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Internet Engineering Task Force (IETF)                          K. GrossRequest for Comments: 7164                                  AVA NetworksUpdates:3550                                         R. van BrandenburgCategory: Standards Track                                            TNOISSN: 2070-1721                                               March 2014RTP and Leap SecondsAbstract   This document discusses issues that arise when RTP sessions span   Coordinated Universal Time (UTC) leap seconds.  It updatesRFC 3550   by describing how RTP senders and receivers should behave in the   presence of leap seconds.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/rfc7164.Copyright 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.Gross & van Brandenburg      Standards Track                    [Page 1]

RFC 7164                  RTP and Leap Seconds                March 2014Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .23.  Leap Seconds  . . . . . . . . . . . . . . . . . . . . . . . .23.1.  UTC Behavior during a Positive Leap Second  . . . . . . .33.2.  NTP Behavior during a Positive Leap Second  . . . . . . .33.3.  POSIX Behavior during a Positive Leap Second  . . . . . .33.4.  Example of Leap-Second Behaviors  . . . . . . . . . . . .44.  Receiver Behavior during a Leap Second  . . . . . . . . . . .55.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .55.1.  Sender Reports  . . . . . . . . . . . . . . . . . . . . .65.2.  RTP Packet Playout  . . . . . . . . . . . . . . . . . . .76.  Security Considerations . . . . . . . . . . . . . . . . . . .77.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .88.  References  . . . . . . . . . . . . . . . . . . . . . . . . .88.1.  Normative References  . . . . . . . . . . . . . . . . . .88.2.  Informative References  . . . . . . . . . . . . . . . . .81.  Introduction   In some media networking applications, RTP streams are referenced to   a wall-clock time (absolute date and time).  This is accomplished   through use of the NTP timestamp field in the sender report (SR) to   create a mapping between RTP timestamps and the wall clock.  When a   wall-clock reference is used, the playout time for RTP packets is   referenced to the wall clock.  Smooth and continuous media playout   requires a smooth and continuous time base.  The time base used by   the wall clock may include leap seconds that are not rendered   smoothly.   This document updatesRFC 3550 [1] by providing recommendations for   smoothly rendering streamed media referenced to common wall clocks   that do not have smooth or continuous behavior in the presence of   leap seconds.2.  Terminology   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described inRFC 2119 [2] and   indicate requirement levels for compliant implementations.3.  Leap Seconds   The world's scientific time standard is International Atomic Time   (TAI), which is based on vibrations of cesium atoms in an atomic   clock.  The world's civil time is based on the rotation of the Earth.Gross & van Brandenburg      Standards Track                    [Page 2]

RFC 7164                  RTP and Leap Seconds                March 2014   In 1972, the civil time standard, Coordinated Universal Time (UTC),   was redefined in terms of TAI and the concept of leap seconds was   introduced to allow UTC to remain synchronized with the rotation of   the Earth.   Leap seconds are scheduled by the International Earth Rotation and   Reference Systems Service.  Leap seconds may be scheduled at the last   day of any month but are preferentially scheduled for December and   June and secondarily March and September [6].  Because Earth's   rotation is unpredictable, leap seconds are typically not scheduled   more than six months in advance.   Leap seconds do not respect local time and always occur at the end of   the UTC day.  Leap seconds can be scheduled to either add or remove a   second from the day.  A leap second that adds an extra second is   known as a positive leap second.  A leap second that skips a second   is known as a negative leap second.   Since their introduction in 1972, all leap seconds have been   scheduled in June or December, and they have all been positive.   NOTE: The ITU is studying a proposal that could eventually eliminate   leap seconds from UTC.  As of January 2012, this proposal is expected   to be decided no earlier than 2015 [7].3.1.  UTC Behavior during a Positive Leap Second   UTC clocks feature a 61st second at the end of the day when a   positive leap second is scheduled.  The leap second is designated   "23h 59m 60s".3.2.  NTP Behavior during a Positive Leap Second   Under NTP [8], a leap second is inserted at the beginning of the last   second of the day.  This results in the clock freezing or slowing for   one second immediately prior to the last second of the affected day.   This results in the last second of the day having a real-time   duration of two seconds.  Timestamp accuracy is compromised during   this period because the clock's rate is not well defined.3.3.  POSIX Behavior during a Positive Leap Second   The POSIX (Portable Operating System Interface) standard [3] requires   that leap seconds be omitted from reported time.  All days are   defined as having 86,400 seconds, but the timebase is defined to be   UTC, a leap-second-bearing reference.  Implementors of POSIX systems   are offered considerable latitude by the standard as to how to map   POSIX time to UTC.Gross & van Brandenburg      Standards Track                    [Page 3]

RFC 7164                  RTP and Leap Seconds                March 2014   In many systems, leap seconds are accommodated by repeating the last   second of the day.  A timestamp within the last second of the day is   therefore ambiguous in that it can refer to a moment in time in   either of the last two seconds of a day containing a leap second.   Other systems use the same technique used by NTP, freezing or slowing   for one second immediately prior to the last second of the affected   day.   In some cases, leap seconds are accommodated by warping time [5] [4];   that is, the length of the second in the vicinity of a leap second is   slightly altered.3.4.  Example of Leap-Second Behaviors   Table 1 illustrates the positive leap second that occurred June 30,   2012 when the offset between TAI and UTC changed from 34 to 35   seconds.  The first column shows RTP timestamps for an 8 kHz audio   stream.  The second column shows the TAI reference.  The following   columns show behavior for the leap-second-bearing wall clocks   described above.  Time values are shown at half-second intervals.   +-------+--------------+--------------+--------------+--------------+   |  RTP  |     TAI      |     UTC      |    POSIX     |     NTP      |   +-------+--------------+--------------+--------------+--------------+   |  8000 | 00:00:32.500 | 23:59:58.500 | 23:59:58.500 | 23:59:58.500 |   | 12000 | 00:00:33.000 | 23:59:59.000 | 23:59:59.000 | 23:59:59.000 |   | 16000 | 00:00:33.500 | 23:59:59.500 | 23:59:59.500 | 23:59:59.500 |   | 20000 | 00:00:34.000 | 23:59:60.000 | 23:59:59.000 | 00:00:00.000 |   | 24000 | 00:00:34.500 | 23:59:60.500 | 23:59:59.500 | 00:00:00.000 |   | 28000 | 00:00:35.000 | 00:00:00.000 | 00:00:00.000 | 00:00:00.000 |   | 32000 | 00:00:35.500 | 00:00:00.500 | 00:00:00.500 | 00:00:00.500 |   +-------+--------------+--------------+--------------+--------------+                  Table 1: Positive Leap-Second Behavior   NOTE: Some NTP implementations do not entirely freeze the clock while   the leap second is inserted.  Successive calls to retrieve system   time return infinitesimally larger (e.g., 1 microsecond or 1   nanosecond larger) time values.  This behavior is designed to satisfy   assumptions applications may make that time increases monotonically.   This behavior occurs in the least-significant bits of the time value   and so is not typically visible in the human-readable format shown in   the table.Gross & van Brandenburg      Standards Track                    [Page 4]

RFC 7164                  RTP and Leap Seconds                March 2014   NOTE: POSIX implementations vary.  The implementation shown here   repeats the last second of the affected day.  Other implementations   mirror NTP behavior or alter the length of a second in the vicinity   of the leap second.4.  Receiver Behavior during a Leap Second   Timestamps generated during a leap second may be ambiguous or   interpreted differently by a sender and receiver or interpreted   differently by different receivers.   Without prior knowledge of the leap-second schedule, NTP servers and   clients may become offset by exactly one second with respect to their   UTC reference.  This potential discrepancy begins when a leap second   occurs and ends when all participants receive a time update from a   server or peer.  Depending on the system implementation, the offset   can last anywhere from a few seconds to a few days.  A long-lived   discrepancy can be particularly disruptive to operation of NTP-   referenced RTP streams.   These discrepancies, depending on direction, may cause receivers to   think they are receiving RTP packets after they should be played or   to attempt to buffer received data an additional second before   playing it.  Either situation can cause an interruption in playback.   Some receivers may automatically recognize an unexpected offset and   resynchronize to the stream to accommodate it.  Once the offset is   resolved, such receivers may need to resynchronize again.5.  Recommendations   Senders and receivers that are not referenced to a wall clock are not   affected by issues associated with leap seconds, and no special   accommodation is required.   RTP implementation using a wall-clock reference is simplified by   using a clock with a timescale that does not include leap seconds.   IEEE 1588 [9], GPS [10], and other systems that use a TAI [11]   reference do not include leap seconds.  NTP time, operating system   clocks, and other systems using a UTC reference include leap seconds.   Note that some TAI-based systems such as IEEE 1588 and GPS, in   addition to the TAI reference clock, deliver TAI to UTC mapping   information.  By combining the delivered TAI reference clock and the   mapping information, some receivers of these systems are able to   synthesize a leap-second-bearing UTC reference clock.  For the   purposes of this document, it is important to recognize that it is   the timescale used, not the delivery mechanism that determines   whether a reference clock is leap-second bearing.Gross & van Brandenburg      Standards Track                    [Page 5]

RFC 7164                  RTP and Leap Seconds                March 2014     +-------------------------+---------------------+---------------+     | Reference clock type    | Examples            | Accommodation |     +-------------------------+---------------------+---------------+     | None                    | Self clocking       | None needed   |     | Non-leap-second-bearing | IEEE 1588, GPS, TAI | None needed   |     | Leap-second-bearing     | NTP                 | Recommended   |     +-------------------------+---------------------+---------------+                     Table 2: Recommendations Summary   All participants generating or consuming timestamps associated with a   leap-second-bearing reference MUST recognize leap seconds and SHOULD   have a working communications channel to receive notifications of   leap-second scheduling.  A working communication channel includes a   protocol means of notifying clocks of an impending leap second such   as the Leap Indicator in the NTP header [8] and also a means for top-   tier clocks to receive leap-second schedule information published by   the International Earth Rotation and Reference Systems Service [12].   Such a communications channel may not be available on all networks.   For security or other reasons, leap-second schedules may be   configured manually for some networks or clocks.  When a device does   not reliably receive leap-second scheduling information, failures as   described inSection 4 may occur.   Because of the timestamp ambiguity that positive leap seconds can   introduce and the inconsistent manner in which different systems   accommodate positive leap seconds, generating or using NTP timestamps   during the entire last second of a day on which a positive leap   second has been scheduled SHOULD be avoided.  Note that the period to   be avoided has a real-time duration of two seconds.  In the Table 1   example, the region to be avoided is indicated by RTP timestamps   12000 through 28000   Negative leap seconds do not introduce timestamp ambiguity or other   complications.  No special treatment is needed to avoid ambiguity   with respect to RTP timestamps in the presence of a negative leap   second.   POSIX clocks that use a warping technique to accommodate leap seconds   (e.g., [4] [5]) are not a good choice for an interoperable timestamp   reference and SHOULD not be used to timestamp RTP streams.5.1.  Sender Reports   In order to avoid generating or using NTP timestamps during positive   leap seconds, RTP senders and receivers need to avoid sending or   using sender reports to synchronize their clocks in the vicinity of aGross & van Brandenburg      Standards Track                    [Page 6]

RFC 7164                  RTP and Leap Seconds                March 2014   leap second and instead rely on their internal clocks to maintain   synchronization until the leap second has passed.   RTP Senders using a leap-second-bearing reference for timestamps   SHOULD NOT generate sender reports containing an originating NTP   timestamp in the vicinity of a positive leap second.  To maintain a   consistent RTCP schedule and avoid the risk of unintentional   timeouts, such senders MAY send receiver reports in place of sender   reports in the vicinity of the leap second.   For the purpose of suspending sender reports in the vicinity of a   leap second, senders MAY assume that a positive leap second occurs at   the end of the last day of every month.   Receivers consuming leap-second-bearing timestamps SHOULD ignore   timestamps in any sender reports generated in the vicinity of a   positive leap second.   For the purpose of ignoring sender reports in the vicinity of a leap   second, receivers MAY assume that a positive leap second occurs at   the end of the last day of every month.5.2.  RTP Packet Playout   Receivers consuming leap-second-bearing timestamps SHOULD take both   positive and negative leap seconds in the reference into account to   determine the playout time based on RTP timestamps for data in RTP   packets.6.  Security Considerations   RTP streams using a wall-clock reference as discussed here present an   additional attack vector compared to self-clocking streams.   Manipulation of the wall clock at either the sender or receiver can   potentially disrupt streaming.   For an RTP stream operating to a leap-second-bearing reference to   operate reliably across a leap second, the sender and receiver must   both be aware of the leap second.  It is possible to disrupt a stream   by blocking or delaying leap second notification to one of the   participants.  Streaming can be similarly affected if one of the   participants can be tricked into believing a leap second has been   scheduled where there is not one.  These vulnerabilities are present   inRFC 3550 [1] and these new recommendations neither heighten nor   diminish them.  Integrity of the leap-second schedule is the   responsibility of the operating system and time distribution   mechanism, both of which are outside the scope ofRFC 3550 [1] and   these recommendations.Gross & van Brandenburg      Standards Track                    [Page 7]

RFC 7164                  RTP and Leap Seconds                March 20147.  Acknowledgements   The authors would like to thank Steve Allen for his valuable comments   that helped to improve this document.8.  References8.1.  Normative References   [1]   Schulzrinne, H., Casner, S., Frederick, R., and V.  Jacobson,         "RTP: A Transport Protocol for Real-Time Applications", STD 64,RFC 3550, July 2003.   [2]   Bradner, S., "Key words for use in RFCs to Indicate Requirement         Levels",BCP 14,RFC 2119, March 1997.8.2.  Informative References   [3]   IEEE, "Portable Operating System Interface (POSIX)", IEEE         Standard 1003.1-2008, December 2008,         <http://standards.ieee.org/findstds/standard/1003.1-2008.html>.   [4]   Google, Inc., "Time, technology and leaping seconds", September         2011, <http://googleblog.blogspot.com/2011/09/time-technology-and-leaping-seconds.html>.   [5]   Kuhn, M., "Coordinated Universal Time with Smoothed Leap         Seconds (UTC-SLS)", Work in Progress, January 2006.   [6]   ITU, "Standard-frequency and time-signal emissions", ITU-R         TF.460-6, February 2002,         <http://www.itu.int/rec/R-REC-TF.460/>.   [7]   ITU, "The future of the UTC time scale", Question ITU-R 236/7,         February 2012, <http://www.itu.int/pub/R-QUE-SG07.236-2001>.   [8]   Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time         Protocol Version 4: Protocol and Algorithms Specification",RFC5905, June 2010.   [9]   IEEE, "IEEE Standard for a Precision Clock Synchronization         Protocol for Networked Measurement and Control Systems", IEEE         Standard 1588-2008, July 2008,         <http://standards.ieee.org/findstds/standard/1588-2008.html>.Gross & van Brandenburg      Standards Track                    [Page 8]

RFC 7164                  RTP and Leap Seconds                March 2014   [10]  Global Positioning Systems Directorate, "Systems Engineering &         Integration Interface Specification", September 2011,         <http://www.navcen.uscg.gov/pdf/IS-GPS-200F.pdf>.   [11]  Bureau International des Poids et Mesures, "International         Atomic Time", Navstar GPS Space Segment/Navigation User Segment         Interfaces IS-GPS-200,         <http://www.bipm.org/en/scientific/tai/tai.html>.   [12]  IERS Earth Orientation Centre, "Bulletin C - Product metadata",         <http://datacenter.iers.org/web/guest/eop/-/somos/5Rgv/product/16>.Authors' Addresses   Kevin Gross   AVA Networks   Boulder, CO   US   EMail: kevin.gross@avanw.com   Ray van Brandenburg   TNO   Brassersplein 2   Delft  2612CT   the Netherlands   Phone: +31-88-866-7000   EMail: ray.vanbrandenburg@tno.nlGross & van Brandenburg      Standards Track                    [Page 9]