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MMUSIC                                                         S. Loreto
Internet-Draft                                              G. Camarillo
Intended status: Standards Track                                Ericsson
Expires: April 25, 2013                                 October 22, 2012


Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
                   Session Description Protocol (SDP)
                     draft-ietf-mmusic-sctp-sdp-02

Abstract

   SCTP (Stream Control Transmission Protocol) is a transport protocol
   used to establish associations between two endpoints.  This document
   describes how to express media transport over SCTP in SDP (Session
   Description Protocol).  This document defines the 'SCTP', 'SCTP/DTLS'
   and 'DTLS/SCTP' protocol identifiers for SDP.

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 April 25, 2013.

Copyright Notice

   Copyright (c) 2012 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
   (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



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Protocol Identifier  . . . . . . . . . . . . . . . . . . . . .  4
   4.  Media Formats  . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  data channels and m-line . . . . . . . . . . . . . . . . .  5
   5.  Streams Attribute  . . . . . . . . . . . . . . . . . . . . . .  6
   6.  Datachannel Attribute  . . . . . . . . . . . . . . . . . . . .  6
   7.  The Setup and Connection Attributes and Association
       Management . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Multihoming  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   9.  Network Address Translation (NAT) Considerations . . . . . . .  8
   10. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     10.1. Actpass/Passive  . . . . . . . . . . . . . . . . . . . . .  8
     10.2. Existing Connection Reuse  . . . . . . . . . . . . . . . .  9
     10.3. SDP description for DTLS Connection  . . . . . . . . . . . 10
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 11
     13.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

























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1.  Introduction

   SDP (Session Description Protocol) [RFC4566] provides a general-
   purpose format for describing multimedia sessions in announcements or
   invitations.  RFC4145 [RFC4145] specifies a general mechanism for
   describing and establishing TCP (Transmission Control Protocol)
   streams.  RFC 4572 [RFC4572] extends RFC4145 [RFC4145] for describing
   TCP-based media streams that are protected using TLS (Transport Layer
   Security) [RFC5246].

   This document defines a new protocol identifier, 'SCTP', to describe
   SCTP-based [RFC4960] media streams.  Additionally, this document
   specifies the use of the 'setup' and 'connection' SDP attributes to
   establish SCTP associations.  These attributes were defined in
   RFC4145 [RFC4145] for TCP.  This document discusses their use with
   SCTP.

   Additionally this document defines two new protocol identifiers:

   SCTP/DTLS :  to allow the usage of the Datagram Transport Layer
      Security (DTLS) [RFC4347] protocol over SCTP, as specified in
      [RFC6083], using SDP.  DTLS over SCTP provides communications
      privacy for applications that use SCTP as their transport
      protocol.

   DTLS/SCTP :  to allow the usage of SCTP on top of the Datagram
      Transport Layer Security (DTLS) protocol, as defined in
      [I-D.tuexen-tsvwg-sctp-dtls-encaps], using SDP.  SCTP over DTLS is
      used by the RTCWeb protocol suite for transporting non- media data
      between browsers.

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

   TLS is designed to run on top of a byte-stream oriented transport
   protocol providing a realible, in-sequence delivery like TCP.  Since
   no-one so far has implemented SCTP over TLS, due to some serious
   limitations described in [RFC6083], this document does not make use
   of TLS over SCTP as described in RFC3436 [RFC3436].


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



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   described in BCP 14, RFC 2119 [RFC2119] and indicate requirement
   levels for compliant implementations.


3.  Protocol Identifier

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

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

   This document defines three new values for the 'proto' field: 'SCTP',
   'SCTP/DTLS' and 'DTLS/SCTP'.

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

   Media described using an 'm' line containing the 'SCTP' protocol
   identifier are carried using SCTP [RFC4960].

   The 'SCTP/DTLS' protocol identifier indicates that the media
   described will use the Datagram Transport Layer Security (DTLS)
   [RFC4347] over SCTP as specified in [RFC6083].

   The 'DTLS/SCTP' protocol identifier indicates that the media
   described will use SCTP on top of the Datagram Transport Layer
   Security (DTLS) protocol as specified in
   [I-D.tuexen-tsvwg-sctp-dtls-encaps].

   An 'm' line that specifies 'SCTP' or 'SCTP/DTLS' or 'DTLS/SCTP' MUST
   further qualify the application-layer protocol using an fmt
   identifier.

   An 'm' line that specifies 'SCTP/DTLS' or 'DTLS/SCTP' MUST further
   provide a certificate fingerprint.  An SDP attribute (an 'a' line) is
   used to transport and exchange end point certificate.  The
   authentication certificates are interpreted and validated as defined
   in [RFC4572].


4.  Media Formats

   The SDP specification, [RFC4566], states that specifications defining
   new proto values, like the SCTP, SCTP/DTLS and DTLS/SCTP proto values
   defined in this RFC, must define the rules by which their media
   format (fmt) namespace is managed.  Use of an existing MIME subtype



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   for the format is encouraged.  If no MIME subtype exists, it is
   RECOMMENDED that a suitable one is registered through the IETF
   process [RFC4288] [RFC4289] by production of, or reference to, a
   standards-track RFC that defines the transport protocol for the
   format.

4.1.  data channels and m-line

   SCTP defines a stream as an unidirectional logical channel existing
   within an SCTP association one to another SCTP endpoint.  The streams
   are used to provide the notion of in-sequence delivery.  Each user
   message is sent on a particular stream, either order or unordered.
   Ordering is preserved only for all ordered messages sent on the same
   stream.

   A datachannel may be:

      unidirectional: formed by one single incoming or outgoing SCTP
      stream

      bidirectional: a pair of one incoming stream and one outgoing SCTP
      stream

   Using the format part of the m line for negotiating datachannel
   allows the intermediaries nodes to become aware of the kind of
   traffic actually exchanged on each datachannel, as well as of the
   number of datachannels established within the association.

   If the <port> sub-field is 'SCTP' or 'SCTP/DTLS' or 'DTLS/SCTP' the
   <fmt> sub-fields contain datachannel number identifiers.  When a list
   of datachannel number identifiers is given, this implies that all of
   these datachannel MUST be used in the association.  For the payload
   type assignments the "a=datachannel:" attribute (see Section 6)
   SHOULD be used to map from a datachannel number to a media encoding
   name that identifies the payload format transported by the
   datachannel.

   [NOTE] The datachannel number identifier does not necessarily map
   directly with the number of the stream used to form the channel.

   [OPEN ISSUE 1]do we need a stream attribute for it?

   [NOTE] The exact definition of datachannel depends from the actual
   media type in use it can be a unidirectional channel formed by a
   unidirectional stream of an SCTP association or a bidirectional
   channel consisting of two SCTP streams one for each side of the
   association.




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   [OPEN ISSUE 2] do we need to register the "RTCWeb" media type in
   accordance with the requirements of RFC 4288 here or in the
   Datachannel draft?.

   [OPEN ISSUE 3] do we need also an attribute to specify the SCTP port
   number for the SCTP over DTLS scenario? that can be useful in the
   case where multiple association are running on top of the same DTLS
   session.

   An example of a datachannel payload type assignment is RTCWeb used
   only in certain defined datachannel of the SCTP association.


           m=application 54111 DTLS/SCTP 0 1 2
           a=streams=3
           a=datachannel:0 RTCWeb label=xxxx;options="dictionary string"
           a=datachannel:1 RTCWeb label=xxxx;options="dictionary string"
           a=datachannel:2 RTCWeb label=xxxx;options="dictionary string"



5.  Streams Attribute

   The 'streams' attribute indicates the number of streams to be
   supported by the association.  If this attribute is not present, the
   implementation should provide a default, with a suggested value of
   16.


         streams-attr        =  "a=streams=" streamsnumbers
         streamsnumbers      =  1*DIGIT



6.  Datachannel Attribute

   This attribute maps from a datachannel number (as used in an "m="
   line) to an encoding name denoting the payload format to be used.



         datachannel-attr        =  "a=datachannel:" datachannel-number
         datachannel-number      =  1*DIGIT








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7.  The Setup and Connection Attributes and Association Management

   The use of the 'setup' and 'connection' attributes in the context of
   an SCTP association is identical to the use of these attributes in
   the context of a TCP connection.  That is, SCTP endpoints MUST follow
   the rules in Sections 4 and 5 of RFC 4145 [RFC4145] when it comes to
   the use of the 'setup' and 'connection' attributes in offer/answer
   [RFC3264] exchanges.

   The management of an SCTP association is identical to the management
   of a TCP connection.  That is, SCTP endpoints MUST follow the rules
   in Section 6 of RFC 4145 [RFC4145] to manage SCTP associations.
   Whether to use the SCTP ordered or unordered delivery service is up
   to the applications using the SCTP association.


8.  Multihoming

   An SCTP endpoint, unlike a TCP endpoint, can be multihomed.  An SCTP
   endpoint is considered to be multihomed if it has more than one IP
   address.  A multihomed SCTP endpoint informs a remote SCTP endpoint
   about all its IP addresses using the address parameters of the INIT
   or the INIT-ACK chunk (depending on whether the multihomed endpoint
   is the one initiating the establishment of the association).
   Therefore, once the address provided in the 'c' line has been used to
   establish the SCTP association (i.e., to send the INIT chunk),
   address management is performed using SCTP.  This means that two SCTP
   endpoints can use addresses that were not listed in the 'c' line but
   that were negotiated using SCTP mechanisms.

   During the lifetime of an SCTP association, the endpoints can add and
   remove new addresses from the association at any point [RFC5061].  If
   an endpoint removes the IP address listed in its 'c' line from the
   SCTP association, the endpoint MUST update the 'c' line (e.g., by
   sending a re-INVITE with a new offer) so that it contains an IP
   address that is valid within the SCTP association.

   In some environments, intermediaries performing firewall control use
   the addresses in offer/answer exchanges to perform media
   authorization.  That is, policy-enforcement network elements do not
   let media through unless it is sent to the address in the 'c' line.

   In such network environments, the SCTP endpoints can only exchange
   media using the IP addresses listed in their 'c' lines.  In these
   environments, an endpoint wishing to use a different address needs to
   update its 'c' line (e.g., by sending a re-INVITE with a new offer)
   so that it contains the new IP address.




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   It is worth to underline that when using SCTP on top of DTLS, only
   single homed SCTP associations can be used, since DTLS does not
   expose any address management to its upper layer.


9.  Network Address Translation (NAT) Considerations

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

   Current NATs do not typically support SCTP.  As an alternative to
   design SCTP specific NATs, Encapsulating SCTP into UDP
   [I-D.tuexen-sctp-udp-encaps] makes it possible to use SCTP in
   networks with legacy NAT and firewalls not supporting SCTP.

   At the time of writing, the work on NAT traversal for SCTP is still
   work in progress.  Additionally, no extension has been defined to
   integrate ICE (Interactive Connectivity Establishment) [RFC5768] with
   SCTP and its multihoming capabilities either.  Therefore, this
   specification does not define how to describe SCTP-over-UDP streams
   in SDP or how to establish and maintain SCTP associations using ICE.
   Should these features be specified for SCTP in the future, there will
   be a need to specify how to use them in an SDP environment as well.


10.  Examples

   The following examples show the use of the 'setup' and 'connection'
   SDP attributes.  As discussed in Section 7, the use of these
   attributes with an SCTP association is identical to their use with a
   TCP connection.  For the purpose of brevity, the main portion of the
   session description is omitted in the examples, which only show 'm'
   lines and their attributes (including 'c' lines).

10.1.  Actpass/Passive

   An offerer at 192.0.2.2 signals its availability for an SCTP
   association at SCTP port 54111.  Additionally, this offerer is also
   willing to initiate the SCTP association:








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           m=application 54111 SCTP 0 1 2
           c=IN IP4 192.0.2.2
           a=setup:actpass
           a=connection:new
           a=streams=3
           a=datachannel:0 RTCWeb label=xxxx;options="dictionary string"
           a=datachannel:1 RTCWeb label=xxxx;options="dictionary string"
           a=datachannel:2 RTCWeb label=xxxx;options="dictionary string"

                                 Figure 1

   The endpoint at 192.0.2.1 responds with the following description:

           m=image 54321 SCTP 0 1 2
           c=IN IP4 192.0.2.1
           a=setup:passive
           a=connection:new
           a=streams=3
           a=datachannel:0 RTCWeb label=xxxx;options="dictionary string"
           a=datachannel:1 RTCWeb label=xxxx;options="dictionary string"
           a=datachannel:2 RTCWeb label=xxxx;options="dictionary string"

                                 Figure 2

   This will cause the offerer (at 192.0.2.2) to initiate an SCTP
   association to port 54321 at 192.0.2.1.

10.2.  Existing Connection Reuse

   Subsequent to the exchange in Section 10.1, another offer/answer
   exchange is initiated in the opposite direction.  The endpoint at
   192.0.2.1, which now acts as the offerer, wishes to continue using
   the existing association:

              m=application 54321 SCTP *
              c=IN IP4 192.0.2.1
              a=setup:passive
              a=connection:existing

                                 Figure 3

   The endpoint at 192.0.2.2 also wishes to use the existing SCTP
   association and responds with the following description:








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              m=application 9 SCTP *
              c=IN IP4 192.0.2.2
              a=setup:active
              a=connection:existing

                                 Figure 4

   The existing SCTP association between 192.0.2.2 and 192.0.2.1 will be
   reused.

10.3.  SDP description for DTLS Connection

   An offerer at 192.0.2.2 signals the availability of a T.38 fax
   session over SCTP/DTLS.

           m=image 54111 SCTP/DTLS 0
           c=IN IP4 192.0.2.2
           a=setup:actpass
           a=connection:new
           a=fingerprint:SHA-1 \
             4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
           a=fmtp:datachannel streams=1
           a=datachannel:0 t38 label=xxxx;options="dictionary string"


                                 Figure 5


11.  Security Considerations

   See RFC 4566 [RFC4566] for security considerations on the use of SDP
   in general.  See RFC 3264 [RFC3264], RFC 4145 [RFC4145] and RFC 4572
   [RFC4572] for security considerations on establishing media streams
   using offer/answer exchanges.  See RFC 4960 [RFC4960] for security
   considerations on SCTP in general and [RFC6083] for security
   consideration using DTLS on top of SCTP.  This specification does not
   introduce any new security consideration in addition to the ones
   discussed in those specifications.


12.  IANA Considerations

   This document defines three new proto values: 'SCTP', 'SCTP/DTLS' and
   'DTLS/SCTP'.  Their formats are defined in Section 3.  These proto
   values should be registered by the IANA under "Session Description
   Protocol (SDP) Parameters" under "proto".

   This document defines two SDP session and media-level attributes:



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      'streams'.  Its format is defined in Section 5.  This attribute
      should be registered by IANA under "Session Description Protocol
      (SDP) Parameters" under "att-field" (both session and media
      level)".

      'datachannel'.  Its format is defined in Section 6.  This
      attribute should be registered by IANA under "Session Description
      Protocol (SDP) Parameters" under "att-field" (both session and
      media level)".


13.  References

13.1.  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.

   [RFC4288]  Freed, N. and J. Klensin, "Media Type Specifications and
              Registration Procedures", BCP 13, RFC 4288, December 2005.

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

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

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



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              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.

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

   [I-D.tuexen-tsvwg-sctp-dtls-encaps]
              Jesup, R., Loreto, S., Stewart, R., and M. Tuexen, "DTLS
              Encapsulation of SCTP Packets for RTCWEB",
              draft-tuexen-tsvwg-sctp-dtls-encaps-01 (work in progress),
              July 2012.

13.2.  Informative References

   [RFC3436]  Jungmaier, A., Rescorla, E., and M. Tuexen, "Transport
              Layer Security over Stream Control Transmission Protocol",
              RFC 3436, December 2002.

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

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

   [I-D.ietf-behave-sctpnat]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation",
              draft-ietf-behave-sctpnat-04 (work in progress),
              December 2010.

   [I-D.tuexen-sctp-udp-encaps]
              Tuexen, M. and R. Stewart, "UDP Encapsulation of SCTP
              Packets", draft-tuexen-sctp-udp-encaps-06 (work in
              progress), January 2011.


Authors' Addresses

   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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