draft-ietf-avtcore-leap-second-01.txt   draft-ietf-avtcore-leap-second-02.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: April 22, 2013 October 19, 2012 Expires: August 23, 2013 February 19, 2013
RTP and Leap Seconds RTP and Leap Seconds
draft-ietf-avtcore-leap-second-01 draft-ietf-avtcore-leap-second-02
Abstract Abstract
This document discusses issues that arise when RTP sessions span This document discusses issues that arise when RTP sessions span
Universal Coordinate Time (UTC) leap seconds. It updates RFC 3550 to Universal Coordinate Time (UTC) leap seconds. It updates RFC 3550 to
describe how RTP senders and receivers should behave in the presence describe how RTP senders and receivers should behave in the presence
of leap seconds. of leap seconds.
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Leap seconds . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Leap seconds . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1. UTC behavior during leap second . . . . . . . . . . . . . . 4 3.1. UTC behavior during leap second . . . . . . . . . . . . . . 4
3.2. NTP behavior during leap second . . . . . . . . . . . . . . 4 3.2. NTP behavior during leap second . . . . . . . . . . . . . . 4
3.3. POSIX behavior during leap second . . . . . . . . . . . . . 4 3.3. POSIX behavior during leap second . . . . . . . . . . . . . 4
3.4. Summary of leap-second behaviors . . . . . . . . . . . . . 4 3.4. Summary of leap-second behaviors . . . . . . . . . . . . . 4
4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Recommendations . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. RTP Sender Reports and Receiver Reports . . . . . . . . . . 5 4.1. RTP Sender Reports . . . . . . . . . . . . . . . . . . . . 6
4.2. RTP Packet Playout . . . . . . . . . . . . . . . . . . . . 5 4.2. RTP Packet Playout . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . . 6 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 8.1. Normative References . . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . . 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 play-out 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 play out 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
the wall clock may include leap seconds which are not rendered the wall clock may include leap seconds which are not rendered
smoothly. smoothly.
This document provides recommendations for smoothly rendering This document updates RFC 3550 [1] providing recommendations for
streamed media referenced to common wall clocks which do not have smoothly rendering streamed media referenced to common wall clocks
smooth or continuous behavior in the presence of leap seconds. which do not have smooth or continuous behavior in the presence of
leap seconds.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [1] and indicate document are to be interpreted as described in RFC 2119 [2] and
requirement levels for compliant implementations. indicate requirement levels for compliant implementations.
3. Leap seconds 3. Leap seconds
The world time standard is International Atomic Time (TAI) which is The world scientific time standard is International Atomic Time (TAI)
based on vibrations of cesium atoms in an atomic clock. The more which is based on vibrations of cesium atoms in an atomic clock. The
common Universal Coordinated Time (UTC) is based on the rotation of world civil time is based on the rotation of the Earth. In 1972 the
the Earth. In 1971 UTC was redefined in terms of TAI and the concept civil time standard, Coordinated Universal Time (UTC), was redefined
of leap seconds was introduced to allow UTC to remain synchronized in terms of TAI and the concept of leap seconds was introduced to
with with the rotation of the Earth. Leap seconds are scheduled by allow UTC to remain synchronized with with the rotation of the Earth.
the International Earth Rotation and Reference Systems Service. Leap
seconds may be scheduled at the last day of any month but are Leap seconds are scheduled by the International Earth Rotation and
preferentially scheduled for December and June and secondarily March Reference Systems Service. Leap seconds may be scheduled at the last
and September.[2] Because Earth's rotation is unpredictable, leap day of any month but are preferentially scheduled for December and
seconds are typically not scheduled more than six months in advance. June and secondarily March and September.[3] Because Earth's rotation
Leap seconds can be scheduled to either add or remove a second from is unpredictable, leap seconds are typically not scheduled more than
the day. All leap second events since their introduction in 1971 six months in advance.
have been scheduled in June or December and all have added seconds.
This is a situation that is expected to but not guaranteed to Leap seconds do not respect local time and always occur at the end of
continue. 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 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.[3] to be decided no earlier than 2015.[4]
3.1. UTC behavior during leap second 3.1. UTC behavior during leap second
UTC clocks insert a 61st second at the end of the day when a leap 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". second is scheduled. The leap second is designated "23h 59m 60s".
3.2. NTP behavior during leap second 3.2. NTP behavior during leap second
Under NTP [4] a leap second is inserted at the beginning of the last Under NTP [5] 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. 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 leap second 3.3. POSIX behavior during leap second
Most POSIX systems insert the leap second at the end of the last 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 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 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 that it can refer to a moment in time in either of the last two
seconds of a day containing a leap second. seconds of a day containing a leap second.
3.4. Summary of leap-second behaviors 3.4. Summary of leap-second behaviors
Table 1 summarizes behavior across a leap second for the wall clocks Table 1 summarizes behavior across a leap second for the wall clocks
discussed above. discussed above.
The table illustrates the leap second that occurred June 30, 2012 Table 1 illustrates the leap second that occurred June 30, 2012 when
when the offset between International Atomic time (TAI) and UTC the offset between International Atomic time (TAI) and UTC changed
changed from 34 to 35 seconds. The first column shows RTP timestamps from 34 to 35 seconds. The first column shows RTP timestamps for an
for an 8 kHz audio stream. The second column shows the TAI 8 kHz audio stream. The second column shows the TAI reference.
reference. Following columns show behavior for the leap-second- Following columns show behavior for the leap-second-bearing wall
bearing wall clocks described above. Time values are shown at half- clocks described above. Time values are shown at half-second
second intervals. intervals.
+-------+--------------+--------------+--------------+--------------+ +-------+--------------+--------------+--------------+--------------+
| RTP | TAI | UTC | POSIX | NTP | | RTP | TAI | UTC | POSIX | NTP |
+-------+--------------+--------------+--------------+--------------+ +-------+--------------+--------------+--------------+--------------+
| 8000 | 00:00:32.500 | 23:59:58.500 | 23:59:58.500 | 23:59:58.500 | | 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 | | 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 | | 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 | | 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 | | 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 | | 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 | | 32000 | 00:00:35.500 | 00:00:00.500 | 00:00:00.500 | 00:00:00.500 |
+-------+--------------+--------------+--------------+--------------+ +-------+--------------+--------------+--------------+--------------+
Table 1 Table 1
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) 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.
4. Recommendations 4. 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 [5], GPS [6] and other TAI [7] references do not include IEEE 1588,[6] GPS [7] and other TAI [8] references do not include
leap seconds. NTP time, operating system clocks and other UTC leap seconds. NTP time, operating system clocks and other UTC
(Coordinated Universal Time) 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. Without prior receive notification of leap second scheduling. Without prior
knowledge of leap second schedule, NTP servers and clients may become knowledge of leap second schedule, NTP servers and clients may become
offset by exactly one second with respect to their UTC reference. offset by exactly one second with respect to their UTC reference.
This potential discrepancy begins when a leap second occurs and ends This potential discrepancy begins when a leap second occurs and ends
when all participants receive a time update from a server or peer. when all participants receive a time update from a server or peer.
Depending on the system implementation, the offset can last anywhere Depending on the system implementation, the offset can last anywhere
from a few seconds to a few days. A long-lived discrepancy can be from a few seconds to a few days. A long-lived discrepancy can be
particularly disruptive to RTP operation. particularly disruptive to RTP operation.
Because of the ambiguity leap seconds can introduce and the Because of the timestamp ambiguity, positive leap seconds can
inconsistent manner in which different systems accommodate leap introduce and the inconsistent manner in which different systems
seconds, generating or using NTP timestamps during the entire last accommodate leap seconds, generating or using NTP timestamps during
second of a day on which a leap second has been scheduled SHOULD be the entire last second of a day on which a positive leap second has
avoided. Note that the period to be avoided has a real-time duration been scheduled SHOULD be avoided. Note that the period to be avoided
of two seconds. In the Table 1 example, the region to be avoided is has a real-time duration of two seconds. In the Table 1 example, the
indicated by RTP timestamps 12000 through 28000 region to be avoided is indicated by RTP timestamps 12000 through
28000
4.1. RTP Sender Reports and Receiver Reports Negative leap seconds do not introduce timestamp ambiguity or other
complications. No special treatment with respect to RTP timestamps
is required in the presence of a negative leap second.
4.1. 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 leap second. Receivers SHOULD ignore timestamps in the vicinity of a positive leap second. To maintain a consistent
any such reports inadvertently generated. 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.
4.2. RTP Packet Playout 4.2. RTP Packet Playout
Receivers working to a leap-second-bearing reference SHOULD take leap Receivers working to a leap-second-bearing reference SHOULD take both
seconds in their reference into account in determining play-out time positive and negative leap seconds in the reference into account in
from RTP timestamps for data in RTP packets. determining playout time based on RTP timestamps for data in RTP
packets.
5. Security Considerations 5. Security Considerations
It is believed that the recommendations herein introduce no new RTP streams using a wall-clock reference as discussed here present an
security considerations beyond those already discussed in [8]. 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-seocnd-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.
6. IANA Considerations 6. IANA Considerations
This document has no actions for IANA. This document has no actions for IANA.
7. Acknowledgements 7. Acknowledgements
The authors would like to thank Steve Allen for his valuable comments The authors would like to thank Steve Allen for his valuable comments
in helping to improve this document. in helping to improve this document.
8. Normative References 8. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement 8.1. Normative References
Levels", March 1997.
[2] ITU-R, "Recommendation ITU-R TF.460-4 - Standard-frequency and [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.
8.2. Informative References
[3] ITU-R, "Recommendation ITU-R TF.460-4 - Standard-frequency and
time-signal emissions", February 2002. time-signal emissions", February 2002.
[3] ITU-R Working Party 7A, "Question SG07.236", February 2012. [4] ITU-R Working Party 7A, "Question SG07.236", February 2012.
[4] Mills, D., Delaware, U., Martin, J., Ed., Burbank, J., and W. [5] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time
Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Protocol Version 4: Protocol and Algorithms Specification",
Specification", June 2010. RFC 5905, June 2010.
[5] IEEE, "IEEE Standard for a Precision Clock Synchronization [6] 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.
[6] Global Positioning Systems Directorate, "Navstar GPS Space [7] Global Positioning Systems Directorate, "Navstar GPS Space
Segment/Navigation User Segment Interfaces", September 2011. Segment/Navigation User Segment Interfaces", September 2011.
[7] BIPM, "Circular T", May 2012. [8] 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.
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
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