Internet DRAFT - draft-gross-leap-second
draft-gross-leap-second
AVTCore K. Gross
Internet-Draft AVA Networks
Updates: RFC3550 (if approved) R. van Brandenburg
Intended status: Standards Track TNO
Expires: November 23, 2012 May 22, 2012
RTP and Leap Seconds
draft-gross-leap-second-01
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.
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This Internet-Draft will expire on November 23, 2012.
Copyright Notice
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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. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
7. 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 [TF.460-6] synchronized
with the rotation of the earth. Leap seconds are scheduled by the
International Earth Rotation and Reference Systems Service. When
they occur, leap seconds are scheduled at the end of the last day of
December and/or June each year. Because earth's rotation is
unpredictable, it is not possible to schedule leap seconds 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 added seconds and 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 "23:59:60".
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3.2. NTP behavior during leap second
Under NTP Section 3.2 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
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.
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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. Acknowledgements
The authors would like to thank Steve Allen for his valuable comments
in helping to improve this document.
7. 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]
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.
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[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, "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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