Internet DRAFT - draft-ietf-avtcore-leap-second
draft-ietf-avtcore-leap-second
AVTCore K. Gross
Internet-Draft AVA Networks
Updates: 3550 (if approved) R. van Brandenburg
Intended status: Standards Track TNO
Expires: July 16, 2014 January 12, 2014
RTP and Leap Seconds
draft-ietf-avtcore-leap-second-08
Abstract
This document discusses issues that arise when RTP sessions span
Coordinated Universal Time (UTC) leap seconds. It updates RFC 3550
to describe how RTP senders and receivers should behave in the
presence of leap seconds.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Leap seconds . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. UTC behavior during positive leap second . . . . . . . . 3
3.2. NTP behavior during positive leap second . . . . . . . . 3
3.3. POSIX behavior during positive leap second . . . . . . . 3
3.4. Example of leap-second behaviors . . . . . . . . . . . . 4
4. Receiver behavior during leap second . . . . . . . . . . . . 5
5. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. Sender Reports . . . . . . . . . . . . . . . . . . . . . 6
5.2. RTP Packet Playout . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. 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 which are not rendered
smoothly.
This document updates RFC 3550 [1] providing recommendations for
smoothly rendering streamed media referenced to common wall clocks
which 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 in RFC 2119 [2] and
indicate requirement levels for compliant implementations.
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3. Leap seconds
The world scientific time standard is International Atomic Time (TAI)
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
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. All leap seconds since their
introduction in 1972 have been scheduled in June or December and all
have been positive.
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.[7]
3.1. UTC behavior during 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 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 positive leap second
The POSIX 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 .
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Implementors of POSIX systems are offered considerable latitude by
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 a leap
second.
3.4. Example of leap-second behaviors
Table 1 illustrates the positive leap second that occurred June 30,
2012 when the offset between International Atomic time (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. 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.
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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
Timestamps generated during a leap second may be ambiguous or
interpreted differently by sender and receiver or interpreted
differently by different receivers.
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.
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 which 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 which 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
synthezise a leap-second-bearing UTC reference clock. For the
purposes of this draft, it is important to recognise that it is the
timescale used, not the delivery mechanism which determines whether a
reference clock is leap-second bearing.
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+-------------------------+----------------+---------------+
| Reference clock type | Examples | Accommodation |
+-------------------------+----------------+---------------+
| None | Self clocking | None needed |
| Non-leap-second-bearing | 1588, GPS, TAI | None needed |
| Leap-second-bearing | NTP | Recommended |
+-------------------------+----------------+---------------+
Recommendations summary
All participants working to a leap-second-bearing reference MUST
recognize leap seconds and SHOULD have a working communications
channel to receive notification 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] but also a means for top-tier clocks to receive leap-
second schedule information published by the International Earth
Rotation and Reference Systems Service. [12]
These recommendations appreciate that 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 in Section 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 which use 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. 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 a
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leap second and instead rely on their internal clocks to maintain
sync until the leap second has passed.
RTP Senders working to a leap-second-bearing reference 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 a positive leap second occurs at the
end of the last day of every month.
Receivers working to a leap-second-bearing reference 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 a positive leap second occurs at the end
of the last day of every month.
5.2. RTP Packet Playout
Receivers working to a leap-second-bearing reference SHOULD take both
positive and negative leap seconds in the reference into account in
determining 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 sender or receiver can
potentially disrupt streaming.
For an RTP stream operating to an leap-second-bearing reference to
operate reliably across a leap second, sender and receive 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
in RFC 3550 [1] and these new recommendations neither heighten or
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 of RFC 3550 [1] and
these recommendations.
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7. IANA Considerations
This document has no actions for IANA.
8. Acknowledgements
The authors would like to thank Steve Allen for his valuable comments
in helping to improve this document.
9. References
9.1. 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.
9.2. Informative References
[3] IEEE, "IEEE Standard for Information Technology - Portable
Operating System Interface (POSIX)", IEEE Standard
1003.1-2008, 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)", draft-kuhn-leapsecond-00 (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, "Question 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", RFC 5905, June 2010.
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[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>.
[10] Global Positioning Systems Directorate, "Navstar GPS Space
Segment/Navigation User Segment Interfaces", September
2011, <http://www.navcen.uscg.gov/pdf/IS-GPS-200F.pdf>.
[11] Bureau International des Poids et Mesures (BIPM),
"International Atomic Time", November 2013,
<http://www.bipm.org/en/scientific/tai/tai.html>.
[12] International Earth Rotation and Reference System Service,
"Bulletin C", November 2013, <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.nl
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