Internet Engineering Task Force (IETF)                       C. Holmberg
Request for Comments: 8841                                      Ericsson
Category: Standards Track                                     R. Shpount
ISSN: 2070-1721                                              TurboBridge
                                                               S. Loreto
                                                            G. Camarillo
                                                                Ericsson
                                                                May 2020

 Session Description Protocol (SDP) Offer/Answer Procedures for Stream
   Control Transmission Protocol (SCTP) over Datagram Transport Layer
                       Security (DTLS) Transport

Abstract

   The Stream Control Transmission Protocol (SCTP) is a transport
   protocol used to establish associations between two endpoints.  RFC
   8261 specifies how SCTP can be used on top of the Datagram Transport
   Layer Security (DTLS) protocol, which is referred to as SCTP-over-
   DTLS.

   This specification defines the following new Session Description
   Protocol (SDP) protocol identifiers (proto values): "UDP/DTLS/SCTP"
   and "TCP/DTLS/SCTP".  This specification also specifies how to use
   the new proto values with the SDP offer/answer mechanism for
   negotiating SCTP-over-DTLS associations.

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/rfc8841.

Copyright Notice

   Copyright (c) 2020 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.  SCTP Terminology
   4.  SDP Media Descriptions
     4.1.  General
     4.2.  Protocol Identifiers
     4.3.  Media-Format Management
     4.4.  Syntax
       4.4.1.  General
       4.4.2.  SDP Media Description Values
     4.5.  Example
   5.  SDP "sctp-port" Attribute
     5.1.  General
     5.2.  Syntax
     5.3.  Mux Category
   6.  SDP "max-message-size" Attribute
     6.1.  General
     6.2.  Syntax
     6.3.  Mux Category
   7.  UDP/DTLS/SCTP Transport Realization
   8.  TCP/DTLS/SCTP Transport Realization
   9.  Association and Connection Management
     9.1.  General
     9.2.  SDP "sendrecv"/"sendonly"/"recvonly"/"inactive" Attributes
     9.3.  SCTP Association
     9.4.  DTLS Association (UDP/DTLS/SCTP and TCP/DTLS/SCTP)
     9.5.  TCP Connection (TCP/DTLS/SCTP)
   10. SDP Offer/Answer Procedures
     10.1.  General
     10.2.  Generating the Initial SDP Offer
     10.3.  Generating the SDP Answer
     10.4.  Offerer Processing of the SDP Answer
     10.5.  Modifying the Session
   11. Multihoming Considerations
   12. NAT Considerations
     12.1.  General
     12.2.  ICE Considerations
   13. Examples
     13.1.  Establishment of UDP/DTLS/SCTP Association
   14. Security Considerations
   15. IANA Considerations
     15.1.  New SDP Proto Values
     15.2.  New SDP Attributes
       15.2.1.  sctp-port
       15.2.2.  max-message-size
     15.3.  association-usage Name Registry
   16. References
     16.1.  Normative References
     16.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Session Description Protocol (SDP) [RFC4566] provides a general-
   purpose format for describing multimedia sessions in announcements or
   invitations.  "TCP-Based Media Transport in the Session Description
   Protocol (SDP)" [RFC4145] specifies a general mechanism for
   describing and establishing TCP [RFC0793] streams.  "Connection-
   Oriented Media Transport over the Transport Layer Security (TLS)
   Protocol in the Session Description Protocol (SDP)" [RFC8122] extends
   [RFC4145] to describe TCP-based media streams that are protected
   using TLS.

   The Stream Control Transmission Protocol (SCTP) [RFC4960] is a
   reliable transport protocol used to transport data between two
   endpoints using SCTP associations.

   [RFC8261] specifies how SCTP can be used on top of the Datagram
   Transport Layer Security (DTLS) protocol, an arrangement referred to
   as SCTP-over-DTLS.

   This specification defines the following new SDP [RFC4566] protocol
   identifiers (proto values): "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP".
   This document also specifies how to use the new proto values with the
   SDP offer/answer mechanism [RFC3264] for negotiating SCTP-over-DTLS
   associations.

      |  NOTE: Due to the characteristics of TCP, while multiple SCTP
      |  streams can still be used, usage of "TCP/DTLS/SCTP" will always
      |  force ordered and reliable delivery of the SCTP packets, which
      |  limits the usage of the SCTP options.  Therefore, it is
      |  RECOMMENDED that TCP is only used in situations where UDP
      |  traffic is blocked.

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.  SCTP Terminology

   SCTP association:  A protocol relationship between SCTP endpoints,
      composed of the two SCTP endpoints and protocol state information
      including verification tags and the currently active set of
      Transmission Sequence Numbers (TSNs), etc.  An association can be
      uniquely identified by the transport addresses used by the
      endpoints in the association.

   SCTP stream:  A unidirectional logical channel established from one
      associated SCTP endpoint to another, within which all user
      messages are delivered in sequence except for those submitted to
      the unordered delivery service.

   SCTP-over-DTLS:  SCTP used on top of DTLS, as specified in [RFC8261].

4.  SDP Media Descriptions

4.1.  General

   This section defines the following new SDP media description ("m="
   line) protocol identifiers (proto values) for describing an SCTP
   association: "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP".  The section also
   describes how an "m=" line associated with the proto values is
   created.

   The following is the format for an "m=" line, as specified in
   [RFC4566]:

   m=<media> <port> <proto> <fmt> ...

   The "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto values are similar to
   both the "UDP" and "TCP" proto values in that they only describe the
   transport-layer protocol and not the upper-layer protocol.

      |  NOTE: When the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto values
      |  are used, the underlying transport protocol is, respectively,
      |  UDP and TCP; SCTP is carried on top of DTLS, which is on top of
      |  those transport-layer protocols.

4.2.  Protocol Identifiers

   The new proto values are defined as below:

   *  The "UDP/DTLS/SCTP" proto value describes an SCTP association on
      top of a DTLS association on top of UDP, as defined in Section 7.

   *  The "TCP/DTLS/SCTP" proto value describes an SCTP association on
      top of a DTLS association on top of TCP, as defined in Section 8.

4.3.  Media-Format Management

   [RFC4566] states that specifications defining new proto values must
   define the rules by which their media format (fmt) namespace is
   managed.

   An "m=" line with a proto value of "UDP/DTLS/SCTP" or "TCP/DTLS/SCTP"
   always describes a single SCTP association.

   In addition, such an "m=" line MUST further indicate the application-
   layer protocol using an "fmt" identifier.  There MUST be exactly one
   fmt value per "m=" line associated with the proto values defined in
   this specification.  The "fmt" namespace associated with those proto
   values describes the generic application usage of the entire SCTP
   association, including the associated SCTP streams.

   When the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto values are used,
   the "m=" line fmt value, which identifies the application-layer
   protocol, MUST be registered by IANA.  Section 15.3 defines the IANA
   registry for the media-format namespace.

      |  NOTE: A mechanism for how to describe and manage individual
      |  SCTP streams within an SCTP association is outside the scope of
      |  this specification.  [RFC8864] defines a mechanism for
      |  negotiating individual SCTP streams used to realize WebRTC data
      |  channels [RFC8831].

4.4.  Syntax

4.4.1.  General

   This section defines the values that can be used within an SDP media
   description ("m=" line) associated with an SCTP-over-DTLS
   association.

   This specification creates an IANA registry for "association-usage"
   values.

4.4.2.  SDP Media Description Values

     +-----------+---------------------------------------------------+
     | "m=" line | parameter value(s)                                |
     | parameter |                                                   |
     +===========+===================================================+
     | <media>   | "application"                                     |
     +-----------+---------------------------------------------------+
     | <proto>   | "UDP/DTLS/SCTP" or "TCP/DTLS/SCTP"                |
     +-----------+---------------------------------------------------+
     | <port>    | UDP port number (for "UDP/DTLS/SCTP") TCP port    |
     |           | number (for "TCP/DTLS/SCTP")                      |
     +-----------+---------------------------------------------------+
     | <fmt>     | A string denoting the association-usage, limited  |
     |           | to the syntax of a "token" as defined in RFC 4566 |
     +-----------+---------------------------------------------------+

                   Table 1: SDP Media Description Values

4.5.  Example

    m=application 12345 UDP/DTLS/SCTP webrtc-datachannel
    a=sctp-port:5000
    a=max-message-size:100000

      |  NOTE: "webrtc-datachannel" indicates the WebRTC Data Channel
      |  Establishment Protocol defined in [RFC8832].

5.  SDP "sctp-port" Attribute

5.1.  General

   This section defines a new SDP media-level attribute, "sctp-port".
   The attribute can be associated with an SDP media description ("m="
   line) with a "UDP/DTLS/SCTP" or a "TCP/DTLS/SCTP" proto value.  In
   that case, the "m=" line port value indicates the port of the
   underlying transport-layer protocol (UDP or TCP), and the "sctp-port"
   value indicates the SCTP port.

   No default value is defined for the SDP "sctp-port" attribute.
   Therefore, if the attribute is not present, the associated "m=" line
   MUST be considered invalid.

      |  NOTE: This specification only defines the usage of the SDP
      |  "sctp-port" attribute when associated with an "m=" line
      |  containing one of the following proto values: "UDP/DTLS/SCTP"
      |  or "TCP/DTLS/SCTP".  Usage of the attribute with other proto
      |  values needs to be defined in a separate specification.

5.2.  Syntax

   The definition of the SDP "sctp-port" attribute is:

   Attribute name:  sctp-port

   Type of attribute:  media

   Mux category:  CAUTION

   Subject to charset:  No

   Purpose:  Indicate the SCTP port value associated with the SDP media
      description.

   Appropriate values:  Integer

   Contact name:  Christer Holmberg

   Contact e-mail:  christer.holmberg@ericsson.com

   Reference:  RFC 8841

   Syntax:  sctp-port-value = 1*5(DIGIT) ; DIGIT defined in RFC 4566

   The SCTP port range is between 0 and 65535 (both included).  Leading
   zeroes MUST NOT be used.

   Example:

      a=sctp-port:5000

5.3.  Mux Category

   The mux category [RFC8859] for the SDP "sctp-port" attribute is
   CAUTION.

   As the usage of multiple SCTP associations on top of a single DTLS
   association is outside the scope of this specification, no mux rules
   are specified for the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto
   values.  Future extensions that define how to negotiate multiplexing
   of multiple SCTP associations of top of a single DTLS association
   need to also define the mux rules for the attribute.

6.  SDP "max-message-size" Attribute

6.1.  General

   This section defines a new SDP media-level attribute, "max-message-
   size".  The attribute can be associated with an "m=" line to indicate
   the maximum SCTP user message size (indicated in bytes) that an SCTP
   endpoint is willing to receive on the SCTP association associated
   with the "m=" line.  Different attribute values can be used in each
   direction.

   An SCTP endpoint MUST NOT send a SCTP user message with a message
   size that is larger than the maximum size indicated by the peer, as
   it cannot be assumed that the peer would accept such a message.

   If the SDP "max-message-size" attribute contains a maximum message
   size value of zero, it indicates that the SCTP endpoint will handle
   messages of any size, subject to memory capacity, etc.

   If the SDP "max-message-size" attribute is not present, the default
   value is 64K.

      |  NOTE: This specification only defines the usage of the SDP
      |  "max-message-size" attribute when associated with an "m=" line
      |  containing one of the following proto values: "UDP/DTLS/SCTP"
      |  or "TCP/DTLS/SCTP".  Usage of the attribute with other proto
      |  values needs to be defined in a separate specification.

6.2.  Syntax

   The definition of the SDP "max-message-size" attribute is:

   Attribute name:  max-message-size

   Type of attribute:  media

   Mux category:  CAUTION

   Subject to charset:  No

   Purpose:  Indicate the maximum message size (indicated in bytes) that
      an SCTP endpoint is willing to receive on the SCTP association
      associated with the SDP media description.

   Appropriate values:  Integer

   Contact name:  Christer Holmberg

   Contact e-mail:  christer.holmberg@ericsson.com

   Reference:  RFC 8841

   Syntax:  max-message-size-value = 1*DIGIT ; DIGIT defined in RFC 4566

   Leading zeroes MUST NOT be used.

   Example:

      a=max-message-size:100000

6.3.  Mux Category

   The mux category for the SDP "max-message-size" attribute is CAUTION.

   As the usage of multiple SCTP associations on top of a single DTLS
   association is outside the scope of this specification, no mux rules
   are specified for the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto
   values.

7.  UDP/DTLS/SCTP Transport Realization

   The UDP/DTLS/SCTP transport is realized as described below:

   *  SCTP on top of DTLS is realized according to the procedures
      defined in [RFC8261]; and

   *  DTLS on top of UDP is realized according to the procedures in
      defined in [RFC6347].

      |  NOTE: While [RFC8261] allows multiple SCTP associations on top
      |  of a single DTLS association, the procedures in this
      |  specification only support the negotiation of a single SCTP
      |  association on top of any given DTLS association.

8.  TCP/DTLS/SCTP Transport Realization

   The TCP/DTLS/SCTP transport is realized as described below:

   *  SCTP on top of DTLS is realized according to the procedures
      defined in [RFC8261]; and

   *  DTLS on top of TCP is realized using the framing method defined in
      [RFC4571], with DTLS packets being sent and received instead of
      RTP/RTCP packets using the shim defined in [RFC4571].  The length
      field defined in [RFC4571] precedes each DTLS message, and SDP
      signaling is done according to the procedures defined in this
      specification.

      |  NOTE: TLS on top of TCP, without using the framing method
      |  defined in [RFC4571], is outside the scope of this
      |  specification.  A separate proto value would need to be
      |  registered for such transport realization.

9.  Association and Connection Management

9.1.  General

   This section describes how to manage an SCTP association, DTLS
   association, and TCP connection using SDP attributes.

   The SCTP association, the DTLS association, and the TCP connection
   are managed independently from each other.  Each can be established
   and closed without impacting others.

   The detailed SDP offer/answer [RFC3264] procedures for the SDP
   attributes are described in Section 10.

9.2.  SDP "sendrecv"/"sendonly"/"recvonly"/"inactive" Attributes

   This specification does not define semantics for the SDP direction
   attributes [RFC4566].  Unless the semantics of these attributes for
   an SCTP association usage have been defined, SDP direction attributes
   MUST be ignored if present.

9.3.  SCTP Association

   When an SCTP association is established, both SCTP endpoints MUST
   initiate the SCTP association (i.e., both SCTP endpoints take the
   "active" role).  In addition, both endpoints MUST use the same SCTP
   port as client port and server port, in order to prevent two separate
   SCTP associations from being established.

   As both SCTP endpoints take the "active" role, the SDP "setup"
   attribute [RFC4145] does not apply to SCTP association establishment.
   However, the "setup" attribute does apply to establishment of the
   underlying DTLS association and TCP connection.

      |  NOTE: The procedure above is different from TCP, where one
      |  endpoint takes the "active" role, the other endpoint takes the
      |  "passive" role, and only the "active" endpoint initiates the
      |  TCP connection [RFC4145].

      |  NOTE: When the SCTP association is established, it is assumed
      |  that any NAT traversal procedures for the underlying transport
      |  protocol (UDP or TCP) have successfully been performed.

   The SDP "connection" attribute [RFC4145] does not apply to the SCTP
   association.  In order to trigger the closure of an existing SCTP
   association and establishment of a new SCTP association, the SDP
   "sctp-port" attribute (Section 5) is used to indicate a new
   (different than the ones currently used) SCTP port.  The existing
   SCTP association is closed, and the new SCTP association is
   established, if one or both endpoints signal a new SCTP port.  The
   "connection" attribute does apply to establishment of underlying TCP
   connections.

   Alternatively, an SCTP association can be closed using the SDP "sctp-
   port" attribute with an attribute value of zero.  Later, a new SCTP
   association can be established using the procedures in this section
   for establishing an SCTP association.

   SCTP associations might be closed without SDP signaling -- for
   example, in case of a failure.  The procedures in this section MUST
   be followed to establish a new SCTP association.  This requires a new
   SDP offer/answer exchange.  New (different than the ones currently
   used) SCTP ports MUST be used by both endpoints.

      |  NOTE: Closing and establishing a new SCTP association using the
      |  SDP "sctp-port" attribute will not affect the state of the
      |  underlying DTLS association.

9.4.  DTLS Association (UDP/DTLS/SCTP and TCP/DTLS/SCTP)

   A DTLS association is managed according to the procedures in
   [RFC8842].  Hence, the SDP "setup" attribute is used to negotiate the
   (D)TLS roles ("client" and "server") [RFC8122].

      |  NOTE: The SDP "setup" attribute is used to negotiate both the
      |  DTLS roles and the TCP roles (Section 9.5).

      |  NOTE: As described in [RFC8445], if the Interactive
      |  Connectivity Establishment (ICE) mechanism [RFC8445] is used,
      |  all ICE candidates associated with a DTLS association are
      |  considered part of the same DTLS association.  Thus, a switch
      |  from one candidate pair to another candidate pair will not
      |  trigger the establishment of a new DTLS association.

9.5.  TCP Connection (TCP/DTLS/SCTP)

   The TCP connection is managed according to the procedures in
   [RFC4145].  Hence, the SDP "setup" attribute is used to negotiate the
   TCP roles ("active" and "passive"), and the SDP "connection"
   attribute is used to indicate whether to use an existing TCP
   connection or create a new one.  The SDP "setup" attribute "holdconn"
   value MUST NOT be used.

      |  NOTE: A change of the TCP roles will also trigger a closure of
      |  the DTLS association and establishment of a new DTLS
      |  association, according to the procedures in [RFC8842].

      |  NOTE: As specified in [RFC8842], usage of the SDP "setup"
      |  attribute "holdconn" value is not allowed.  Therefore, this
      |  specification also forbids usage of the attribute value for
      |  TCP, as DTLS is transported on top of TCP.

10.  SDP Offer/Answer Procedures

10.1.  General

   This section defines the SDP Offer/Answer [RFC3264] procedures for
   negotiating and establishing an SCTP-over-DTLS association.  Unless
   explicitly stated, the procedures apply to both the "UDP/DTLS/SCTP"
   and "TCP/DTLS/SCTP" "m=" line proto values.

   Each endpoint MUST associate one or more certificate fingerprints
   using the SDP "fingerprint" attribute with the "m=" line, following
   the procedures in [RFC8122].

   The authentication certificates are interpreted and validated as
   defined in [RFC8122].  Self-signed certificates can be used securely,
   provided that the integrity of the SDP description is assured, as
   defined in [RFC8122].

   Each endpoint MUST associate an SDP "tls-id" attribute with the "m="
   line, following the procedures in [RFC8842].

10.2.  Generating the Initial SDP Offer

   When the offerer creates an initial offer, the offerer:

   *  MUST associate an SDP "setup" attribute with the "m=" line;

   *  MUST associate an SDP "sctp-port" attribute with the "m=" line;

   *  MUST, in the case of TCP/DTLS/SCTP, associate an SDP "connection"
      attribute, with a "new" attribute value, with the "m=" line; and

   *  MAY associate an SDP "max-message-size" attribute (Section 6) with
      the "m=" line.

10.3.  Generating the SDP Answer

   When the answerer receives an offer that contains an "m=" line
   describing an SCTP-over-DTLS association, if the answerer accepts the
   association, the answerer:

   *  MUST insert a corresponding "m=" line in the answer, with an "m="
      line proto value [RFC3264] identical to the value in the offer;

   *  MUST associate an SDP "setup" attribute with the "m=" line;

   *  MUST associate an SDP "sctp-port" attribute with the "m=" line.
      If the offer contained a new (different than the one currently
      used) SCTP port value, the answerer MUST also associate a new SCTP
      port value.  If the offer contained a zero SCTP port value, or if
      the answerer does not accept the SCTP association, the answerer
      MUST also associate a zero SCTP port value; and

   *  MAY associate an SDP "max-message-size" attribute (Section 6) with
      the "m=" line.  The attribute value in the answer is independent
      of the value (if present) in the corresponding "m=" line of the
      offer.

   Once the answerer has sent the answer:

   *  in the case of TCP/DTLS/SCTP, if a TCP connection has not yet been
      established or an existing TCP connection is to be closed and
      replaced by a new one, the answerer MUST follow the procedures in
      [RFC4145] for closing and establishing a TCP connection;

   *  if a DTLS association has not yet been established or an existing
      DTLS association is to be closed and replaced by a new one, the
      answerer MUST follow the procedures in [RFC8842] for closing the
      currently used DTLS association and establishing a new one; and

   *  if an SCTP association has not yet been established or an existing
      SCTP association is to be closed and replaced by a new one, the
      answerer MUST initiate the closing of the existing SCTP
      association (if applicable) and establishment of the new
      association.

   If the SDP "sctp-port" attribute in the answer contains an attribute
   value of zero, the answerer MUST NOT establish an SCTP association.
   If an SCTP association exists, the offerer MUST close it.

   If the answerer does not accept the "m=" line in the offer, it MUST
   assign a zero port value to the corresponding "m=" line in the
   answer, following the procedures in [RFC3264].  In addition, the
   answerer MUST NOT initiate the establishment of a TCP connection, a
   DTLS association, or a DTLS association associated with the "m="
   line.

10.4.  Offerer Processing of the SDP Answer

   Once the offerer has received the answer:

   *  in the case of TCP/DTLS/SCTP, if a TCP connection has not yet been
      established or an existing TCP connection is to be closed and
      replaced by a new one, the offerer MUST follow the procedures in
      [RFC4145] for closing and establishing a TCP connection;

   *  if a DTLS association has not yet been established or an existing
      DTLS association is to be closed and replaced by a new one, the
      offerer MUST follow the procedures in [RFC8842] for closing and
      establishing a DTLS association; and

   *  if an SCTP association has not yet been established or an existing
      SCTP association is to be closed and replaced by a new one, the
      offerer MUST initiate the closing of the existing SCTP association
      (if applicable) and establishment of the new association.

   If the SDP "sctp-port" attribute in the answer contains an attribute
   value of zero, the offerer MUST NOT establish an SCTP association.
   If, in addition, an SCTP association exists, the offerer MUST close
   it.

   If the "m=" line in the answer contains a zero port value, the
   offerer MUST NOT initiate the establishment of a TCP connection, a
   DTLS association, or an SCTP association associated with the "m="
   line.  If, in addition, a TCP connection, DTLS association, or SCTP
   association exists, the offerer MUST close it.

10.5.  Modifying the Session

   When an offerer sends an updated offer, in order to modify a
   previously established SCTP association, it follows the procedures in
   Section 10.2, with the following exceptions:

   *  If the offerer wants to close an SCTP association and immediately
      establish a new SCTP association, it MUST associate an SDP "sctp-
      port" attribute with a new (different than the one currently used)
      attribute value.  This will not impact the underlying DTLS
      association (or TCP connection, in the case of TCP/DTLS/SCTP).

   *  If the offerer wants to close an SCTP association without
      immediately establishing a new SCTP association, it MUST associate
      an SDP "sctp-port" attribute with an attribute value of zero.
      This will not impact the underlying DTLS association (or TCP
      connection, in the case of TCP/DTLS/SCTP).

   *  If the offerer wants to establish an SCTP association, and another
      SCTP association was previously closed, the offerer MUST associate
      an SDP "sctp-port" attribute with a new attribute value (different
      than the value associated with the previous SCTP association).  If
      the previous SCTP association was closed successfully following
      use of an SDP "sctp-port" attribute with an attribute value of
      zero, the offerer MAY use the same attribute value for the new
      SCTP association that was used with the previous SCTP association
      before it was closed.  This will not impact the underlying DTLS
      association (or TCP connection, in the case of TCP/DTLS/SCTP).

   *  If the offerer wants to close an existing SCTP association and the
      underlying DTLS association (and the underlying TCP connection, in
      the case of TCP/DTLS/SCTP), it MUST assign a zero port value to
      the "m=" line associated with the SCTP and DTLS associations (and
      TCP connection, in the case of TCP/DTLS/SCTP), following the
      procedures in [RFC3264].

   *  NOTE: This specification does not define a mechanism for
      explicitly closing a DTLS association while maintaining the
      overlying SCTP association.  However, if a DTLS association is
      closed and replaced with a new DTLS association as a result of
      some other action [RFC8842], the state of the SCTP association is
      not affected.

   The offerer follows the procedures in [RFC8842] regarding the DTLS
   association impacts when modifying a session.

   In the case of TCP/DTLS/SCTP, the offerer follows the procedures in
   [RFC4145] regarding the TCP connection impacts when modifying a
   session.

11.  Multihoming Considerations

   Multihoming is not supported when sending SCTP on top of DTLS, as
   DTLS does not expose address management of the underlying transport
   protocols (UDP or TCP) to its upper layer.

12.  NAT Considerations

12.1.  General

   When SCTP-over-DTLS is used in a NAT environment, it relies on the
   NAT traversal procedures for the underlying transport protocol (UDP
   or TCP).

12.2.  ICE Considerations

   When SCTP-over-DTLS is used with UDP-based ICE candidates [RFC8445],
   then the procedures for UDP/DTLS/SCTP (Section 7) are used.

   When SCTP-over-DTLS is used with TCP-based ICE candidates [RFC6544],
   then the procedures for TCP/DTLS/SCTP (Section 8) are used.

   In ICE environments, during the nomination process, endpoints go
   through multiple ICE candidate pairs until the most preferred
   candidate pair is found.  During the nomination process, data can be
   sent as soon as the first working candidate pair is found, but the
   nomination process still continues, and selected candidate pairs can
   still change while data is sent.  Furthermore, if endpoints roam
   between networks -- for instance, when a mobile endpoint switches
   from mobile connection to WiFi -- endpoints will initiate an ICE
   restart.  This will trigger a new nomination process between the new
   set of candidates, which will likely result in the new nominated
   candidate pair.

   Implementations MUST treat all ICE candidate pairs associated with an
   SCTP association on top of a DTLS association as part of the same
   DTLS association.  Thus, there will only be one SCTP handshake and
   one DTLS handshake even if there are multiple valid candidate pairs;
   shifting from one candidate pair to another, including switching
   between UDP and TCP candidate pairs, will not impact the SCTP or DTLS
   associations.  If new candidates are added, they will also be part of
   the same SCTP and DTLS associations.  When transitioning between
   candidate pairs, different candidate pairs can be currently active in
   different directions, and implementations MUST be ready to receive
   data on any of the candidates, even if this means sending and
   receiving data using UDP/DTLS/SCTP and TCP/DTLS/SCTP at the same time
   in different directions.

   In order to maximize the likelihood of interoperability between the
   endpoints, all ICE-enabled SCTP-over-DTLS endpoints SHOULD implement
   support for UDP/DTLS/SCTP.

   When an SDP offer or answer is sent with multiple ICE candidates
   during initial connection negotiation or after ICE restart, UDP-based
   candidates SHOULD be included, and the default candidate SHOULD be
   chosen from one of those UDP candidates.  The proto value MUST match
   the transport protocol associated with the default candidate.  If UDP
   transport is used for the default candidate, then the "UDP/DTLS/SCTP"
   proto value MUST be used.  If TCP transport is used for the default
   candidate, then the "TCP/DTLS/SCTP" proto value MUST be used.  Note
   that under normal circumstances, the proto value for offers and
   answers sent during ICE nomination SHOULD be "UDP/DTLS/SCTP".

   When a subsequent SDP offer or answer is sent after ICE nomination is
   complete, and it does not initiate ICE restart, it will contain only
   the nominated ICE candidate pair.  In this case, the proto value MUST
   match the transport protocol associated with the nominated ICE
   candidate pair.  If UDP transport is used for the nominated pair,
   then the "UDP/DTLS/SCTP" proto value MUST be used.  If TCP transport
   is used for the nominated pair, then the "TCP/DTLS/SCTP" proto value
   MUST be used.  Please note that if an endpoint switches between TCP-
   based and UDP-based candidates during the nomination process, the
   endpoint is not required to send an SDP offer for the sole purpose of
   keeping the proto value of the associated "m=" line in sync.

      |  NOTE: The text in the paragraph above only applies when the
      |  usage of ICE has been negotiated.  If ICE is not used, the
      |  proto value MUST always reflect the transport protocol used at
      |  any given time.

13.  Examples

13.1.  Establishment of UDP/DTLS/SCTP Association

   SDP Offer:

       m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
       c=IN IP6 2001:DB8::A8FD
       a=tls-id:abc3de65cddef001be82
       a=setup:actpass
       a=sctp-port:5000
       a=max-message-size:100000

   *  The offerer indicates that the usage of the UDP/DTLS/SCTP
      association will be as defined for the "webrtc-datachannel" format
      value.

   *  The offerer UDP port value is 54111.

   *  The offerer SCTP port value is 5000.

   *  The offerer indicates that it can take either the client or the
      server DTLS role.

   SDP Answer:

       m=application 64300 UDP/DTLS/SCTP webrtc-datachannel
       c=IN IP6 2001:DB8::001D
       a=tls-id:dbc8de77cddef001be90
       a=setup:passive
       a=sctp-port:6000
       a=max-message-size:100000

   *  The answerer UDP port value is 64300.

   *  The answerer SCTP port value is 6000.

   *  The answerer takes the server DTLS role.

14.  Security Considerations

   [RFC4566] defines general SDP security considerations, while
   [RFC3264], [RFC4145], and [RFC8122] define security considerations
   when using the SDP offer/answer mechanism to negotiate media streams.

   [RFC4960] defines general SCTP security considerations, and [RFC8261]
   defines security considerations when using SCTP on top of DTLS.

   This specification does not introduce new security considerations in
   addition to those defined in the specifications listed above.

15.  IANA Considerations

15.1.  New SDP Proto Values

   This document updates the "Session Description Protocol (SDP)
   Parameters" registry, following the procedures in [RFC4566], by
   adding the following values to the table in the SDP "proto" field
   registry:

                   +-------+---------------+-----------+
                   |  Type |    SDP Name   | Reference |
                   +=======+===============+===========+
                   | proto | UDP/DTLS/SCTP |  RFC 8841 |
                   +-------+---------------+-----------+
                   | proto | TCP/DTLS/SCTP |  RFC 8841 |
                   +-------+---------------+-----------+

                     Table 2: SDP "proto" Field Values

15.2.  New SDP Attributes

15.2.1.  sctp-port

   This document defines a new SDP media-level attribute,"sctp-port".
   The details of the attribute are defined in Section 5.2.

15.2.2.  max-message-size

   This document defines a new SDP media-level attribute,"max-message-
   size".  The details of the attribute are defined in Section 6.2.

15.3.  association-usage Name Registry

   Per this specification, a new IANA registry has been created,
   following the procedures in [RFC8126], for the namespace associated
   with the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" protocol identifiers.
   Each fmt value describes the usage of an entire SCTP association,
   including all SCTP streams associated with the SCTP association.

      |  NOTE: Usage indication of individual SCTP streams is outside
      |  the scope of this specification.

   The fmt value "association-usage" used with these "proto" values is
   required.  It is defined in Section 4.

   As part of this registry, IANA maintains the following information:

   association-usage name:  The identifier of the subprotocol, as will
      be used as the fmt value.

   association-usage reference:  A reference to the document in which
      the association-usage is defined.

   association-usage names are to be subject to the "First Come First
   Served" IANA registration policy [RFC8126].

   IANA has added the following initial values to the registry.

                +--------------------+--------------------+
                | Name               | Reference          |
                +====================+====================+
                | webrtc-datachannel | RFC 8832, RFC 8841 |
                +--------------------+--------------------+
                +--------------------+--------------------+

                        Table 3: IANA Initial Values

16.  References

16.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

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

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

   [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>.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              DOI 10.17487/RFC4145, September 2005,
              <https://www.rfc-editor.org/info/rfc4145>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC4571]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
              and RTP Control Protocol (RTCP) Packets over Connection-
              Oriented Transport", RFC 4571, DOI 10.17487/RFC4571, July
              2006, <https://www.rfc-editor.org/info/rfc4571>.

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

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

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

   [RFC6544]  Rosenberg, J., Keranen, A., Lowekamp, B. B., and A. B.
              Roach, "TCP Candidates with Interactive Connectivity
              Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
              March 2012, <https://www.rfc-editor.org/info/rfc6544>.

   [RFC8122]  Lennox, J. and C. Holmberg, "Connection-Oriented Media
              Transport over the Transport Layer Security (TLS) Protocol
              in the Session Description Protocol (SDP)", RFC 8122,
              DOI 10.17487/RFC8122, March 2017,
              <https://www.rfc-editor.org/info/rfc8122>.

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

   [RFC8842]  Holmberg, C. and R. Shpount, "Session Description Protocol
              (SDP) Offer/Answer Considerations for Datagram Transport
              Layer Security (DTLS) and Transport Layer Security (TLS)",
              RFC 8842, DOI 10.17487/RFC8842, May 2020,
              <https://www.rfc-editor.org/info/rfc8842>.

   [RFC8859]  Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", RFC 8859, DOI 10.17487/RFC8859, May 2020,
              <https://www.rfc-editor.org/info/rfc8859>.

16.2.  Informative References

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

   [RFC8831]  Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data
              Channels", RFC 8831, DOI 10.17487/RFC8831, May 2020,
              <https://www.rfc-editor.org/info/rfc8831>.

   [RFC8864]  Drage, K., Makaraju, M., Ejzak, R., Marcon, J., and R.
              Even, "SDP-Based Data Channel Negotiation", RFC 8864,
              DOI 10.17487/RFC8864, May 2020,
              <https://www.rfc-editor.org/info/rfc8864>.

   [RFC8832]  Jesup, R., Loreto, S., and M. Tüxen, "WebRTC Data Channel
              Establishment Protocol", RFC 8832, DOI 10.17487/RFC8832,
              May 2020, <https://www.rfc-editor.org/info/rfc8832>.

Acknowledgements

   The authors wish to thank Harald Alvestrand, Randell Jesup, Paul
   Kyzivat, Michael Tüxen, Juergen Stoetzer-Bradler, Flemming Andreasen,
   and Ari Keränen for their comments and useful feedback.  Ben Campbell
   provided comments as part of his Area Director review.  Brian
   Carpenter performed the Gen-ART review.

Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   FI-02420 Jorvas
   Finland

   Email: christer.holmberg@ericsson.com

   Roman Shpount
   TurboBridge
   4905 Del Ray Avenue, Suite 300
   Bethesda, MD 20814
   United States of America

   Phone: +1 (240) 292-6632
   Email: rshpount@turbobridge.com

   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   FI-02420 Jorvas
   Finland
   Grönlandsgatan 31
   SE- Kista
   Sweden

   Email: Salvatore.Loreto@ericsson.com

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   FI-02420 Jorvas
   Finland

   Email: Gonzalo.Camarillo@ericsson.com