wdiff rfc7273v2.txt rfc7273.txt

Internet Engineering Task Force (IETF) A. Williams
Request for Comments: 7273 Audinate
Category: Standards Track K. Gross
ISSN: 2070-1721 AVA Networks
R. van Brandenburg
H. Stokking
TNO
June 2014

RTP Clock Source Signalling

Abstract

NTP format timestamps are used by several RTP protocols for
synchronisation and statistical measurements. This memo specifies
Session Description Protocol (SDP) signalling that identifies
timestamp reference clock sources and SDP signalling that identifies
the media clock sources in a multimedia session.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7273.

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
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Applications . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Timestamp Reference Clock Source Signalling . . . . . . . . . 5
4.1. Clock Synchronisation . . . . . . . . . . . . . . . . . . 6 5
4.2. Identifying NTP Reference Clocks . . . . . . . . . . . . 6
4.3. Identifying PTP Reference Clocks . . . . . . . . . . . . 6
4.4. Identifying Global Reference Clocks . . . . . . . . . . . 8
4.5. Private Reference Clocks . . . . . . . . . . . . . . . . 8
4.6. Local Reference Clocks . . . . . . . . . . . . . . . . . 8
4.7. Traceable Reference Clocks . . . . . . . . . . . . . . . 8
4.8. SDP Signalling of Timestamp Reference Clock Source . . . 9
4.8.1. Examples . . . . . . . . . . . . . . . . . . . . . . 11
5. Media Clock Source Signalling . . . . . . . . . . . . . . . . 12
5.1. Asynchronously Generated Media Clock . . . . . . . . . . 12
5.2. Direct-Referenced Media Clock . . . . . . . . . . . . . . 13 12
5.3. Stream-Referenced Media Clock . . . . . . . . . . . . . . 14
5.4. SDP Signalling of Media Clock Source . . . . . . . . . . 15
5.5. Examples . . . . . . . . . . . . . . . . . . . . . . . . 17
6. Signalling Considerations . . . . . . . . . . . . . . . . . . 19
6.1. Usage in Offer/Answer . . . . . . . . . . . . . . . . . . 19
6.1.1. Indicating Support for Clock Source Signalling . . . 20
6.1.2. Timestamp Reference Clock . . . . . . . . . . . . . . 20
6.1.3. Media Clock . . . . . . . . . . . . . . . . . . . . . 20
6.2. Usage Outside of Offer/Answer . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8.1. Reference Clock SDP Parameter . . . . . . . . . . . . . . 22
8.2. Media Clock SDP Parameter . . . . . . . . . . . . . . . . 23
8.3. Timestamp Reference Clock Source Parameters Registry . . 23
8.4. Media Clock Source Parameters Registry . . . . . . . . . 24
8.5. Source-Level Attributes . . . . . . . . . . . . . . . . . 25
8.5.1. Source-Level Timestamp Reference Clock Attribute . . 25
8.5.2. Source-Level Media Clock Attribute . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . 27

1. Introduction

RTP protocols use NTP format timestamps to facilitate multimedia
session synchronisation and to provide estimates of round-trip time
(RTT) and other statistical parameters.

Information about media clock timing exchanged in NTP format
timestamps may come from a clock that is synchronised to a global
time reference, but this cannot be assumed, nor is there a
standardised mechanism available to indicate that timestamps are
derived from a common reference clock. Therefore, RTP
implementations typically assume that NTP timestamps are taken using
unsynchronised clocks and must compensate for absolute time
differences and rate differences. Without a shared reference clock,
RTP can time align flows from the same source at a given receiver
using relative timing; however, tight synchronisation between two or
more different receivers (possibly with different network paths) or
between two or more senders is not possible.

High performance AV systems often use a reference media clock
distributed to all devices in the system. The reference media clock
is often
may be distinct from the reference clock used to provide timestamps.
A reference media clock may be provided along with an audio or video
signal interface, or via a dedicated clock signal (e.g., genlock
[SMPTE-318M-1999] or audio word clock [AES11-2009]). If sending and
receiving media clocks are known to be synchronised to a common
reference clock, performance can be improved by minimising buffering
and avoiding rate conversion.

This specification defines SDP signalling of timestamp reference
clock sources and media reference clock sources.

1.1. Requirements Language

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].

2. Applications

Timestamp reference clock source and media clock signalling benefit
applications requiring synchronised media capture or playout and low
latency operation.

Examples include, but are not limited to:

Social TV: "Inter-Destination Media Synchronization (IDMS) Using the
RTP Control Protocol (RTCP)" [RFC7272] defines social TV as the
combination of media content consumption by two or more users at
different devices and locations and real-time communication
between those users. An example of social TV is where two or more
users are watching the same television broadcast at different
devices and/or locations while communicating with each other using
text, audio, and/or video. A skew in the media playout of the two
or more users can have adverse effects on their experience. A
well-known use case here is one friend experiencing a goal in a
football match well before or after other friends.

Video Walls: A video wall consists of multiple computer monitors,
video projectors, or television sets tiled together contiguously
or overlapped in order to form one large screen. Each of the
screens reproduces a portion of the larger picture. In some
implementations, each screen or projector may be individually
connected to the network and receive its portion of the overall
image from a network-connected video server or video scaler.
Screens are refreshed at 50 or 60 hertz or potentially faster. If
the refresh is not synchronised, the effect of multiple screens
acting as one is broken.

Networked Audio: Networked loudspeakers, amplifiers, and analogue
input/output (I/O) devices transmitting or receiving audio signals
via RTP can be connected to various parts of a building or campus
network. Such situations can, for example, be found in large
conference rooms, legislative chambers, classrooms (especially
those supporting distance learning), and other large-scale
environments such as stadiums. Since humans are more susceptible sensitive to
differences in audio delay, this use case needs even more accuracy
than the video wall use case. Depending on the exact application,
the need for accuracy can then be in the range of microseconds
[Olsen].

Sensor Arrays: Sensor arrays contain many synchronised measurement
elements producing signals that are then combined to form an
overall measurement. Accurate capture of the phase relationships
between the various signals arriving at each element of the array
is critically important for proper operation. Examples include
towed or fixed sonar arrays, seismic arrays, and phased arrays
used in radar applications, for instance.

3. Definitions

The following definitions are used in this draft:

media level: Media-level information applies to a single SDP media
stream. In an SDP description, media-level information appears
after each "m"-line.

multimedia session: A set of multimedia senders and receivers as
well as the data streams flowing from senders to receivers. SDP
[RFC4566] describes multimedia sessions.

RTP media stream: A single stream of RTP packets identified by an
RTP Synchronisation Source (SSRC).

RTP media sender: The device generating an associated RTP media stream.

SDP media stream: An RTP session potentially containing more than
one RTP source. SDP media descriptions beginning with an "m"-line
define the parameters of an SDP media stream.

session level: Session-level information applies to an entire
multimedia session. In an SDP description, session-level
information appears before the first "m"-line.

source level: Source-level information applies to a specific RTP
media stream. "Source-Specific Media Attributes in the Session
Description Protocol (SDP)" [RFC5576] defines how source-level
information is included into an SDP session description.

traceable time: A clock is considered to provide traceable time if
it can be proven to be synchronised to International Atomic Time
(TAI). Coordinated Universal Time (UTC) is a time standard
synchronised to TAI. UTC is, therefore, also considered traceable
time once leap seconds have been taken into account. GPS
[IS-GPS-200F] is commonly used to provide a TAI traceable time
reference. Some network time synchronisation protocols (e.g.,
Precision Time Protocol (PTP) [IEEE1588-2008] and NTP) can
explicitly indicate that the master clock is providing a traceable
time reference over the network.

4. Timestamp Reference Clock Source Signalling

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
synchronised to another clock (time source) by some means, or it may
be unsynchronised. A variety of methods are available to synchronise
local clocks to a reference time source, including network time
protocols (e.g., NTP [RFC5905] and PTP [IEEE1588-2008]) and radio
clocks (e.g., GPS [IS-GPS-200F]).

The following sections describe and define SDP signalling, indicating
whether and how the local timestamping clock in an RTP sender/ sender or
receiver is synchronised to a reference clock.

4.1. Clock Synchronisation

Two or more local clocks that are sufficiently synchronised will
produce timestamps for a given RTP event that can be used as if they
came from the same clock. Providing they are sufficiently
synchronised, timestamps Timestamps produced in one RTP sender or
receiver can be directly compared to a local clock in another RTP
sender or receiver.

The accuracy of synchronisation required is application dependent.
See "Applications" (Section 2) for a discussion of applications and
their corresponding requirements. To serve as a reference clock,
clocks must minimally be "syntonised" (exactly frequency matched) to
one another.

Sufficient synchronisation can typically be achieved by using a
network time protocol (e.g., NTP, 802.1AS, and IEEE 1588-2008) to
synchronise all devices to a single master clock.

Another approach is to use clocks providing a global time reference
(e.g., GPS, Galileo, and GLONASS). This concept may be used in
conjunction with network time protocols as some protocols (e.g., PTP
and NTP) allow master clocks to indicate explicitly that they are
providing traceable time.

4.2. Identifying NTP Reference Clocks

A single NTP server is identified by a hostname (or IP address) and
an optional port number. If the port number is not indicated, it is
assumed to be the standard NTP port (123).

Two or more NTP servers MAY be listed at the same level in the
session description to indicate that all of the listed servers
deliver the same reference time and may be used interchangeably. RTP
senders and receivers are assured proper synchronisation regardless
of which server they choose and, in support of fault tolerance, may
switch servers while streaming.

4.3. Identifying PTP Reference Clocks

The Precision Time Protocol (PTP) as standardised in IEEE 1588 PTP family of clock synchronisation protocols
provides a shared reference clock in a network -- typically a LAN. network. IEEE 1588 provides
sub-microsecond synchronisation between devices on a LAN and
typically locks within seconds at startup. With support from
Ethernet switches, IEEE 1588 protocols can achieve nanosecond timing
accuracy in LANs. Network interface chips and cards supporting
hardware timestamping of timing critical timing-critical protocol messages are also
available.

Three flavours of IEEE 1588 are in use today:

o IEEE 1588-2002 [IEEE1588-2002]: the original "Standard for a
Precision Clock Synchronization Protocol for Networked Measurement
and Control Systems". This is also known as IEEE1588v1 or PTPv1.

o IEEE 1588-2008 [IEEE1588-2008]: the second version of the
"Standard for a Precision Clock Synchronization Protocol for
Networked Measurement and Control Systems". This is a revised
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 profile of IEEE 1588-2008 that is Layer 2 only and is 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 64-bit Extended Unique
Identifier (EUI-64) called a "ClockIdentity". A slave clock using
one of the IEEE 1588 family of network time protocols acquires the
ClockIdentity/EUI-64
ClockIdentity of the grandmaster clock that is the ultimate source of
timing information for the network. A boundary clock, which is
itself slaved to another boundary clock, or the grandmaster passes
the grandmaster ClockIdentity through to its slaves.

Several instances of the IEEE 1588 protocol may operate independently
on a single network, forming distinct PTP domains, each of which may
have a different grandmaster clock. As the IEEE 1588 standards have
developed,
evolved, the definition of PTP domains has changed. IEEE 1588-2002
identifies protocol subdomains by a textual name, but IEEE 1588-2008
identifies protocol domains using a numeric domain number. 802.1AS is
a Layer 2 profile of IEEE 1588-2008 supporting a single numeric clock
domain (0).

When PTP domains are signalled via SDP, senders and receivers SHOULD
check that both grandmaster ClockIdentity and the PTP domain match
when 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 synchronisation regardless of which
synchronisation source they choose and, in support of fault
tolerance, may switch the reference clock source while streaming.

The PTP protocols employ a distributed election protocol called the
"Best Master Clock Algorithm" (BMCA) to determine the active clock
master. The clock master choices available to BMCA can be restricted
or biased by configuration parameters to influence the election
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
reference clock sources when the clock master changes.

4.4. Identifying Global Reference Clocks

Global reference clocks provide a source of traceable time, typically
via a hardware radio receiver interface. Examples include GPS,
Galileo, and GLONASS. Apart from the name of the reference clock
system, no further identification is required.

4.5. Private Reference Clocks

In other systems, all RTP senders and receivers may use a timestamp
reference clock that is not provided by one of the methods listed
above. Examples may include the reference time information provided
by digital television or cellular services. These sources are
identified as "private" reference clocks. All RTP senders and
receivers in a session using a private reference clock are assumed to
have a mechanism outside this specification for determining whether
their timestamp reference clocks are equivalent.

4.6. Local Reference Clocks

[RFC3550] allows senders and receivers to either use a local
wallclock reference for their NTP timestamps or, by setting the
timestamp field to 0, supply no timestamps at all. Both are common
practice in embedded RTP implementations. These clocks are
identified as "local" and can only can, at best, be assumed to be equivalent
to clocks originating from the same device.

4.7. Traceable Reference Clocks

A timestamp reference clock source may be labelled "traceable" if it
is known to be delivering traceable time, providing provided adjustments are
made for differing epochs, timezones, and leap seconds. Timestamps
taken using clocks synchronised to a traceable time source can be
directly compared even if the clocks are synchronised to different
sources or via different mechanisms.

Marking a clock as traceable allows additional information (e.g., IP
addresses, PTP master identifiers, and the like) to be omitted from
the SDP since any traceable clock available at the answerer is
considered to be an appropriate timestamp reference clock. For
example, an offerer could specify ts-refclk:ntp=/traceable/ and the
answerer could use GPS as a reference clock since GPS is a source of
traceable time.

4.8. SDP Signalling of Timestamp Reference Clock Source

Specification of the timestamp reference clock source may be at any
or all levels (session, media, or source) of an SDP description (see
level definitions (Section 3) in Section 3 earlier in this document for more
information).

Timestamp reference clock source signalling included at the session
level provides default parameters for all RTP sessions and sources in
the session description. More specific signalling included at the
media level overrides default session-level signalling. More specific
signalling included at the source level overrides default media-level
signalling.

If timestamp reference clock source signalling is included anywhere
in an SDP description, it must be properly defined for all levels in
the description. This may simply be achieved by providing default
signalling at the session level.

Timestamp reference clock parameters may be repeated at a given level
(i.e., for a session or source) to provide information about
additional servers or clock sources. If the attribute is repeated at
a given level, all clocks described at that level are assumed to be
equivalent. Traceable time sources MUST NOT be mixed with non-
traceable time sources at any given level.

Note that clock source parameters may change from time to time, for
example, as a result of a PTP clock master grandmaster election. SIP [RFC3261]
supports the re-signalling of updated SDP information; however, other
protocols may require additional notification mechanisms.

General forms of usage:

session level: a=ts-refclk:<clksrc>

media level: a=ts-refclk:<clksrc>

source level: a=ssrc:<ssrc-id> ts-refclk:<clksrc>

ABNF [RFC5234] grammar for the timestamp reference clock attribute:

; external references:
POS-DIGIT = <See RFC 4566>
token = <See RFC 4566>
byte-string = <See RFC 4566>
DIGIT = <See RFC 5234>
HEXDIG = <See RFC 5234>
CRLF = <See RFC 5234>
hostport = <See RFC 3261, with revisions from RFC 5954>

timestamp-refclk = "ts-refclk:" clksrc CRLF

clksrc = ntp / ptp / gps / gal / glonass / local / private /
clksrc-ext

clksrc-ext = clksrc-param-name clksrc-param-value
clksrc-param-name = token
clksrc-param-value = ["=" byte-string ]

ntp = "ntp=" ntp-server-addr
ntp-server-addr = hostport / "/traceable/"

ptp = "ptp=" ptp-version ":" ptp-server
ptp-version = "IEEE1588-2002"
/ "IEEE1588-2008"
/ "IEEE802.1AS-2011"
/ ptp-version-ext
ptp-version-ext = token

ptp-server = ptp-gmid [":" ptp-domain]
/ "traceable"
ptp-gmid = EUI64
ptp-domain = ptp-domain-name / ptp-domain-nmbr

; PTP domain allowed characters: 0x21-0x7E (IEEE 1588-2002)
ptp-domain-name = "domain-name=" 1*16ptp-domain-char
ptp-domain-char = %x21-7E

; PTP domain allowed number range: 0-127 (IEEE 1588-2008)
ptp-domain-nmbr = "domain-nmbr=" ptp-domain-dgts
ptp-domain-dgts = ptp-domain-n1 / ptp-domain-n2 / ptp-domain-n3
ptp-domain-n1 = DIGIT ; 0-9
ptp-domain-n2 = POS-DIGIT DIGIT ; 10-99
ptp-domain-n3 = ("10"/"11") DIGIT ; 100-119
/ "12" %x30-37 ; 120-127

gps = "gps"
gal = "gal"
glonass = "glonass"
local = "local"
private = "private" [ ":traceable" ]

EUI64 = 7(2HEXDIG "-") 2HEXDIG

Figure 1: Timestamp Reference Clock Source Signalling

4.8.1. Examples

Figure 2 shows an example SDP description with a timestamp reference
clock source defined at the session level.

v=0
o=jdoe 2890844526 2890842807 IN IP4 192.0.2.1
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 233.252.0.1/64
t=2873397496 2873404696
a=recvonly
a=ts-refclk:ntp=/traceable/
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000

Figure 2: Timestamp Reference Clock Definition at the Session Level

Figure 3 shows an example SDP description with timestamp reference
clock definitions at the media level overriding the session-level
defaults.

v=0
o=jdoe 2890844526 2890842807 IN IP4 192.0.2.1
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 233.252.0.1/64
t=2873397496 2873404696
a=recvonly
a=ts-refclk:local
m=audio 49170 RTP/AVP 0
a=ts-refclk:ntp=203.0.113.10
a=ts-refclk:ntp=198.51.100.22
m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000
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 4 shows an example SDP description with a timestamp reference
clock definition at the source level overriding the session-level
default.

v=0
o=jdoe 2890844526 2890842807 IN IP4 192.0.2.1
s=SDP Seminar
i=A Seminar on the session description protocol
u=http://www.example.com/seminars/sdp.pdf
e=j.doe@example.com (Jane Doe)
c=IN IP4 233.252.0.1/64
t=2873397496 2873404696
a=recvonly
a=ts-refclk:local
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 99
a=rtpmap:99 h263-1998/90000
a=ssrc:12345 ts-refclk:ptp=IEEE802.1AS-2011:39-A7-94-FF-FE-07-CB-D0

Figure 4: Timestamp Reference Clock Signalling at the Source Level

5. Media Clock Source Signalling

The media clock source for a stream determines the timebase used to
advance the RTP timestamps included in RTP packets. The media clock
may be asynchronously generated by the sender, it may be generated in
fixed relationship to the reference clock, or it may be generated
with respect to another stream on the network (which is presumably
being received by the sender).

5.1. Asynchronously Generated Media Clock

In the simplest sender implementation, the sender generates media by
sampling audio or video according to a free-running local clock. The
RTP timestamps in media packets are advanced according to this media
clock, and packet transmission is typically timed to regular
intervals on this timeline. The sender may or may not include an NTP
timestamp in sender reports to allow mapping of this asynchronous
media clock to a reference clock.

The asynchronously generated media clock is the assumed mode of
operation when there is no signalling of the media clock source.
Alternatively, an asynchronous media clock may be explicitly
signalled.

a=mediaclk:sender

5.2. Direct-Referenced Media Clock

A media clock may be directly derived from a reference clock. For
this case, it is required that a reference clock be specified with an
a=ts-refclk attribute (Section 4.8).

The signalling optionally indicates a media clock offset value. The
offset indicates the RTP timestamp value at the epoch (time of
origin) of the reference clock. To use the offset, implementations
need to compute RTP timestamps from reference clocks. To simplify
these calculations, streams utilizing offset signalling SHOULD use a
TAI timestamp reference clock to avoid complications introduced by
leap seconds. See [RFC7164] for further discussion of leap-second
issues in timestamp reference clocks.

To compute the RTP timestamp against an IEEE 1588 (TAI-based)
reference, the time elapsed between the 00:00:00 1 January 1970 IEEE
1588 epoch and the current time must be computed. Between the epoch
and 1 January 2013, there were 15,706 days (including extra days
during leap years). Since there are no leap seconds in a TAI
reference, there are exactly 86,400 seconds during each of these days
or a total of 1,356,998,400 seconds from the epoch to 00:00:00 1
January 2013. A 90 kHz RTP clock for a video stream would have
advanced 122,129,856,000,000 units over this period. With a
signalled offset of 0, the RTP clock value modulo the 32-bit unsigned
RTP timestamp representation in the RTP header would have been
2,460,938,240 at 00:00:00 1 January 2013. If an offset of 23,465 had
been signalled, the clock value would have been 2,460,961,705.

In order to use an NTP reference, the actual time elapsed between the
00:00:00 1 January 1900 NTP epoch to the current time must be
computed. 2,208,988,800 seconds elapsed between the NTP epoch and
00:00:00 1 January 1970 [RFC0868]. Between the beginning of 1970 and
2013, there were 15,706 days elapsed (including extra days during
leap years) and 25 leap seconds inserted. There is, therefore, a
total of 3,565,987,225 seconds from the NTP epoch to 00:00:00 1
January 2013. A 90 kHz RTP clock for a video stream would have
advanced 320,938,850,250,000 units over this period. With a
signalled offset of 0, the RTP clock value modulo the 32-bit unsigned
representation would have been 1,714,023,696 at 00:00:00 1 January
2013.

If no offset is signalled, the offset can be inferred at the receiver
by examining RTCP sender reports that contain NTP and RTP timestamps,
which combined define a mapping. The NTP/RTP timestamp mapping
provided by RTCP sender reports (SRs) takes precedence over that
signalled through SDP; however, the media clock rate implied by the
SRs MUST be consistent with the rate signalled.

A rate modifier may be specified. The modifier is expressed as the
ratio of two integers and modifies the rate specified or implied by
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
example, without a rate specification, the RTP clock for an 8 kHz
G.711 audio stream will advance exactly 8000 units for each second
advance in the reference clock from which it is derived.

The rate modifier is primarily useful for accommodating certain
"oddball" audio sample rates associated with NTSC video (see
Figure 7). Modified rates are not advised for video streams that
generally use a 90 kHz RTP clock regardless of frame rate or sample
rate used for embedded audio.

a=mediaclk:direct[=<offset>] [rate=<rate numerator>/<rate
denominator>]

5.3. Stream-Referenced Media Clock

A common synchronisation architecture for audio/visual systems
involves distributing a reference media clock from a master device to
a number of slave devices, typically by means of a cable. Examples
include audio word clock distribution and video black burst
distribution. In this case, the media clock is locally generated,
often by a crystal oscillator, and is not locked to a timestamp
reference clock.

To support this architecture across a network, a master clock
identifier is associated with an RTP media stream carrying media
clock timing information from a master device. The master clock
identifier represents a media clock source in the master device.
Slave devices in turn associate the master media clock identifier
with streams they transmit, signalling the synchronisation
relationship between the master and the transmitter's media clock.

Slave devices recover media clock timing from the clock master
stream, using it to synchronise the slave slave's media clock with the
master. Timestamps If a common reference clock is available, NTP timestamps in
the master clock RTP media stream are taken using the timestamp reference clock shared by the master and slave
devices.
reference clock. The NTP timestamps communicate information about
media clock timing (rate and phase) from the master to the slave
devices.
Timestamps NTP timestamps are communicated in the usual RTP fashion
via RTCP SRs, or via the [RFC6051] header extension. The stream If no shared
reference clock is available or signalled, a slave can synchronise to
the master's media format may
indicate other clock information, such using RTP timestamps alone as the nominal rate. described in
Section 5.1 of [RFC3550].

Note that the slaving of a device media clock to a master device does
not affect the usual RTP lip-sync/time alignment algorithms. Time-
aligned inter-media synchronisation. Time-aligned playout of
two or more RTP sources still relies upon NTP timestamps supplied via
RTCP SRs or by the RFC 6051 timestamp header extension.

In a given system, master clock identifiers must uniquely identify a
single media clock source. Such identifiers MAY be manually
configured; however, identifiers SHOULD be generated according to the
"short-term persistent RTCP CNAME" algorithm as described in
[RFC7022]. Master clock identifiers not already in base64 format
MUST be encoded as base64 strings when used in SDP. Although the
RTCP Canonical Name (CNAME)
CNAME algorithm is used to generate the master clock identifier, it
is used to tag RTP sources in SDP descriptions and does not appear in
RTCP as a CNAME.

A reference stream can be an RTP stream or an Audio Video Bridging
(AVB) stream based on the [IEEE1722] standard.

An RTP clock master stream SHOULD be identified at the source level
by an SSRC [RFC5576] and master clock identifier. An RTP stream that
provides media clock timing directly from a reference media clock
(e.g., internal crystal, audio word clock, or video black burst
signal) SHOULD tag the stream as a master clock source using the
"src:" prefix. If master clock identifiers are declared at the media
or session level, all RTP sources at or below the level of
declaration MUST provide equivalent timing to a slave receiver.

a=ssrc:<ssrc> mediaclk:id=src:<media-clktag> sender

a=mediaclk:id=src:<media-clktag> sender

A transmitted RTP stream slaved to the media clock master is
signalled by including a master clock identifier:

a=mediaclk:id=<media-clktag> sender

An RTP media sender indicates that it is slaved to an IEEE 1722 clock
master via a stream identifier (an EUI-64):

a=mediaclk:IEEE1722=<StreamID>

An RTP media sender may gateway IEEE 1722 media clock timing to RTP:

a=mediaclk:id=src:<media-clktag> IEEE1722=<StreamID>

5.4. SDP Signalling of Media Clock Source

Specification of the media clock source may be at any or all levels
(session, media, or source) of an SDP description (see level
definitions (Section 3) earlier in this document for more
information).

Media clock source signalling included at session level provides
default parameters for all RTP sessions and sources in the session
description. More specific signalling included at the media level
overrides default session-level signalling. Further, source-level signalling
overrides media clock source signalling at the enclosing media level
and session level.

Media clock source signalling may be present or absent on a per-
stream basis. In the absence of media clock source signals,
receivers assume an asynchronous media clock generated by the sender.

Media clock source parameters may be repeated at a given level (i.e.,
for a session or source) to provide information about additional
clock sources. If the attribute is repeated at a given level, all
clocks described at that level are comparable clock sources and may
be used interchangeably.

General forms of usage:

session level: a=mediaclk:<mediaclock>

media level: a=mediaclk:<mediaclock>

source level: a=ssrc:<ssrc-id> mediaclk:<mediaclock>
ABNF [RFC5234] grammar for the media clock reference attribute:

; external references:
integer = <See RFC 4566>
token = <See RFC 4566>
byte-string = <See RFC 4566>
base64 = <See RFC 4566>
SP = <See RFC 5234>
DIGIT = <See RFC 5234>
HEXDIG = <See RFC 5234>

media-clksrc = "mediaclk:" [media-clkid SP] mediaclock

media-clkid = "id=" [ "src:" ] media-clktag
media-clktag = base64

mediaclock = sender / direct / ieee1722-streamid / mediaclock-ext

mediaclock-ext = mediaclock-param-name mediaclock-param-value
mediaclock-param-name = token
mediaclock-param-value = [ "=" byte-string ]

sender = "sender"
direct = "direct" [ "=" 1*DIGIT ] [SP rate]
rate = "rate=" integer "/" integer

ieee1722-streamid = "IEEE1722=" avb-stream-id
avb-stream-id = EUI64
EUI64 = 7(2HEXDIG "-") 2HEXDIG

Figure 5: Media Clock Source Signalling

5.5. Examples

Figure 6 shows an example SDP description -- 8 channels of 24-bit, 48
kHz audio transmitted as a multicast stream. Media clock is derived
directly from an IEEE 1588-2008 reference.

v=0
o=- 1311738121 1311738121 IN IP4 192.0.2.1
c=IN IP4 233.252.0.1/64
s=
t=0 0
m=audio 5004 RTP/AVP 96
a=rtpmap:96 L24/48000/8
a=sendonly
a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
a=mediaclk:direct=963214424

Figure 6: Media Clock Directly Referenced to IEEE 1588-2008

Figure 7 shows an example SDP description -- 2 channels of 24-bit,
44056 kHz NTSC "pull-down" media clock derived directly from an IEEE
1588-2008 reference clock.

v=0
o=- 1311738121 1311738121 IN IP4 192.0.2.1
c=IN IP4 233.252.0.1/64
s=
t=0 0
m=audio 5004 RTP/AVP 96
a=rtpmap:96 L24/44100/2
a=sendonly
a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
a=mediaclk:direct=963214424 rate=1000/1001

Figure 7: "Oddball" Sample Rate Directly Referenced to IEEE 1588-2008

Figure 8 shows the same 48 kHz audio transmission from Figure 6 with
media clock derived from another RTP stream.

v=0
o=- 1311738121 1311738121 IN IP4 192.0.2.1
c=IN IP4 233.252.0.1/64
s=
t=0 0
m=audio 5004 RTP/AVP 96
a=rtpmap:96 L24/48000/2
a=sendonly
a=ts-refclk:ptp=IEEE1588-2008:39-A7-94-FF-FE-07-CB-D0:0
a=mediaclk:id=MDA6NjA6MmI6MjA6MTI6MWY= sender

Figure 8: RTP Stream with Media Clock Slaved to a Master

Figure 9 shows the same 48 kHz audio transmission from Figure 6 with
media clock derived from an IEEE 1722 AVB stream.

Figure 9: RTP Stream with Media Clock Slaved to an IEEE 1722 Master
Device

6. Signalling Considerations

Signalling of timestamp reference clock source (Section 4.8) and
media clock source (Section 5.4) is defined to be used either by
applications that implement the SDP Offer/Answer model [RFC3264] or
by applications that use SDP to describe media and transport
configurations.

A description SHOULD include both reference clock signalling and
media clock signalling. If no reference clock is available, this
SHOULD be signalled as a local reference (Section 4.6).

When no media clock signalling is present, an asynchronous media
clock (Section 5.1) MUST be assumed. When no reference clock
signalling is present, a local reference clock (Section 4.6) MUST be
assumed.

If a reference clock is not signalled or a local reference is
specified, the corresponding media clock may be established as rate
synchronised with no assurance of time synchronisation.

When the description signals a direct-referenced media clock
(Section 5.2), reference clock signalling is REQUIRED. Asynchronous
and stream-referenced media clocks (Section 5.3) MAY be specified
with or without a reference clock signalling.

6.1. Usage in Offer/Answer

During offer/answer, clock source signalling via SDP uses a
declarative model. Supported media and/or reference clocks are
specified in the offered SDP description. The answerer may accept or
reject the offer in an application-specific way depending on the
clocks that are available and the clocks that are offered. For
example, an answerer may choose to accept an offer that lacks a
common clock by falling back to a lower-performance mode of operation
(e.g., by assuming reference or media clocks are local rather than
shared). Conversely, the answerer may choose to reject the offer
when the offered clock specifications indicate that the available
reference and/or media clocks are incompatible.

While negotiation of reference clock and media clock attributes is
not defined in this document, negotiation MAY be accomplished using
the capabilities negotiation procedures defined in [RFC5939].

6.1.1. Indicating Support for Clock Source Signalling

An offerer or answerer indicates support for media clock signalling
by including a reference or media clock specification in the SDP
description. An offerer or answerer without specific reference or
media clocks to signal SHOULD indicate support for clock source
signalling by including a local reference clock (Section 4.6)
specification in the SDP description.

6.1.2. Timestamp Reference Clock

If one or more of the reference clocks specified in the offer are
usable by the answerer, the answerer SHOULD respond with an answer
containing the subset of reference clock specifications in the offer
that are usable by the answerer. If the answerer rejects the offer
because the available reference clocks are incompatible, the
rejection MUST contain at least one timestamp reference clock
specification usable by the answerer so that appropriate information
is available for debugging. diagnostics. If no external reference clock is
available to the answerer, a local reference clock (Section 4.6)
specification SHOULD be included in the rejection.

In both offers and answers, multiple reference clock specifications
indicate equivalent clocks from different sources that may be used
interchangeably. RTP senders and receivers are assured proper
synchronisation regardless of which of the specified sources is
chosen and, in support of fault tolerance, may switch clock sources
while streaming.

6.1.3. Media Clock

If the media clock mode specified in the offer is acceptable to the
answerer, the answerer SHOULD respond with an answer containing the
same media clock specification as the offer. If the answerer rejects
the offer because the available reference clocks are incompatible,
the rejection MUST contain a media clock specification supported by
the answerer so that appropriate information is available for
debugging.
diagnostics. If no shared media clocks are available to the
answerer, an asynchronous media clock (Section 5.1) specification
SHOULD be included in the rejection.

6.2. Usage Outside of Offer/Answer

SDP can be employed outside of the offer/answer context, for
instance, for multimedia sessions that are announced through the
Session Announcement Protocol (SAP) [RFC2974] or streamed through the
Real Time Streaming Protocol (RTSP) [RFC2326].

Devices using published descriptions to join sessions SHOULD assess
their synchronisation compatibility with the described session based
on the clock source signalling and SHOULD NOT attempt to join a
session with incompatible reference or media clocks.

7. Security Considerations

Entities receiving and acting upon an SDP message should note that a
session description cannot be trusted unless it has been obtained by
an authenticated transport protocol from a known and trusted source.
Many different transport protocols may be used to distribute a
session description, and the nature of the authentication will differ
from transport to transport. For some transports, security features
are often not deployed. In case a session description has not been
obtained in a trusted manner, the endpoint should exercise care
because, among other attacks, the media sessions received may not be
the intended ones, the destination where media is sent to may not be
the expected one, and any of the parameters of the session may be
incorrect.

Incorrect reference or media clock parameters may cause devices or
streams to synchronise to unintended clock sources. Normally, this
simply results in failure to establish a session or failure to
synchronise once connected. Enough devices fraudulently assigned to
a specific clock source (e.g., a particular IEEE 1588 grandmaster)
may, however, constitute a successful denial-of-service attack on
that source. Devices MAY wish to validate the integrity of the clock
description through some means before connecting to unfamiliar clock
sources.

The timestamp reference clocks negotiated by this protocol are used
to provide media timing information to RTP. Negotiated timestamp
reference clocks should not be relied upon to provide a secure time
reference for security critical operations (e.g., the expiration of
public key certificates).

8. IANA Considerations

This document defines two new SDP attributes: "ts-refclk" and
"mediaclk", within the existing Internet Assigned Numbers Authority
(IANA) registry of SDP Parameters.

This document also defines a new IANA registry subordinate to the
IANA SDP Parameters registry: the Media Clock Source Parameters
registry. Within this new registry, this document defines an initial
set of three media clock source parameters. Further, this document
defines a second new IANA registry subordinate to the IANA SDP
Parameters registry: the Timestamp Reference Clock Source Parameters
registry. Within this new registry, this document defines an initial
six parameters.

8.1. Reference Clock SDP Parameter

The SDP attribute "ts-refclk" defined by this document is registered
with the IANA registry of SDP Parameters as follows:

SDP Attributes ( "att-field (both session and media level)" &
"att-field (source level)" ):

Attribute name: ts-refclk

Long form: Timestamp reference clock source

Type of name: att-field

Type of attribute: Session, media, and source level

Subject to charset: No

Purpose: See Section 4 of this document

Reference: This document

Values: See Section 8.3 of this document

Figure 10: Reference Clock SDP Parameter IANA Registration

The attribute has an extensible parameter field; therefore, a
registry for these parameters is required. This new registry is
defined in Section 8.3.

8.2. Media Clock SDP Parameter

The SDP attribute "mediaclk" defined by this document is registered
with the IANA registry of SDP Parameters as follows:

SDP Attributes ( "att-field (both session and media level)" &
"att-field (source level)" ):

Attribute name: mediaclk

Long form: Media clock source

Type of name: att-field

Type of attribute: Session, media, and source level

Subject to charset: No

Purpose: See Section 5 of this document

Reference: This document

Values: See Section 8.4 of this document

Figure 11: Media Clock SDP Parameter IANA Registration

The attribute has an extensible parameter field; therefore, a
registry for these parameters is required. The new registry is
defined in Section 8.4.

8.3. Timestamp Reference Clock Source Parameters Registry

This document creates a new IANA subregistry called the Timestamp
Reference Clock Source Parameters registry, subordinate to the IANA
SDP Parameters registry. Each entry in the Timestamp Reference Clock
Source Parameters registry contains:

Name: Token used in the SDP description (clksrc-param-name)

Long name: Descriptive name for the timestamp reference clock source

Reference: Reference to the document describing the
SDP token (clksrc-param-name) and syntax for the optional
value associated with the token (mediaclock-param-value)

Initial values for the Timestamp Reference Clock Source Parameters
registry are given below.

Future assignments are to be made through the Specification Required
policy [RFC5226]. The Name field in the table corresponds to a new
value corresponding to clksrc-param-name. The Reference must specify
a syntax corresponding to clksrc-param-value.

8.4. Media Clock Source Parameters Registry

This document creates a new IANA subregistry called the Media Clock
Source Parameters registry, subordinate to the IANA SDP Parameters
registry. Each entry in the Media Clock Source Parameters registry
contains:

Name: Token used in the SDP description (mediaclock-param-name)

Long name: Descriptive name for the media clock source type

Reference: Reference to the document describing the SDP token
(mediaclock-param-name) and syntax for the optional
value associated with the token (mediaclock-param-value)

Initial values for the Media Clock Source Parameters registry are
given below.

Future assignments are to be made through the Specification Required
policy [RFC5226]. The Name field in the table corresponds to a new
value corresponding to mediaclock-param-name. The Reference must
specify a syntax corresponding to mediaclock-param-value.

8.5. Source-Level Attributes

[RFC5576] requires new source-level attributes to be registered with
the IANA registry named "att-field (source level)".

8.5.1. Source-Level Timestamp Reference Clock Attribute

The source-level SDP attribute "ts-refclk" defined by this document
is registered with the "att-field (source level)" IANA registry of
SDP Parameters, according to Figure 10.

8.5.2. Source-Level Media Clock Attribute

The source-level SDP attribute "mediaclk" defined by this document is
registered with the "att-field (source level)" IANA registry of SDP
Parameters, according to Figure 11.

9. Acknowledgements

The authors would like to thank Magnus Westerlund and Paul Kyzivat
for valuable comments that resulted in important improvements to this
document.

10. References

10.1. Normative References

[IEEE1588-2002]
Institute of Electrical and Electronics Engineers,
"1588-2002 - IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", October 2002,
<http://standards.ieee.org/findstds/
standard/1588-2002.html>.

[IEEE1588-2008]
IEEE, "1588-2008 - IEEE Standard for a Precision Clock
Synchronization Protocol for Networked Measurement and
Control Systems", July 2008,
<http://standards.ieee.org/findstds/
standard/1588-2008.html>.

[IEEE1722]
IEEE, "1722-2001 - IEEE Standard for Layer 2 Transport
Protocol for Time Sensitive Applications in a Bridged
Local Area Network", May 2011,
<http://standards.ieee.org/findstds/
standard/1722-2011.html>.

[IEEE802.1AS-2011]
IEEE, "802.1AS-2011 - IEEE Standard for Local and
Metropolitan Area Networks - Timing and Synchronization
for Time-Sensitive Applications in Bridged Local Area
Networks", February 2011,
<http://standards.ieee.org/findstds/
standard/802.1AS-2011.html>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.

[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific
Media Attributes in the Session Description Protocol
(SDP)", RFC 5576, June 2009.

[RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.

[RFC6051] Perkins, C. and T. Schierl, "Rapid Synchronisation of RTP
Flows", RFC 6051, November 2010.

[RFC7022] Begen, A., Perkins, C., Wing, D., and E. Rescorla,
"Guidelines for Choosing RTP Control Protocol (RTCP)
Canonical Names (CNAMEs)", RFC 7022, September 2013.

10.2. Informative References

[AES11-2009]
Audio Engineering Society, "AES11-2009: AES recommended
practice for digital audio engineering - Synchronization
of digital audio equipment in studio operations", February
2010, <http://www.aes.org/standards/>.

[IEEE802.1BA-2011]
IEEE, "802.1BA-2011 - IEEE Standard for Local and
metropolitan area networks -- Audio Video Bridging (AVB)
Systems", September 2011,
<http://standards.ieee.org/findstds/
standard/802.1BA-2011.html>.

[IS-GPS-200F]
Global Positioning Systems Directorate, "Navstar GPS Space
Segment/Navigation User Segment Interfaces", IS-GPS-200F ,
September 2011,
<http://www.navcen.uscg.gov/pdf/IS-GPS-200F.pdf>.

[Olsen] Olsen, D., "Time Accuracy Requirements in Audio Networks",
April 2007,
<http://www.ieee802.org/1/files/public/docs2007/
as-dolsen-time-accuracy-0407.pdf>.

[RFC0868] Postel, J. and K. Harrenstien, "Time Protocol", STD 26,
RFC 868, May 1983.

[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.

[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.

[RFC5939] Andreasen, F., "Session Description Protocol (SDP)
Capability Negotiation", RFC 5939, September 2010.

[RFC7164] Gross, K. and R. Brandenburg, "RTP and Leap Seconds", RFC
7164, March 2014.

[RFC7272] Brandenburg, R., Stokking, H., Deventer, O., Boronat, F.,
Montagud, M., and K. Gross, "Inter-Destination Media
Synchronization (IDMS) Using the RTP Control Protocol
(RTCP)", RFC 7272, June 2014.

[SMPTE-318M-1999]
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", ST 318:1999,
<http://standards.smpte.org/>.

Authors' Addresses

Aidan Williams
Audinate
Level 1, 458 Wattle St
Ultimo, NSW 2007
Australia

Phone: +61 2 8090 1000
Fax: +61 2 8090 1001
EMail: aidan.williams@audinate.com
URI: http://www.audinate.com/

Kevin Gross
AVA Networks
Boulder, CO
US

EMail: kevin.gross@avanw.com
URI: http://www.avanw.com/
Ray van Brandenburg
TNO
Brassersplein 2
Delft 2612CT
The Netherlands

Phone: +31-88-866-7000
EMail: ray.vanbrandenburg@tno.nl

Hans Stokking
TNO
Brassersplein 2
Delft 2612CT
The Netherlands

EMail: hans.stokking@tno.nl