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rfc:rfc7273

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.

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Williams, et al. Standards Track [Page 1] RFC 7273 RTP Clock Source Signalling June 2014

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 . . . . . . . . . . . . . . . . . .   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 . . . . . . . . . . . . . .  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

Williams, et al. Standards Track [Page 2] RFC 7273 RTP Clock Source Signalling June 2014

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
 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.

Williams, et al. Standards Track [Page 3] RFC 7273 RTP Clock Source Signalling June 2014

 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 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.

Williams, et al. Standards Track [Page 4] RFC 7273 RTP Clock Source Signalling June 2014

3. Definitions

 The following definitions are used in this document:
 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 sender:  The device generating an associated RTP 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]).

Williams, et al. Standards Track [Page 5] RFC 7273 RTP Clock Source Signalling June 2014

 The following sections describe and define SDP signalling, indicating
 whether and how the local timestamping clock in an RTP 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.  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
 provides a shared reference clock in a 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

Williams, et al. Standards Track [Page 6] RFC 7273 RTP Clock Source Signalling June 2014

 accuracy in LANs.  Network interface chips and cards supporting
 hardware timestamping of 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 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
 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

Williams, et al. Standards Track [Page 7] RFC 7273 RTP Clock Source Signalling June 2014

 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, 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, 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.

Williams, et al. Standards Track [Page 8] RFC 7273 RTP Clock Source Signalling June 2014

 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 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 session-level signalling.  More specific
 signalling included at the source level overrides 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 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>

Williams, et al. Standards Track [Page 9] RFC 7273 RTP Clock Source Signalling June 2014

 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"

Williams, et al. Standards Track [Page 10] RFC 7273 RTP Clock Source Signalling June 2014

 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

Williams, et al. Standards Track [Page 11] RFC 7273 RTP Clock Source Signalling June 2014

 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

Williams, et al. Standards Track [Page 12] RFC 7273 RTP Clock Source Signalling June 2014

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

Williams, et al. Standards Track [Page 13] RFC 7273 RTP Clock Source Signalling June 2014

 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>]

Williams, et al. Standards Track [Page 14] RFC 7273 RTP Clock Source Signalling June 2014

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's media clock with the
 master.  If a common reference clock is available, NTP timestamps in
 the master clock RTP media stream are taken using the shared
 reference clock.  The NTP timestamps communicate information about
 media clock timing (rate and phase) from the master to the slave
 devices.  NTP timestamps are communicated in the usual RTP fashion
 via RTCP SRs, or via the [RFC6051] header extension.  If no shared
 reference clock is available or signalled, a slave can synchronise to
 the master's media clock using RTP timestamps alone as described in
 Section 5.1 of [RFC3550].
 Note that the slaving of a device media clock to a master device does
 not affect RTP 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
 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.

Williams, et al. Standards Track [Page 15] RFC 7273 RTP Clock Source Signalling June 2014

 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 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

Williams, et al. Standards Track [Page 16] RFC 7273 RTP Clock Source Signalling June 2014

 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

Williams, et al. Standards Track [Page 17] RFC 7273 RTP Clock Source Signalling June 2014

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

Williams, et al. Standards Track [Page 18] RFC 7273 RTP Clock Source Signalling June 2014

 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.
 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:IEEE1722=38-D6-6D-8E-D2-78-13-2F
          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).

Williams, et al. Standards Track [Page 19] RFC 7273 RTP Clock Source Signalling June 2014

 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

Williams, et al. Standards Track [Page 20] RFC 7273 RTP Clock Source Signalling June 2014

 rejection MUST contain at least one timestamp reference clock
 specification usable by the answerer so that appropriate information
 is available for 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
 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

Williams, et al. Standards Track [Page 21] RFC 7273 RTP Clock Source Signalling June 2014

 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.

Williams, et al. Standards Track [Page 22] RFC 7273 RTP Clock Source Signalling June 2014

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.

Williams, et al. Standards Track [Page 23] RFC 7273 RTP Clock Source Signalling June 2014

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)

Williams, et al. Standards Track [Page 24] RFC 7273 RTP Clock Source Signalling June 2014

 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.
 +---------+------------------------------+--------------------------+
 | Name    | Long Name                    | Reference                |
 +---------+------------------------------+--------------------------+
 | ntp     | Network Time Protocol        | This document, Section 4 |
 |         |                              |                          |
 | ptp     | Precision Time Protocol      | This document, Section 4 |
 |         |                              |                          |
 | gps     | Global Positioning System    | This document, Section 4 |
 |         |                              |                          |
 | gal     | Galileo                      | This document, Section 4 |
 |         |                              |                          |
 | glonass | Global Navigation Satellite  | This document, Section 4 |
 |         | System                       |                          |
 |         |                              |                          |
 | local   | Local Clock                  | This document, Section 4 |
 |         |                              |                          |
 | private | Private Clock                | This document, Section 4 |
 +---------+------------------------------+--------------------------+

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.

Williams, et al. Standards Track [Page 25] RFC 7273 RTP Clock Source Signalling June 2014

 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.
 +----------+-----------------------------+--------------------------+
 | Name     | Long Name                   | Reference                |
 +----------+-----------------------------+--------------------------+
 | sender   | Asynchronously Generated    | This document, Section 5 |
 |          | Media Clock                 |                          |
 |          |                             |                          |
 | direct   | Direct-Referenced Media     | This document, Section 5 |
 |          | Clock                       |                          |
 |          |                             |                          |
 | IEEE1722 | IEEE1722 Media Stream       | This document, Section 5 |
 |          | Identifier                  |                          |
 +----------+-----------------------------+--------------------------+

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.

Williams, et al. Standards Track [Page 26] RFC 7273 RTP Clock Source Signalling June 2014

10. References

10.1. Normative References

 [IEEE1588-2002]
            IEEE, "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.

Williams, et al. Standards Track [Page 27] RFC 7273 RTP Clock Source Signalling June 2014

 [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>.

Williams, et al. Standards Track [Page 28] RFC 7273 RTP Clock Source Signalling June 2014

 [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/>.

Williams, et al. Standards Track [Page 29] RFC 7273 RTP Clock Source Signalling June 2014

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
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Williams, et al. Standards Track [Page 30]

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