MMUSIC                                                       C. Holmberg
Internet-Draft                                                 S. Loreto
Intended status: Standards Track                            G. Camarillo
Expires: June 8, 22, 2015                                          Ericsson
                                                       December 5, 19, 2014

Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
                   Session Description Protocol (SDP)
                     draft-ietf-mmusic-sctp-sdp-09
                     draft-ietf-mmusic-sctp-sdp-10

Abstract

   SCTP (Stream Control Transmission Protocol) is a transport protocol
   used to establish associations between two endpoints.

   This specification describes how to describe SCTP associations using
   the Session Description Protocol (SDP), and defines the following new
   SDP Media Description protocol identifiers (proto values):'SCTP',
   'SCTP/DTLS'
   'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'DTLS/SCTP'. 'TCP/DTLS/SCTP'.

   The specification also describes how to use the new proto values
   together with the SDP Offer/Answer mechanism in order to negotiate
   and establish SCTP associations, and how to indicate the SCTP
   application usage.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on June 8, 22, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  SCTP Terminology  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  SDP Media Descriptions  . . . . . . . . . . . . . . . . . . .   4
     4.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Protocol Identifiers  . . . . . . . . . . . . . . . . . .   5
     4.3.  Media Format Management . . . . . . . . . . . . . . . . .   5
     4.4.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  SDP 'sctp-port' Attribute . . . . . . . . . . . . . . . . . .   6
     5.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   7   6
   6.  SDP 'max-message-size' Attribute  . . . . . . . . . . . . . .   7
     6.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  SDP 'fmtp' Attribute  . . . . . . . . . . . . . . . . . . . .   7
     7.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   8.  UDP/DTLS/SCTP Transport Realization . . . . . . . . . . . . .   8
   9.  TCP/DTLS/SCTP Transport Realization . . . . . . . . . . . . .   8
   10. SCTP Association Management . . . . . . . . . . . . . . . . .   8
     8.1.
     10.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.2.
     10.2.  SDP sendrecv/sendonly/sendrecv/inactive Attribute  . . . .   8
     8.3.   9
     10.3.  SDP setup Attribute  . . . . . . . . . . . . . . . . . . .   8
       8.3.1.   9
       10.3.1.  General  . . . . . . . . . . . . . . . . . . . . . . .   8
       8.3.2.   9
       10.3.2.  SCTP Association Initiation  . . . . . . . . . . . . .   9
       8.3.3.
       10.3.3.  TLS Role Determination . . . . . . . . . . . . . . .   9
     8.4.
     10.4.  SDP connection Attribute . . . . . . . . . . . . . . . .   9
   9.  10
   11. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . .  10
     9.1.
     11.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.2.
     11.2.  Generating the Initial SDP Offer . . . . . . . . . . . .  11
     9.3.
     11.3.  Generating the SDP Answer  . . . . . . . . . . . . . . . .  11
     9.4.
     11.4.  Offerer Processing of the SDP Answer . . . . . . . . . .  12
     9.5.
     11.5.  Modifying the Session  . . . . . . . . . . . . . . . . . .  12
   10.  13
   12. Multihoming Considerations  . . . . . . . . . . . . . . . . .  13
   11.
   13. NAT Considerations  . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  14
     13.1.  General  . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.2.  14
     13.2.  ICE Considerations . . . . . . . . . . . . . . . . . . .  13

   12.  14
   14. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   13.
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   14.
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     14.1.
     16.1.  New SDP proto values . . . . . . . . . . . . . . . . . .  14
     14.2.
     16.2.  New SDP Attribute  . . . . . . . . . . . . . . . . . . .  14
     14.3.  15
     16.3.  association-usage Name Registry  . . . . . . . . . . . .  15
   15.
   17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   16.
   18. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   17.
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     17.1.  17
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     17.2.  17
     19.2.  Informative References . . . . . . . . . . . . . . . . .  17  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18  19

1.  Introduction

   SDP (Session Description Protocol) [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 (Transmission Control Protocol) [RFC5246]
   streams.  Connection-Oriented Media Transport over the Transport
   Layer Security (TLS) Protocol in the Session Description Protocol
   (SDP) [RFC4572] extends RFC4145 [RFC4145] for describing TCP-based
   media streams that are protected using TLS.

   SCTP (Stream Control Transmission Protocol) is a transport protocol
   used to establish associations between two endpoints.

   This specification describes how to describe SCTP associations using
   the Session Description Protocol (SDP) [RFC4566], and defines the
   following new SDP Media Description [RFC4566] protocol identifiers
   (proto values):'SCTP', 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'DTLS/SCTP'. 'TCP/DTLS/
   SCTP'.

   The specification also describes how to use the new proto values
   together with the SDP Offer/Answer mechanism [RFC3264] in order to
   negotiate and establish SCTP associations, and how to indicate the
   SCTP application usage.

   NOTE: TLS is designed to run on top of a byte-stream oriented
   transport protocol providing a reliable, in-sequence delivery like
   TCP.  [RFC6083] presents serious limitations with transporting SCTP
   on top of TLS.  Therefore, defining a mechanism to negotiate media
   streams transported using SCTP on top of TLS is outside the scope of
   this specification.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in BCP 14, RFC 2119 [RFC2119] and indicate requirement
   levels for compliant implementations.

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.  Two SCTP endpoints MUST NOT have more than one
   SCTP association between them at any given time.

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

   SCTP Transport address: A transport address is traditionally defined
   by a network-layer address, a transport-layer protocol, and a
   transport-layer port number.  In the case of SCTP running over IP, a
   transport address is defined by the combination of an IP address and
   an SCTP port number (where SCTP is the transport protocol).

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: 'SCTP', 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'DTLS/SCTP'. '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
   [RFC4566]:

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

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

   NOTE: When the 'DTLS/SCTP' 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP' proto value is values are
   used, the underlying transport protocol is either UDP or TCP. TCP, SCTP is
   carried on top of either of those transport-layer protocols.

   The m- line fmt value, identifying the application-layer protocol,
   MUST be registered by IANA.

4.2.  Protocol Identifiers

   The new proto values are defined as below:

   o  The 'SCTP' proto value describes an SCTP association, as defined
      in [RFC4960].

   o  The 'SCTP/DTLS' proto value describes a Datagram Transport Layer
      Security (DTLS) [RFC6347] connection on top of an SCTP
      association, as defined in [RFC6083].

   o  The 'DTLS/SCTP' 'UDP/DTLS/SCTP' proto value describes an SCTP association on
      top of a DTLS connection, connection on top of UDP, as defined in
      [I-D.ietf-tsvwg-sctp-dtls-encaps].

   NOTE: In the case Section 8.

   o  The 'TCP/DTLS/SCTP' proto value describes an SCTP association on
      top of 'DTLS/SCTP', the actual transport protocol below a DTLS is either UDP or TCP.

   OPEN ISSUE #1: It is FFS whether separate proto values will be used,
   depending connection on whether the underlying transport protocol is UDP (e.g.
   'UDP/DTLS/SCTP') or TCP (e.g.  'TCP/DTLS/SCTP'). top of TCP, as defined in Section 9.

4.3.  Media Format Management

   [RFC4566] defines that specifications defining new proto values must
   define the rules by which their media format (fmt) namespace is
   managed.  Use of an existing MIME subtype for the format is
   encouraged.  If no MIME subtype exists, it is recommended that a
   suitable one is registered through the IETF process [RFC6838]
   [RFC4289] by production of, or reference to, a standards-track RFC
   that defines the transport protocol for the format.

   An m- line with a proto value of 'SCTP', 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP'
   or 'DTLS/SCTP' 'TCP/DTLS/SCTP' always describe a single SCTP association.

   In addition, such 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.

   NOTE: A mechanism on how to describe, and manage, individual SCTP
   streams within an SCTP association, is outside the scope of this
   specification.

4.4.  Syntax

     sctp-m-line = %x6d "="
      ("application" SP sctp-port SP "SCTP"  SP sctp-fmt CRLF) /
      ("application" SP sctp-port SP "SCTP/DTLS" SP sctp-fmt CRLF) /
      ("application" SP udp-port  SP "DTLS/SCTP" "UDP/DTLS/SCTP" SP sctp-fmt CRLF) /
      ("application" SP udp-port  SP "TCP/DTLS/SCTP" SP sctp-fmt CRLF)

     sctp-port = port

     udp-port = port

     sctp-fmt = association-usage

     association-usage = token

4.5.  Example

    TEMP:
    m=application 12345 DTLS/SCTP webrtc-datachannel
    a=max-message-size=100000

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

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 descriptor (m-
   line) with a 'DTLS/SCTP' 'UDP/DTLS/SCTP' or a 'TCP/DTLS/SCTP' proto value, in
   which case the m- line port value indicates the port of the underlying transport
   transport-layer protocol (UDP or
   TCP).

   TEMP: TCP), on which SCTP is carried.

   If the SDP sctp-port attribute is not present, the m- line
   SHOULD MUST be
   discarded.  If the SDP sctp-port attribute is not present,
   the default value is 5000.

   Usage of the SDP sctp-port attribute with other proto values is not
   specified, and MUST be discarded if received.

5.2.  Syntax

       sctp-port-attr  =  "a=sctp-port:" port
       port            =  1*DIGIT

6.  SDP 'max-message-size' Attribute

6.1.  General

   The SDP 'max-message-size' attribute can be associated with an m-
   line to indicate the maximum message size that an SCTP endpoint is
   willing to receive on the SCTP association associated with the m-
   line.

   The remote peer MUST assume that larger messages will be rejected by
   the SCTP endpoint.  SCTP endpoints need to decide on appropriate
   behaviour
   behavior in case a message that exceeds the maximum size needs to be
   sent.

   If the SDP 'max-message-size' attribute contains a maximum message
   size value of zero, it indicates 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.

6.2.  Syntax

       max-message-size-attr =  "a=max-message-size:" max-message-size
       max-message-size      =  1*DIGIT

7.  SDP 'fmtp' Attribute

7.1.  General

   The SDP 'fmtp' attribute can be used with an m- line, associated with
   an SCTP association, to indicate the maximum message size that an
   SCTP endpoint is willing to receive, for a particular SCTP
   association usage, on that SCTP association.

   The remote peer MUST assume that larger messages will be rejected by
   the SCTP endpoint.  SCTP endpoints need to decide on appropriate
   behaviour
   behavior in case a message that exceeds the maximum size needs to be
   sent.

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

   If the SDP 'fmtp' 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 field values: 'SCTP', 'SCTP/DTLS' 'SCTP/DTLS',
   'UDP/DTLS/SCTP' or
   'DTLS/SCTP'. 'TCP/DTLS/SCTP'.  Usage of the attribute with
   other proto field values needs to be defined in a separate specification.

7.2.  Syntax

     sctpmap-attr      =  "a=fmtp:" association-usage [max-message-size]
     max-message-size  =  "max-message-size" EQUALS 1*DIGIT

8.  SCTP Association Management

8.1.  General  UDP/DTLS/SCTP Transport Realization

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

   o  SCTP association on top of DTLS is identical realized according to the management
   of a TCP connection.  An SCTP endpoints MUST follow the rules procedures
      defined in
   Section 6 [I-D.ietf-tsvwg-sctp-dtls-encaps]; and

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

9.  TCP/DTLS/SCTP Transport Realization

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

   o  SCTP on top of DTLS is realized according to the procedures
      defined in [I-D.ietf-tsvwg-sctp-dtls-encaps]; and

   o  DTLS on top of TCP is realized using the framing method defined in
      [RFC4571].

   NOTE: DTLS 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.

10.  SCTP Association Management

10.1.  General

   The management of an SCTP association is identical to the management
   of a TCP connection.  An SCTP endpoints MUST follow the rules in
   Section 6 of [RFC4145] to manage SCTP associations.  Whether to use
   the SCTP ordered or unordered delivery service is up to the
   applications using the SCTP association, and this specification does
   not define a mechanism to indicate the type of delivery service using
   SDP.

8.2.

10.2.  SDP sendrecv/sendonly/sendrecv/inactive Attribute

   This specification does not define any semantics for the SDP direction
   attributes [RFC4566].  Specifications for an individual SCTP
   association usage MAY define how the attributes can be used with that association
   usage.  If the  Unless semantics of these attributes for an SCTP association
   usage has not have been defined, SDP direction attributes MUST be discarded
   if present.

8.3.

10.3.  SDP setup Attribute

8.3.1.

10.3.1.  General

   The SDP setup attribute is used to determine the 'active/passive'
   status of the endpoints, following the procedures for TCP in
   [RFC4145].

8.3.2.

10.3.2.  SCTP Association Initiation

   Both the 'active' and 'passive' endpoint MUST initiate the SCTP
   association, and MUST use the same SCTP port as client port and
   server port (in order to prevent two separate SCTP associations from
   being established).

   NOTE: The procedure above is different from TCP, where only the
   'active' endpoint initiates the TCP connection [RFC4145].

   If the m- line proto field value is 'DTLS/SCTP', and if SCTP is
   carried on top of TCP, 'TCP/DTLS/SCTP', only the 'active'
   endpoint will initiate the TCP connection, following the procedures
   in [RFC4145], while both [RFC4145].  Both endpoints will still initiate the SCTP
   association carried on top of the TCP connection.

8.3.3.

10.3.3.  TLS Role Determination

   If the m- line proto field value is 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'DTLS/SCTP',
   'TCP/DTLS/SCTP', the 'active/passive' status is used to determine the
   TLS roles. roles of the endpoints.  Following the procedures in [RFC4572],
   the 'active' endpoint will take the TLS client role.

   Once a DTLS connection has been established, if the 'active/passive'
   status of the endpoints change (as result of an offer/answer
   transaction) during a session, a new DTLS connection MUST be
   established.  Therefore, endpoints SHOULD NOT change the 'active/passive' 'active/
   passive' status in subsequent offers and answers, during a session, unless they want to establish a new
   DTLS connection.

   If the transport parameters or the key fingerprints change, the
   endpoints MUST establish a new DTLS connection.  In such case the
   'active/passive' status of the endpoints will again be determined
   following the procedures in [RFC4145], and the new status will be
   used to determine the TLS roles of the endpoints associated with the
   new DTLS connection.

   NOTE: The procedure above is identical to the one defined for SRTP-
   DTLS in [RFC5763].

   NOTE: A new DTLS connection needs to be established if the transport
   parameters or the key fingerprints change.

8.4.

10.4.  SDP connection Attribute

   The SDP connection attribute is used following the procedures in
   [RFC4145], with the additional SCTP specific considerations described
   in this section.

   The SDP connection attribute only applies to an SCTP association and,
   if

   If the m- line proto field value is 'DTLS/SCTP', also to the TCP 'TCP/DTLS/SCTP', an SDP connection which is used
   attribute associated with that m- line applies to carry both the SCTP association.  An
   association and the TCP connection.  Therefore, an attribute 'new'
   value indicates that both a new SCTP association (and, if
   applicable, the association, and a new TCP
   connection, have to be established, following the procedures in
   [RFC4145].

   OPEN ISSUE #3: We need to determine whether the SDP connection
   attribute only applies to the transport-layer protocol, and

   NOTE: This specification does not e.g.
   to an SCTP assocation carried on top define a mechanism which allows re-
   establishing of UDP or TCP. a new SCTP association, while maintaining the TCP
   connection.

   The SDP connection attribute value does not automatically impact an
   existing DTLS connection.  Section 8.3.3 10.3.3 describes in which cases a
   new DTLS connections will be established.

   NOTE: if If the m- line proto field value is 'SCTP/DTLS', and if the SCTP
   association is re-established, the DTLS connection also needs to be
   re-established.

   OPEN ISSUE #2: Verify that the above statement regarding 'SCTP/DTLS'
   is correct.

9.

11.  SDP Offer/Answer Procedures

9.1.

11.1.  General

   This section defines the SDP Offer/Answer [RFC3264] procedures for
   negotiating and establishing an SCTP association.  Unless explicitly
   stated, the procedures apply to all m- line proto values ('SCTP',
   'SCTP/DTLS'
   'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'DTLS/SCTP') 'TCP/DTLS/SCTP') defined in this
   specification.

   If the m- line proto value is 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP' or 'DTLS/SCTP',
   'TCP/DTLS/SCTP', each endpoint MUST provide a certificate
   fingerprint, using the SDP 'fingerprint' attribute [RFC4145], if the
   endpoint supports, and is willing to use, a cipher suite with an
   associated certificate.

   The authentication certificates are interpreted and validated as
   defined in [RFC4572].  Self-signed certificates can be used securely,
   provided that the integrity of the SDP description is assured as
   defined in [RFC4572].

   NOTE: The procedures apply to a specific m- line describing an SCTP
   association.  If an offer or answer contains multiple m- line lines
   describing SCTP associations, the procedures are applied separately
   to each m- line.

9.2.

11.2.  Generating the Initial SDP Offer

   When the offerer creates an initial offer, the offerer:

   o  MUST, if the m- line proto field value is 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP'
      or 'DTLS/
      SCTP', 'TCP/DTLS/SCTP', associate an SDP setup attribute
      [Section 8.3], 10.3], with an 'actpass' value, with the m- line;

   o  MUST, if the m- line proto field is 'DTLS/SCTP', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP',
      associate an SDP
      sctp-port attribute[Section 'sctp-port' attribute [Section 5] with the m-
      line;

   o  MUST associate an SDP 'connection' attribute [Section 8.4], 10.4], with
      a 'new' value, with the m- line; and

   o  MAY associate an SDP 'max-message-size' attribute [Section 7] with
      the m- line.

9.3.

11.3.  Generating the SDP Answer

   When the answerer receives an offer, which contains an m- line
   describing an SCTP association, if the answerer accepts the m- line
   it:

   o  MUST insert a corresponiding corresponding m- line in the answer, with an
      identical m- line proto value [RFC3264];

   o  MUST, if the m- line proto field value is 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP'
      or 'DTLS/
      SCTP', 'TCP/DTLS/SCTP', associate an SDP setup 'setup' attribute
      [Section 8.3], 10.3], with an 'active' or 'passive' value, with the m-
      line;

   o  MUST, if the m- line proto field is 'DTLS/SCTP', 'UDP/DTLS/SCTP' or 'TCP/DTLS/SCTP',
      associate an SDP
      sctp-port 'sctp-port' attribute[Section 5] with the m-
      line; and

   o  MAY associate an SDP 'max-message-size' attribute [Section 7] with
      the m- line.

   Once the answerer has sent the answer, the answerer:

   o  MUST, if an SCTP association associated with the m- line has yet
      not been established, or if an existing SCTP association is to be
      re-established, initiate the establishing of the SCTP association;
      and

   o  MUST, if the answerer is the 'active' endpoint, and if an DTLS
      connection associated with the m- line is to be established (or
      re-established), initiate the establishing of the DTLS connection
      (by sending a ClientHello message).

   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.
   In addition, the answerer MUST NOT establish an SCTP association, or
   a DTLS connection, associated with the m- line.

9.4.

11.4.  Offerer Processing of the SDP Answer

   When the offerer receives an answer, which contains an m- line with a
   non-zero port value, describing an SCTP association, the offerer:

   o  MUST, if the offerer is the 'active' endpoint, if the m- line
      proto is 'TCP/DTLS/SCTP', and if a TCP connection used to carry
      the SCTP association has yet not been established (or if an
      existing TCP connection is to be re-established), initiate the
      establishing of the TCP connection;

   o  MUST, if an SCTP association associated with the m- line has yet
      not been established, or established (or if an existing SCTP association is to be
      re-established,
      re-established), initiate the establishing of the SCTP
      association; and

   o  MUST, if the offerer is the 'active' endpoint, and if an DTLS
      connection associated with the m- line is to be established (or if
      an existing DTLS connection is to be re-established), initiate the
      establishing of the DTLS connection (by sending a ClientHello
      message).

   If the m- line in the answer contains a zero port value, the offerer
   MUST NOT establish a TCP connection, an SCTP association, or a DTLS
   connection, associated with the m- line.

9.5.

11.5.  Modifying the Session

   When an offerer sends an updated offer, in order to modify a
   previously established SCTP assiciation, association, it follows the procedures in
   Section 9.2, 11.2, with the following exceptions:

   o  Unless the offerer wants to re-establish an existing SCTP
      association, the offerer MUST associate an SDP connection
      attribute, with an 'existing' value, with the m- line; and

   o  If the offerer wants to disable a previously established SCTP
      association, it MUST assign a zero port value to the m- line
      associated with the SCTP association, following the procedures in
      [RFC3264].

   NOTE: Different SCTP association usages might define protocol
   procedures etc that need to be performed before an SCTP association
   is terminated.  Such procedures are outside the scope of this
   specification.

10.

12.  Multihoming Considerations

   SCTP supports multihoming.  An SCTP endpoint is considered multihomed
   if it has more than one IP address on which SCTP can be used.  An
   SCTP endpoint inform the remote peer about its IP addresses using the
   address parameters in the INIT/INIT-ACK chunk.  Therefore, when SDP
   is used to describe an SCTP association, while the "c=" line contains
   the address which was used to negotiate the SCTP association,
   multihomed SCTP endpoints might end up using other IP addresses.

   If an endpoint removes the IP address [RFC5061] that it offered in
   the SDP "c=" line associated with the SCTP association, it MUST send
   a new Offer, in which the "c=" line contains an IP address with is
   valid within the SCTP association.

   NOTE: In some network environments, intermediaries performing gate-
   and firewall control use the address information in the SDP "c=" and
   "m=" lines to authorize media, and will not pass media sent using
   other addresses.  In such network environment, if an SCTP endpoints
   wants to change the address information on which media is sent and
   received, it needs to send an updated Offer, in which the SDP "c="
   and "m=" lines contain the new address information.

   Multihoming is not supported when sending SCTP on top of DTLS, as
   DTLS does not expose address management to its upper layer.

11.

13.  NAT Considerations

11.1.

13.1.  General

   SCTP features not present in UDP or TCP, including the checksum
   (CRC32c) value calculated on the whole packet (rather than just the
   header), and multihoming, introduce new challenges for NAT traversal.
   [I-D.ietf-behave-sctpnat] defines an SCTP specific variant of NAT,
   which provides similar features of Network Address and Port
   Translation (NAPT).

   Current NATs typically do not support SCTP.  [RFC6951] defines a
   mechanism for sending SCTP on top of UDP, which makes it possible to
   use SCTP with NATs and firewalls that do not support SCTP.

11.2.

13.2.  ICE Considerations

   At the time of writing this specification, no procedures have been
   defined for using ICE ICE (Interactive Connectivity Establishment)
   [RFC5768] together with SCTP.  Such procedures, including the
   associated SDP Offer/Answer procedures, are outside the scope of this
   specification, and might be defined in a future specification.

12.

14.  Examples

   TODO: ADD EXAMPLES HERE

13.

15.  Security Considerations

   [RFC4566] defines general SDP security considerations, while
   [RFC3264], [RFC4145] and [RFC4572] define security considerations
   when using the SDP Offer/Answer offer/answer mechanism to negotiate media streams.

   [RFC4960] defines general SCTP security considerations.  security
   considerations on SCTP in general, while [RFC6083] defines security
   considerations when using DTLS on top of SCTP.

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

14.

16.  IANA Considerations

14.1.

16.1.  New SDP proto values

   [RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
   document.]
   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     |      SCTP     | [RFCXXXX] |
               |     proto     |   SCTP/DTLS   | [RFCXXXX] |
               |     proto     | DTLS/SCTP UDP/DTLS/SCTP | [RFCXXXX] |
                     +-------+-----------+-----------+
               | TCP/DTLS/SCTP |   [RFCXXXX]   |
               +---------------+---------------+-----------+

                     Table 1: SDP "proto" field values

14.2.

16.2.  New SDP Attribute

   [RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
   document.]

   This document defines a new SDP media-level attribute,'sctp-port', as
   follows:

       Attribute name:     sctp-port
       Type of attribute:  media
       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:          RFCXXXX

14.3.

16.3.  association-usage Name Registry

   [RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
   document.]

   This specification creates a new IANA registry, following the
   procedures in [RFC5226], for the "fmt" namespace associated with the
   'SCTP', 'SCTP/DTLS' 'SCTP/DTLS', 'UDP/DTLS/SCTP' and 'DTLS/SCTP' '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" is
   required.  It is defined in section Section 4. [Section 4].

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

   association-usage Name:  .The identifier of the subprotocol, as will
      be used in the <sctp-fmtp> subfield.

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

   association-usage names are to be subject to the "First Come First
   Served" IANA registration policy [RFC5226].

   IANA is asked to add initial values to the registry.

        |         name       |           Reference                 |
       -+--------------------+-------------------------------------+
        | webrtc-datachannel | draft-ietf-rtcweb-data-protocol-xx  |
       -+----------------------------------------------------------|

                                 Figure 1

15.

17.  Acknowledgments

   The authors wish to thank Harald Alvestrand, Randell Jesup, Paul
   Kyzivat, Michael Tuexen for their comments and useful feedback.

16.

18.  Change Log

   [RFC EDITOR NOTE: Please remove this section when publishing]

   Changes from draft-ietf-mmusic-sctp-sdp-09

   o  'DTLS/SCTP' split into 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'

   o  Procedures for realizing UDP/DTLS/SCTP- and TCP/DTLS/SCTP
      transports added.

   Changes from draft-ietf-mmusic-sctp-sdp-08

   o  Default SCTP port removed removed:

   o  - Usage of SDP sctp-port attribute mandatory mandatory.

   o  SDP max-message-size attribute defined defined:

   o  - Attribute definition definition.

   o  - SDP Offer/Answer procedures procedures.

   o  Text about SDP direction attributes added added.

   o  Text about TLS role determination added

17. added.

19.  References

17.1.

19.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June
              2002.

   [RFC4145]  Yon, D. and G. Camarillo, "TCP-Based Media Transport in
              the Session Description Protocol (SDP)", RFC 4145,
              September 2005.

   [RFC4289]  Freed, N. and J. Klensin, "Multipurpose Internet Mail
              Extensions (MIME) Part Four: Registration Procedures", BCP
              13, RFC 4289, December 2005.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4571]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
              and RTP Control Protocol (RTCP) Packets over Connection-
              Oriented Transport", RFC 4571, July 2006.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572, July 2006.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol", RFC
              4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061, September
              2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

   [I-D.ietf-tsvwg-sctp-dtls-encaps]
              Tuexen, M., Stewart, R., Jesup, R., and S. Loreto, "DTLS
              Encapsulation of SCTP Packets", draft-ietf-tsvwg-sctp-
              dtls-encaps-06 (work in progress), November 2014.

17.2.

19.2.  Informative References

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, May 2010.

   [RFC5768]  Rosenberg, J., "Indicating Support for Interactive
              Connectivity Establishment (ICE) in the Session Initiation
              Protocol (SIP)", RFC 5768, April 2010.

   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083, January 2011.

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951, May 2013.

   [I-D.ietf-behave-sctpnat]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation",
              draft-ietf-behave-sctpnat-09 (work in progress), September
              2013.

Authors' Addresses

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com

   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Salvatore.Loreto@ericsson.com

   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com