AVTCore                                                         K. Gross
Internet-Draft                                              AVA Networks
Updates: 3550 (if approved)                           R. van Brandenburg
Intended status: Standards Track                                     TNO
Expires: December 23, 2012                                 June 21, 2012


                          RTP and Leap Seconds
                   draft-ietf-avtcore-leap-second-00

Abstract

   This document discusses issues that arise when RTP sessions span
   (UTC) leap seconds.  It updates RFC 3550 to describe how RTP senders
   and receivers should behave in the presence of leap seconds.

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   This Internet-Draft will expire on December 23, 2012.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Leap seconds  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     3.1.  UTC behavior during leap second . . . . . . . . . . . . . . 3
     3.2.  NTP behavior during leap second . . . . . . . . . . . . . . 4
     3.3.  POSIX behavior during leap second . . . . . . . . . . . . . 4
   4.  Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 4
     4.1.  RTP Sender Reports and Receiver Reports . . . . . . . . . . 5
     4.2.  RTP Packet Playout  . . . . . . . . . . . . . . . . . . . . 5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 6



































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.  Introduction

   In some applications, RTP streams are referenced to a walllock time
   (absolute date and time).  This is typically accomplished through use
   of the NTP timestamp field in the RTCP sender report (SR) to create a
   mapping between RTP timestamps and the wallclock.  When a wallclock
   reference is used, the playout time for RTP packets is referenced to
   the wallclock.  Smooth and continuous media playout requires a smooth
   and continuous timebase.  The timebase used by the wallclock may
   include leap seconds which, in many cases, are not rendered smoothly.

   This document provides recommendations for smoothly rendering
   streamed media referenced to common wallclocks which may not have
   smooth or continuous behavior in the presence of leap seconds.


.  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 in [RFC2119] and indicate
   requirement levels for compliant implementations.


.  Leap seconds

   Leap seconds are intended to keep UTC time 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.[TF.460-6] Because earth's rotation is unpredictable,
   leap seconds are typically not scheduled more than six months in
   advance.  Leap seconds can be scheduled to either add or remove a
   second from the day.  All leap second events thus far have been
   scheduled in June or December and have all added seconds.  This is a
   situation that is expected but not guaranteed to continue.

   NOTE- The ITU is studying a proposal which could eventually eliminate
   leap seconds from UTC.  As of January 2012, this proposal is expected
   to be decided no earlier than 2015.

3.1.  UTC behavior during leap second

   UTC clocks insert a 61st second at the end of the day when a leap
   second is scheduled.  The leap second is designated "23h 59m 60s".
   The sequence of the second markers near the UTC leap second
   transition are:



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      Day 0, 23h 59m 59s

      Day 0, 23h 59m 60s <-- leap second

      Day 1, 0h 0m 0s

3.2.  NTP behavior during leap second

   Under NTP [RFC5905] 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.

3.3.  POSIX behavior during leap second

   Most POSIX systems insert the leap second at the end of the last
   second of the day.  This results in repetition of the last second.  A
   timestamp within the last second of the day is therefore ambiguous in
   that it can refer to either of the last two seconds of a day
   containing a leap second.


.  Recommendations

   Senders and receivers which are not referenced to a wallclock are not
   affected by issues associated with leap seconds and no special
   accommodation is required.

   RTP implementation using a wallclock reference is simplified by using
   a clock with a timescale which does not include leap seconds.  IEEE
   1588 [IEEE1588-2008], GPS [IS-GPS-200F] and other TAI (International
   Atomic Time) [CircularT] references do not include leap seconds.  NTP
   time, operating system clocks and other UTC (Coordinated Universal
   Time) references include leap seconds.

   All participants working to a leap-second-bearing reference SHOULD
   recognize leap seconds and have a working communications channel to
   receive notification of leap second scheduling.  Without prior
   knowledge of 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 RTP operation.

   Because of the ambiguity leap seconds can introduce and the



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   inconsistent manner in which different systems accommodate leap
   seconds, generating or using NTP timestamps during the entire last
   second of a day on which a leap second has been scheduled SHOULD be
   avoided.  Note that the period to be avoided has a real-time duration
   of two seconds.

4.1.  RTP Sender Reports and Receiver Reports

   RTP Senders working to a leap-second-bearing reference SHOULD NOT
   generate sender reports containing an originating NTP timestamp in
   the vicinity of a leap second.  Receivers SHOULD ignore timestamps in
   any such reports inadvertently generated.

4.2.  RTP Packet Playout

   Receivers working to a leap-second-bearing reference SHOULD take leap
   seconds in their reference into account in determining playout time
   from RTP timestamps for data in RTP packets.


.  Security Considerations

   It is believed that the recommendations herein introduce no new
   security considerations beyond those already discussed in [RFC3550].


.  IANA Considerations

   This document has no actions for IANA."


.  Acknowledgements

   The authors would like to thank Steve Allen for his valuable comments
   in helping to improve this document.


.  Normative References

   [CircularT]
              BIPM, "Circular T", May 2012.

   [IEEE1588-2008]
              IEEE, "IEEE Standard for a Precision Clock Synchronization
              Protocol for Networked Measurement and Control Systems",
              July 2008.

   [IS-GPS-200F]



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              Global Positioning Systems Directorate, "Navstar GPS Space
              Segment/Navigation User Segment Interfaces",
              September 2011.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997.

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

   [RFC5905]  Mills, D., Delaware, U., Martin, J., Ed., Burbank, J., and
              W. Kasch, "Network Time Protocol Version 4: Protocol and
              Algorithms Specification", June 2010.

   [TF.460-6]
              ITU-R, "Recommendation ITU-R TF.460-4 - Standard-frequency
              and time-signal emissions", February 2002.


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.nl













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