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MMUSIC                                                         S. Loreto
Internet-Draft                                              G. Camarillo
Intended status: Standards Track                                Ericsson
Expires: September 15, 2011                               March 14, 2011


Stream Control Transmission Protocol (SCTP)-Based Media Transport in the
                   Session Description Protocol (SDP)
                    draft-loreto-mmusic-sctp-sdp-07

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' and 'SCTP/
   DTLS' protocol identifiers for SDP.

Status of this Memo

   This Internet-Draft is submitted to IETF 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 September 15, 2011.

Copyright Notice

   Copyright (c) 2011 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
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   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 . . . . . . . . . . . . . . . . . . . . . . 3
   4.  The Setup and Connection Attributes and Association
       Management  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Multihoming . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     6.1.  Actpass/Passive . . . . . . . . . . . . . . . . . . . . . . 5
     6.2.  Existing Connection Reuse . . . . . . . . . . . . . . . . . 6
     6.3.  SDP description for DTLS Connection . . . . . . . . . . . . 6
   7.  Network Address Translators (NAT) Considerations  . . . . . . . 7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   10. Normative References  . . . . . . . . . . . . . . . . . . . . . 8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 9































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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 a new protocol identifier, 'SCTP/
   DTLS', to establish secure SCTP-based media streams over DTLS
   (Datagram Transport Layer Security) [RFC4347], as specified in
   [RFC6083], using SDP.  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
   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> ...




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

   The 'SCTP' protocol identifier is similar to both the 'UDP' and 'TCP'
   protocol identifiers in that it only describes 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].

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

   An 'm' line that specifies 'SCTP/DTLS' 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.  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.


5.  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 or not 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),



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   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.

   Some intermediaries performing firewall control use the addresses in
   offer/answer exchanges to perform media authorization.  That is, they
   will not let media through unless it is sent to the address in the
   'c' line.

   The SCTP endpoints MAY choose to use the main address all the time
   (e.g., they do not send retransmissions to a backup address) and to
   send a re-INVITE every time they change that address.

   However not using non-primary paths for retransmission means not to
   utilize the multi-homing feature of SCTP for resilience.  Therefore,
   the SCTP endpoints MAY use the 'candidate' SDP attribute to
   disclosure, to intermediaries performing firewall control, all its
   alternative IP addresses; this will avoid the need for the SCTP
   endpoints to send a re-INVITE every time they change the primary
   path.


6.  Examples

   The following examples show the use of the 'setup' and 'connection'
   SDP attributes.  As discussed in Section 4, 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).

6.1.  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:

              m=image 54111 SCTP *
              c=IN IP4 192.0.2.2
              a=setup:actpass
              a=connection:new

                                 Figure 1

   The endpoint at 192.0.2.1 responds with the following description:






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              m=image 54321 SCTP *
              c=IN IP4 192.0.2.1
              a=setup:passive
              a=connection:new

                                 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.

6.2.  Existing Connection Reuse

   Subsequent to the exchange in Section 6.1, 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:new

                                 Figure 3

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

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

                                 Figure 4

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

6.3.  SDP description for DTLS Connection

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










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           m=image 54111 SCTP/DTLS t38
           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

                                 Figure 5


7.  Network Address Translators (NAT) Considerations

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

   As an alternative to design SCTP specific NAT, SCTP-over-UDP
   [I-D.tuexen-sctp-udp-encaps] makes possible, encapsulating SCTP
   packets into UDP packets, to use SCTP in networks with legacy NAT and
   firewalls not supporting SCTP.

   OPEN ISSUE: do we want to include in SDP the ability to signal SCTP-
   over-UDP ?

   TBD: How to use Interactive Connectivity Establishment (ICE) to
   establish SCTP streams.

   OPENT ISSUE: Do we want ICE to only use SCTP over IP candidates, or
   we ant ICE to use SCTP over UDP candidates as well?


8.  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.







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9.  IANA Considerations

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

   The SDP specification, [RFC4566], states that specifications defining
   new proto values, like the SCTP and SCTP/DTLS proto values defined in
   this RFC, must define the rules by which their media format (fmt)
   namespace is managed.  For the SCTP protocol, new formats SHOULD have
   an associated MIME registration.  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 [RFC4288] [RFC4289] by production of, or reference to, a
   standards-track RFC that defines the transport protocol for the
   format.


10.  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.

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



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   [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.

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

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

   [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


   Gonzalo Camarillo
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: Gonzalo.Camarillo@ericsson.com







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