Audio/Video Transport Core                                   A. Williams
Maintenance                                                     Audinate
Internet-Draft                                                  K. Gross
Intended status: Standards Track                            AVA Networks
Expires: January 4, April 25, 2013                               R. van Brandenburg
                                                             H. Stokking
                                                                     TNO
                                                            July 3,
                                                        October 22, 2012

                      RTP Clock Source Signalling
                      draft-ietf-avtcore-clksrc-00
                      draft-ietf-avtcore-clksrc-01

Abstract

   NTP timestamps are used by several RTP protocols for synchronisation
   and statistical measurement.  This memo specifies SDP signalling
   identifying NTP timestamp clock sources and SDP signalling
   identifying the media clock sources in a multimedia session.

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 RFC 2119 [1].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 4, April 25, 2013.

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   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Timestamp Reference Clock Source Signalling  . . . . . . . . .  5
     4.1.  Clock synchronization  . . . . . . . . . . . . . . . . . .  5
     4.2.  Identifying NTP Reference Clocks . . . . . . . . . . . . .  6
     4.3.  Identifying PTP Reference Clocks . . . . . . . . . . . . .  6
     4.4.  Identifying Global Reference Clocks  . . . . . . . . . . .  7
     4.5.  Other Reference Clocks . . . . . . . . . . . . . . . . . .  8
     4.6.  Traceable Reference Clocks . . . . . . . . . . . . . . . .  8
     4.7.  Synchronisation Confidence Quality  . . . . . . . . . . . . . . . . .  8
     4.8.  SDP Signalling of Timestamp Clock Source . . . . . . . . .  9
       4.8.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Media Clock Source Signalling  . . . . . . . . . . . . . . . . 12
     5.1.  Asynchronously Generated Media Clock . . . . . . . . . . . 13
     5.2.  Direct-Referenced Media Clock  . . . . . . . . . . . . . . 13
     5.3.  Stream-Referenced Media Clock  . . . . . . . . . . . . . . 13 14
     5.4.  Signalling Grammar . . . . . . . . . . . . . . . . . . . . 14 15
     5.5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17 18
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 19 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20 21

1.  Introduction

   RTP protocols use NTP format timestamps to facilitate media stream
   synchronisation and for providing 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 which 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 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 [12] [16] or audio word clock [13]). [17]).  If sending and
   receiving media clocks are known to be synchronised to a common
   reference clock, performance can improved by minimising buffering and
   avoiding rate conversion.

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

2.  Applications

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

   Examples include, but are not limited to:

   Social TV  RTCP for inter-destination media synchronization [6] [8]
      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 synchronized, the effect of multiple screens
      acting as one is broken.

   Networked Audio  Networked loudspeakers, amplifiers and analogue 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 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 [14]. [18].

   Sensor Arrays  Sensor arrays contain many synchronised measurement
      elements producing signals which 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 definitions of streams, sources and levels of information in SDP
   descriptions follow the definitions found in Source-Specific Media
   Attributes in the Session Description Protocol (SDP) [2].

   multimedia session  A set of multimedia senders and receivers as well
      as the data streams flowing from senders to receivers.  The
      Session Description Protocol (SDP) [3] describes multimedia
      sessions.

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

   media source  A media source is single stream of RTP packets,
      identified by an RTP SSRC.

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

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

   source level  Source level information applies to a single stream of
      RTP packets, identified by an RTP SSRC Source-Specific Media
      Attributes in the Session Description Protocol (SDP) [2] 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 a global time reference.  GPS
      [7]
      [9] is commonly used to provide a traceable time reference.  Some
      network time synchronisation protocols (e.g.  PTP [8], [10], 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 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 [9]) [11]) and radio clocks like (e.g.  GPS [7]. [9]).

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

4.1.  Clock synchronization

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

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

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

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

4.2.  Identifying NTP Reference Clocks

   A single NTP server is identified by hostname (or IP address) and an
   optional port number.  If the port number is not indicated, it is
   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 they are interchangeable.  RTP
   senders/receivers
   senders or receivers can use any of the listed NTP servers to govern
   a local clock that is equivalent to a local clock slaved to a
   different server.

4.3.  Identifying PTP Reference Clocks

   The IEEE 1588 Precision Time Protocol (PTP) family of clock
   synchronisation protocols provides a shared reference clock in an
   network - typically a LAN.  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 time-stamping of timing
   critical protocol messages are also available.

   Three flavours of IEEE 1588 are in use today:

   o  IEEE 1588-2002 [10]: [12]: 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 [8]: [10]: 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 [11]: [13]: "Timing and Synchronization for Time Sensitive
      Applications in Bridged Local Area Networks".  This is a Layer-2
      only profile of IEEE 1588-2008 for use in Audio/Video Bridged
      LANs. LANs
      as described in IEEE 802.1BA-2011 [14].

   Each IEEE 1588 clock is identified by a globally unique EUI-64 called
   a "ClockIdentity".  A slave clock using one of the IEEE 1588 family
   of network time protocols acquires the ClockIdentity/EUI-64 of the
   grandmaster clock that is the ultimate source of timing information
   for the network.  A master boundary clock which is itself slaved to another
   master
   boundar 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 network protocol domains, each of which may
   have a different grandmaster clock.  As the IEEE 1588 standards have
   developed, 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 subdomains domains are signalled via SDP, senders and receivers SHOULD
   check that both grandmaster ClockIdentity and PTP subdomain domain match when
   determining clock equivalence.

   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 favourably biased by setting stratum values, preferred master
   clock bits, or other 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 re-signalling the need to re-signal timestamp
   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 and
   Galileo.  Apart from the name of the reference clock system, no
   further identification is required.

4.5.  Other Reference Clocks

   At the time of writing, it is common for RTP senders/receivers not

   RFC 3550 allows senders and receivers to
   synchronise their either use a local wallclock
   reference for their NTP timestamps or, by setting the timestamp clocks field
   to a shared master.  An
   unsynchronised clock such as a quartz oscillator is 0, to supply no timstamps at all.  Both are common practice in
   embedded RTP implementations.  These clocks are identified as a "local" reference clock.
   and can only be assumed to be equivalent to clocks originating from
   the same device.

   In some other systems, all RTP senders/receivers senders and receivers may use a timestamp
   clock synchronised to a 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/receivers
   senders and receivers in a session using a private reference clock
   are assumed to have a mechanism outside this specification confirming
   that for
   determining whether their local timestamp clocks are equivalent.

4.6.  Traceable Reference Clocks

   A timestamp clock source may be labelled "traceable" if it is known
   to be sourced from a global time reference such as TAI or UTC.
   Providing adjustments are made for differing time bases, 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.

   Since all NTP and PTP servers providing traceable time can be
   directly compared, it is not necessary to identify traceable time
   servers by protocol address or other identifiers.

4.7.  Synchronisation Confidence Quality

   Network time protocol services periodically exchange timestamped
   messages between servers and clients.  Assuming RTP sender/receiver device clocks are
   based on commonly available quartz crystal hardware which is subject
   to drift, tight synchronisation requires frequent exchange of
   synchronisation messages.

   Unfortunately, in some implementations, it is not possible to control
   the frequency of synchronisation messages nor is it possible to
   discover when the last synchronisation message occurred.  In order to
   provide a measure of confidence that the timestamp clock is
   sufficiently synchronised, synchronization quality, an optional timestamp
   may be included in the SDP clock source signalling.  In addition, the
   frequency of synchronisation message messages may also be signalled.

   The optional timestamp and synchronisation frequency parameters
   provide an indication of synchronisation quality to the receiver of
   those parameters.  If the synchronisation confidence quality timestamp is far
   from the timestamp clock at the receiver of the parameters, it can be
   assumed that synchronisation has not occurred recently or recently, the timestamp
   reference clock source cannot be contacted.  In this case, contacted or the SDP information has
   not been recently refreshed.  To eliminate the latter possiblity,
   updated SDP information should be retrieved.  If the synchronisation
   quality timestamp is still far from the timestamp clock the receiver
   can take action to prevent unsynchronised playout or may fall back to
   assuming that the timestamp clocks are not synchronised.

   Synchronisation frequency is expressed as a signed (two's-compliment)
   8-bit field which is the base-2 logarithm of the frequency in Hz.
   The synchronisation frequencies represented by this field range from
   2^-128 Hz to 2^+127 Hz.  The field value of 0 corresponds to an
   update frequency of 1 Hz.

   When multiple reference clocks are specified, a sender or receiver
   may wish to base selection of the clock used based on most recent or
   most frequent update as indicated by the synchronization quality
   signalling.

4.8.  SDP Signalling of Timestamp 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) earlier in this document for more
   information).

   Timestamp 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 timestamp clock source signalling at the
   enclosing media level and session level.

   If timestamp 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 clock sources MUST NOT be mixed with non-
   traceable clock sources at any given level.  Unless synchronisation
   confidence
   quality information is available for each of the reference clocks
   listed at a given level, it SHOULD only be included with the first
   reference clock source attribute at that level.

   Note that clock source parameters may change from time to time, for
   example, as a result of a PTP clock master election.  The SIP [4]
   protocol supports re-signalling of updated SDP information, however
   other protocols may require additional notification mechanisms.

   Figure 1 shows the ABNF [5] grammar for the SDP reference clock
   source information.

    timestamp-refclk = "a=ts-refclk:" clksrc [ SP sync-confidence sync-quality ] CRLF

    clksrc = ntp / ptp / gps / gal / local / private

    ntp             =  "ntp=" ntp-server-addr
    ntp-server-addr =  host [ ":" port ]
    ntp-server-addr =/ "traceable" )

    ptp             =  "ptp=" ptp-version ":" ptp-gmid [":" ptp-domain] ptp-server
    ptp-version     =  "IEEE1588-2002"
    ptp-version     =/ "IEEE1588-2008"
    ptp-version     =/ "IEEE802.1AS-2011"
    ptp-gmid
    ptp-server      =  EUI64  ptp-gmid        =/ [":" ptp-domain] / "traceable"
    ptp-gmid        =  EUI64
    ptp-domain      =  ptp-domain-name / ptp-domain-nmbr
    ptp-domain-name =  "domain-name=" 16ptp-domain-char
    ptp-domain-char =  %x21-7E / %x00
                       ; allowed characters: 0x21-0x7E (IEEE 1588-2002)
    ptp-domain-nmbr =  "domain-nmbr=" %x00-7f
                       ; allowed number range: 0-127 (IEEE 1588-2008)

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

    sync-confidence

    sync-quality    = sync-timestamp [SP sync-frequency]
    sync-timestamp  = sync-date SP sync-time SP sync-UTCoffset
    sync-date       = 4DIGIT "-" 2DIGIT "-" 2DIGIT
                      ; yyyy-mm-dd (e.g., 1982-12-02)
    sync-time       = 2DIGIT ":" 2DIGIT ":" 2DIGIT "." 3DIGIT
                      ; 00:00:00.000 - 23:59:59.999
    sync-UTCoffset  = ( "+" / "-" ) 2DIGIT ":" 2DIGIT
                      ; +HH:MM or -HH:MM
    sync-frequency  = 2HEXDIG
                      ; If N is the field value, HZ=2^(N-127)

    host          = hostname / IPv4address / IPv6reference
    hostname      =  *( domainlabel "." ) toplabel [ "." ]
    toplabel      =  ALPHA / ALPHA *( alphanum / "-" ) alphanum
    domainlabel   =  alphanum
                  =/ alphanum *( alphanum / "-" ) alphanum
    IPv4address   =  1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
    IPv6reference =  "[" IPv6address "]"
    IPv6address   =  hexpart [ ":" IPv4address ]
    hexpart       =  hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
    hexseq        =  hex4 *( ":" hex4)
    hex4          =  1*4HEXDIG

    port = 1*DIGIT

    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 10.47.16.5
              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 224.2.17.12/127
              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.  Note that the synchronisation confidence timestamp appears
   on the first attribute at the media level only.

         v=0
         o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
         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 224.2.17.12/127
         t=2873397496 2873404696
         a=recvonly
         a=ts-refclk:local
         m=audio 49170 RTP/AVP 0
         a=ts-refclk:ntp=203.0.113.10 2011-02-19 21:03:20.345+01:00
         a=ts-refclk:ntp=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 10.47.16.5
    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 224.2.17.12/127
    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 media clock source.
   Alternatively, asynchronous media clock me may be signaled. 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. specified with an
   a=ts-refclk attribute (Section 4.8).

   The signalling optionally indicates a media clock offset value at the epoch value.  The
   offset indicates the RTP timestamp value at the epoch (time of
   origin) of the reference clock.  If no offset is signalled, the
   offset can be inferred at the receiver by examining RTCP sender
   reports which contain NTP and RTP timestamps which combined define a
   mapping.

   A rate for the media clock modifier may also be specified.  If include,  The modifier is expressed as the
   ratio of two integers and modifies the rate specification here overrides that specified or implied by
   the media description. description by this ratio.  If omitted, the rate is assumed
   to be the exact rate used specified or implied by the media format.  For
   example, without a rate specification, the media 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.

   A

   The rate modifier may optionally be expressed as the ratio of two
   integers.  This provision is primarily useful for accommodating certain
   "oddball rates"
   "oddball" audio sample rates associated with NTSC video (see
   Figure 7).

      a=mediaclk:offset=<offset>[ rate=<rate  Modified rates are not advised for video streams which
   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> ]
      denominator>]

5.3.  Stream-Referenced Media Clock

   The media clock

   A common synchronisation architecture for an outgoing stream may be generated based on the audio/visual systems
   involves distributing a reference media clock received with an incoming stream. 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 media sources 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 identifer with streams they
   transmit, signalling identifies the session synchronisation relationship between the
   master and slave devices.

   Slave devices recover media clock timing from the clock master
   stream, using it to synchronise the slave media clock with the
   master.  Timestamps in the master clock media stream source. are taken using
   the timestamp reference clock shared by the master and slave devices.
   The
   received timestamps communicate information about media clock may be converted to a real-time clock which can
   then be used timing
   (rate, phase) from the master to generate outgoing the slave devices.  Timestamps are
   communicated in the usual RTP fashion via RTCP SRs, or via the
   RFC6051 [6] header extension.  The stream media clocks.  In this way, format may indicate
   other clock information, such as the
   format nominal rate.

   Note that slaving of the reference stream a device media clock to a master device does not need to match
   affect the format usual RTP lip sync / time alignment algorithms.  Time
   aligned playout of two or more RTP sources still relies upon NTP
   timestamps supplied via RTCP SRs or by the outgoing stream. RFC6051 timestamp header
   extension.

   In a given system, master clock identifiers must be unique.  Such
   identifiers MAY be manually configured, however 17 octet string
   identifiers SHOULD be generated according to the "short-term
   persistent RTCP CNAME" algorithm as described in RFC6222 [7].

   A reference stream can be either another an RTP stream or AVB stream based on the
   IEEE 1722 [15] standard.

   An RTP clock master stream is SHOULD be identified at the source level
   by
   destination IP address (for a multicast stream) an SSRC and master clock identifier.  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:12345 mediaclk:master id=<media-clock-master-id>

   An RTP media source IP address
   (for indicates that it is slaved to a unicast stream), destination port number and CNAME of the
   source.

      a=mediaclk:rtp=<connection address>:<port> <CNAME> clock master via
   a clock master identifier:

      a=mediaclk:slave id=<media-clock-master-id>

   An RTP media source indicates that it is slaved to an IEEE 1722 clock
   master via a stream is identified by its StreamID, an EUI-64. identifier (an EUI-64):

      a=mediaclk:IEEE1722=<StreamID>

5.4.  Signalling Grammar

   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.

   Figure 5 shows the ABNF [5] grammar for the SDP media clock source
   information.

   timestamp-mediaclk = "a=mediaclk:" mediaclock

   mediaclock = refclk / rtp / streamid / sender

   refclk = "offset=" 1*DIGIT [ SP "rate=" 1*DIGIT "/" 1*DIGIT ]

   rtp = "rtp=" nettype SP addrtype SP connection-address SP port SP cname

   streamid = "IEEE1722=" EUI64
   sender = "sender"

   cname = non-ws-string

   nettype = token
           ;typically "IN"

   addrtype = token
           ;typically "IP4" or "IP6"

   token-char = %x21 / %x23-27 / %x2A-2B / %x2D-2E / %x30-39 / %x41-5A
                / %x5E-7E

   token = 1*(token-char)

   connection-address =  multicast-address / unicast-address

   unicast-address = IP4-address / IP6-address / FQDN / extn-addr

   multicast-address = IP4-multicast / IP6-multicast / FQDN / extn-addr

   IP4-multicast = m1 3( "." decimal-uchar ) "/" ttl [ "/" integer ]
           ; IPv4 multicast addresses 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 in used interchangeably.

   Figure 5 shows the
           ; range 224.0.0.0 to 239.255.255.255

   m1 = ("22" ("4"/"5"/"6"/"7"/"8"/"9")) / ("23" DIGIT )

   IP6-multicast ABNF [5] grammar for the SDP media clock source
   information.

               timestamp-mediaclk = hexpart [ "/" integer ]
           ; IPv6 address starting with FF

   FQDN "a=mediaclk:" mediaclock

               mediaclock = 4*(alpha-numeric refclk / "-" streamid / ".")
           ; fully qualified domain name as specified
           ; in RFC 1035 (and updates)

   IP4-address = b1 3("." decimal-uchar)

   b1 = decimal-uchar
           ; less than "224"

   ; The following is consistent with RFC 2373 [30], Appendix B.
   IP6-address sender

               rate = hexpart [ ":" IP4-address SP "rate=" 1*DIGIT "/" 1*DIGIT ]

   hexpart

               refclk = hexseq / hexseq "::" [ hexseq ] / "::" "direct" [ hexseq "=" 1*DIGIT ]

   hexseq = hex4 *( ":" hex4)

   hex4 rate

               streamid = 1*4HEXDIG
   ; Generic for other address families
   extn-addr "master-id=" clk-master-id

               streamid =/ "slave-to=" clk-master-id

               streamid =/ "IEEE1722=" EUI64

               clk-master-id = non-ws-string

   non-ws-string EUI48

               sender = 1*(VCHAR/%x80-FF)
           ;string of visible characters

   port "sender"

               EUI48 = 1*DIGIT 5(2HEXDIG ":") 2HEXDIG
               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 192.168.1.1
          c=IN IP4 239.0.0.2/255
          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:offset=963214424
          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 192.168.1.1
          c=IN IP4 239.0.0.2/255
          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:offset=963214424
          a=mediaclk:direct=963214424 rate=1000/1001

   Figure 7: "Oddball" sample rate directly refernced referenced to IEEE 1588-2008

   Figure 8 shows the same 48 kHz audio transmission from Figure 6 with
   media clock derived from another RTP multicast stream.  The stream
   providing the media clock must use the same reference clock as this
   stream that references it.

          v=0
          o=- 1311738121 1311738121 IN IP4 192.168.1.1
          c=IN IP4 224.2.228.230/32
          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:rtp=IN IP4 239.0.0.1 5004 00:60:2b:20:12:if
          a=mediaclk:slave id=00:60:2b:20:12:1f

      Figure 8: Stream media clock derived from another RTP multicast stream with media clock slaved to a master device

   Figure 9 shows the same 48 kHz audio transmission from Figure 6 with
   media clock derived from an IEEE 1722 AVB stream.  The stream
   providing the media clock must be synchronized with the IEEE 1588-
   2008 reference clock used by this stream.

          v=0
          o=- 1311738121 1311738121 IN IP4 192.168.1.1
          c=IN IP4 224.2.228.230/32
          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: Stream media clock derived from another RTP multicast stream with media clock slaved to an IEEE1722 master
                                  device

6.  IANA Considerations

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

   SDP Attribute ("att-field"):

     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 this document and registrations below

   The attribute has an extensible parameter field and therefore a
   registry for these parameters is required.  This document creates an
   IANA registry called the Timestamp Reference Clock Source Parameters
   Registry.  It contains the six parameters defined in Figure 1: "ntp",
   "ptp", "gps", "gal", "local", "private".

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

   SDP Attribute ("att-field"):

     Attribute name:     mediaclk

     Long form:          Media clock source

     Type of name:       att-field

     Type of attribute:  session and media level

     Subject to charset: no

     Purpose:            See section 6 of this document

     Reference:          This document

     Values:             see this document and registrations below

   The attribute has an extensible parameter field and therefore a
   registry for these parameters is required.  This document creates an
   IANA registry called the Media Clock Source Parameters Registry.  It
   contains the three parameters defined in Figure 5: "refclk", "ssrc",
   "sender". "sender",
   "direct", "master", "slave" and "IEEE1722".

7.  References

7.1.  Normative References

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

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

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

   [4]   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.

   [5]   Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

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

   [7]   Begen, A., Perkins, C., and D. Wing, "Guidelines for Choosing
         RTP Control Protocol (RTCP) Canonical Names (CNAMEs)",
         RFC 6222, April 2011.

7.2.  Informative References

   [6]

   [8]   Brandenburg, R., Stokking, H., Deventer, O., Boronat, F.,
         Montagud, M., and K. Gross, "RTCP for inter-destination media synchronization",
         draft-ietf-avtcore-idms-04 "Inter-destination Media
         Synchronization using the RTP Control Protocol (RTCP)",
         draft-ietf-avtcore-idms-07 (work in progress), May October 2012.

   [7]

   [9]   Global Positioning Systems Directorate, "Navstar GPS Space
         Segment/Navigation User Segment Interfaces", September 2011.

   [8]

   [10]  Institute of Electrical and Electronics Engineers, "1588-2008 -
         IEEE Standard for a Precision Clock Synchronization Protocol
         for Networked Measurement and Control Systems",  IEEE Std 1588-
         2008, 2008,
         <http://standards.ieee.org/findstds/standard/1588-2008.html>.

   [9]

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

   [10]

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

   [11]

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

   [14]  "Audio Video Bridging (AVB) Systems",
         <http://standards.ieee.org/findstds/standard/
         802.1BA-2011.html>.

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

URIs

   [12]

   [16]  <http://en.wikipedia.org/wiki/Genlock>

   [13]

   [17]  <http://en.wikipedia.org/wiki/Word_clock>

   [14]

   [18]  <http://www.ieee802.org/1/files/public/docs2007/
         as-dolsen-time-accuracy-0407.pdf>

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

   Phone:
   Email: stokking@tno.nl