draft-ietf-avtcore-leap-second-04.txt   draft-ietf-avtcore-leap-second-05.txt 
AVTCore K. Gross AVTCore K. Gross
Internet-Draft AVA Networks Internet-Draft AVA Networks
Updates: 3550 (if approved) R. van Brandenburg Updates: 3550 (if approved) R. van Brandenburg
Intended status: Standards Track TNO Intended status: Standards Track TNO
Expires: February 28, 2014 August 27, 2013 Expires: April 04, 2014 October 01, 2013
RTP and Leap Seconds RTP and Leap Seconds
draft-ietf-avtcore-leap-second-04 draft-ietf-avtcore-leap-second-05
Abstract Abstract
This document discusses issues that arise when RTP sessions span This document discusses issues that arise when RTP sessions span
Coordinated Universal Time (UTC) leap seconds. It updates RFC 3550 Coordinated Universal Time (UTC) leap seconds. It updates RFC 3550
to describe how RTP senders and receivers should behave in the to describe how RTP senders and receivers should behave in the
presence of leap seconds. presence of leap seconds.
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on February 28, 2014. This Internet-Draft will expire on April 04, 2014.
Copyright Notice Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Provisions Relating to IETF Documents Provisions Relating to IETF Documents
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Leap seconds . . . . . . . . . . . . . . . . . . . . . . . . 2 3. Leap seconds . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. UTC behavior during positive leap second . . . . . . . . 3 3.1. UTC behavior during positive leap second . . . . . . . . 3
3.2. NTP behavior during positive leap second . . . . . . . . 3 3.2. NTP behavior during positive leap second . . . . . . . . 3
3.3. POSIX behavior during positive leap second . . . . . . . 3 3.3. POSIX behavior during positive leap second . . . . . . . 3
3.4. Example of leap-second behaviors . . . . . . . . . . . . 4 3.4. Example of leap-second behaviors . . . . . . . . . . . . 4
4. Receiver behavior during leap second . . . . . . . . . . . . 4 4. Receiver behavior during leap second . . . . . . . . . . . . 5
5. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 5 5. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. RTP Sender Reports . . . . . . . . . . . . . . . . . . . 6 5.1. RTP Sender Reports . . . . . . . . . . . . . . . . . . . 6
5.2. RTP Packet Playout . . . . . . . . . . . . . . . . . . . 6 5.2. RTP Packet Playout . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7 9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
In some media networking applications, RTP streams are referenced to In some media networking applications, RTP streams are referenced to
a wall-clock time (absolute date and time). This is accomplished a wall-clock time (absolute date and time). This is accomplished
through use of the NTP timestamp field in the RTCP sender report (SR) through use of the NTP timestamp field in the RTCP sender report (SR)
to create a mapping between RTP timestamps and the wall clock. When 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 a wall-clock reference is used, the playout time for RTP packets is
referenced to the wall clock. Smooth and continuous media playout referenced to the wall clock. Smooth and continuous media playout
requires a smooth and continuous time base. The time base used by requires a smooth and continuous time base. The time base used by
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The world scientific time standard is International Atomic Time (TAI) The world scientific time standard is International Atomic Time (TAI)
which is based on vibrations of cesium atoms in an atomic clock. The which is based on vibrations of cesium atoms in an atomic clock. The
world civil time is based on the rotation of the Earth. In 1972 the world civil time is based on the rotation of the Earth. In 1972 the
civil time standard, Coordinated Universal Time (UTC), was redefined civil time standard, Coordinated Universal Time (UTC), was redefined
in terms of TAI and the concept of leap seconds was introduced to in terms of TAI and the concept of leap seconds was introduced to
allow UTC to remain synchronized with the rotation of the Earth. allow UTC to remain synchronized with the rotation of the Earth.
Leap seconds are scheduled by the International Earth Rotation and Leap seconds are scheduled by the International Earth Rotation and
Reference Systems Service. Leap seconds may be scheduled at the last Reference Systems Service. Leap seconds may be scheduled at the last
day of any month but are preferentially scheduled for December and day of any month but are preferentially scheduled for December and
June and secondarily March and September.[3] Because Earth's rotation June and secondarily March and September.[6] Because Earth's rotation
is unpredictable, leap seconds are typically not scheduled more than is unpredictable, leap seconds are typically not scheduled more than
six months in advance. six months in advance.
Leap seconds do not respect local time and always occur at the end of 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 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 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 known as a positive leap second. A leap second that skips a second
is known as a negative leap second. All leap seconds since their is known as a negative leap second. All leap seconds since their
introduction in 1972 have been scheduled in June or December and all introduction in 1972 have been scheduled in June or December and all
have been positive. have been positive.
NOTE- The ITU is studying a proposal which could eventually eliminate NOTE- The ITU is studying a proposal which could eventually eliminate
leap seconds from UTC. As of January 2012, this proposal is expected leap seconds from UTC. As of January 2012, this proposal is expected
to be decided no earlier than 2015.[4] to be decided no earlier than 2015.[7]
3.1. UTC behavior during positive leap second 3.1. UTC behavior during positive leap second
UTC clocks feature a 61st second at the end of the day when a UTC clocks feature a 61st second at the end of the day when a
positive leap second is scheduled. The leap second is designated positive leap second is scheduled. The leap second is designated
"23h 59m 60s". "23h 59m 60s".
3.2. NTP behavior during positive leap second 3.2. NTP behavior during positive leap second
Under NTP[5] a leap second is inserted at the beginning of the last 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 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. 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 This results in the last second of the day having a real-time
duration of two seconds. Timestamp accuracy is compromised during duration of two seconds. Timestamp accuracy is compromised during
this period because the clock's rate is not well defined. this period because the clock's rate is not well defined.
3.3. POSIX behavior during positive leap second 3.3. POSIX behavior during positive leap second
Most POSIX systems insert the positive leap second at the end of the The POSIX standard [3] requires that leap seconds be omitted from
last second of the day. This results in repetition of the last reported time. All days are defined as having 86,400 seconds but the
second. A timestamp within the last second of the day is therefore timebase is defined to be UTC, a leap-second-bearing reference .
ambiguous in that it can refer to a moment in time in either of the Implementors of POSIX systems are offered considerable latitude by
last two seconds of a day containing a leap second. the standard as to how to map POSIX time to UTC.
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 [5] [4] leap seconds are accommodated by warping time,
slightly altering the length of the second in the vicinity of the
leap second.
3.4. Example of leap-second behaviors 3.4. Example of leap-second behaviors
Table 1 illustrates the positive leap second that occurred June 30, Table 1 illustrates the positive leap second that occurred June 30,
2012 when the offset between International Atomic time (TAI) and UTC 2012 when the offset between International Atomic time (TAI) and UTC
changed from 34 to 35 seconds. The first column shows RTP timestamps changed from 34 to 35 seconds. The first column shows RTP timestamps
for an 8 kHz audio stream. The second column shows the TAI for an 8 kHz audio stream. The second column shows the TAI
reference. Following columns show behavior for the leap-second- reference. Following columns show behavior for the leap-second-
bearing wall clocks described above. Time values are shown at half- bearing wall clocks described above. Time values are shown at half-
second intervals. second intervals.
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NOTE- Some NTP implementations do not entirely freeze the clock while NOTE- Some NTP implementations do not entirely freeze the clock while
the leap second is inserted. Successive calls to retrieve system the leap second is inserted. Successive calls to retrieve system
time return infinitesimally larger (e.g. 1 microsecond or 1 time return infinitesimally larger (e.g. 1 microsecond or 1
nanosecond larger) time values. This behavior is designed to satisfy nanosecond larger) time values. This behavior is designed to satisfy
assumptions applications may make that time increases monotonically. assumptions applications may make that time increases monotonically.
This behavior occurs in the least-significant bits of the time value 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 and so is not typically visible in the human-readable format shown in
the table. the table.
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 leap second 4. Receiver behavior during leap second
Timestamps generated during a leap second may be ambiguous or Timestamps generated during a leap second may be ambiguous or
interpreted differently by sender and receiver or interpreted interpreted differently by sender and receiver or interpreted
differently by different receivers. differently by different receivers.
Without prior knowledge of leap-second schedule, NTP servers and Without prior knowledge of leap-second schedule, NTP servers and
clients may become offset by exactly one second with respect to their clients may become offset by exactly one second with respect to their
UTC reference. This potential discrepancy begins when a leap second UTC reference. This potential discrepancy begins when a leap second
occurs and ends when all participants receive a time update from a occurs and ends when all participants receive a time update from a
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resolved, such receivers may need to resynchronize again. resolved, such receivers may need to resynchronize again.
5. Recommendations 5. Recommendations
Senders and receivers which are not referenced to a wall clock are Senders and receivers which are not referenced to a wall clock are
not affected by issues associated with leap seconds and no special not affected by issues associated with leap seconds and no special
accommodation is required. accommodation is required.
RTP implementation using a wall-clock reference is simplified by RTP implementation using a wall-clock reference is simplified by
using a clock with a timescale which does not include leap seconds. using a clock with a timescale which does not include leap seconds.
IEEE 1588,[6] GPS [7] and other TAI [8] references do not include IEEE 1588,[9] GPS [10] and other TAI [11] references do not include
leap seconds. NTP time, operating system clocks and other UTC leap seconds. NTP time, operating system clocks and other UTC
references include leap seconds. references include leap seconds.
All participants working to a leap-second-bearing reference SHOULD All participants working to a leap-second-bearing reference SHOULD
recognize leap seconds and have a working communications channel to recognize leap seconds and have a working communications channel to
receive notification of leap-second scheduling. Note that a working receive notification of leap-second scheduling. Note that a working
communication channel includes a protocol means of notifying clocks communication channel includes a protocol means of notifying clocks
of an impending leap second such as the Leap Indicator in the NTP of an impending leap second such as the Leap Indicator in the NTP
header [5] but also a means for top-tier clocks to receive leap- header [8] but also a means for top-tier clocks to receive leap-
second schedule information published by the International Earth second schedule information published by the International Earth
Rotation and Reference Systems Service. Rotation and Reference Systems Service.
Because of the timestamp ambiguity, positive leap seconds can Because of the timestamp ambiguity, positive leap seconds can
introduce and the inconsistent manner in which different systems introduce and the inconsistent manner in which different systems
accommodate positive leap seconds, generating or using NTP timestamps accommodate positive leap seconds, generating or using NTP timestamps
during the entire last second of a day on which a positive leap 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 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 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 example, the region to be avoided is indicated by RTP timestamps
12000 through 28000 12000 through 28000
Negative leap seconds do not introduce timestamp ambiguity or other Negative leap seconds do not introduce timestamp ambiguity or other
complications. No special treatment is needed to avoid ambiguity complications. No special treatment is needed to avoid ambiguity
with respect to RTP timestamps in the presence of a negative leap with respect to RTP timestamps in the presence of a negative leap
second. second.
POSIX clocks which use the a warping technique to accommodate leap
seconds (e.g. [5] [4]) are not a good choice for an interoperable
timestamp reference and SHOULD be avoided for this application.
5.1. RTP Sender Reports 5.1. RTP Sender Reports
RTP Senders working to a leap-second-bearing reference SHOULD NOT RTP Senders working to a leap-second-bearing reference SHOULD NOT
generate sender reports containing an originating NTP timestamp in generate sender reports containing an originating NTP timestamp in
the vicinity of a positive leap second. To maintain a consistent the vicinity of a positive leap second. To maintain a consistent
RTCP schedule and avoid the risk of unintentional timeouts, such RTCP schedule and avoid the risk of unintentional timeouts, such
senders MAY send receiver reports in place of sender reports in the senders MAY send receiver reports in place of sender reports in the
vicinity of the leap second. vicinity of the leap second.
For the purpose of suspending sender reports in the vicinity of a For the purpose of suspending sender reports in the vicinity of a
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[1] Schulzrinne, H., Casner, S., Frederick, R., and V. [1] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003. Applications", STD 64, RFC 3550, July 2003.
[2] Bradner, S., "Key words for use in RFCs to Indicate [2] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References 9.2. Informative References
[3] ITU-R, "Recommendation ITU-R TF.460-6 - Standard-frequency [3] Institute of Electrical and Electronics Engineers,
"Portable Operating System Interface (POSIX)", IEEE
Standard 1003.1, 2008.
[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)", draft-kuhn-leapsecond-00 (work in
progress), January 2006.
[6] ITU-R, "Recommendation ITU-R TF.460-6 - Standard-frequency
and time-signal emissions", February 2002. and time-signal emissions", February 2002.
[4] ITU-R Working Party 7A, "Question SG07.236", February [7] ITU-R Working Party 7A, "Question SG07.236", February
2012. 2012.
[5] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network [8] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010. Specification", RFC 5905, June 2010.
[6] IEEE, "IEEE Standard for a Precision Clock Synchronization [9] IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems", Protocol for Networked Measurement and Control Systems",
July 2008. July 2008.
[7] Global Positioning Systems Directorate, "Navstar GPS Space [10] Global Positioning Systems Directorate, "Navstar GPS Space
Segment/Navigation User Segment Interfaces", September Segment/Navigation User Segment Interfaces", September
2011. 2011.
[8] BIPM, "Circular T", May 2012. [11] BIPM, "Circular T", May 2012.
Authors' Addresses Authors' Addresses
Kevin Gross Kevin Gross
AVA Networks AVA Networks
Boulder, CO Boulder, CO
US US
Email: kevin.gross@avanw.com Email: kevin.gross@avanw.com
Ray van Brandenburg Ray van Brandenburg
TNO TNO
Brassersplein 2 Brassersplein 2
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