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Internet Engineering Task Force (IETF)                          A. BegenRequest for Comments: 7197                                         CiscoCategory: Standards Track                                         Y. CaiISSN: 2070-1721                                                Microsoft                                                                   H. Ou                                                                   Cisco                                                              April 2014Duplication Delay Attribute in the Session Description ProtocolAbstract   A straightforward approach to provide protection against packet   losses due to network outages with a longest duration of T time units   is to duplicate the original packets and send each copy separated in   time by at least T time units.  This approach is commonly referred to   as "time-shifted redundancy", "temporal redundancy", or simply   "delayed duplication".  This document defines an attribute to   indicate the presence of temporally redundant media streams and the   duplication delay in the Session Description Protocol.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/rfc7197.Begen, et al.                Standards Track                    [Page 1]

RFC 7197           Duplication Delay Attribute in SDP         April 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 ...........................................43. The 'duplication-delay' Attribute ...............................54. SDP Examples ....................................................65. Security Considerations .........................................76. IANA Considerations .............................................86.1. Registration of SDP Attributes .............................97. Acknowledgements ................................................98. References ......................................................98.1. Normative References .......................................98.2. Informative References ....................................101.  Introduction   Inside an IP network, packet delivery may be interrupted due to   failure of a physical link, interface, or device.  To reduce the   impact of such interruptions, some networks are built in a resilient   manner, allowing for multiple alternative paths between two   endpoints.  However, if there is no resiliency in the network or the   failure happens in a non-resilient part of the network, a temporary   outage will occur (i.e., packets will get dropped).  The outage will   last until network reconvergence takes place (i.e., until   connectivity is restored) around the failure.  Typically, network   reconvergence takes between tens and hundreds of milliseconds,   depending on the size and features of the network.   There are a number of network-reconvergence technologies available   today, such as IP Fast Convergence, MPLS Traffic Engineering Fast   Reroute, and Multicast Only Fast Reroute.  These technologies can be   augmented by different types of application-layer loss-repair methods   such as Forward Error Correction (FEC), retransmission, temporalBegen, et al.                Standards Track                    [Page 2]

RFC 7197           Duplication Delay Attribute in SDP         April 2014   redundancy, and spatial redundancy to minimize (and sometimes totally   eliminate) the impact of outages.  Each combination has its distinct   requirements in terms of bandwidth consumption and results in a   different network complexity.  Thus, a network operator has to   carefully consider what combination to deploy for different parts of   a network (e.g., core vs. edge).  A detailed overview of network-   convergence technologies and loss-repair methods is provided in   [IC2011].   One of the loss-repair methods is temporal redundancy, also known as   delayed duplication.  A media sender using this method transmits an   original source packet and transmits its duplicate after a certain   delay following the original transmission.  If a network outage hits   the original transmission, the expectation is that the second   transmission arrives at the receiver (with a high probability).   Alternatively, the second transmission may be hit by an outage and so   gets dropped, and the original transmission completes successfully.   Also, both transmissions can arrive on the receiver side; in that   case, the receiver (or the node that does the duplicate suppression)   needs to identify the duplicate packets and discard them   appropriately, thereby producing a duplicate-free stream.   Delayed duplication can be used in a variety of multimedia   applications where there is sufficient bandwidth for the duplicated   traffic and the application can tolerate the introduced delay.   However, it must be used with care, since it might easily result in a   new series of denial-of-service attacks.  Delayed duplication is   harmful in cases where the primary cause of packet loss is   congestion, rather than a network outage due to a temporary link or   network element failure.  Duplication should only be used by   endpoints that want to protect against network failures; protection   against congestion must be achieved through other means, as   duplication will only make congestion worse.   One particular use case for delayed duplication is to improve the   reliability of real-time video feeds inside a core IP network where   bandwidth is plentiful and maximum reliability (preferably zero loss)   is desired [IC2011].  Compared to other redundancy approaches such as   FEC [RFC6363] and redundant data encoding (e.g., [RFC2198]), delayed   duplication is easy to implement, since it does not require any   special type of encoding or decoding.Begen, et al.                Standards Track                    [Page 3]

RFC 7197           Duplication Delay Attribute in SDP         April 2014   For duplicate suppression, the receiver has to be able to identify   the identical packets.  This is straightforward for media packets   that carry one or more unique identifiers such as the sequence number   field in the RTP header [RFC3550].  In non-RTP applications, the   receiver can use unique sequence numbers if available or other   alternative approaches to compare the incoming packets and discard   the duplicate ones.   This specification introduces a new Session Description Protocol   (SDP) [RFC4566] attribute for applications/services using the delayed   duplication method to indicate the relative delay for each additional   duplication.  The attribute is used with the duplication grouping   semantics defined in [RFC7104].   This specification does not explain how to select the duplication   delay that a sender should use; the selection technique depends on   the underlying network and the reconvergence technologies used inside   such a network.  This specification does not explain how the receiver   should suppress the duplicate packets and merge the incoming streams   to produce a loss-free and duplication-free output stream (a process   commonly called "stream merging"), either.  An application or a   transport service that will use the delayed duplication method must   determine its own rules about stream merging.   In practice, more than two redundant streams are unlikely to be used,   since the additional delay and increased overhead are not easily   justified.  However, we define the new attribute in a general way so   that it could be used with more than two redundant streams (i.e.,   multiple duplications), if needed.  While the primary focus in this   specification is the RTP-based transport, the new attribute is   applicable to both RTP and non-RTP streams.  Protocol issues and   details on duplicating RTP streams are presented in [RFC7198].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].Begen, et al.                Standards Track                    [Page 4]

RFC 7197           Duplication Delay Attribute in SDP         April 20143.  The 'duplication-delay' Attribute   The following ABNF [RFC5234] syntax formally describes the   'duplication-delay' attribute:      delaying-attribute     = "a=duplication-delay:" periods CRLF      periods                = period *( SP period)      period                 = 1*DIGIT ; in milliseconds             ABNF Syntax for the 'duplication-delay' Attribute   The 'duplication-delay' attribute is defined as both a media-level   and session-level attribute.  It specifies the relative delay with   respect to the previous transmission of each duplication in   milliseconds (ms) at the time of transmission.  The following rules   apply:   o  If used as a media-level attribute, it MUST be used with the      'ssrc-group' attribute and "DUP" grouping semantics as defined in      [RFC7104].  When used as a media-level attribute, the relative      delay value(s) it specifies SHALL apply to every Synchronization      Source (SSRC)-based duplication grouping in the same media      description.  In other words, one cannot specify different      duplication delay values for different duplication groups in the      same media description.   o  If used as a session-level attribute, it MUST be used with 'group'      attribute and "DUP" grouping semantics as defined in [RFC7104].      When used as a session-level attribute, the relative delay      value(s) it specifies SHALL apply to every duplication grouping in      the same SDP description.  In other words, one cannot specify      different duplication delay values for different duplication      groups in the same SDP description.  If one needs to specify      different duplication delay values for different duplication      groups, then one MUST use different SDP descriptions for each or      MUST use the 'duplication-delay' attribute at the media level.  In      that case, the 'duplication-delay' attribute MUST NOT be used at      the session level.   o  For offer/answer model considerations, refer to [RFC7104].Begen, et al.                Standards Track                    [Page 5]

RFC 7197           Duplication Delay Attribute in SDP         April 20144.  SDP Examples   In the first example below, the multicast stream consists of two RTP   streams, each duplicated once, resulting in two sets of two-stream   groups.  The same duplication delay of 100 ms is applied to each   grouping.  The first set's streams have SSRCs of 1000 and 1010, and   the second set's streams have SSRCs of 1020 and 1030.      v=0      o=ali 1122334455 1122334466 IN IP4 dup.example.com      s=Delayed Duplication      t=0 0      m=video 30000 RTP/AVP 100 101      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=rtpmap:101 MP2T/90000      a=ssrc:1020 cname:ch1b@example.com      a=ssrc:1030 cname:ch1b@example.com      a=ssrc-group:DUP 1020 1030      a=duplication-delay:100      a=mid:Ch1   Note that in actual use, SSRC values, which are random 32-bit   numbers, could be much larger than the ones shown in this example.   In the second example below, the multicast stream is duplicated   twice.  50 ms after the original transmission, the first duplicate is   transmitted, and 100 ms after that, the second duplicate is   transmitted.  In other words, the same packet is transmitted three   times over a period of 150 ms.      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:ch1c@example.com      a=ssrc:1010 cname:ch1c@example.com      a=ssrc:1020 cname:ch1c@example.comBegen, et al.                Standards Track                    [Page 6]

RFC 7197           Duplication Delay Attribute in SDP         April 2014      a=ssrc-group:DUP 1000 1010 1020      a=duplication-delay:50 100      a=mid:Ch1   In the third example below, the multicast UDP stream is duplicated   with a duplication delay of 50 ms.  Redundant streams are sent in   separate source-specific multicast (SSM) sessions, so the receiving   host has to join both SSM sessions if it wants to receive both   streams.      v=0      o=ali 1122334455 1122334466 IN IP4 dup.example.com      s=Delayed Duplication      t=0 0      a=group:DUP S1a S1b      a=duplication-delay:50      m=audio 30000 udp mp4      c=IN IP4 233.252.0.1/127      a=source-filter:incl IN IP4 233.252.0.1 198.51.100.1      a=mid:S1a      m=audio 40000 udp mp4      c=IN IP4 233.252.0.2/127      a=source-filter:incl IN IP4 233.252.0.2 198.51.100.1      a=mid:S1b5.  Security Considerations   The 'duplication-delay' attribute is not believed to introduce any   significant security risk to multimedia applications.  A malevolent   third party could use this attribute to misguide the receiver(s)   about the duplication delays and/or the number of redundant streams.   For example, if the malevolent third party increases the value of the   duplication delay, the receiver(s) will unnecessarily incur a longer   delay, since they will have to wait for the entire period.  Or, if   the duplication delay is reduced by the malevolent third party, the   receiver(s) might not wait long enough for the duplicated   transmission and incur unnecessary packet losses.  However, these   require intercepting and rewriting the packets carrying the SDP   description; if an interceptor can do that, many more attacks are   also possible.   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 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.Begen, et al.                Standards Track                    [Page 7]

RFC 7197           Duplication Delay Attribute in SDP         April 2014   Another security risk is due to possible software misconfiguration or   a software bug where a large number of duplicates could be   unwillingly signaled in the 'duplication-delay' attribute.   Similarly, an attacker can use this attribute to start a denial-of-   service attack by signaling and sending too many duplicated streams.   In applications where this attribute is to be used, it is a good   practice to put a hard limit on both the number of duplicate streams   and the total delay introduced due to duplication, regardless of what   the SDP description specifies.   Since this mechanism causes duplication of media packets, if those   packets are also cryptographically protected (e.g., encrypted) then   such duplication could act as an accelerator if any Million Message   [RFC3218] or similar attack such as Lucky 13 [Lucky13] exists against   the security mechanism that is in use.  Such acceleration could turn   an otherwise infeasible attack into one that is practical; however,   assuming that the amount of duplication is small and that such weak   or broken security mechanisms should really not be used, the overall   security impact of the duplication should be minimal.  If, however, a   bad actor were in control of the SDP but did not have access to the   keying material used for media, then such a bad actor could   potentially use the SDP to cause the media handling to use a weak or   broken mechanism with a lot of duplication, in which case the   duplication could be significant.  Deployments where the SDP is   controlled by an actor who should not have access to the media keying   material should therefore be cautious in their use of this   duplication mechanism.   If this mechanism were used in conjunction with a source description   (SDES) and if the key being used for media protection is derived from   a human-memorable or otherwise dictionary-attackable secret, then the   duplication done here could allow for a more efficient dictionary   attack against the media.  The right countermeasure is to use proper   keying or, if using an SDES, to ensure that the keys used are not   dictionary-attackable.6.  IANA Considerations   The following contact information shall be used for the registration   in this document:      Ali Begen      abegen@cisco.comBegen, et al.                Standards Track                    [Page 8]

RFC 7197           Duplication Delay Attribute in SDP         April 20146.1.  Registration of SDP Attributes   This document registers a new attribute name in SDP.      SDP Attribute ("att-field"):         Attribute name:     duplication-delay         Long form:          Duplication delay for temporally redundant                             streams         Type of name:       att-field         Type of attribute:  Media or session level         Subject to charset: No         Purpose:            Specifies the relative duplication delay(s)                             for redundant stream(s)         Reference:          [RFC7197]         Values:             See [RFC7197]7.  Acknowledgements   The authors would like to thank Colin Perkins, Paul Kyzivat, and   Stephen Farrell for their suggestions and reviews.8.  References8.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session              Description Protocol",RFC 4566, July 2006.   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.              Jacobson, "RTP: A Transport Protocol for Real-Time              Applications", STD 64,RFC 3550, July 2003.   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF", STD 68,RFC 5234, January 2008.   [RFC7104]  Begen, A., Cai, Y., and H. Ou, "Duplication Grouping              Semantics in the Session Description Protocol",RFC 7104,              January 2014.Begen, et al.                Standards Track                    [Page 9]

RFC 7197           Duplication Delay Attribute in SDP         April 20148.2.  Informative References   [RFC6363]  Watson, M., Begen, A., and V. Roca, "Forward Error              Correction (FEC) Framework",RFC 6363, October 2011.   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,              Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-              Parisis, "RTP Payload for Redundant Audio Data",RFC 2198,              September 1997.   [RFC7198]  Begen, A. and C. Perkins, "Duplicating RTP Streams",RFC 7198, April 2014.   [IC2011]   Evans, J., Begen, A., Greengrass, J., and C. Filsfils,              "Toward Lossless Video Transport", IEEE Internet              Computing, Vol. 15, No. 6, pp. 48-57, November 2011.   [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.   [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.   [RFC3218]  Rescorla, E., "Preventing the Million Message Attack on              Cryptographic Message Syntax",RFC 3218, January 2002.   [Lucky13]  AlFardan, N. and K. Paterson, "Lucky Thirteen: Breaking              the TLS and DTLS Record Protocols", IEEE Symposium on              Security and Privacy, May 2013,              <http://ieeexplore.ieee.org/xpl/articleDetails.jsp?              tp=&arnumber=6547131&queryText%3DLucky+Thirteen>.Begen, et al.                Standards Track                   [Page 10]

RFC 7197           Duplication Delay Attribute in SDP         April 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 11]