MMUSIC A. Begen Internet-Draft Cisco Intended status: Standards Track Y. Cai Expires: June 6, 2014 Microsoft H. Ou Cisco December 3, 2013 Delayed Duplication Attribute in the Session Description Protocol draft-ietf-mmusic-delayed-duplication-03 Abstract 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 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 June 6, 2014. Copyright Notice Copyright (c) 2013 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 Begen, et al. Expires June 6, 2014 [Page 1] Internet-Draft Delayed Duplication Attribute in SDP December 2013 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 . . . . . . . . . . . . . . . . . . . . 4 3. The 'duplication-delay' Attribute . . . . . . . . . . . . . . 4 4. SDP Examples . . . . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6.1. Registration of SDP Attributes . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . 9 8.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10 1. Introduction Inside an IP network, packet delivery may be interrupted due to a physical link, interface or device failure. 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) lasting until connectivity is restored around the failure (which is called network reconvergence). Typically, network reconvergence takes between tens and hundreds of milliseconds depending on the size and features of the network. Begen, et al. Expires June 6, 2014 [Page 2] Internet-Draft Delayed Duplication Attribute in SDP December 2013 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, temporal 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 gets dropped, and the original transmission completes successfully. On the receiver side, both transmissions can also arrive and in that case, the receiver (or the node that does the duplicate suppression) needs to identify the duplicate packets and discard them appropriately, 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 make congestion only 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. Expires June 6, 2014 [Page 3] Internet-Draft Delayed Duplication Attribute in SDP December 2013 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 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 [I-D.ietf-mmusic-duplication-grouping]. This specification does not explain how to select the duplication delay that a sender should use, which depends on the underlying network and reconvergence technologies used inside this 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 [I-D.ietf-avtext-rtp-duplication]. 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. 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) Begen, et al. Expires June 6, 2014 [Page 4] Internet-Draft Delayed Duplication Attribute in SDP December 2013 period = 1*DIGIT ; in milliseconds Figure 1: 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 [I-D.ietf-mmusic-duplication-grouping]. 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 to 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 [I-D.ietf-mmusic-duplication-grouping]. 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 to 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 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 [I-D.ietf-mmusic-duplication-grouping]. 4. 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 Begen, et al. Expires June 6, 2014 [Page 5] Internet-Draft Delayed Duplication Attribute in SDP December 2013 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.com 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 Begen, et al. Expires June 6, 2014 [Page 6] Internet-Draft Delayed Duplication Attribute in SDP December 2013 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:S1b 5. 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; and 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. 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 both on 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 Begen, et al. Expires June 6, 2014 [Page 7] Internet-Draft Delayed Duplication Attribute in SDP December 2013 such duplication could act as an accelerator if any million-message [RFC3218] or similar Lucky13 attack 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 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 SDES to ensure that the keys used are not dictionary-attackable. 6. IANA Considerations The following contact information shall be used for all registrations in this document: Ali Begen abegen@cisco.com Note to the RFC Editor: In the following, replace "XXXX" with the number of this document prior to publication as an RFC. 6.1. Registration of SDP Attributes This document registers a new attribute name in SDP. Begen, et al. Expires June 6, 2014 [Page 8] Internet-Draft Delayed Duplication Attribute in SDP December 2013 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: [RFCXXXX] Values: See [RFCXXXX] 7. Acknowledgements Authors would like to thank Colin Perkins and Paul Kyzivat for their suggestions and reviews. 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. [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. [I-D.ietf-mmusic-duplication-grouping] Begen, A., Cai, Y., and H. Ou, "Duplication Grouping Semantics in the Session Description Protocol", draft- ietf-mmusic-duplication-grouping-04 (work in progress), November 2013. 8.2. Informative References [RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error Correction (FEC) Framework", RFC 6363, October 2011. Begen, et al. Expires June 6, 2014 [Page 9] Internet-Draft Delayed Duplication Attribute in SDP December 2013 [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. [I-D.ietf-avtext-rtp-duplication] Begen, A. and C. Perkins, "Duplicating RTP Streams", draft-ietf-avtext-rtp-duplication-04 (work in progress), October 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. [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. 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 Begen, et al. Expires June 6, 2014 [Page 10] Internet-Draft Delayed Duplication Attribute in SDP December 2013 Heidi Ou Cisco 170 W. Tasman Dr. San Jose, CA 95134 USA Email: hou@cisco.com Begen, et al. Expires June 6, 2014 [Page 11]