoperability. If the
subprotocol has defined offer/answer procedures when used outside of
dcsa but no specification exists for the offer/answer procedures for
the subprotocol when used with dcsa, implementations SHOULD assume
the use of the default values for all otherwise-negotiable and
applicable subprotocol parameters.

5.2.1. DCSA Attribute Syntax

"a=dcsa:" is a media-level attribute having the following definition
and ABNF (Augmented Backus-Naur Form) syntax [RFC5234].

+================================+
| "a=dcsa:" Attribute |
+===================+============+
| Name | dcsa |
+-------------------+------------+
| Value | dcsa-value |
+-------------------+------------+
| Usage Level | media |
+-------------------+------------+
| Charset Dependent | No |
+-------------------+------------+

Table 2: "a=dcsa:" Attribute
Definition

Formal syntax:

dcsa-value = stream-id SP attribute
stream-id = 1*5DIGIT
attribute = <from RFC 8866>

Example:

a=dcmap:2 subprotocol="msrp";ordered=true;label="msrp"

a=dcsa:2 accept-types:text/plain

The reference to [RFC8866] defines where the attribute definition can
be found; it does not provide any limitations on support of
attributes defined in other documents in accordance with this
attribute definition. However, not all SDP attributes are suitable
as an "a=dcsa:" parameter. The registry of IANA SDP parameters
contains the lists of IANA-registered session-level and media-level
or media-level-only SDP attributes.

Thus, in the example above, the original attribute line
"a=accept-types:text/plain" is represented by the attribute line
"a=dcsa:2 accept-types:text/plain", which specifies that this
instance of the MSRP subprotocol being transported on the SCTP
association using the data channel with stream id 2 accepts plaintext
files.

As opposed to the data channel "a=dcmap:" attribute parameters, these
parameters are subject to offer/answer negotiation, following the
procedures defined in the subprotocol-specific documents.

It is assumed that in general the usages of subprotocol-related
media-level attributes are independent from the subprotocol's
transport protocol. Such transport-protocol-independent subprotocol-
related attributes are used in the same way as defined in the
original subprotocol specification, also if the subprotocol is
transported over a data channel and if the attribute is
correspondingly embedded in an "a=dcsa:" attribute.

There may be cases where the usage of a subprotocol-related media-
level attribute depends on the subprotocol's transport protocol. In
such cases, the subprotocol-related usage of the attribute is
expected to be described for the data channel transport. A data-
channel-specific usage of a subprotocol attribute is expected to be
specified in the same document that registers the subprotocol's
identifier for data channel usage as described in Section 9.1.

5.2.2. DCSA Multiplexing Category

The multiplexing category of the "a=dcsa:" attribute is SPECIAL.

As the usage of multiple SCTP associations on top of a single DTLS
association is outside the scope of [RFC8841], no "a=dcsa:" attribute
multiplexing rules are specified for the UDP/DTLS/SCTP and
TCP/DTLS/SCTP proto values. If future extensions of [RFC8841] define
how to negotiate multiplexing of multiple SCTP associations on top of
a single DTLS association or how to add multiple SCTP associations to
one BUNDLE group, then multiplexing rules for the "a=dcsa:" attribute
need to be defined as well -- for instance, in an extension of this
specification.

6. SDP Offer/Answer Procedures

This section defines how data channels can be negotiated using the
SDP offer/answer mechanism. A given media description can describe
multiple data channels (each represented by a separate SDP dcmap
attribute) that can be created, modified, and closed using different
offer/answer exchanges. The procedures in this section apply for a
given data channel.

The generic offer/answer procedures for negotiating the SCTP
association used to realize data channels are defined in [RFC8841].
This section only defines the data-channel-specific procedures.

"Initial offer" refers to the offer in which a data channel is
opened. It can be either the initial offer or a subsequent offer of
the associated SDP session.

The detailed offer/answer procedures for the dcsa attribute are
dependent on the associated subprotocol; see Section 5.2.

6.1. Managing Stream Identifiers

In order to avoid SCTP Stream identifier collisions, in alignment
with [RFC8832], the endpoint acting as a DTLS client (for the SCTP
association used to realize data channels) MUST use even identifier
values, and the endpoint acting as a DTLS server MUST use odd
identifier values.

SCTP stream identifiers associated with data channels that have been
negotiated using DCEP MUST NOT be included in SDP offers and answers.

6.2. Negotiating Data Channel Parameters

The data channel types defined in [RFC8832] are mapped to the dcmap
SDP attribute parameters in the following manner, where
"ordered=true" is the default and may be omitted:

DATA_CHANNEL_RELIABLE
ordered=true

DATA_CHANNEL_RELIABLE_UNORDERED
ordered=false

DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT
ordered=true;max-retr=<number of retransmissions>

DATA_CHANNEL_PARTIAL_RELIABLE_REXMIT_UNORDERED
ordered=false;max-retr=<number of retransmissions>

DATA_CHANNEL_PARTIAL_RELIABLE_TIMED
ordered=true;max-time=<lifetime in milliseconds>

DATA_CHANNEL_PARTIAL_RELIABLE_TIMED_UNORDERED
ordered=false;max-time=<lifetime in milliseconds>

By definition, 'max-retr' and 'max-time' are mutually exclusive, so
both MUST NOT be present in the "a=dcmap:" attribute line. If an SDP
offer contains both of these parameters, then the receiver of such an
SDP offer MUST reject the SDP offer. If an SDP answer contains both
of these parameters, then the offerer MUST treat the associated SDP
offer/answer as failed.

6.3. Generating the Initial Offer for a Data Channel

When an offerer sends an initial offer, in order to negotiate an SCTP
stream for a data channel, the offerer

* SHALL include an SDP dcmap attribute (Sections 5.1 and 6.2)
associated with the data channel in the "m=" section representing
the SCTP association used to realize the data channel, and

* MAY include one or more SDP dcsa attributes (Section 5.2)
associated with the data channel. The value of the 'stream-id'
part of each attribute SHALL match the 'dcmap-stream-id' value of
the dcmap attribute.

6.4. Generating the SDP Answer

When an answerer receives an offer that includes an "m=" section for
an SCTP association, the offer describes an SCTP stream for a data
channel, if the answerer accepts the data channel, it

* SHALL include an SDP dcmap attribute (Sections 5.1 and 6.2)
associated with the data channel in the "m=" section representing
the SCTP association used to realize the data channel. The value
of the 'dcmap-stream-id', 'max-retr', and 'max-time' values of the
dcmap attribute SHALL be identical to the value used for the data
channel in the offer, and

* MAY include one or more SDP dcsa attributes (Section 5.2)
associated with the data channel.

6.5. Offerer Processing of the SDP Answer

An offerer receiving an SDP answer performs the following:

* It SHALL close any created data channels as described in
Section 6.6.1 for which the expected "a=dcmap:" attributes are not
present in the SDP answer. If the SDP answer has no "a=dcmap:"
attributes, either the peer does not support "a=dcmap:" attributes
or it rejected all the data channels. In either case, the offerer
closes all the data channels offered by SDP that were open at the
time of the offer. The DTLS association and SCTP association will
still be set up. At this point, the offerer may use DCEP
negotiation [RFC8832] to open data channels.

Each agent application MUST wait to send data until it has
confirmation that the data channel at the peer is instantiated. For
WebRTC, this is when both data channel stacks have channel parameters
instantiated and occurs as follows:

* At both peers when a data channel is created without a previously
established SCTP association, as soon as the SCTP association is
successfully established.

* At the agent receiving an SDP offer for which there is an
established SCTP association, as soon as it creates the negotiated
data channel based on information signaled in the SDP offer.

* At the agent sending an SDP offer to create a new data channel for
which there is an established SCTP association, when it receives
the SDP answer confirming acceptance of the data channel or when
it begins to receive data on the data channel from the peer,
whichever occurs first.

6.6. Modifying the Session

When an offerer sends a subsequent offer that includes information
for a previously negotiated data channel, unless the offerer intends
to close the data channel (Section 6.6.1), the offerer SHALL include
the previously negotiated SDP attributes and attribute values
associated with the data channel. The answerer may reject the offer.
The means for rejecting an offer are dependent on the higher-layer
protocol. The offer/answer exchange is atomic; if the answer is
rejected, the session reverts to the state prior to the offer
[RFC3264].

6.6.1. Closing a Data Channel

In order to close a data channel, the endpoint that wants to close
the data channel SHALL send an SCTP SSN Reset message [RFC6525],
following the procedure in Section 6.7 of [RFC8831]. In addition, if
the closed data channel was negotiated using the offer/answer
mechanism (Section 6.3), the endpoint that closed the data channel
SHALL send a subsequent offer in which it does one of the following:

* Removes the SDP dcmap attribute and SDP dcsa attributes associated
with the closed data channel. Once the endpoint receives a
successful answer, the SCTP stream identifier value can later be
used for a new data channel (negotiated using either SCTP or the
offer/answer mechanism), or

* After a reset has been performed, reuses the SCTP stream used for
the closed data channel for a new data channel, following the
procedure in Section 6.3. The offerer SHALL use a different SDP
dcmap attribute value for the data channel using the same SCTP
stream.

6.7. Various SDP Offer/Answer Considerations

An SDP offer or answer has no "a=dcmap:" attributes but has "a=dcsa:"
attributes:

* This is considered an error case. In this case, the receiver of
such an SDP offer or answer MUST discard the "a=dcsa:" attributes.

An SDP offer or answer has an "a=dcsa:" attribute whose subprotocol
attribute is unknown:

* The receiver of such an SDP offer or answer SHOULD ignore this
entire "a=dcsa:" attribute line.

An SDP offer or answer has an "a=dcsa:" attribute whose subprotocol
attribute is known but whose subprotocol attribute semantic is not
known for the data channel transport case:

* The receiver of such an SDP offer or answer SHOULD ignore this
entire "a=dcsa:" attribute line.

7. Examples

Figure 1 shows an example of an SDP offer and answer where the SDP
answerer rejects the data channel with stream id 0 either for
explicit reasons or because it does not understand the "a=dcmap:"
attribute. As a result, the offerer will close the data channel
created with the SDP offer/answer negotiation option. The SCTP
association will still be set up over DTLS. At this point, the
offerer or the answerer may use DCEP negotiation to open data
channels.

m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:db8::3
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=tls-id:abc3de65cddef001be82
a=dcmap:0 subprotocol="bfcp";label="bfcp"
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:db8::1
a=max-message-size:100000
a=sctp-port:5002
a=setup:passive
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=tls-id:dcb3ae65cddef0532d42

Figure 1: Example 1

Figure 2 shows an example of an SDP offer and answer where the SDP
offer contains data channels for BFCP and MSRP subprotocols. The SDP
answer rejects BFCP and accepts MSRP. So, the offerer closes the
data channel for BFCP, and both the offerer and the answerer may
start using the MSRP data channel (after the SCTP association is
set up). The data channel with stream id 0 is free and can be used
for future DCEP or SDP offer/answer negotiation.

m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 192.0.2.1
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=tls-id:abc3de65cddef001be82
a=dcmap:0 subprotocol="bfcp";label="bfcp"
a=dcmap:2 subprotocol="msrp";label="msrp"
a=dcsa:2 accept-types:message/cpim text/plain
a=dcsa:2 path:msrp://alice.example.com:10001/2s93i93idj;dc
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 192.0.2.2
a=max-message-size:100000
a=sctp-port:5002
a=setup:passive
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=tls-id:dcb3ae65cddef0532d42
a=dcmap:2 subprotocol="msrp";label="msrp"
a=dcsa:2 accept-types:message/cpim text/plain
a=dcsa:2 path:msrp://bob.example.com:10002/si438dsaodes;dc

Figure 2: Example 2

The example in Figure 3 is a continuation of the example in Figure 2.
The SDP offerer now removes the MSRP data channel with stream id 2
but opens a new MSRP data channel with stream id 4. The answerer
accepts the entire offer. As a result, the offerer closes the
previously negotiated MSRP-related data channel, and both the offerer
and the answerer may start using the new MSRP-related data channel.

m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 192.0.2.1
a=max-message-size:100000
a=sctp-port:5000
a=setup:actpass
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
a=tls-id:abc3de65cddef001be82
a=dcmap:4 subprotocol="msrp";label="msrp"
a=dcsa:4 accept-types:message/cpim text/plain
a=dcsa:4 path:msrp://alice.example.com:10001/2s93i93idj;dc
m=application 10002 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 192.0.2.2
a=max-message-size:100000
a=sctp-port:5002
a=setup:passive
a=fingerprint:SHA-1 \
5B:AD:67:B1:3E:82:AC:3B:90:02:B1:DF:12:5D:CA:6B:3F:E5:54:FA
a=tls-id:dcb3ae65cddef0532d42
a=dcmap:4 subprotocol="msrp";label="msrp"
a=dcsa:4 accept-types:message/cpim text/plain
a=dcsa:4 path:msrp://bob.example.com:10002/si438dsaodes;dc

Figure 3: Example 3

8. Security Considerations

This document specifies new SDP attributes used in the negotiation of
data channel parameters.

These parameters are negotiated as part of opening an SCTP channel
over DTLS as specified in [RFC8841]. Each subprotocol may come with
its own security considerations that need to be documented as part of
the subprotocol definition. Otherwise, this document does not add
any security considerations to those specified in [RFC8841].

Error cases such as the use of unknown parameter values or violations
of the odd/even rule (Section 6.1) MUST be handled by closing the
corresponding data channel.

9. IANA Considerations

9.1. Subprotocol Identifiers

Registration of new subprotocol identifiers is performed using the
existing IANA "WebSocket Subprotocol Name Registry" table.

The following text has been added below the title of the table.

"This table also includes subprotocol identifiers specified for usage
within a WebRTC data channel."

This document (RFC 8864) has been added to the "Reference" list for
the registry.

This document assigns no new values to this table.

A subprotocol may simultaneously be defined for data channel
transport and for WebSocket transport. In such a case, the
"Subprotocol Definition" and "Reference" cells in the subprotocol's
row of the IANA "WebSocket Subprotocol Name Registry" table should
contain two entries. One entry in each of these cells should refer
to the WebSocket-related subprotocol specification, and the other
entry should refer to the data-channel-related subprotocol
specification.

9.2. New SDP Attributes

9.2.1. dcmap

This document defines a new SDP media-level attribute, "a=dcmap:", as
follows:

+==================================================================+
| "a=dcmap:" |
+=====================+============================================+
| Contact name | IESG |
+---------------------+--------------------------------------------+
| Contact email | [email protected] |
+---------------------+--------------------------------------------+
| Attribute name | dcmap |
+---------------------+--------------------------------------------+
| Attribute syntax | As per Section 5.1.1 |
+---------------------+--------------------------------------------+
| Attribute semantics | As per Section 5.1.1 |
+---------------------+--------------------------------------------+
| Usage level | media |
+---------------------+--------------------------------------------+
| Charset dependent | No |
+---------------------+--------------------------------------------+
| Purpose | To define data-channel-specific parameters |
+---------------------+--------------------------------------------+
| Appropriate values | As per Section 5.1.1 |
+---------------------+--------------------------------------------+
| O/A procedures | SDP offer/answer procedures as per |
| | Section 6 |
+---------------------+--------------------------------------------+
| Mux category | SPECIAL. See Section 5.1.9 |
+---------------------+--------------------------------------------+
| Reference | RFC 8864 |
+---------------------+--------------------------------------------+

Table 3: New "a=dcmap:" Attribute

9.2.2. dcsa

This document defines a new SDP media-level attribute, "a=dcsa:", as
follows:

+=============================================================+
| "a=dcsa:" |
+=====================+=======================================+
| Contact name | IESG |
+---------------------+---------------------------------------+
| Contact email | [email protected] |
+---------------------+---------------------------------------+
| Attribute name | dcsa |
+---------------------+---------------------------------------+
| Attribute syntax | As per Section 5.2.1 |
+---------------------+---------------------------------------+
| Attribute semantics | As per Section 5.2.1 |
+---------------------+---------------------------------------+
| Usage level | media |
+---------------------+---------------------------------------+
| Charset dependent | No |
+---------------------+---------------------------------------+
| Purpose | To define attributes that are |
| | specific to data channel subprotocols |
+---------------------+---------------------------------------+
| Appropriate values | As per Section 5.2.1 |
+---------------------+---------------------------------------+
| O/A procedures | SDP offer/answer procedures as per |
| | Section 6 |
+---------------------+---------------------------------------+
| Mux category | SPECIAL. See Section 5.2.2 |
+---------------------+---------------------------------------+
| Reference | RFC 8864 |
+---------------------+---------------------------------------+

Table 4: New "a=dcsa:" Attribute

9.3. Registering Attributes for Use with Data Channels

When a subprotocol is defined for use over data channels with the SDP
offer/answer mechanism, any SDP attributes that may be negotiated
using the "a=dcsa:" attribute MUST be added to the IANA "attribute-
name registry (formerly "att-field")", as specified in [RFC8866],
Section 8.2.4. This document specifies that new Usage Levels of the
form "dcsa (foo)" (where "foo" is a placeholder for the subprotocol
name) should be registered by documents that specify negotiation of
particular subprotocols.

IANA has updated the "attribute-name (formerly "att-field")" registry
to point to this document.

10. References

10.1. 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>.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<https://www.rfc-editor.org/info/rfc3264>.

[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>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[RFC6525] Stewart, R., Tuexen, M., and P. Lei, "Stream Control
Transmission Protocol (SCTP) Stream Reconfiguration",
RFC 6525, DOI 10.17487/RFC6525, February 2012,
<https://www.rfc-editor.org/info/rfc6525>.

[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>.

[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>.

[RFC8832] Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data Channel
Establishment Protocol", RFC 8832, DOI 10.17487/RFC8832,
January 2021, <https://www.rfc-editor.org/info/rfc8832>.

[RFC8841] Holmberg, C., Shpount, R., Loreto, S., and G. Camarillo,
"Session Description Protocol (SDP) Offer/Answer
Procedures for Stream Control Transmission Protocol (SCTP)
over Datagram Transport Layer Security (DTLS) Transport",
RFC 8841, DOI 10.17487/RFC8841, January 2021,
<https://www.rfc-editor.org/info/rfc8841>.

[RFC8859] Nandakumar, S., "A Framework for Session Description
Protocol (SDP) Attributes When Multiplexing", RFC 8859,
DOI 10.17487/RFC8859, January 2021,
<https://www.rfc-editor.org/info/rfc8859>.

[RFC8866] Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP:
Session Description Protocol", RFC 8866,
DOI 10.17487/RFC8866, January 2021,
<https://www.rfc-editor.org/info/rfc8866>.

10.2. Informative References

[RFC4975] Campbell, B., Ed., Mahy, R., Ed., and C. Jennings, Ed.,
"The Message Session Relay Protocol (MSRP)", RFC 4975,
DOI 10.17487/RFC4975, September 2007,
<https://www.rfc-editor.org/info/rfc4975>.

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

[RFC8850] Holmberg, C., "Controlling Multiple Streams for
Telepresence (CLUE) Protocol Data Channel", RFC 8850,
DOI 10.17487/RFC8850, January 2021,
<https://www.rfc-editor.org/info/rfc8850>.

[RFC8855] Camarillo, G., Drage, K., Kristensen, T., Ott, J., and C.
Eckel, "The Binary Floor Control Protocol (BFCP)",
RFC 8855, DOI 10.17487/RFC8855, January 2021,
<https://www.rfc-editor.org/info/rfc8855>.

[RFC8873] Recio, JM., Ed. and C. Holmberg, "Message Session Relay
Protocol (MSRP) over Data Channels", RFC 8873,
DOI 10.17487/RFC8873, January 2021,
<https://www.rfc-editor.org/info/rfc8873>.

[T38] International Telecommunication Union, "Procedures for
real-time Group 3 facsimile communication over IP
networks", ITU-T Recommendation T.38, November 2015,
<https://www.itu.int/rec/T-REC-T.38-201511-I/en>.

[WebRtcAPI]
Jennings, C., Boström, H., and J-I. Bruaroey, "WebRTC 1.0:
Real-time Communication Between Browsers", W3C Proposed
Recommendation, <https://www.w3.org/TR/webrtc/>.

Appendix A. Generic Data Channel Negotiation Aspects when Not Using
DCEP

This appendix summarizes how data channels work in general and
discusses some key aspects that should be considered for the out-of-
band negotiation of data channels if DCEP is not used.

A WebRTC application creates a data channel by providing a number of
setup parameters (subprotocol, label, maximal number of
retransmissions, maximal retransmission time, order of delivery,
priority). The application also specifies whether it wants to make
use of the negotiation using DCEP [RFC8832] or intends to negotiate
data channels using the SDP offer/answer protocol.

In any case, the SDP offer generated by the application is per
[RFC8841]. In brief, it contains one "m=" line for the SCTP
association on top of which the data channels will run:

m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP4 192.0.2.1
a=max-message-size:100000
a=sctp-port:5000
a=tls-id:abc3de65cddef001be82
a=setup:actpass
a=fingerprint:SHA-1 \
4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB

| Note: A WebRTC application will only use the "m=" line format
| "webrtc-datachannel" and will not use other formats in the "m="
| line for other protocols such as T.38 [T38]. [RFC8841]
| supports only one SCTP association to be established on top of
| a DTLS association.

| Note: The above SDP media description does not contain any
| channel-specific information.

A.1. Stream Identifier Numbering

Independently from the requested type of negotiation, the application
creating a data channel can either (1) pass the stream identifier to
the data channel stack to assign to the data channel or (2) let the
data channel stack pick one identifier from the unused ones.

Moreover, to avoid glare situations [RFC3264], each endpoint can own
an exclusive set of stream identifiers, in which case an endpoint can
only create a data channel with a stream identifier it owns.

Which set of stream identifiers is owned by which endpoint is
determined by convention or other means.

| Note: For data channels negotiated with DCEP, one endpoint owns
| by convention the even stream identifiers, whereas the other
| owns the odd stream identifiers, as defined in [RFC8832].

| Note: For data channels negotiated via a protocol other than
| DCEP, no convention is defined by default.

A.2. Generic Data Channel Negotiation Not Using DCEP

A.2.1. Overview

DCEP negotiation only provides for negotiation of data channel
transport parameters and does not provide for negotiation of
subprotocol-specific parameters. Non-DCEP data channel negotiation
can be defined to allow negotiation of parameters beyond those
handled by DCEP, e.g., parameters specific to the subprotocol
instantiated on a particular data channel.

The following procedures are common to all methods of data channel
negotiation not using DCEP, whether in-band (communicated using
proprietary means on an already-established data channel) or out-of-
band (using the SDP offer/answer mechanism or some other protocol
associated with the signaling channel).

A.2.2. Opening a Data Channel

In the case of non-DCEP negotiation, the endpoint application has the
option to fully control the stream identifier assignments. However,
these assignments have to coexist with the assignments controlled by
the data channel stack for data channels negotiated using DCEP (if
any). It is the responsibility of the application to ensure
consistent assignment of stream identifiers.

When the application requests that the creation of a new data channel
be set up via non-DCEP negotiation, the data channel stack creates
the data channel locally without sending any DATA_CHANNEL_OPEN
messages in-band. However, even if the ICE (Interactive Connectivity
Establishment), DTLS, and SCTP procedures were already successfully
completed, the application can't send data on this data channel until
the negotiation with the peer is complete. This is because the peer
needs to be aware of and accept the usage of this data channel. The
peer, after accepting the data channel offer, can start sending data
immediately. This implies that the offerer may receive data channel
subprotocol messages before the negotiation is complete, and the
application should be ready to handle it.

If the peer rejects the data channel part of the offer, then it
doesn't have to do anything, as the data channel was not created
using the stack. The offerer, on the other hand, needs to close the
data channel that was opened by invoking relevant data channel stack
API procedures.

It is also worth noting that a data channel stack implementation may
not provide any APIs to create and close data channels; instead, the
data channels may be used on the fly as needed, just by communicating
via non-DCEP means or even by having some local configuration/
assumptions on both of the peers.

The application then negotiates the data channel properties and
subprotocol properties with the peer's application using a mechanism
different from DCEP.

The peer then symmetrically creates a data channel with these
negotiated data channel properties. This is the only way for the
peer's data channel stack to know which properties to apply when
transmitting data on this channel. The data channel stack must allow
data channel creation with any nonconflicting stream identifier so
that both peers can create the data channel with the same stream
identifier.

A.2.3. Closing a Data Channel

When the application requests the closing of a data channel
negotiated without DCEP, the data channel stack always performs an
SCTP SSN Reset for this channel.

Depending upon the method used for non-DCEP negotiation and the
subprotocol associated with the data channel, the closing of the data
channel might also be signaled to the peer via SDP offer/answer
negotiation.

Acknowledgements

The authors wish to acknowledge the borrowing of ideas from other
draft documents by Salvatore Loreto, Gonzalo Camarillo, Peter
Dunkley, and Gavin Llewellyn. The authors also wish to thank
Flemming Andreasen, Christian Groves, Gunnar Hellström, Paul Kyzivat,
Jonathan Lennox, Uwe Rauschenbach, and Roman Shpount for their
invaluable comments.

Special thanks to Christer Holmberg for helping finish the document
and cleaning up Section 6.

Contributors

Juergen Stoetzer-Bradler made significant contributions to this
document and should be considered a coauthor.

Authors' Addresses

Keith Drage
Unaffiliated

Email: [email protected]

Maridi R. Makaraju (Raju)
Unaffiliated

Email: [email protected]

Richard Ejzak
Unaffiliated

Email: [email protected]

Jerome Marcon
Unaffiliated

Email: [email protected]

Roni Even (editor)

Email: [email protected]