





Internet Engineering Task Force (IETF)                          A. Begen
Request for Comments: 7104                                         Cisco
Category: Standards Track                                         Y. Cai
ISSN: 2070-1721                                                Microsoft
                                                                   H. Ou
                                                                   Cisco
                                                            January 2014


   Duplication Grouping Semantics in the Session Description Protocol

Abstract

   Packet loss is undesirable for real-time multimedia sessions, but it
   can occur due to congestion or other unplanned network outages.  This
   is especially true for IP multicast networks, where packet loss
   patterns can vary greatly between receivers.  One technique that can
   be used to recover from packet loss without incurring unbounded delay
   for all the receivers is to duplicate the packets and send them in
   separate redundant streams.  This document defines the semantics for
   grouping redundant streams in the Session Description Protocol (SDP).
   The semantics defined in this document are to be used with the SDP
   Grouping Framework.  Grouping semantics at the Synchronization Source
   (SSRC) level are also defined in this document for RTP streams using
   SSRC multiplexing.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7104.












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Copyright Notice

   Copyright (c) 2014 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
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Requirements Notation ...........................................3
   3. Duplication Grouping ............................................3
      3.1. "DUP" Grouping Semantics ...................................3
      3.2. Duplication Grouping for SSRC-Multiplexed RTP Streams ......3
      3.3. SDP Offer/Answer Model Considerations ......................4
   4. SDP Examples ....................................................5
      4.1. Separate Source Addresses ..................................5
      4.2. Separate Destination Addresses .............................6
      4.3. Temporal Redundancy ........................................7
   5. Security Considerations .........................................7
   6. IANA Considerations .............................................8
   7. Acknowledgments .................................................8
   8. References ......................................................8
      8.1. Normative References .......................................8
      8.2. Informative References .....................................9


















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1.  Introduction

   The Real-time Transport Protocol (RTP) [RFC3550] is widely used today
   for delivering IPTV traffic and other real-time multimedia sessions.
   Many of these applications support very large numbers of receivers
   and rely on intra-domain UDP/IP multicast for efficient distribution
   of traffic within the network.

   While this combination has proved successful, there does exist a
   weakness.  As [RFC2354] noted, packet loss is not avoidable, even in
   a carefully managed network.  This loss might be due to congestion;
   it might also be a result of an unplanned outage caused by a flapping
   link, a link or interface failure, a software bug, or a maintenance
   person accidentally cutting the wrong fiber.  Since UDP/IP flows do
   not provide any means for detecting loss and retransmitting packets,
   it is left up to the RTP layer and the applications to detect, and
   recover from, packet loss.

   One technique to recover from packet loss without incurring unbounded
   delay for all the receivers is to duplicate the packets and send them
   in separate redundant streams.  Variations on this idea have been
   implemented and deployed today [IC2011].  [RTP-DUP] explains how
   duplication can be achieved for RTP streams without breaking the RTP
   and RTP Control Protocol (RTCP) functionality.  In this document, we
   describe the semantics needed in the Session Description Protocol
   (SDP) [RFC4566] to support this technique.

2.  Requirements Notation

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

3.  Duplication Grouping

3.1.  "DUP" Grouping Semantics

   Each "a=group" line is used to indicate an association relationship
   between the redundant streams.  The streams included in one "a=group"
   line are called a "Duplication Group".

   Using the SDP Grouping Framework in [RFC5888], this document defines
   "DUP" as the grouping semantics for redundant streams.







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   The "a=group:DUP" semantics MUST be used to group the redundant
   streams, except when the streams are specified in the same media
   description, i.e., in the same "m" line (see Section 3.2).  In an
   "a=group:DUP" line, the order of the listed redundant streams does
   not strictly indicate the order of transmission, although it is
   RECOMMENDED that the stream listed first be sent first, with the
   other stream(s) being the (time-delayed) duplicate(s).

3.2.  Duplication Grouping for SSRC-Multiplexed RTP Streams

   [RFC5576] defines an SDP media-level attribute, called "ssrc-group",
   for grouping the RTP streams that are SSRC multiplexed and carried in
   the same RTP session.  The grouping is based on the SSRC identifiers.
   Since SSRC-multiplexed RTP streams are defined in the same "m" line,
   the "group" attribute cannot be used.

   This section explains how duplication is used with SSRC-multiplexed
   streams using the "ssrc-group" attribute [RFC5576].

   The semantics of "DUP" for the "ssrc-group" attribute are the same as
   the one defined for the "group" attribute, except that the SSRC
   identifiers are used to designate the duplication grouping
   associations: a=ssrc-group:DUP *(SP ssrc-id) [RFC5576].  As above,
   while in an "a=ssrc-group:DUP" line, the order of the listed
   redundant streams does not necessarily indicate the order of
   transmission, but it is RECOMMENDED that the stream listed first be
   sent first, with the other stream(s) being the (time-delayed)
   duplicate(s).

3.3.  SDP Offer/Answer Model Considerations

   When offering duplication grouping using SDP in an offer/answer model
   [RFC3264], the following considerations apply.

   A node that is receiving an offer from a sender may or may not
   understand line grouping.  It is also possible that the node
   understands line grouping but does not understand the "DUP"
   semantics.  From the viewpoint of the sender of the offer, these
   cases are indistinguishable.

   When a node is offered a session with the "DUP" grouping semantics
   but it does not support line grouping or the duplication grouping
   semantics, as per [RFC5888], the node responds to the offer either
   (1) with an answer that omits the grouping attribute or (2) with a
   refusal to the request (e.g., "488 Not Acceptable Here" or "606 Not
   Acceptable in SIP").





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   In the first case, the original sender of the offer must send a new
   offer without any duplication grouping.  In the second case, if the
   sender of the offer still wishes to establish the session, it should
   retry the request with an offer without the duplication grouping.
   This behavior is specified in [RFC5888].

4.  SDP Examples

4.1.  Separate Source Addresses

   In this example, the redundant streams use the same IP destination
   address (232.252.0.1), but they are sourced from different addresses
   (198.51.100.1 and 198.51.100.2).  Thus, the receiving host needs to
   join both source-specific multicast (SSM) sessions separately.

       v=0
       o=ali 1122334455 1122334466 IN IP4 dup.example.com
       s=DUP Grouping Semantics
       t=0 0
       m=video 30000 RTP/AVP 100
       c=IN IP4 233.252.0.1/127
       a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1 198.51.100.2
       a=rtpmap:100 MP2T/90000
       a=ssrc:1000 cname:ch1@example.com
       a=ssrc:1010 cname:ch1@example.com
       a=ssrc-group:DUP 1000 1010
       a=mid:Ch1

   Note that in actual use, SSRC values, which are random 32-bit
   numbers, can be much larger than the ones shown in this example.
   Also, note that this SDP description does not use the "duplication-
   delay" attribute (defined in [DELAYED-DUP]) since the sender does not
   apply any delay between the redundant streams upon transmission.
   Alternatively, one MAY explicitly insert an "a=duplication-delay:0"
   line before the "a=mid:Ch1" line for informational purposes.
















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4.2.  Separate Destination Addresses

   In this example, the redundant streams have different IP destination
   addresses.  The example shows the same UDP port number and IP source
   address for each stream, but either or both could have been different
   for the two streams.

        v=0
        o=ali 1122334455 1122334466 IN IP4 dup.example.com
        s=DUP Grouping Semantics
        t=0 0
        a=group:DUP S1a S1b
        m=video 30000 RTP/AVP 100
        c=IN IP4 233.252.0.1/127
        a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
        a=rtpmap:100 MP2T/90000
        a=mid:S1a
        m=video 30000 RTP/AVP 101
        c=IN IP4 233.252.0.2/127
        a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1
        a=rtpmap:101 MP2T/90000
        a=mid:S1b

   Optionally, one could be more explicit and insert an
   "a=duplication-delay:0" line before the first "m" line.


























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4.3.  Temporal Redundancy

   In this example, the redundant streams have the same IP source and
   destination addresses (i.e., they are transmitted in the same SSM
   session).  Due to the same source and destination addresses, the
   packets in both streams will be routed over the same path.  To
   provide resiliency against packet loss, the duplicate of an original
   packet is transmitted 50 milliseconds (ms) later as indicated by the
   "duplication-delay" attribute (defined in [DELAYED-DUP]).

        v=0
        o=ali 1122334455 1122334466 IN IP4 dup.example.com
        s=Delayed Duplication
        t=0 0
        m=video 30000 RTP/AVP 100
        c=IN IP4 233.252.0.1/127
        a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1
        a=rtpmap:100 MP2T/90000
        a=ssrc:1000 cname:ch1a@example.com
        a=ssrc:1010 cname:ch1a@example.com
        a=ssrc-group:DUP 1000 1010
        a=duplication-delay:50
        a=mid:Ch1

5.  Security Considerations

   In general, the security considerations of [RFC4566] apply to this
   document as well.

   There is a weak threat for the receiver that the duplication grouping
   can be modified to indicate relationships that do not exist.  Such
   attacks might result in failure of the duplication mechanisms and/or
   mishandling of the media streams by the receivers.

   In order to avoid attacks of this sort, the SDP description needs to
   be integrity protected and provided with source authentication.  This
   can, for example, be achieved on an end-to-end basis using S/MIME
   [RFC5652] [RFC5751] when the SDP is used in a signaling packet using
   MIME types (application/sdp).  Alternatively, HTTPS [RFC2818] or the
   authentication method in the Session Announcement Protocol (SAP)
   [RFC2974] could be used as well.  As for the confidentiality, if it
   is desired, it can be useful to use a secure, encrypted transport
   method to carry the SDP description.








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6.  IANA Considerations

   This document registers the following semantics with IANA in the
   "Semantics for the "group" SDP Attribute" subregistry (under the
   "Session Description Protocol (SDP) Parameters" registry:

   Semantics                              Token   Reference
   -------------------------------------  ------  ---------
   Duplication                            DUP     [RFC7104]

   This document also registers the following semantics with IANA in the
   "Semantics for the "ssrc-group" SDP Attribute" subregistry under the
   "Session Description Protocol (SDP) Parameters" registry:

   Token    Semantics                      Reference
   -------  -----------------------------  ---------
   DUP      Duplication                    [RFC7104]

7.  Acknowledgments

   The authors would like to thank Colin Perkins, Bill Ver Steeg, Dave
   Oran, and Toerless Eckert for their input and suggestions.

8.  References

8.1.  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.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

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

   [RFC5576]  Lennox, J., Ott, J., and T. Schierl, "Source-Specific
              Media Attributes in the Session Description Protocol
              (SDP)", RFC 5576, June 2009.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888, June 2010.




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8.2.  Informative References

   [DELAYED-DUP]
              Begen, A., Cai, Y., and H. Ou, "Delayed Duplication
              Attribute in the Session Description Protocol", Work in
              Progress, December 2013.

   [IC2011]   Evans, J., Begen, A., Greengrass, J., and C. Filsfils,
              "Toward Lossless Video Transport, IEEE Internet Computing,
              vol. 15/6, pp. 48-57", November 2011.

   [RFC2354]  Perkins, C. and O. Hodson, "Options for Repair of
              Streaming Media", RFC 2354, June 1998.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session
              Announcement Protocol", RFC 2974, October 2000.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

   [RTP-DUP]  Begen, A. and C. Perkins, "Duplicating RTP Streams", Work
              in Progress, October 2013.























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Authors' Addresses

   Ali Begen
   Cisco
   181 Bay Street
   Toronto, ON  M5J 2T3
   Canada

   EMail: abegen@cisco.com


   Yiqun Cai
   Microsoft
   1065 La Avenida
   Mountain View, CA  94043
   USA

   EMail: yiqunc@microsoft.com


   Heidi Ou
   Cisco
   170 W. Tasman Dr.
   San Jose, CA  95134
   USA

   EMail: hou@cisco.com
























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