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MMUSIC                                                       C. Holmberg
Internet-Draft                                                 S. Loreto
Intended status: Standards Track                            G. Camarillo
Expires: March 3, 2017                                          Ericsson
                                                         August 30, 2016


Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
                   Session Description Protocol (SDP)
                     draft-ietf-mmusic-sctp-sdp-17

Abstract

   The Stream Control Transmission Protocol (SCTP) 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',
   'UDP/DTLS/SCTP' and '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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 3, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   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  . . . . . . . . . . . . . . . . . . . . . . . . .   5
       4.4.1.  General . . . . . . . . . . . . . . . . . . . . . . .   6
       4.4.2.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  SDP 'sctp-port' Attribute . . . . . . . . . . . . . . . . . .   6
     5.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Mux Category  . . . . . . . . . . . . . . . . . . . . . .   7
   6.  SDP 'max-message-size' Attribute  . . . . . . . . . . . . . .   7
     6.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Syntax  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.3.  Mux Category  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  UDP/DTLS/SCTP Transport Realization . . . . . . . . . . . . .   8
   8.  TCP/DTLS/SCTP Transport Realization . . . . . . . . . . . . .   9
   9.  Association And Connection Management . . . . . . . . . . . .   9
     9.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.2.  SDP sendrecv/sendonly/recvonly/inactive Attribute . . . .   9
     9.3.  SCTP Association  . . . . . . . . . . . . . . . . . . . .   9
     9.4.  DTLS Association (UDP/DTLS/SCTP And TCP/DTLS/SCTP)  . . .  10
     9.5.  TCP Connection (TCP/DTLS/SCTP)  . . . . . . . . . . . . .  10
   10. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . .  11
     10.1.  General  . . . . . . . . . . . . . . . . . . . . . . . .  11
     10.2.  Generating the Initial SDP Offer . . . . . . . . . . . .  11
     10.3.  Generating the SDP Answer  . . . . . . . . . . . . . . .  12
     10.4.  Offerer Processing of the SDP Answer . . . . . . . . . .  12
     10.5.  Modifying the Session  . . . . . . . . . . . . . . . . .  13
   11. Multihoming Considerations  . . . . . . . . . . . . . . . . .  13
   12. NAT Considerations  . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  General  . . . . . . . . . . . . . . . . . . . . . . . .  14



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     12.2.  ICE Considerations . . . . . . . . . . . . . . . . . . .  14
   13. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     13.1.  Establishment of UDP/DTLS/SCTP association . . . . . . .  15
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
     15.1.  New SDP proto values . . . . . . . . . . . . . . . . . .  16
     15.2.  New SDP Attributes . . . . . . . . . . . . . . . . . . .  16
       15.2.1.  sctp-port  . . . . . . . . . . . . . . . . . . . . .  16
       15.2.2.  max-message-size . . . . . . . . . . . . . . . . . .  17
     15.3.  association-usage Name Registry  . . . . . . . . . . . .  17
   16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   17. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     18.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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 [RFC0793] streams.  Connection-Oriented Media
   Transport over the Transport Layer Security (TLS) Protocol in SDP
   [RFC4572] extends RFC4145 [RFC4145] for describing TCP-based media
   streams that are protected using TLS.

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

   This specification defines 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', 'UDP/DTLS/SCTP' and '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 TLS on
   top of SCTP.  Therefore, defining a mechanism to negotiate media
   streams transported using TLS on top of SCTP, i.e. 'SCTP/TLS', is
   outside the scope of this specification.



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2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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 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', '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
   [RFC4566]:

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

   The 'SCTP', '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;



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   SCTP is carried on top of DTLS which is on top 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 'UDP/DTLS/SCTP' proto value describes an SCTP association on
      top of a DTLS association on top of UDP, as defined in Section 7.

   o  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] 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', 'UDP/DTLS/SCTP' or
   '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







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4.4.1.  General

   This section defines the ABNF [RFC5234] for the SDP media description
   when associated with any of the proto values defined in this
   document.

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

4.4.2.  ABNF

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

       sctp-port = port

       udp-port = port

       tcp-port = port

       fmt = association-usage

       association-usage = token

           token and port as defined in RFC4566

4.5.  Example


    m=application 12345 UDP/DTLS/SCTP webrtc-datachannel
    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 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.






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   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.

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

5.2.  Syntax

   The ABNF for the SDP 'sctp-port' attribute is:


       sctp-port-attr  =  "a=sctp-port:" port
       port            =  (1*5)DIGIT

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


5.3.  Mux Category

   The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the SDP
   sctp-port' attribute is SPECIAL.  Usage of the attribute is only
   applicable when associated with 'UDP/DTLS/SCTP' and 'TCP/DTLS/SCTP'
   proto value m- lines.

   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 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.

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




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   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.

   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: 'SCTP', '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 ABNF for the SDP 'max-message-size' attribute is:


       max-message-size-attr =  "a=max-message-size:" max-message-size
       max-message-size      =  1*40DIGIT

             Leading zeroes MUST NOT be used.


6.3.  Mux Category

   The mux category for the SDP 'max-message-size' attribute is SPECIAL.
   The mux rules depends on the proto value of the associated m- line.
   If the proto value is 'SCTP' the rules are identical to the rules
   associated with the TRANSPORT mux category.

   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:

   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 UDP is realized according to the procedures in
      defined in [RFC6347].

   NOTE: While [I-D.ietf-tsvwg-sctp-dtls-encaps] allows multiple SCTP
   associations on top of a single DTLS association, the procedures in



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   this specification only supports 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:

   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], with DTLS packets being sent instead of RTP/RTCP
      packets, and SDP signaling according to the procedures defined in
      this specification.

   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.

9.  Association And Connection Management

9.1.  General

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

   In case of UDP/DTLS/SCTP and TCP/DTLS/SCTP the SCTP association, DTLS
   association and TCP connection are managed independetly for each
   other.  An assocation/connection can be re-established without
   impacting other assocations/connections.

   The detailed SDP Offer/Answer [RFC3264] procedures for the SDP
   attributes are described in (Section 10).

9.2.  SDP sendrecv/sendonly/recvonly/inactive Attribute

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

9.3.  SCTP Association

   When an SCTP association is established/re-established, both SCTP
   endpoints MUST initiate the SCTP association (i.e. both SCTP
   endpoints take the 'active' role), and MUST use the same SCTP port as




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   client port and server port (in order to prevent two separate SCTP
   associations from being established).

   As both SCTP endpoints take the 'active' role, this specification
   does not define usage of the SDP 'setup' attribute [RFC4145] for
   SCTP.

   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: If the underlying transport protocol is UDP or TCP (UDP/DTLS/
   SCTP or TCP/DTLS/SCTP), when the SCTP association is established it
   is assumed that any NAT traversal procedures for the underlying
   transport protocol has successfully been performed.

   Usage of the SDP 'connection' attribute [RFC4145] is not defined for
   SCTP.  In order to trigger the re-establishment of an SCTP
   association, the SDP 'sctp-port' attribute (Section 5) is used to
   indicate a new (different than the ones currently used) SCTP ports.

   NOTE: This specification does not define procedures for re-
   establishing an SCTP association using the SCTP ports currently used
   for the SCTP association.

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

   An DTLS association is managed according to the procedures in
   [I-D.ietf-mmusic-dtls-sdp].  Hence, the SDP 'setup' attribute is used
   to negotiate the (D)TLS roles ('client' and 'server') [RFC4572].

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

   NOTE: As described in [RFC5245], if the Interactive Connectivity
   Establishment (ICE) mechanism [RFC5245] is used, all ICE candidates
   associated with an DTLS association as 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'




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   attribute is used to indicate whether to use an existing TCP
   connection, or create a new one.

   NOTE: A change of the TCP roles will also trigger a re-establishment
   of the DTLS association, according to the procedures in
   [I-D.ietf-mmusic-dtls-sdp].

10.  SDP Offer/Answer Procedures

10.1.  General

   This section defines the SDP Offer/Answer [RFC3264] procedures for
   negotiating and establishing an SCTP association.  Unless explicitly
   stated, the procedures apply to SCTP, UDP/DTLS/SCTP and TCP/DTLS/
   SCTP.

   In case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, each endpoint MUST
   associate one or more certificate fingerprints, using the SDP
   'fingerprint' attribute with the m- line, following the procedures in
   [RFC4572] and [I-D.ietf-mmusic-4572-update].

   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].

   In case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, each endpoint MUST
   associate an SDP 'dtls-id' attribute with the m- line, following the
   procedures in [I-D.ietf-mmusic-dtls-sdp].

10.2.  Generating the Initial SDP Offer

   When the offerer creates an initial offer, the offerer:

   o  MUST, in the case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, associate an
      SDP setup attribute, with an 'actpass' value, with the m- line;

   o  MUST, in the case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, associate an
      SDP 'sctp-port' attribute with the m- line;

   o  MUST, in the case of TCP/DTLS/SCTP, associate an SDP 'connection'
      attribute, with a 'new' attribute value, with the m- line; and

   o  MAY associate an SDP 'max-message-size' attribute (Section 6) with
      the m- line.






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10.3.  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 corresponding m- line in the answer, with an
      identical m- line proto value [RFC3264];

   o  MUST, in the case of UDP/DTLS/SCTP or TCP/DTLS/SCTP associate an
      SDP 'setup' attribute, with an 'active' or 'passive' value, with
      the m- line;

   o  MUST, in the case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, associate an
      SDP 'sctp-port' attribute with the m- line; and

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

   Once the answerer has sent the answer, the answerer MUST, if an SCTP
   association has yet not been established, or if an existing SCTP
   association is to be re-established, initiate the establishement of
   the SCTP association.

   In the case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, the answerer follows
   the procedures in [I-D.ietf-mmusic-dtls-sdp] regarding the
   establishment/re-establishemnt of the DTLS association.

   In the case of TCP/DTLS/SCTP, the answerer follows the procedures in
   [RFC4145] regarding the establishment/re-establishemnt of the TCP
   conection association.

   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 an SCTP 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, which contains an m- line
   with a non-zero port value, the offerer MUST, if an SCTP association
   has yet not been established, or if an existing SCTP association is
   to be re-established, initiate the establishement of the SCTP
   association.




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   In the case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, the offerer follows
   the procedures in [I-D.ietf-mmusic-dtls-sdp] regarding the
   establishment/re-establishemnt of the DTLS association.

   In the case of TCP/DTLS/SCTP, the offerer follows the procedures in
   [RFC4145] regarding the establishment/re-establishemnt of the TCP
   conection association.

   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
   association, associated with the m- line.

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:

   o  Unless the offerer wants to re-establish an SCTP association, the
      offerer MUST associate an SDP 'sctp-port' attribute, with a new
      attribute 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].

   In the case of UDP/DTLS/SCTP or TCP/DTLS/SCTP, the offer follows the
   procedures in [I-D.ietf-mmusic-dtls-sdp] regarding the DTLS
   association impacts when modifying a session.

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

11.  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




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   a new Offer, in which the "c=" line contains an IP address which is
   valid within the SCTP association.

   NOTE: In some network environments, intermediaries performing gate-
   and firewall control using 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 environments, 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 of the underlying transport
   protocols (UDP or TCP) to its upper layer.

12.  NAT Considerations

12.1.  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.

12.2.  ICE Considerations

   At the time of writing this specification, no procedures have been
   defined for using ICE [RFC5245] together with SCTP as transport layer
   protocol.  Such procedures, including the associated SDP Offer/Answer
   procedures, are outside the scope of this specification, and might be
   defined in a future specification.

   When the transport layer protocol is UDP (UDP/DTLS/SCTP), if ICE is
   used, the ICE procedures defined in [RFC5245] are used.

   When the transport layer protocol is TCP (TCP/DTLS/SCTP), if ICE is
   used, the ICE procedures defined in [RFC6544] are used.

   Implementations MUST treat all ICE candidate pairs associated with a
   an SCTP association on top of a DTLS association as part of the same
   DTLS association.  Thus, there will only be one DTLS handshake even
   if there are multiple valid candidate pairs, and shifting from one



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   candidate pair to another will not impact the DTLS association.  If
   new candidates are added, they will also be part of the same DTLS
   association.

13.  Examples

13.1.  Establishment of UDP/DTLS/SCTP association


    SDP Offer:

       m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
       c=IN IP4 192.0.2.1
       a=dtls-id:abc3dl
       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 IP4 192.0.2.2
       a=dtls-id:ggr4rd
       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 [RFC4572] define security considerations
   when using the SDP offer/answer mechanism to negotiate media streams.




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   [RFC4960] defines general SCTP security considerations, while
   [RFC6083] defines security considerations when using DTLS on top of
   SCTP, and [I-D.ietf-tsvwg-sctp-dtls-encaps] 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

   [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 | UDP/DTLS/SCTP | [RFCXXXX] |
                   | proto | TCP/DTLS/SCTP | [RFCXXXX] |
                   +-------+---------------+-----------+

                     Table 1: SDP "proto" field values

15.2.  New SDP Attributes

15.2.1.  sctp-port

   [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:












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           Attribute name:     sctp-port
           Type of attribute:  media
           Mux category:       SPECIAL
           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


15.2.2.  max-message-size

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

   This document defines a new SDP media-level attribute,'max-message-
   size', as follows:


           Attribute name:     max-message-size
           Type of attribute:  media
           Mux category:       SPECIAL
           Subject to charset: No
           Purpose:            Indicate the maximum message size 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:          RFCXXXX


15.3.  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', '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.



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   The "fmt" value, "association-usage", used with these "proto" 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 [RFC5226].

   IANA is asked to add initial values to the registry.


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

         [RFC EDITOR NOTE: Please hold the publication of this draft
     until draft-ietf-rtcweb-data-protocol has been published as an RFC.
     Then, replace the reference to draft-ietf-rtcweb-data-protocol
         with the RFC number.]


                                 Figure 1

16.  Acknowledgments

   The authors wish to thank Roman Shpount, Harald Alvestrand, Randell
   Jesup, Paul Kyzivat, Michael Tuexen, Juergen Stoetzer-Bradler,
   Flemming Andreasen and Ari Keranen for their comments and useful
   feedback.

17.

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

   Changes from draft-ietf-mmusic-sctp-sdp-15

   o  Chapter about SCTP, DTLS and TCP association/connection management
      modified.

   o  Removal of SCTP/DTLS.



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   Changes from draft-ietf-mmusic-sctp-sdp-14

   o  Changes based on WGLC comments from Magnus Westerlund.

   o  - ABNF clarification that token and port are defined in RFC4566.

   o  - Specify 40 as maximum digit character length for the SDP max-
      message-size value.

   o  - Editorial clarification.

   o  Changes based on discussions at IETF#92.

   o  - Specify that all ICE candidate pairs belong to the same DTLS
      association.

   Changes from draft-ietf-mmusic-sctp-sdp-13

   o  Changes based on comments from Paul Kyzivat.

   o  - Text preventing usage of well-known ports removed.

   o  - Editorial clarification.

   Changes from draft-ietf-mmusic-sctp-sdp-12

   o  Mux category rules added for new SDP attributes.

   o  Reference to draft-ietf-mmusic-sdp-mux-attributes added.

   o  Changes based on comments from Roman Shpount:

   o  - Specify that fingerprint or setup roles must not be modified,
      unless underlying transport protocol is also modified.

   o  Changes based on comments from Ari Keranen:

   o  - Editorial corrections.

   o  Changes based on comments from Flemming Andreasen:

   o  - Clarify that, if UDP/DTLS/SCTP or TCP/DTLS/SCTP is used, the
      DTLS association is established before the SCTP association.

   o  - Clarify that max-message-size value is given in bytes, and that
      different values can be used per direction.

   o  - Section on fmtp attribute removed.



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   o  - Editorial corrections.

   Changes from draft-ietf-mmusic-sctp-sdp-11

   o  Example added.

   Changes from draft-ietf-mmusic-sctp-sdp-10

   o  SDP max-message-size attribute added to IANA considerations.

   o  Changes based on comments from Paul Kyzivat:

   o  - Text about max message size removed from fmtp attribute section.

   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:

   o  - Usage of SDP sctp-port attribute mandatory.

   o  SDP max-message-size attribute defined:

   o  - Attribute definition.

   o  - SDP Offer/Answer procedures.

   o  Text about SDP direction attributes added.

   o  Text about TLS role determination added.

18.  References

18.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.






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   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4145>.

   [RFC4289]  Freed, N. and J. Klensin, "Multipurpose Internet Mail
              Extensions (MIME) Part Four: Registration Procedures",
              BCP 13, RFC 4289, DOI 10.17487/RFC4289, December 2005,
              <http://www.rfc-editor.org/info/rfc4289>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <http://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, <http://www.rfc-editor.org/info/rfc4571>.

   [RFC4572]  Lennox, J., "Connection-Oriented Media Transport over the
              Transport Layer Security (TLS) Protocol in the Session
              Description Protocol (SDP)", RFC 4572,
              DOI 10.17487/RFC4572, July 2006,
              <http://www.rfc-editor.org/info/rfc4572>.

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

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)
              Dynamic Address Reconfiguration", RFC 5061,
              DOI 10.17487/RFC5061, September 2007,
              <http://www.rfc-editor.org/info/rfc5061>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.







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

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

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <http://www.rfc-editor.org/info/rfc6838>.

   [I-D.ietf-mmusic-4572-update]
              Holmberg, C., "Updates to RFC 4572", draft-ietf-mmusic-
              4572-update-05 (work in progress), June 2016.

   [I-D.ietf-mmusic-dtls-sdp]
              Holmberg, C. and R. Shpount, "Using the SDP Offer/Answer
              Mechanism for DTLS", draft-ietf-mmusic-dtls-sdp-14 (work
              in progress), July 2016.

   [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-09 (work in progress), January 2015.

   [I-D.ietf-mmusic-sdp-mux-attributes]
              Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-13
              (work in progress), June 2016.

18.2.  Informative References

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

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <http://www.rfc-editor.org/info/rfc5245>.







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   [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
              Transport Layer Security (DTLS) for Stream Control
              Transmission Protocol (SCTP)", RFC 6083,
              DOI 10.17487/RFC6083, January 2011,
              <http://www.rfc-editor.org/info/rfc6083>.

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

   [RFC6951]  Tuexen, M. and R. Stewart, "UDP Encapsulation of Stream
              Control Transmission Protocol (SCTP) Packets for End-Host
              to End-Host Communication", RFC 6951,
              DOI 10.17487/RFC6951, May 2013,
              <http://www.rfc-editor.org/info/rfc6951>.

   [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










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   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com












































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