draft-ietf-avtcore-clksrc-03.txt   draft-ietf-avtcore-clksrc-04.txt 
Audio/Video Transport Core A. Williams Audio/Video Transport Core Maintenance A.M. Williams
Maintenance Audinate Internet-Draft Audinate
Internet-Draft K. Gross Intended status: Standards Track K. Gross
Intended status: Standards Track AVA Networks Expires: November 10, 2013 AVA Networks
Expires: September 19, 2013 R. van Brandenburg R. van Brandenburg
H. Stokking H.M. Stokking
TNO TNO
March 18, 2013 May 09, 2013
RTP Clock Source Signalling RTP Clock Source Signalling
draft-ietf-avtcore-clksrc-03 draft-ietf-avtcore-clksrc-04
Abstract Abstract
NTP format timestamps are used by several RTP protocols for NTP format timestamps are used by several RTP protocols for
synchronisation and statistical measurements. This memo specifies synchronisation and statistical measurements. This memo specifies
SDP signalling identifying timestamp reference clock sources and SDP SDP signalling identifying timestamp reference clock sources and SDP
signalling identifying the media clock sources in a multimedia signalling identifying the media clock sources in a multimedia
session. session.
Requirements Language Requirements Language
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 RFC 2119 [1]. document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 19, 2013. This Internet-Draft will expire on November 10, 2013.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Applications . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Timestamp Reference Clock Source Signalling . . . . . . . . . 5 4. Timestamp Reference Clock Source Signalling . . . . . . . . . 5
4.1. Clock synchronization . . . . . . . . . . . . . . . . . . 5 4.1. Clock synchronization . . . . . . . . . . . . . . . . . . 5
4.2. Identifying NTP Reference Clocks . . . . . . . . . . . . . 6 4.2. Identifying NTP Reference Clocks . . . . . . . . . . . . 6
4.3. Identifying PTP Reference Clocks . . . . . . . . . . . . . 6 4.3. Identifying PTP Reference Clocks . . . . . . . . . . . . 6
4.4. Identifying Global Reference Clocks . . . . . . . . . . . 7 4.4. Identifying Global Reference Clocks . . . . . . . . . . . 8
4.5. Other Reference Clocks . . . . . . . . . . . . . . . . . . 8 4.5. Other Reference Clocks . . . . . . . . . . . . . . . . . 8
4.6. Traceable Reference Clocks . . . . . . . . . . . . . . . . 8 4.6. Traceable Reference Clocks . . . . . . . . . . . . . . . 8
4.7. SDP Signalling of Timestamp Clock Source . . . . . . . . . 8 4.7. SDP Signalling of Timestamp Reference Clock Source . . . 8
4.7.1. Examples . . . . . . . . . . . . . . . . . . . . . . . 11 4.7.1. Examples . . . . . . . . . . . . . . . . . . . . . . 10
5. Media Clock Source Signalling . . . . . . . . . . . . . . . . 12 5. Media Clock Source Signalling . . . . . . . . . . . . . . . . 11
5.1. Asynchronously Generated Media Clock . . . . . . . . . . . 12 5.1. Asynchronously Generated Media Clock . . . . . . . . . . 12
5.2. Direct-Referenced Media Clock . . . . . . . . . . . . . . 12 5.2. Direct-Referenced Media Clock . . . . . . . . . . . . . . 12
5.3. Stream-Referenced Media Clock . . . . . . . . . . . . . . 13 5.3. Stream-Referenced Media Clock . . . . . . . . . . . . . . 13
5.4. SDP Signalling of Media Clock Source . . . . . . . . . . . 14 5.4. SDP Signalling of Media Clock Source . . . . . . . . . . 14
5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 15
6. Signalling considerations . . . . . . . . . . . . . . . . . . 17 6. Signalling considerations . . . . . . . . . . . . . . . . . . 17
6.1. Usage in Offer/Answer . . . . . . . . . . . . . . . . . . 18 6.1. Usage in Offer/Answer . . . . . . . . . . . . . . . . . . 17
6.2. Usage Outside of Offer/Answer . . . . . . . . . . . . . . 18 6.2. Usage Outside of Offer/Answer . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18 7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8.1. Reference clock registry . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . . 20 8.2. Media clock registry . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . . 21 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 9.1. Normative References . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction 1. Introduction
RTP protocols use NTP format timestamps to facilitate multimedia RTP protocols use NTP format timestamps to facilitate multimedia
session synchronisation and for providing estimates of round trip session synchronisation and for providing estimates of round trip
time (RTT) and other statistical parameters. time (RTT) and other statistical parameters.
Information about media clock timing exchanged in NTP format Information about media clock timing exchanged in NTP format
timestamps may come from a clock which is synchronised to a global timestamps may come from a clock which is synchronised to a global
time reference, but this cannot be assumed nor is there a time reference, but this cannot be assumed nor is there a
skipping to change at page 3, line 29 skipping to change at page 3, line 23
RTP can time align flows from the same source at a given receiver RTP can time align flows from the same source at a given receiver
using relative timing, however tight synchronisation between two or using relative timing, however tight synchronisation between two or
more different receivers (possibly with different network paths) or more different receivers (possibly with different network paths) or
between two or more senders is not possible. between two or more senders is not possible.
High performance AV systems often use a reference media clock High performance AV systems often use a reference media clock
distributed to all devices in the system. The reference media clock distributed to all devices in the system. The reference media clock
is often distinct from the reference clock used to provide is often distinct from the reference clock used to provide
timestamps. A reference media clock may be provided along with an timestamps. A reference media clock may be provided along with an
audio or video signal interface, or via a dedicated clock signal audio or video signal interface, or via a dedicated clock signal
(e.g. genlock [9] or audio word clock [10]). If sending and (e.g. genlock [SMPTE-318-1999] or audio word clock [AES11-2009]).
receiving media clocks are known to be synchronised to a common If sending and receiving media clocks are known to be synchronised to
reference clock, performance can improved by minimising buffering and a common reference clock, performance can improved by minimising
avoiding rate conversion. buffering and avoiding rate conversion.
This specification defines SDP signalling of timestamp clock sources This specification defines SDP signalling of timestamp reference
and media reference clock sources. clock sources and media reference clock sources.
2. Applications 2. Applications
Timestamp clock source and reference media clock signalling benefit Timestamp reference clock source and media clock signalling benefit
applications requiring synchronised media capture or playout and low applications requiring synchronised media capture or playout and low
latency operation. latency operation.
Examples include, but are not limited to: Examples include, but are not limited to:
Social TV : RTCP for inter-destination media synchronization [11] Social TV : RTCP for inter-destination media synchronization
defines social TV as the combination of media content consumption [I-D.ietf-avtcore-idms] defines social TV as the combination of
by two or more users at different devices and locations and real- media content consumption by two or more users at different
time communication between those users. An example of Social TV, devices and locations and real-time communication between those
is where two or more users are watching the same television users. An example of Social TV, is where two or more users are
broadcast at different devices and/or locations, while watching the same television broadcast at different devices and/or
communicating with each other using text, audio and/or video. A locations, while communicating with each other using text, audio
skew in the media playout of the two or more users can have and/or video. A skew in the media playout of the two or more
adverse effects on their experience. A well-known use case here users can have adverse effects on their experience. A well-known
is one friend experiencing a goal in a football match well before use case here is one friend experiencing a goal in a football
or after other friends. match well before or after other friends.
Video Walls : A video wall consists of multiple computer monitors, Video Walls : A video wall consists of multiple computer monitors,
video projectors, or television sets tiled together contiguously video projectors, or television sets tiled together contiguously
or overlapped in order to form one large screen. Each of the or overlapped in order to form one large screen. Each of the
screens reproduces a portion of the larger picture. In some screens reproduces a portion of the larger picture. In some
implementations, each screen or projector may be individually implementations, each screen or projector may be individually
connected to the network and receive its portion of the overall connected to the network and receive its portion of the overall
image from a network-connected video server or video scaler. image from a network-connected video server or video scaler.
Screens are refreshed at 50 or 60 hertz or potentially faster. If Screens are refreshed at 50 or 60 hertz or potentially faster. If
the refresh is not synchronized, the effect of multiple screens the refresh is not synchronized, the effect of multiple screens
acting as one is broken. acting as one is broken.
Networked Audio : Networked loudspeakers, amplifiers and analogue Networked Audio : Networked loudspeakers, amplifiers and analogue I/
I/O devices transmitting or receiving audio signals via RTP can be O devices transmitting or receiving audio signals via RTP can be
connected to various parts of a building or campus network. Such connected to various parts of a building or campus network. Such
situations can for example be found in large conference rooms, situations can for example be found in large conference rooms,
legislative chambers, classrooms (especially those supporting legislative chambers, classrooms (especially those supporting
distance learning) and other large-scale environments such as distance learning) and other large-scale environments such as
stadiums. Since humans are more susceptible to differences in stadiums. Since humans are more susceptible to differences in
audio delay, this use case needs even more accuracy than the video audio delay, this use case needs even more accuracy than the video
wall use case. Depending on the exact application, the need for wall use case. Depending on the exact application, the need for
accuracy can then be in the range of microseconds [21]. accuracy can then be in the range of microseconds [1].
Sensor Arrays : Sensor arrays contain many synchronised measurement Sensor Arrays : Sensor arrays contain many synchronised measurement
elements producing signals which are then combined to form an elements producing signals which are then combined to form an
overall measurement. Accurate capture of the phase relationships overall measurement. Accurate capture of the phase relationships
between the various signals arriving at each element of the array between the various signals arriving at each element of the array
is critically important for proper operation. Examples include is critically important for proper operation. Examples include
towed or fixed sonar arrays, seismic arrays and phased arrays used towed or fixed sonar arrays, seismic arrays and phased arrays used
in radar applications, for instance. in radar applications, for instance.
3. Definitions 3. Definitions
The following definitions are used in this draft: The following definitions are used in this draft:
media level : Media level information applies to a single SDP media media level : Media level information applies to a single SDP media
stream. In an SDP description, media-level information appears stream. In an SDP description, media-level information appears
after each "m"-line. after each "m"-line.
multimedia session : A set of multimedia senders and receivers as multimedia session : A set of multimedia senders and receivers as
well as the data streams flowing from senders to receivers. The well as the data streams flowing from senders to receivers. The
Session Description Protocol (SDP) [2] describes multimedia Session Description Protocol (SDP) [RFC4566] describes multimedia
sessions. sessions.
RTP media stream : A single stream of RTP packets identified by an RTP media stream : A single stream of RTP packets identified by an
RTP SSRC. RTP SSRC.
RTP media sender : The device generating an associated RTP media RTP media sender : The device generating an associated RTP media
stream stream
SDP media stream : An RTP session potentially containing more than SDP media stream : An RTP session potentially containing more than
one RTP source. SDP media descriptions beginning with an "m"-line one RTP source. SDP media descriptions beginning with an "m"-line
define the parameters of an SDP media stream. define the parameters of an SDP media stream.
session level : Session level information applies to an entire session level : Session level information applies to an entire
multimedia session. In an SDP description, session-level multimedia session. In an SDP description, session-level
information appears before the first "m"-line. information appears before the first "m"-line.
source level : Source level information applies to a RTP media source level : Source level information applies to a RTP media
stream Source-Specific Media Attributes in the Session Description stream Source-Specific Media Attributes in the Session Description
Protocol (SDP) [3] defines how source-level information is Protocol (SDP) [RFC5576] defines how source-level information is
included into an SDP session description. included into an SDP session description.
traceable time : A clock is considered to provide traceable time if traceable time : A clock is considered to provide traceable time if
it can be proven to be synchronised to International Atomic Time it can be proven to be synchronised to International Atomic Time
(TAI). Coordinated Universal Time (UTC) is a time standard (TAI). Coordinated Universal Time (UTC) is a time standard
synchronized to TAI. UTC is therefore also considered traceable synchronized to TAI. UTC is therefore also considered traceable
time once leap seconds have been taken unto account. GPS [12] is time once leap seconds have been taken unto account. GPS
commonly used to provide a TAI traceable time reference. Some [IS-GPS-200F] is commonly used to provide a TAI traceable time
network time synchronisation protocols (e.g. PTP [13], NTP) can reference. Some network time synchronisation protocols (e.g. PTP
explicitly indicate that the master clock is providing a traceable [IEEE1588-2008], NTP) can explicitly indicate that the master
time reference over the network. clock is providing a traceable time reference over the network.
4. Timestamp Reference Clock Source Signalling 4. Timestamp Reference Clock Source Signalling
The NTP format timestamps used by RTP are taken by reading a local The NTP format timestamps used by RTP are taken by reading a local
real-time clock at the sender or receiver. This local clock may be real-time clock at the sender or receiver. This local clock may be
synchronised to another clock (time source) by some means or it may synchronised to another clock (time source) by some means or it may
be unsynchronised. A variety of methods are available to synchronise be unsynchronised. A variety of methods are available to synchronise
local clocks to a reference time source, including network time local clocks to a reference time source, including network time
protocols (e.g. NTP [14], PTP [13]) and radio clocks (e.g. GPS protocols (e.g. NTP [RFC5905], PTP [IEEE1588-2008]) and radio clocks
[12]). (e.g. GPS [IS-GPS-200F]).
The following sections describe and define SDP signalling, indicating The following sections describe and define SDP signalling, indicating
whether and how the local timestamping clock in an RTP sender/ whether and how the local timestamping clock in an RTP sender/
receiver is synchronised to a reference clock. receiver is synchronised to a reference clock.
4.1. Clock synchronization 4.1. Clock synchronization
Two or more local clocks that are sufficiently synchronised will Two or more local clocks that are sufficiently synchronised will
produce timestamps for a given RTP event can be used as if they came produce timestamps for a given RTP event can be used as if they came
from the same clock. Providing they are sufficiently synchronised, from the same clock. Providing they are sufficiently synchronised,
skipping to change at page 6, line 30 skipping to change at page 6, line 23
network time protocols as some protocols (e.g. PTP, NTP) allow network time protocols as some protocols (e.g. PTP, NTP) allow
master clocks to indicate explicitly that they are providing master clocks to indicate explicitly that they are providing
traceable time. traceable time.
4.2. Identifying NTP Reference Clocks 4.2. Identifying NTP Reference Clocks
A single NTP server is identified by hostname (or IP address) and an A single NTP server is identified by hostname (or IP address) and an
optional port number. If the port number is not indicated, it is optional port number. If the port number is not indicated, it is
assumed to be the standard NTP port (123). assumed to be the standard NTP port (123).
Two or more NTP servers may be listed at the same level in the Two or more NTP servers MAY be listed at the same level in the
session description to indicate that they are interchangeable. RTP session description to indicate that all of the listed servers
senders or receivers can use any of the listed NTP servers to govern deliver the same reference time and may be used interchangeably. RTP
a local clock that is equivalent to a local clock slaved to a senders and receivers are assured proper synchronization regardless
different server. of which server they choose and, in support of fault tolerance, may
switch servers while streaming.
4.3. Identifying PTP Reference Clocks 4.3. Identifying PTP Reference Clocks
The IEEE 1588 Precision Time Protocol (PTP) family of clock The IEEE 1588 Precision Time Protocol (PTP) family of clock
synchronisation protocols provides a shared reference clock in an synchronisation protocols provides a shared reference clock in an
network - typically a LAN. IEEE 1588 provides sub-microsecond network - typically a LAN. IEEE 1588 provides sub-microsecond
synchronisation between devices on a LAN and typically locks within synchronisation between devices on a LAN and typically locks within
seconds at startup. With support from Ethernet switches, IEEE 1588 seconds at startup. With support from Ethernet switches, IEEE 1588
protocols can achieve nanosecond timing accuracy in LANs. Network protocols can achieve nanosecond timing accuracy in LANs. Network
interface chips and cards supporting hardware time-stamping of timing interface chips and cards supporting hardware time-stamping of timing
critical protocol messages are also available. critical protocol messages are also available.
Three flavours of IEEE 1588 are in use today: Three flavours of IEEE 1588 are in use today:
o IEEE 1588-2002 [15]: the original "Standard for a Precision Clock o IEEE 1588-2002 [IEEE1588-2002]: the original "Standard for a
Synchronization Protocol for Networked Measurement and Control
Systems". This is also known as IEEE1588v1 or PTPv1.
o IEEE 1588-2008 [13]: the second version of the "Standard for a
Precision Clock Synchronization Protocol for Networked Measurement Precision Clock Synchronization Protocol for Networked Measurement
and Control Systems". This is a revised version of the original and Control Systems". This is also known as IEEE1588v1 or PTPv1.
IEEE1588-2002 standard and is also known as IEEE1588v2 or PTPv2.
IEEE 1588-2008 is not protocol compatible with IEEE 1588-2002.
o IEEE 802.1AS [16]: "Timing and Synchronization for Time Sensitive o IEEE 1588-2008 [IEEE1588-2008]: the second version of the
Applications in Bridged Local Area Networks". This is a Layer-2 "Standard for a Precision Clock Synchronization Protocol for
only profile of IEEE 1588-2008 for use in Audio/Video Bridged LANs Networked Measurement and Control Systems". This is a revised
as described in IEEE 802.1BA-2011 [17]. version of the original IEEE1588-2002 standard and is also known
as IEEE1588v2 or PTPv2. IEEE 1588-2008 is not protocol compatible
with IEEE 1588-2002.
o IEEE 802.1AS [IEEE802.1AS-2011]: "Timing and Synchronization for
Time Sensitive Applications in Bridged Local Area Networks". This
is a Layer-2 only profile of IEEE 1588-2008 for use in Audio/Video
Bridged LANs as described in IEEE 802.1BA-2011 [IEEE802.1BA-2011].
Each IEEE 1588 clock is identified by a globally unique EUI-64 called Each IEEE 1588 clock is identified by a globally unique EUI-64 called
a "ClockIdentity". A slave clock using one of the IEEE 1588 family a "ClockIdentity". A slave clock using one of the IEEE 1588 family
of network time protocols acquires the ClockIdentity/EUI-64 of the of network time protocols acquires the ClockIdentity/EUI-64 of the
grandmaster clock that is the ultimate source of timing information grandmaster clock that is the ultimate source of timing information
for the network. A boundary clock which is itself slaved to another for the network. A boundary clock which is itself slaved to another
boundary clock or the grandmaster passes the grandmaster boundary clock or the grandmaster passes the grandmaster
ClockIdentity through to its slaves. ClockIdentity through to its slaves.
Several instances of the IEEE 1588 protocol may operate independently Several instances of the IEEE 1588 protocol may operate independently
on a single network, forming distinct PTP domains, each of which may on a single network, forming distinct PTP domains, each of which may
have a different grandmaster clock. As the IEEE 1588 standards have have a different grandmaster clock. As the IEEE 1588 standards have
developed, the definition of PTP domains has changed. IEEE 1588-2002 developed, the definition of PTP domains has changed. IEEE 1588-2002
identifies protocol subdomains by a textual name, but IEEE 1588-2008 identifies protocol subdomains by a textual name, but IEEE 1588-2008
identifies protocol domains using a numeric domain number. 802.1AS is identifies protocol domains using a numeric domain number. 802.1AS
a Layer-2 profile of IEEE 1588-2008 supporting a single numeric clock is a Layer-2 profile of IEEE 1588-2008 supporting a single numeric
domain (0). clock domain (0).
When PTP domains are signalled via SDP, senders and receivers SHOULD When PTP domains are signalled via SDP, senders and receivers SHOULD
check that both grandmaster ClockIdentity and PTP domain match when check that both grandmaster ClockIdentity and PTP domain match when
determining clock equivalence. determining clock equivalence.
Two or more IEEE 1588 clocks MAY be listed at the same level in the
session description to indicate that all of the listed clocks are
candidate grandmaster clocks for the domain or deliver the same
reference time and may be used interchangeably. RTP senders and
receivers are assured proper synchronization regardless of which
synchronization source they choose and, in support of fault
tolerance, may switch refence clock source while streaming.
The PTP protocols employ a distributed election protocol called the The PTP protocols employ a distributed election protocol called the
"Best Master Clock Algorithm" (BMCA) to determine the active clock "Best Master Clock Algorithm" (BMCA) to determine the active clock
master. The clock master choices available to BMCA can be restricted master. The clock master choices available to BMCA can be restricted
or biased by configuration parameters to influence the election or biased by configuration parameters to influence the election
process. In some systems it may be desirable to limit the number of process. In some systems it may be desirable to limit the number of
possible PTP clock masters to avoid the need to re-signal timestamp possible PTP clock masters to avoid the need to re-signal timestamp
clock sources when the clock master changes. reference clock sources when the clock master changes.
4.4. Identifying Global Reference Clocks 4.4. Identifying Global Reference Clocks
Global reference clocks provide a source of traceable time, typically Global reference clocks provide a source of traceable time, typically
via a hardware radio receiver interface. Examples include GPS and via a hardware radio receiver interface. Examples include GPS and
Galileo. Apart from the name of the reference clock system, no Galileo. Apart from the name of the reference clock system, no
further identification is required. further identification is required.
4.5. Other Reference Clocks 4.5. Other Reference Clocks
RFC 3550 allows senders and receivers to either use a local wallclock RFC 3550 allows senders and receivers to either use a local wallclock
reference for their NTP timestamps or, by setting the timestamp field reference for their NTP timestamps or, by setting the timestamp field
to 0, to supply no timestamps at all. Both are common practice in to 0, to supply no timestamps at all. Both are common practice in
embedded RTP implementations. These clocks are identified as "local" embedded RTP implementations. These clocks are identified as "local"
and can only be assumed to be equivalent to clocks originating from and can only be assumed to be equivalent to clocks originating from
the same device. the same device.
In other systems, all RTP senders and receivers may use a timestamp In other systems, all RTP senders and receivers may use a timestamp
clock synchronised to a reference clock that is not provided by one reference clock that is not provided by one of the methods listed
of the methods listed above. Examples may include the reference time above. Examples may include the reference time information provided
information provided by digital television or cellular services. by digital television or cellular services. These sources are
These sources are identified as "private" reference clocks. All RTP identified as "private" reference clocks. All RTP senders and
senders and receivers in a session using a private reference clock receivers in a session using a private reference clock are assumed to
are assumed to have a mechanism outside this specification for have a mechanism outside this specification for determining whether
determining whether their timestamp clocks are equivalent. their timestamp reference clocks are equivalent.
4.6. Traceable Reference Clocks 4.6. Traceable Reference Clocks
A timestamp clock source may be labelled "traceable" if it is known A timestamp reference clock source may be labelled "traceable" if it
to be to delivering traceable time. Providing adjustments are made is known to be to delivering traceable time. Providing adjustments
for differing epochs, timezones and leap seconds, timestamps taken are made for differing epochs, timezones and leap seconds, timestamps
using clocks synchronised to a traceable time source can be directly taken using clocks synchronised to a traceable time source can be
compared even if the clocks are synchronised to different sources or directly compared even if the clocks are synchronised to different
via different mechanisms. sources or via different mechanisms.
Since all NTP and PTP servers providing traceable time can be Since all NTP and PTP servers providing traceable time can be
directly compared, it is not necessary to identify traceable time directly compared, it is not necessary to identify traceable time
servers by protocol address or other identifiers. servers by protocol address or other identifiers.
4.7. SDP Signalling of Timestamp Clock Source 4.7. SDP Signalling of Timestamp Reference Clock Source
Specification of the timestamp reference clock source may be at any Specification of the timestamp reference clock source may be at any
or all levels (session, media or source) of an SDP description (see or all levels (session, media or source) of an SDP description (see
level definitions (Section 3) earlier in this document for more level definitions (Section 3) earlier in this document for more
information). information).
Timestamp clock source signalling included at session-level provides Timestamp reference clock source signalling included at session-level
default parameters for all RTP sessions and sources in the session provides default parameters for all RTP sessions and sources in the
description. More specific signalling included at the media level session description. More specific signalling included at the media
overrides default session level signalling. More specific signalling level overrides default session level signalling. More specific
included at the source level overrides default media level signalling included at the source level overrides default media level
signalling. signalling.
If timestamp clock source signalling is included anywhere in an SDP If timestamp reference clock source signalling is included anywhere
description, it must be properly defined for all levels in the in an SDP description, it must be properly defined for all levels in
description. This may simply be achieved by providing default the description. This may simply be achieved by providing default
signalling at the session level. signalling at the session level.
Timestamp reference clock parameters may be repeated at a given level Timestamp reference clock parameters may be repeated at a given level
(i.e. for a session or source) to provide information about (i.e. for a session or source) to provide information about
additional servers or clock sources. If the attribute is repeated at additional servers or clock sources. If the attribute is repeated at
a given level, all clocks described at that level are assumed to be a given level, all clocks described at that level are assumed to be
equivalent. Traceable time sources MUST NOT be mixed with non- equivalent. Traceable time sources MUST NOT be mixed with non-
traceable time sources at any given level. traceable time sources at any given level.
Note that clock source parameters may change from time to time, for Note that clock source parameters may change from time to time, for
example, as a result of a PTP clock master election. The SIP [4] example, as a result of a PTP clock master election. The SIP
protocol supports re-signalling of updated SDP information, however [RFC3261] protocol supports re-signalling of updated SDP information,
other protocols may require additional notification mechanisms. however other protocols may require additional notification
mechanisms.
Figure 1 shows the ABNF [5] grammar for the SDP reference clock Figure 1 shows the ABNF [RFC5234] grammar for the SDP reference clock
source information. source information.
timestamp-refclk = "a=ts-refclk:" clksrc CRLF timestamp-refclk = "a=ts-refclk:" clksrc CRLF
clksrc = ntp / ptp / gps / gal / local / private / clksrc-ext clksrc = ntp / ptp / gps / gal / local / private / clksrc-ext
ntp = "ntp=" ntp-server-addr ntp = "ntp=" ntp-server-addr
ntp-server-addr = host [ ":" port ] ntp-server-addr = host [ ":" port ]
ntp-server-addr =/ "traceable" ntp-server-addr =/ "traceable"
ptp = "ptp=" ptp-version ":" ptp-server ptp = "ptp=" ptp-version ":" ptp-server
skipping to change at page 10, line 33 skipping to change at page 10, line 10
ptp-domain-char = %x21-7E / %x00 ptp-domain-char = %x21-7E / %x00
; allowed characters: 0x21-0x7E (IEEE 1588-2002) ; allowed characters: 0x21-0x7E (IEEE 1588-2002)
ptp-domain-nmbr = "domain-nmbr=" %x00-7f ptp-domain-nmbr = "domain-nmbr=" %x00-7f
; allowed number range: 0-127 (IEEE 1588-2008) ; allowed number range: 0-127 (IEEE 1588-2008)
gps = "gps" gps = "gps"
gal = "gal" gal = "gal"
local = "local" local = "local"
private = "private" [ ":" "traceable" ] private = "private" [ ":" "traceable" ]
clksrc-ext = token clksrc-ext = token
host = hostname / IPv4address / IPv6reference host = hostname / IPv4address / IPv6reference
hostname = *( domainlabel "." ) toplabel [ "." ] hostname = *( domainlabel "." ) toplabel [ "." ]
toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum toplabel = ALPHA / ALPHA *( alphanum / "-" ) alphanum
domainlabel = alphanum domainlabel = alphanum
/ alphanum *( alphanum / "-" ) alphanum / alphanum *( alphanum / "-" ) alphanum
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
IPv6reference = "[" IPv6address "]" IPv6reference = "[" IPv6address "]"
IPv6address = hexpart [ ":" IPv4address ] IPv6address = hexpart [ ":" IPv4address ]
hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ] hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
skipping to change at page 11, line 28 skipping to change at page 11, line 7
a=recvonly a=recvonly
a=ts-refclk:ntp=traceable a=ts-refclk:ntp=traceable
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 99 m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000 a=rtpmap:99 h263-1998/90000
Figure 2: Timestamp reference clock definition at the session level Figure 2: Timestamp reference clock definition at the session level
Figure 3 shows an example SDP description with timestamp reference Figure 3 shows an example SDP description with timestamp reference
clock definitions at the media level overriding the session level clock definitions at the media level overriding the session level
defaults. Note that the synchronisation confidence timestamp appears defaults.
on the first attribute at the media level only.
v=0 v=0
o=jdoe 2890844526 2890842807 IN IP4 192.0.2.1 o=jdoe 2890844526 2890842807 IN IP4 192.0.2.1
s=SDP Seminar s=SDP Seminar
i=A Seminar on the session description protocol i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe) e=j.doe@example.com (Jane Doe)
c=IN IP4 233.252.0.1/64 c=IN IP4 233.252.0.1/64
t=2873397496 2873404696 t=2873397496 2873404696
a=recvonly a=recvonly
a=ts-refclk:local a=ts-refclk:local
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=ts-refclk:ntp=203.0.113.10 2011-02-19 21:03:20.345+01:00 a=ts-refclk:ntp=203.0.113.10
a=ts-refclk:ntp=198.51.100.22 a=ts-refclk:ntp=198.51.100.22
m=video 51372 RTP/AVP 99 m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000 a=rtpmap:99 h263-1998/90000
a=ts-refclk:ptp=IEEE802.1AS-2011:39-A7-94-FF-FE-07-CB-D0 a=ts-refclk:ptp=IEEE802.1AS-2011:39-A7-94-FF-FE-07-CB-D0
Figure 3: Timestamp reference clock definition at the media level Figure 3: Timestamp reference clock definition at the media level
Figure 4 shows an example SDP description with a timestamp reference Figure 4 shows an example SDP description with a timestamp reference
clock definition at the source level overriding the session level clock definition at the source level overriding the session level
default. default.
skipping to change at page 13, line 21 skipping to change at page 12, line 47
A rate modifier may be specified. The modifier is expressed as the A rate modifier may be specified. The modifier is expressed as the
ratio of two integers and modifies the rate specified or implied by ratio of two integers and modifies the rate specified or implied by
the media description by this ratio. If omitted, the rate is assumed the media description by this ratio. If omitted, the rate is assumed
to be the exact rate specified or implied by the media format. For to be the exact rate specified or implied by the media format. For
example, without a rate specification, the media clock for an 8 kHz example, without a rate specification, the media clock for an 8 kHz
G.711 audio stream will advance exactly 8000 units for each second G.711 audio stream will advance exactly 8000 units for each second
advance in the reference clock from which it is derived. advance in the reference clock from which it is derived.
The rate modifier is primarily useful for accommodating certain The rate modifier is primarily useful for accommodating certain
"oddball" audio sample rates associated with NTSC video (see "oddball" audio sample rates associated with NTSC video (see Figure
Figure 7). Modified rates are not advised for video streams which 7). Modified rates are not advised for video streams which generally
generally use a 90 kHz RTP clock regardless of frame rate or sample use a 90 kHz RTP clock regardless of frame rate or sample rate used
rate used for embedded audio. for embedded audio.
a=mediaclk:direct[=<offset>] [rate=<rate numerator>/<rate a=mediaclk:direct[=<offset>] [rate=<rate numerator>/<rate
denominator>] denominator>]
5.3. Stream-Referenced Media Clock 5.3. Stream-Referenced Media Clock
A common synchronisation architecture for audio/visual systems A common synchronisation architecture for audio/visual systems
involves distributing a reference media clock from a master device to involves distributing a reference media clock from a master device to
a number of slave devices, typically by means of a cable. Examples a number of slave devices, typically by means of a cable. Examples
include audio word clock distribution and video black burst include audio word clock distribution and video black burst
skipping to change at page 14, line 4 skipping to change at page 13, line 32
Slave devices in turn associate the master media clock identifier Slave devices in turn associate the master media clock identifier
with streams they transmit, signalling the synchronisation with streams they transmit, signalling the synchronisation
relationship between the master and slave devices. relationship between the master and slave devices.
Slave devices recover media clock timing from the clock master Slave devices recover media clock timing from the clock master
stream, using it to synchronise the slave media clock with the stream, using it to synchronise the slave media clock with the
master. Timestamps in the master clock RTP media stream are taken master. Timestamps in the master clock RTP media stream are taken
using the timestamp reference clock shared by the master and slave using the timestamp reference clock shared by the master and slave
devices. The timestamps communicate information about media clock devices. The timestamps communicate information about media clock
timing (rate, phase) from the master to the slave devices. timing (rate, phase) from the master to the slave devices.
Timestamps are communicated in the usual RTP fashion via RTCP SRs, or Timestamps are communicated in the usual RTP fashion via RTCP SRs, or
via the RFC6051 [6] header extension. The stream media format may via the RFC6051 [RFC6051] header extension. The stream media format
indicate other clock information, such as the nominal rate. may indicate other clock information, such as the nominal rate.
Note that slaving of a device media clock to a master device does not Note that slaving of a device media clock to a master device does not
affect the usual RTP lip sync / time alignment algorithms. Time affect the usual RTP lip sync / time alignment algorithms. Time
aligned playout of two or more RTP sources still relies upon NTP aligned playout of two or more RTP sources still relies upon NTP
timestamps supplied via RTCP SRs or by the RFC6051 timestamp header timestamps supplied via RTCP SRs or by the RFC6051 timestamp header
extension. extension.
In a given system, master clock identifiers must be unique. Such In a given system, master clock identifiers must be unique. Such
identifiers MAY be manually configured, however 17 octet string identifiers MAY be manually configured, however 17 octet string
identifiers SHOULD be generated according to the "short-term identifiers SHOULD be generated according to the "short-term
persistent RTCP CNAME" algorithm as described in RFC6222 [7]. persistent RTCP CNAME" algorithm as described in RFC6222 [RFC6222].
A reference stream can be an RTP stream or AVB stream based on the A reference stream can be an RTP stream or AVB stream based on the
IEEE 1722 [18] standard. IEEE 1722 [IEEE1722] standard.
An RTP clock master stream SHOULD be identified at the source level An RTP clock master stream SHOULD be identified at the source level
by an SSRC and master clock identifier. If master clock identifiers by an SSRC and master clock identifier. If master clock identifiers
are declared at the media or session level, all RTP sources at or are declared at the media or session level, all RTP sources at or
below the level of declaration MUST provide equivalent timing to a below the level of declaration MUST provide equivalent timing to a
slave receiver. slave receiver.
a=ssrc:<media-clock-master-ssrc-id> mediaclk:master-id=<media- a=ssrc:<media-clock-master-ssrc-id> mediaclk:master-id=<media-
clock-master-id> clock-master-id>
skipping to change at page 15, line 17 skipping to change at page 14, line 44
Media clock source signalling may be present or absent on a per- Media clock source signalling may be present or absent on a per-
stream basis. In the absence of media clock source signals, stream basis. In the absence of media clock source signals,
receivers assume an asynchronous media clock generated by the sender. receivers assume an asynchronous media clock generated by the sender.
Media clock source parameters may be repeated at a given level (i.e. Media clock source parameters may be repeated at a given level (i.e.
for a session or source) to provide information about additional for a session or source) to provide information about additional
clock sources. If the attribute is repeated at a given level, all clock sources. If the attribute is repeated at a given level, all
clocks described at that level are comparable clock sources and may clocks described at that level are comparable clock sources and may
be used interchangeably. be used interchangeably.
Figure 5 shows the ABNF [5] grammar for the SDP media clock source Figure 5 shows the ABNF [RFC5234] grammar for the SDP media clock
information. source information.
mediaclk-master = "a=ssrc:" integer SP clk-master-id mediaclk-master = "a=ssrc:" integer SP clk-master-id
clk-master-id = "mediaclk:master-id=" master-id clk-master-id = "mediaclk:master-id=" master-id
timestamp-mediaclk = "a=mediaclk:" mediaclock timestamp-mediaclk = "a=mediaclk:" mediaclock
mediaclock = sender / refclk / streamid / mediaclock-ext
mediaclock = sender / refclk / streamid / mediaclock-ext sender = "sender" sender-ext
sender = "sender" sender-ext sender-ext = token
sender-ext = token refclk = "direct" [ "=" 1*DIGIT ] [rate] [direct-ext]
refclk = "direct" [ "=" 1*DIGIT ] [rate] [direct-ext] rate = [ SP "rate=" integer "/" integer ]
rate = [ SP "rate=" integer "/" integer ] direct-ext = token
direct-ext = token streamid = "master-id=" master-id
streamid =/ "IEEE1722=" avb-stream-id
streamid =/ streamid-ext
streamid = "master-id=" master-id master-id = EUI48
streamid =/ "IEEE1722=" avb-stream-id avb-stream-id = EUI64
streamid =/ streamid-ext
master-id = EUI48 EUI48 = 5(2HEXDIG ":") 2HEXDIG
avb-stream-id = EUI64 EUI64 = 7(2HEXDIG ":") 2HEXDIG
EUI48 = 5(2HEXDIG ":") 2HEXDIG streamid-ext = token
EUI64 = 7(2HEXDIG ":") 2HEXDIG
streamid-ext = token mediaclock-ext = token [SP byte-string]
mediaclock-ext = token
Figure 5: Media Clock Source Signalling Figure 5: Media Clock Source Signalling
5.5. Examples 5.5. Examples
Figure 6 shows an example SDP description 8 channels of 24-bit, 48 Figure 6 shows an example SDP description 8 channels of 24-bit, 48
kHz audio transmitted as a multicast stream. Media clock is derived kHz audio transmitted as a multicast stream. Media clock is derived
directly from an IEEE 1588-2008 reference. directly from an IEEE 1588-2008 reference.
v=0 v=0
o=- 1311738121 1311738121 IN IP4 192.0.2.1 o=- 1311738121 1311738121 IN IP4 192.0.2.1
skipping to change at page 16, line 26 skipping to change at page 16, line 6
t=0 0 t=0 0
m=audio 5004 RTP/AVP 96 m=audio 5004 RTP/AVP 96
a=rtpmap:96 L24/48000/8 a=rtpmap:96 L24/48000/8
a=sendonly a=sendonly
a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0 a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
a=mediaclk:direct=963214424 a=mediaclk:direct=963214424
Figure 6: Media clock directly referenced to IEEE 1588-2008 Figure 6: Media clock directly referenced to IEEE 1588-2008
Figure 7 shows an example SDP description 2 channels of 24-bit, 44056 Figure 7 shows an example SDP description 2 channels of 24-bit, 44056
kHz NTSC "pull-down" media clock derived directly from an IEEE 1588- kHz NTSC "pull-down" media clock derived directly from an IEEE
2008 reference clock 1588-2008 reference clock
v=0 v=0
o=- 1311738121 1311738121 IN IP4 192.0.2.1 o=- 1311738121 1311738121 IN IP4 192.0.2.1
c=IN IP4 233.252.0.1/64 c=IN IP4 233.252.0.1/64
s= s=
t=0 0 t=0 0
m=audio 5004 RTP/AVP 96 m=audio 5004 RTP/AVP 96
a=rtpmap:96 L24/44100/2 a=rtpmap:96 L24/44100/2
a=sendonly a=sendonly
a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0 a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
skipping to change at page 17, line 37 skipping to change at page 17, line 21
a=rtpmap:96 L24/48000/2 a=rtpmap:96 L24/48000/2
a=sendonly a=sendonly
a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0 a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
a=mediaclk:IEEE1722=38-D6-6D-8E-D2-78-13-2F a=mediaclk:IEEE1722=38-D6-6D-8E-D2-78-13-2F
Figure 9: RTP stream with media clock slaved to an IEEE1722 master Figure 9: RTP stream with media clock slaved to an IEEE1722 master
device device
6. Signalling considerations 6. Signalling considerations
Signaling for timestamp clock source (Section 4.7) and media clock Signaling for timestamp reference clock source (Section 4.7) and
source (Section 5.4) is defined to be used either by applications media clock source (Section 5.4) is defined to be used either by
that implement the SDP Offer/Answer model [8] or by applications that applications that implement the SDP Offer/Answer model [RFC3264] or
use SDP to describe media and transport configurations. by applications that use SDP to describe media and transport
configurations.
A description or offer may include reference clock signalling, media A description or offer may include reference clock signalling, media
clock signalling or both. If no reference clock is specified, the clock signalling neither or both. When a direct-referenced media
direct-referenced media clock (Section 5.2) is not allowed. If no clock (Section 5.2) is specified, reference clock signalling is
media clock is specified, an asynchronous media clock (Section 5.1) REQUIRED. If no media clock is signalled, an asynchronous media
is assumed. stream-referenced media clock (Section 5.3) may be used clock (Section 5.1) is assumed. Asynchronous and stream-referenced
with or without a reference clock specification. If a reference media clocks (Section 5.3) may be used with or without reference
clock is not signalled, the stream may be established as rate clock signalling. If a reference clock is not signalled, the
synchronized however time synchronisation is not guaranteed. corresponding media clock may be established as rate synchronized
with no assurance of time synchronisation.
6.1. Usage in Offer/Answer 6.1. Usage in Offer/Answer
An answer to an offer with direct-referenced media clock and Usage of reference clock and media clock signalling in offer/answer
reference clock specification must include the same media clock and allows the offerer to declare the reference clock and media clock in
reference clock signalling in which case a connection is established use and allows the answerer to acknowledge that a connection
using the specified synchronisation. Alternatively the answer may according to these declarations will be successful or, in limited
omit both the signals or return only the reference clock cases described below, specify a simplified synchronization mode.
specification. In this case, a connection is established assuming an
asynchronous media clock.
An answer to an offer with media-referenced media clock specification While full negotiation of reference clock and media clock attributes
must include the same media clock specification. The answer MUST is not directly supported in the clock source signalling defined in
include the same reference clock signalling or may drop the reference this document, negotiation MAY be accomplished using capabilities
negotiation procedures defined in [RFC5939].
An answer to an offer with direct-referenced media clock SHOULD
include the same media clock and reference clock signalling in which
case a connection is established using the specified synchronisation.
Alternatively the answer MAY omit both signals or include only the
reference clock specified in the offer. In this case, a connection
with asynchronous media clock is established.
An answer to an offer with stream-referenced media clock signalling
SHOULD include the same media clock specification. If the offer
contains reference clock signalling, the answer SHOULD include the
same reference clock specification. The answer MAY omit reference
clock signalling. If reference clock signalling is not present in clock signalling. If reference clock signalling is not present in
the answer, either due to not being present in the offer or due to the answer, either due to not being present in the offer or due to
being dropped by the answerer, the stream may be established as rate being dropped in the answer, the stream may be established as rate
synchronized but not time synchronized. synchronised but not time synchronised.
An asynchronous media clock is the default media clock mode. This An asynchronous media clock is the default media clock mode. This
mode may be explicitly signalled or presumed due to lack of mode may be explicitly signalled (a=mediaclk:sender) or presumed due
signalling. Asynchronous media clocking does not require reference to lack of signalling. If explicit signalling of asynchronous media
clock signalling. An offer with asynchronous media clocking MAY clock is included in the offer, it SHOULD also be included in the
include reference clock signalling. Because the asynchronous media answer. An offer with asynchronous media clocking MAY include
clock is the default mode, the answerer is not required to explicitly reference clock signalling. An answer to an asynchronous media clock
signal this even if it is explicitly signalled in the offer. offer with reference clock signalling SHOULD include the same
reference clock specification. Alternatively, the answer MAY drop
reference clock signalling in which case the connection may be
established as rate synchronised but not time synchronised.
6.2. Usage Outside of Offer/Answer 6.2. Usage Outside of Offer/Answer
SDP can be employed outside of the Offer/Answer context, for instance SDP can be employed outside of the Offer/Answer context, for instance
for multimedia sessions that are announced through the Session for multimedia sessions that are announced through the Session
Announcement Protocol (SAP) [19], or streamed through the Real Time Announcement Protocol (SAP) [RFC2974], or streamed through the Real
Streaming Protocol (RTSP) [20]. The signaling model is simpler, as Time Streaming Protocol (RTSP) [RFC2326]. The signaling model is
the sender does not negotiate parameters, but the functionality simpler, as the sender does not negotiate parameters, but the
expected from specifying medial clock and reference clock attributes functionality expected from specifying media clock and reference
is the same as in Offer/Answer. clock attributes is the same as in Offer/Answer.
7. Security Considerations 7. Security Considerations
Entities receiving and acting upon an SDP message SHOULD be aware Entities receiving and acting upon an SDP message SHOULD be aware
that a session description cannot be trusted unless it has been that a session description cannot be trusted unless it has been
obtained by an authenticated transport protocol from a known and obtained by an authenticated transport protocol from a known and
trusted source. Many different transport protocols may be used to trusted source. Many different transport protocols may be used to
distribute session description, and the nature of the authentication distribute session description, and the nature of the authentication
will differ from transport to transport. For some transports, will differ from transport to transport. For some transports,
security features are often not deployed. In case a session security features are often not deployed. In case a session
description has not been obtained in a trusted manner, the endpoint description has not been obtained in a trusted manner, the endpoint
SHOULD exercise care because, among other attacks, the media sessions SHOULD exercise care because, among other attacks, the media sessions
received may not be the intended ones, the destination where media is received may not be the intended ones, the destination where media is
sent to may not be the expected one, any of the parameters of the sent to may not be the expected one, any of the parameters of the
session may be incorrect. session may be incorrect.
Incorrect reference or media clock parameters may cause devices or Incorrect reference or media clock parameters may cause devices or
streams to synchronize to unintended clock sources. Normally this streams to synchronize to unintended clock sources. Normally this
simply results in failure to make a media connection or failure to simply results in failure to make a media connection or failure to
synchronize once connected. Enough devices fraudulently assigned to synchronize once connected. Enough devices fraudulently assigned to
a specific clock source (e.g. a particular IEEE 1588 grandmaster) a specific clock source (e.g. a particular IEEE 1588 grandmaster)
may, however, constitute a successful a denial of service attack on may, however, constitute a successful a denial of service attack on
that source. Devices MAY wish to validate the integrity of the clock that source. Devices MAY wish to validate the integrity of the clock
description through some means before connecting to unfamiliar clock description through some means before connecting to unfamiliar clock
sources. sources.
8. IANA Considerations 8. IANA Considerations
8.1. Reference clock registry
The SDP attribute "ts-refclk" defined by this document is registered The SDP attribute "ts-refclk" defined by this document is registered
with the IANA registry of SDP Parameters as follows: with the IANA registry of SDP Parameters as follows:
SDP Attribute ("att-field"): SDP Attribute ("att-field"):
Attribute name: ts-refclk Attribute name: ts-refclk
Long form: Timestamp reference clock source Long form: Timestamp reference clock source
Type of name: att-field Type of name: att-field
Type of attribute: session, media and source level Type of attribute: Session, media and source level
Subject to charset: no Subject to charset: No
Purpose: See section 4 of this document Purpose: See section 4 of this document
Reference: This document Reference: This document
Values: see this document and registrations below Values: See this document and registrations below
The attribute has an extensible parameter field and therefore a The attribute has an extensible parameter field and therefore a
registry for these parameters is required. This document creates an registry for these parameters is required. This document creates an
IANA registry called the Timestamp Reference Clock Source Parameters IANA registry called the Timestamp Reference Clock Source Parameters
Registry. It contains the six parameters defined in Figure 1: "ntp", Registry. It contains the six parameters defined in Figure 1: "ntp",
"ptp", "gps", "gal", "local", "private". "ptp", "gps", "gal", "local", "private". New entries MAY be added to
this registry according to well-known Specification Required policy
[RFC5226].
8.2. Media clock registry
The SDP attribute "mediaclk" defined by this document is registered The SDP attribute "mediaclk" defined by this document is registered
with the IANA registry of SDP Parameters as follows: with the IANA registry of SDP Parameters as follows:
SDP Attribute ("att-field"): SDP Attribute ("att-field"):
Attribute name: mediaclk Attribute name: mediaclk
Long form: Media clock source Long form: Media clock source
Type of name: att-field Type of name: att-field
Type of attribute: session and media level Type of attribute: session and media level
Subject to charset: no Subject to charset: No
Purpose: See section 5 of this document Purpose: See section 5 of this document
Reference: This document Reference: This document
Values: see this document and registrations below Values: See this document and registrations below
The attribute has an extensible parameter field and therefore a The attribute has an extensible parameter field and therefore a
registry for these parameters is required. This document creates an registry for these parameters is required. This document creates an
IANA registry called the Media Clock Source Parameters Registry. It IANA registry called the Media Clock Source Parameters Registry. It
contains the three parameters defined in Figure 5: "sender", contains the three parameters defined in Figure 5: "sender",
"direct", "master", "slave" and "IEEE1722". "direct", "master", "slave" and "IEEE1722". New entries MAY be added
to this registry according to well-known Specification Required
policy [RFC5226].
9. References 9. References
9.1. Normative References 9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
Description Protocol", RFC 4566, July 2006. A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[3] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
Attributes in the Session Description Protocol (SDP)", with Session Description Protocol (SDP)", RFC 3264, June
RFC 5576, June 2009. 2002.
[4] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Description Protocol", RFC 4566, July 2006.
Session Initiation Protocol", RFC 3261, June 2002.
[5] Crocker, D. and P. Overell, "Augmented BNF for Syntax [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
Specifications: ABNF", STD 68, RFC 5234, January 2008. IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[6] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Flows", RFC 6051, November 2010. Specifications: ABNF", STD 68, RFC 5234, January 2008.
[7] Begen, A., Perkins, C., and D. Wing, "Guidelines for Choosing [RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
RTP Control Protocol (RTCP) Canonical Names (CNAMEs)", Media Attributes in the Session Description Protocol
RFC 6222, April 2011. (SDP)", RFC 5576, June 2009.
[8] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with [RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Session Description Protocol (SDP)", RFC 3264, June 2002. Capability Negotiation", RFC 5939, September 2010.
9.2. Informative References [RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.
[9] Society of Motion Picture & Television Engineers, "Television [RFC6222] Begen, A., Perkins, C., and D. Wing, "Guidelines for
and Audio - Synchronization of 59.94- or 50-Hz Related Video Choosing RTP Control Protocol (RTCP) Canonical Names
and Audio Systems in Analog and Digital Areas - Reference (CNAMEs)", RFC 6222, April 2011.
Signals", <http://standards.smpte.org/>.
[10] Audio Engineering Society, "AES11-2009: AES recommended 9.2. Informative References
practice for digital audio engineering - Synchronization of
digital audio equipment in studio operations",
<http://www.aes.org/standards/>.
[11] Brandenburg, R., Stokking, H., Deventer, O., Boronat, F., [AES11-2009]
Montagud, M., and K. Gross, "Inter-destination Media Audio Engineering Society, "AES11-2009: AES recommended
Synchronization using the RTP Control Protocol (RTCP)", practice for digital audio engineering - Synchronization
draft-ietf-avtcore-idms-07 (work in progress), October 2012. of digital audio equipment in studio operations ", ,
<http://www.aes.org/standards/>.
[12] Global Positioning Systems Directorate, "Navstar GPS Space [I-D.ietf-avtcore-idms]
Segment/Navigation User Segment Interfaces", September 2011. Brandenburg, R., Stokking, H., Deventer, O., Boronat, F.,
Montagud, M., and K. Gross, "Inter-destination Media
Synchronization using the RTP Control Protocol (RTCP)",
draft-ietf-avtcore-idms-09 (work in progress), March 2013.
[13] Institute of Electrical and Electronics Engineers, "1588-2008 - [IEEE1588-2002]
IEEE Standard for a Precision Clock Synchronization Protocol Institute of Electrical and Electronics Engineers,
for Networked Measurement and Control Systems", IEEE Std 1588- "1588-2002 - IEEE Standard for a Precision Clock
2008, 2008, Synchronization Protocol for Networked Measurement and
<http://standards.ieee.org/findstds/standard/1588-2008.html>. Control Systems", IEEE Std 1588-2002, 2002, <http://
standards.ieee.org/findstds/standard/1588-2002.html>.
[14] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time [IEEE1588-2008]
Protocol Version 4: Protocol and Algorithms Specification", Institute of Electrical and Electronics Engineers,
RFC 5905, June 2010. "1588-2008 - IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", IEEE Std 1588-2008, 2008, <http://
standards.ieee.org/findstds/standard/1588-2008.html>.
[15] Institute of Electrical and Electronics Engineers, "1588-2002 - [IEEE1722]
IEEE Standard for a Precision Clock Synchronization Protocol Institute of Electrical and Electronics Engineers, "IEEE
for Networked Measurement and Control Systems", IEEE Std 1588- Standard for Layer 2 Transport Protocol for Time Sensitive
2002, 2002, Applications in a Bridged Local Area Network", , <http://
<http://standards.ieee.org/findstds/standard/1588-2002.html>. standards.ieee.org/findstds/standard/1722-2011.html>.
[16] Institute of Electrical and Electronics Engineers, "Timing and [IEEE802.1AS-2011]
Synchronization for Time-Sensitive Applications in Bridged Institute of Electrical and Electronics Engineers, "Timing
Local Area Networks", and Synchronization for Time-Sensitive Applications in
<http://standards.ieee.org/findstds/standard/ Bridged Local Area Networks", , <http://standards.ieee.org
802.1AS-2011.html>. /findstds/standard/802.1AS-2011.html>.
[17] Institute of Electrical and Electronics Engineers, "Audio Video [IEEE802.1BA-2011]
Bridging (AVB) Systems", Institute of Electrical and Electronics Engineers, "Audio
<http://standards.ieee.org/findstds/standard/ Video Bridging (AVB) Systems", , <http://
802.1BA-2011.html>. standards.ieee.org/findstds/standard/802.1BA-2011.html>.
[18] Institute of Electrical and Electronics Engineers, "IEEE [IS-GPS-200F]
Standard for Layer 2 Transport Protocol for Time Sensitive Global Positioning Systems Directorate, "Navstar GPS Space
Applications in a Bridged Local Area Network", Segment/Navigation User Segment Interfaces", September
<http://standards.ieee.org/findstds/standard/1722-2011.html>. 2011.
[19] Handley, M., Perkins, C., and E. Whelan, "Session Announcement [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Protocol", RFC 2974, October 2000. Streaming Protocol (RTSP)", RFC 2326, April 1998.
[20] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming [RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Protocol (RTSP)", RFC 2326, April 1998. Announcement Protocol", RFC 2974, October 2000.
URIs [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
[21] <http://www.ieee802.org/1/files/public/docs2007/ [SMPTE-318-1999]
as-dolsen-time-accuracy-0407.pdf> Society of Motion Picture & Television Engineers,
"Television and Audio - Synchronization of 59.94- or 50-Hz
Related Video and Audio Systems in Analog and Digital
Areas - Reference Signals", ,
<http://standards.smpte.org/>.
Authors' Addresses Authors' Addresses
Aidan Williams Aidan Williams
Audinate Audinate
Level 1, 458 Wattle St Level 1, 458 Wattle St
Ultimo, NSW 2007 Ultimo, NSW 2007
Australia Australia
Phone: +61 2 8090 1000 Phone: +61 2 8090 1000
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