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Versions: (draft-williams-avtcore-clksrc) 00 01 03 04 05 06 07 08 09 10 11 RFC 7273

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


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

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 4, 2013.

Copyright Notice

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




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   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.


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 . . . . . . . . . . . . . . . .  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
     5.4.  Signalling Grammar . . . . . . . . . . . . . . . . . . . . 14
     5.5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20














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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] or audio word clock [13]).  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]
      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



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      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].

   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.





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   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] is commonly used to provide a traceable time reference.  Some
      network time synchronisation protocols (e.g.  PTP [8], 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]) and radio clocks like GPS [7].

   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
   from the same clock.  Providing they are sufficiently synchronised,
   timestamps produced in one RTP sender/receiver can be directly



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   compared to a local clock in another RTP sender/receiver.  The
   timestamps produced by synchronized local clocks in two or more RTP
   senders/receivers can be directly compared.

   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 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]: the original "Standard for a Precision Clock
      Synchronization Protocol for Networked Measurement and Control
      Systems".  This is also known as IEEE1588v1 or PTPv1.



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   o  IEEE 1588-2008 [8]: 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]: "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.

   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 clock which is itself slaved to another
   master clock 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 are signalled via SDP, senders and receivers
   SHOULD check that both grandmaster ClockIdentity and PTP subdomain
   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 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 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.




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4.5.  Other Reference Clocks

   At the time of writing, it is common for RTP senders/receivers not to
   synchronise their local timestamp clocks to a shared master.  An
   unsynchronised clock such as a quartz oscillator is identified as a
   "local" reference clock.

   In some systems, all RTP senders/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 in a session using a private reference clock are
   assumed to have a mechanism outside this specification confirming
   that 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

   Network time protocol services periodically exchange timestamped
   messages between servers and clients.  Assuming RTP sender/receiver
   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, an optional timestamp may be included in
   the SDP clock source signalling.  In addition, the frequency of
   synchronisation message 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 timestamp is far



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   from the timestamp clock at the receiver of the parameters, it can be
   assumed that synchronisation has not occurred recently or the
   timestamp reference clock source cannot be contacted.  In this case,
   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.

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 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.



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    timestamp-refclk = "a=ts-refclk:" clksrc [ SP sync-confidence ] 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-version     =  "IEEE1588-2002"
    ptp-version     =/ "IEEE1588-2008"
    ptp-version     =/ "IEEE802.1AS-2011"
    ptp-gmid        =  EUI64
    ptp-gmid        =/ "traceable"
    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-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



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    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.









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         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



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   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 be signaled.

      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.  The
   signalling indicates a media clock offset value at the epoch (time of
   origin) of the reference clock.  A rate for the media clock may also
   be specified.  If include, the rate specification here overrides that
   specified or implied by the media description.  If omitted, the rate
   is assumed to be the exact rate used by the media format.  For
   example, 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 rate modifier may optionally be expressed as the ratio of two
   integers.  This provision is useful for accommodating certain
   "oddball rates" associated with NTSC video (see Figure 7).

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

5.3.  Stream-Referenced Media Clock

   The media clock for an outgoing stream may be generated based on the
   media clock received with an incoming stream.  In this case, the
   signalling identifies the session and the stream source.  The
   received media clock may be converted to a real-time clock which can
   then be used to generate outgoing media clocks.  In this way, the
   format of the reference stream does not need to match the format of
   the outgoing stream.




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   A reference stream can be either another RTP stream or AVB stream
   based on the IEEE 1722 standard.  An RTP stream is identified by
   destination IP address (for a multicast stream) or source IP address
   (for a unicast stream), destination port number and CNAME of the
   source.

      a=mediaclk:rtp=<connection address>:<port> <CNAME>

   An IEEE 1722 stream is identified by its StreamID, 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



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   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 may be in the
           ; range 224.0.0.0 to 239.255.255.255

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

   IP6-multicast = hexpart [ "/" integer ]
           ; IPv6 address starting with FF

   FQDN = 4*(alpha-numeric / "-" / ".")
           ; 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 = hexpart [ ":" IP4-address ]

   hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]

   hexseq = hex4 *( ":" hex4)

   hex4 = 1*4HEXDIG




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   ; Generic for other address families
   extn-addr = non-ws-string

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

   port = 1*DIGIT

   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

        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 rate=1000/1001

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




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   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

      Figure 8: Stream media clock derived from another RTP multicast
                                  stream

   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


6.  IANA Considerations

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








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   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



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   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".


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.

7.2.  Informative References

   [6]   Brandenburg, R., Stokking, H., Boronat, F., Montagud, M., and
         K. Gross, "RTCP for inter-destination media synchronization",
         draft-ietf-avtcore-idms-04 (work in progress), May 2012.

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

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

   [10]  Institute of Electrical and Electronics Engineers, "1588-2002 -
         IEEE Standard for a Precision Clock Synchronization Protocol



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         for Networked Measurement and Control Systems",  IEEE Std 1588-
         2002, 2002,
         <http://standards.ieee.org/findstds/standard/1588-2002.html>.

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

URIs

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

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

   [14]  <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/











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   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

































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