AVTCore K. Gross
Internet-Draft AVA Networks
Updates: 3550 (if approved) R. van Brandenburg
Intended status: Standards Track TNO
Expires: April 22, 2013 October 19, 2012
RTP and Leap Seconds
draft-ietf-avtcore-leap-second-01
Abstract
This document discusses issues that arise when RTP sessions span
Universal Coordinate 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Leap seconds . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. UTC behavior during leap second . . . . . . . . . . . . . . 4
3.2. NTP behavior during leap second . . . . . . . . . . . . . . 4
3.3. POSIX behavior during leap second . . . . . . . . . . . . . 4
3.4. Summary of leap-second behaviors . . . . . . . . . . . . . 4
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. RTP Sender Reports and Receiver Reports . . . . . . . . . . 5
4.2. RTP Packet Playout . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
8. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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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 RTCP sender report (SR)
to create a mapping between RTP timestamps and the wall clock. When
a wall-clock reference is used, the play-out time for RTP packets is
referenced to the wall clock. Smooth and continuous media play out
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 provides 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 [1] and indicate
requirement levels for compliant implementations.
3. Leap seconds
The world time standard is International Atomic Time (TAI) which is
based on vibrations of cesium atoms in an atomic clock. The more
common Universal Coordinated Time (UTC) is based on the rotation of
the Earth. In 1971 UTC was redefined in terms of TAI and the concept
of leap seconds was introduced to allow UTC to remain synchronized
with 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.[2] 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 since their introduction in 1971
have been scheduled in June or December and all have added seconds.
This is a situation that is expected to 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]
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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".
3.2. NTP behavior during leap second
Under NTP [4] 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 a moment in time in either of the last two
seconds of a day containing a leap second.
3.4. Summary of leap-second behaviors
Table 1 summarizes behavior across a leap second for the wall clocks
discussed above.
The table illustrates the 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
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4. 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 [5], GPS [6] and other TAI [7] 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
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. In the Table 1 example, the region to be avoided is
indicated by RTP timestamps 12000 through 28000
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 play-out time
from RTP timestamps for data in RTP packets.
5. Security Considerations
It is believed that the recommendations herein introduce no new
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security considerations beyond those already discussed in [8].
6. IANA Considerations
This document has no actions for IANA.
7. Acknowledgements
The authors would like to thank Steve Allen for his valuable comments
in helping to improve this document.
8. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", March 1997.
[2] ITU-R, "Recommendation ITU-R TF.460-4 - Standard-frequency and
time-signal emissions", February 2002.
[3] ITU-R Working Party 7A, "Question SG07.236", February 2012.
[4] Mills, D., Delaware, U., Martin, J., Ed., Burbank, J., and W.
Kasch, "Network Time Protocol Version 4: Protocol and Algorithms
Specification", June 2010.
[5] IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
July 2008.
[6] Global Positioning Systems Directorate, "Navstar GPS Space
Segment/Navigation User Segment Interfaces", September 2011.
[7] BIPM, "Circular T", May 2012.
[8] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications, RFC3550",
July 2003.
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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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