Negotiation Data Channels Using the Session Description Protocol (SDP)
RFC 8864



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       | iesg@ietf.org                              |
   +---------------------+--------------------------------------------+
   | 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       | iesg@ietf.org                         |
      +---------------------+---------------------------------------+
      | 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: drageke@ntlworld.com

   Maridi R. Makaraju (Raju)
   Unaffiliated

   Email: mmraju@gmail.com

   Richard Ejzak
   Unaffiliated

   Email: richard.ejzak@gmail.com

   Jerome Marcon
   Unaffiliated

   Email: jeromee.marcon@free.fr

   Roni Even (editor)

   Email: ron.even.tlv@gmail.com