RTP and Leap Seconds
draft-gross-leap-second-00

AVTCore                                                         K. Gross
Internet-Draft                                              AVA Networks
Updates: RFC3550 (if approved)                        R. van Brandenburg
Intended status: Standards Track                                     TNO
Expires: November 8, 2012                                    May 7, 2012

                          RTP and Leap Seconds
                       draft-gross-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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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.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5

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1.  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 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.

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

3.  Leap seconds

   Leap seconds are intended to keep UTC time synchronizaed with the
   rotation of the earth.  Leap seconds are scheduled by the
   International Earth Rotation and Reference Systems Service.  When
   they occur, leap seconds are scheduled at the end of the last day of
   December and/or June each year.  Because earth's rotation is
   unpredictable, it is not possible to schedule leap seconds 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 added seconds and 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 "23:59:60".

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3.2.  NTP behavior during leap second

   Uner NTP 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.

4.  Recommendations

   Senders and receivers which are not referenced to a wallclock are not
   affected by issues associated with leap seconds and no special
   accomodation 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, GPS and other TAI (Inernational Atomic Time) 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
   recognise 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 offest 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 seonds can introduce and the
   inconsistent manner in which different systems accomodate leap
   seconds, generating or using NTP timestamps during the entire last
   second of a day on which a leap second has been scheduled SNOULD be
   avoided.  Note that the period to be avoided has a real-time duration
   of two seconds.

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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.

5.  Security Considerations

   It is beleived that the recommendataions herein indroduce no new
   security considerations beyond those already discussed in [RFC3550].

6.  Normative References

   [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.

Authors' Addresses

   Kevin Gross
   AVA Networks
   Boulder, CO
   US

   Email: kevin.gross@avanw.com

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   Ray van Brandenburg
   TNO
   Brassersplein 2
   Delft  2612CT
   the Netherlands

   Phone: +31-88-866-7000
   Email: ray.vanbrandenburg@tno.nl

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