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Versions: (draft-gross-avtcore-leap-second)
00 01 02 03 04 05 06 07 08 RFC 7164
AVTCore K. Gross
Internet-Draft AVA Networks
Updates: 3550 (if approved) R. van Brandenburg
Intended status: Standards Track TNO
Expires: December 23, 2012 June 21, 2012
RTP and Leap Seconds
draft-ietf-avtcore-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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 23, 2012.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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described in the Simplified BSD License.
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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. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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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 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 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.[TF.460-6] 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 thus far have been
scheduled in June or December and have all added seconds. 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 "23h 59m 60s".
The sequence of the second markers near the UTC leap second
transition are:
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Day 0, 23h 59m 59s
Day 0, 23h 59m 60s <-- leap second
Day 1, 0h 0m 0s
3.2. NTP behavior during leap second
Under NTP [RFC5905] 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
accommodation 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 [IEEE1588-2008], GPS [IS-GPS-200F] and other TAI (International
Atomic Time) [CircularT] 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
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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.
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 believed that the recommendations herein introduce no new
security considerations beyond those already discussed in [RFC3550].
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
[CircularT]
BIPM, "Circular T", May 2012.
[IEEE1588-2008]
IEEE, "IEEE Standard for a Precision Clock Synchronization
Protocol for Networked Measurement and Control Systems",
July 2008.
[IS-GPS-200F]
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Global Positioning Systems Directorate, "Navstar GPS Space
Segment/Navigation User Segment Interfaces",
September 2011.
[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.
[RFC5905] Mills, D., Delaware, U., Martin, J., Ed., Burbank, J., and
W. Kasch, "Network Time Protocol Version 4: Protocol and
Algorithms Specification", June 2010.
[TF.460-6]
ITU-R, "Recommendation ITU-R TF.460-4 - Standard-frequency
and time-signal emissions", February 2002.
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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