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Internet Engineering Task Force (IETF)                          A. BegenRequest for Comments: 7104                                         CiscoCategory: Standards Track                                         Y. CaiISSN: 2070-1721                                                Microsoft                                                                   H. Ou                                                                   Cisco                                                            January 2014Duplication Grouping Semantics in the Session Description ProtocolAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7104.Begen, et al.                Standards Track                    [Page 1]

RFC 7104          Duplication Grouping Semantics in SDP     January 2014Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1. Introduction ....................................................22. Requirements Notation ...........................................33. Duplication Grouping ............................................33.1. "DUP" Grouping Semantics ...................................33.2. Duplication Grouping for SSRC-Multiplexed RTP Streams ......33.3. SDP Offer/Answer Model Considerations ......................44. SDP Examples ....................................................54.1. Separate Source Addresses ..................................54.2. Separate Destination Addresses .............................64.3. Temporal Redundancy ........................................75. Security Considerations .........................................76. IANA Considerations .............................................87. Acknowledgments .................................................88. References ......................................................88.1. Normative References .......................................88.2. Informative References .....................................9Begen, et al.                Standards Track                    [Page 2]

RFC 7104          Duplication Grouping Semantics in SDP     January 20141.  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 Grouping3.1.  "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.Begen, et al.                Standards Track                    [Page 3]

RFC 7104          Duplication Grouping Semantics in SDP     January 2014   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 (seeSection 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").Begen, et al.                Standards Track                    [Page 4]

RFC 7104          Duplication Grouping Semantics in SDP     January 2014   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 Examples4.1.  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.Begen, et al.                Standards Track                    [Page 5]

RFC 7104          Duplication Grouping Semantics in SDP     January 20144.2.  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.Begen, et al.                Standards Track                    [Page 6]

RFC 7104          Duplication Grouping Semantics in SDP     January 20144.3.  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:Ch15.  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.Begen, et al.                Standards Track                    [Page 7]

RFC 7104          Duplication Grouping Semantics in SDP     January 20146.  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.  References8.1.  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.Begen, et al.                Standards Track                    [Page 8]

RFC 7104          Duplication Grouping Semantics in SDP     January 20148.2.  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.Begen, et al.                Standards Track                    [Page 9]

RFC 7104          Duplication Grouping Semantics in SDP     January 2014Authors' 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.comBegen, et al.                Standards Track                   [Page 10]