[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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

Audio/Video Transport Core                                   A. Williams
Maintenance                                                     Audinate
Internet-Draft                                                  K. Gross
Intended status: Standards Track                            AVA Networks
Expires: September 26, 2014                           R. van Brandenburg
                                                             H. Stokking
                                                                     TNO
                                                          March 25, 2014


                      RTP Clock Source Signalling
                      draft-ietf-avtcore-clksrc-11

Abstract

   NTP format timestamps are used by several RTP protocols for
   synchronisation and statistical measurements.  This memo specifies
   SDP signalling identifying timestamp reference 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 [RFC2119].

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 September 26, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.



Williams, et al.       Expires September 26, 2014               [Page 1]

Internet-Draft         RTP Clock Source Signalling            March 2014


   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.       Expires September 26, 2014               [Page 2]

Internet-Draft         RTP Clock Source Signalling            March 2014


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Timestamp Reference Clock Source Signalling  . . . . . . . . .  6
     4.1.  Clock synchronization  . . . . . . . . . . . . . . . . . .  6
     4.2.  Identifying NTP Reference Clocks . . . . . . . . . . . . .  7
     4.3.  Identifying PTP Reference Clocks . . . . . . . . . . . . .  7
     4.4.  Identifying Global Reference Clocks  . . . . . . . . . . .  9
     4.5.  Private Reference Clocks . . . . . . . . . . . . . . . . .  9
     4.6.  Local Reference Clocks . . . . . . . . . . . . . . . . . .  9
     4.7.  Traceable Reference Clocks . . . . . . . . . . . . . . . .  9
     4.8.  SDP Signalling of Timestamp Reference Clock Source . . . . 10
       4.8.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Media Clock Source Signalling  . . . . . . . . . . . . . . . . 13
     5.1.  Asynchronously Generated Media Clock . . . . . . . . . . . 13
     5.2.  Direct-Referenced Media Clock  . . . . . . . . . . . . . . 13
     5.3.  Stream-Referenced Media Clock  . . . . . . . . . . . . . . 15
     5.4.  SDP Signalling of Media Clock Source . . . . . . . . . . . 16
     5.5.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . 18
   6.  Signalling Considerations  . . . . . . . . . . . . . . . . . . 20
     6.1.  Usage in Offer/Answer  . . . . . . . . . . . . . . . . . . 20
       6.1.1.  Indicating Support for Clock Source Signalling . . . . 21
       6.1.2.  Timestamp Reference Clock  . . . . . . . . . . . . . . 21
       6.1.3.  Media Clock  . . . . . . . . . . . . . . . . . . . . . 21
     6.2.  Usage Outside of Offer/Answer  . . . . . . . . . . . . . . 22
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  Reference Clock SDP Parameter  . . . . . . . . . . . . . . 23
     8.2.  Media Clock SDP Parameter  . . . . . . . . . . . . . . . . 24
     8.3.  Timestamp Reference Clock Source Parameters Registry . . . 24
     8.4.  Media Clock Source Parameters Registry . . . . . . . . . . 25
     8.5.  Source-level Attributes  . . . . . . . . . . . . . . . . . 26
       8.5.1.  Source-level Timestamp Reference Clock Attribute . . . 26
       8.5.2.  Source-level Media Clock Attribute . . . . . . . . . . 26
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     10.2. Informative References . . . . . . . . . . . . . . . . . . 28
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29










Williams, et al.       Expires September 26, 2014               [Page 3]

Internet-Draft         RTP Clock Source Signalling            March 2014


1.  Introduction

   RTP protocols use NTP format timestamps to facilitate multimedia
   session 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 [SMPTE-318-1999] or audio word clock [AES11-2009]).  If
   sending and receiving media clocks are known to be synchronised to a
   common reference clock, performance can improved by minimising
   buffering and avoiding rate conversion.

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


2.  Applications

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

   Examples include, but are not limited to:

   Social TV  : RTCP for inter-destination media synchronization
      [I-D.ietf-avtcore-idms] 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



Williams, et al.       Expires September 26, 2014               [Page 4]

Internet-Draft         RTP Clock Source Signalling            March 2014


      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 [Olsen].

   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 following definitions are used in this draft:

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

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



Williams, et al.       Expires September 26, 2014               [Page 5]

Internet-Draft         RTP Clock Source Signalling            March 2014


   RTP media stream  : A single stream of RTP packets identified by an
      RTP SSRC.

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

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

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

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

   traceable time  : A clock is considered to provide traceable time if
      it can be proven to be synchronised to International Atomic Time
      (TAI).  Coordinated Universal Time (UTC) is a time standard
      synchronized to TAI.  UTC is therefore also considered traceable
      time once leap seconds have been taken unto account.  GPS
      [IS-GPS-200F] is commonly used to provide a TAI traceable time
      reference.  Some network time synchronisation protocols (e.g.  PTP
      [IEEE1588-2008], 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], PTP [IEEE1588-2008]) and radio clocks
   (e.g.  GPS [IS-GPS-200F]).

   The following sections describe and define SDP signalling, indicating
   whether and how the local timestamping clock in an RTP sender/
   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 came



Williams, et al.       Expires September 26, 2014               [Page 6]

Internet-Draft         RTP Clock Source Signalling            March 2014


   from the same clock.  Providing they are sufficiently synchronised,
   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) 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 synchronisation 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, GLONASS).  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
   providing traceable time.

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 all of the listed servers
   deliver the same reference time and may be used interchangeably.  RTP
   senders and receivers are assured proper synchronization regardless
   of which server they choose and, in support of fault tolerance, may
   switch servers while streaming.

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 [IEEE1588-2002]: the original "Standard for a
      Precision Clock Synchronization Protocol for Networked Measurement



Williams, et al.       Expires September 26, 2014               [Page 7]

Internet-Draft         RTP Clock Source Signalling            March 2014


      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 Layer-2 only profile of IEEE 1588-2008 for use in Audio/Video
      Bridged LANs as described in IEEE 802.1BA-2011 [IEEE802.1BA-2011].

   Each IEEE 1588 clock is identified by an 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 boundary clock which is itself slaved to another
   boundary clock or the grandmaster passes the grandmaster
   ClockIdentity through to its slaves.

   Several instances of the IEEE 1588 protocol may operate independently
   on a single network, forming distinct PTP domains, each of which may
   have a different grandmaster clock.  As the IEEE 1588 standards have
   developed, 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 PTP domain match when
   determining clock equivalence.

   Two or more IEEE 1588 clocks MAY be listed at the same level in the
   session description to indicate that all of the listed clocks are
   candidate grandmaster clocks for the domain or deliver the same
   reference time and may be used interchangeably.  RTP senders and
   receivers are assured proper synchronization regardless of which
   synchronization source they choose and, in support of fault
   tolerance, may switch 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



Williams, et al.       Expires September 26, 2014               [Page 8]

Internet-Draft         RTP Clock Source Signalling            March 2014


   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

   RFC 3550 allows senders and receivers to either use a local wall
   clock reference for their NTP timestamps or, by setting the timestamp
   field to 0, to supply no timestamps at all.  Both are common practice
   in embedded RTP implementations.  These clocks are identified as
   "local" and can only 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 to delivering traceable time.  Providing adjustments
   are made for differing epochs, timezones and leap seconds, timestamps
   taken using clocks synchronised to a traceable time source can be
   directly compared even if the clocks are synchronised to different
   sources or via different mechanisms.

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





Williams, et al.       Expires September 26, 2014               [Page 9]

Internet-Draft         RTP Clock Source Signalling            March 2014


4.8.  SDP Signalling of Timestamp Reference Clock Source

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

   Timestamp reference 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.  More specific
   signalling included at the source level overrides default media level
   signalling.

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

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

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

   General forms of usage:

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

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

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

   ABNF [RFC5234] grammar for the timestamp reference clock attribute:
   ; external references:
   POS-DIGIT   = <See RFC 4566>
   token       = <See RFC 4566>
   byte-string = <See RFC 4566>
   DIGIT       = <See RFC 5324>
   HEXDIG      = <See RFC 5324>
   CRLF        = <See RFC 5324>



Williams, et al.       Expires September 26, 2014              [Page 10]

Internet-Draft         RTP Clock Source Signalling            March 2014


   hostport    = <See RFC 3261, with revisions from RFC 5954>

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

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

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

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

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

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

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

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

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

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


           Figure 1: Timestamp Reference Clock Source Signalling




Williams, et al.       Expires September 26, 2014              [Page 11]

Internet-Draft         RTP Clock Source Signalling            March 2014


4.8.1.  Examples

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

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

    Figure 2: Timestamp reference clock definition at the session level

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

          v=0
          o=jdoe 2890844526 2890842807 IN IP4 192.0.2.1
          s=SDP Seminar
          i=A Seminar on the session description protocol
          u=http://www.example.com/seminars/sdp.pdf
          e=j.doe@example.com (Jane Doe)
          c=IN IP4 233.252.0.1/64
          t=2873397496 2873404696
          a=recvonly
          a=ts-refclk:local
          m=audio 49170 RTP/AVP 0
          a=ts-refclk:ntp=203.0.113.10
          a=ts-refclk:ntp=198.51.100.22
          m=video 51372 RTP/AVP 99
          a=rtpmap:99 h263-1998/90000
          a=ts-refclk:ptp=IEEE802.1AS-2011:39-A7-94-FF-FE-07-CB-D0

     Figure 3: Timestamp reference clock definition at the media level

   Figure 4 shows an example SDP description with a timestamp reference
   clock definition at the source level overriding the session level
   default.




Williams, et al.       Expires September 26, 2014              [Page 12]

Internet-Draft         RTP Clock Source Signalling            March 2014


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

    Figure 4: Timestamp reference clock signalling at the source level


5.  Media Clock Source Signalling

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

5.1.  Asynchronously Generated Media Clock

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

   The asynchronously generated media clock is the assumed mode of
   operation when there is no signalling of media clock source.
   Alternatively, 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).



Williams, et al.       Expires September 26, 2014              [Page 13]

Internet-Draft         RTP Clock Source Signalling            March 2014


   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 [I-D.ietf-avtcore-leap-second] for further
   discussion of leap-second issues in timestamp reference clocks.

   To compute the RTP timestamp against an IEEE 1588 (TAI-based)
   reference, the time elapsed between the 00:00:00 1 January 1970 IEEE
   1588 epoch and the current time must be computed.  Between the epoch
   and 1 January 2013, there were 15,706 days (including extra days
   during leap years).  Since there are no leap seconds in a TAI
   reference, there are exactly 86,400 seconds during each of these days
   or a total of 1,356,998,400 seconds from the epoch to 00:00:00 1
   January 2013.  A 90 kHz RTP clock for a video stream would have
   advanced 122,129,856,000,000 units over this period.  With a
   signalled offset of 0, the RTP clock value modulo the 32-bit unsigned
   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 which contain NTP and RTP timestamps
   which combined define a mapping.  The NTP/RTP timestamp mapping
   provided by RTCP SRs takes precedence over that singaled 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



Williams, et al.       Expires September 26, 2014              [Page 14]

Internet-Draft         RTP Clock Source Signalling            March 2014


   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 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>]

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 media clock with the
   master.  Timestamps in the master clock RTP media stream are taken
   using the timestamp reference clock shared by the master and slave
   devices.  The timestamps communicate information about media clock
   timing (rate, phase) from the master to the slave devices.
   Timestamps are communicated in the usual RTP fashion via RTCP SRs, or
   via the RFC6051 [RFC6051] header extension.  The stream media format
   may indicate other clock information, such as the nominal rate.

   Note that slaving of a device media clock to a master device does not
   affect the 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 RFC6051 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



Williams, et al.       Expires September 26, 2014              [Page 15]

Internet-Draft         RTP Clock Source Signalling            March 2014


   "short-term persistent RTCP CNAME" algorithm as described in RFC7022
   [RFC7022].  Master clock identifiers not already in base64 format
   MUST be encoded as a base64 strings when used in SDP.  Although the
   RTCP 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 AVB stream based on the
   IEEE 1722 [IEEE1722] standard.

   An RTP clock master stream SHOULD be identified at the source level
   by an SSRC [RFC5576] and master clock identifier.  An RTP stream that
   provides media clock timing directly from a reference media clock
   (e.g. internal crystal, audio word clock or video blackburst 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 media clock master is signalled by
   including master clock identifier:

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

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

      a=mediaclk:IEEE1722=<StreamID>

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

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

5.4.  SDP Signalling of Media Clock Source

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

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



Williams, et al.       Expires September 26, 2014              [Page 16]

Internet-Draft         RTP Clock Source Signalling            March 2014


   signalling overrides media clock source signalling at the enclosing
   media level and session level.

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

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

   General forms of usage:

   session level:  a=mediaclk:<mediaclock>

   media level:  a=mediaclk:<mediaclock>

   source level:  a=ssrc:<ssrc-id> mediaclk:<mediaclock>































Williams, et al.       Expires September 26, 2014              [Page 17]

Internet-Draft         RTP Clock Source Signalling            March 2014


   ABNF [RFC5234] grammar for the media clock reference attribute:
   ; external references:
   integer     = <See RFC 4566>
   token       = <See RFC 4566>
   byte-string = <See RFC 4566>
   base64      = <See RFC 4566>
   SP          = <See RFC 5234>
   DIGIT       = <See RFC 5234>
   HEXDIG      = <See RFC 5234>

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

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

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

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

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

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


                  Figure 5: Media Clock Source Signalling

5.5.  Examples

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














Williams, et al.       Expires September 26, 2014              [Page 18]

Internet-Draft         RTP Clock Source Signalling            March 2014


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

        Figure 6: Media clock directly referenced to IEEE 1588-2008

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

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

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

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

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

         Figure 8: RTP stream with media clock slaved to a master

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



Williams, et al.       Expires September 26, 2014              [Page 19]

Internet-Draft         RTP Clock Source Signalling            March 2014


          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 IEEE1722 master
                                  device


6.  Signalling Considerations

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

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

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

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

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

6.1.  Usage in Offer/Answer

   During offer/answer, clock source signalling via SDP uses a
   declarative model.  Supported media and/or reference clocks are
   specified in the offered SDP description.  The answerer may accept or
   reject the offer in an application-specific way depending on the
   clocks that are available and the clocks that are offered.  For



Williams, et al.       Expires September 26, 2014              [Page 20]

Internet-Draft         RTP Clock Source Signalling            March 2014


   example, an answerer may choose to accept an offer that lacks a
   common clock by falling back to a lower performance mode of operation
   (e.g. by assuming reference or media clocks are local rather than
   shared).  Conversely, the answerer may choose to reject the offer
   when the offered clock specifications indicate that the available
   reference and/or media clocks are incompatible.

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

6.1.1.  Indicating Support for Clock Source Signalling

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

6.1.2.  Timestamp Reference Clock

   If one or more of the reference clocks specified in the offer are
   usable by the answerer, the answerer SHOULD respond with an answer
   containing the subset of reference clock specifications in the offer
   that are usable by the answerer.  If the answerer rejects the offer
   because the available reference clocks are incompatible, the
   rejection MUST contain at least one timestamp reference clock
   specification usable by the answerer so that appropriate information
   is available for debugging.  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 which may be used
   interchangeably.  RTP senders and receivers are assured proper
   synchronization 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



Williams, et al.       Expires September 26, 2014              [Page 21]

Internet-Draft         RTP Clock Source Signalling            March 2014


   debugging.  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 synchronization 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 session
   description, and the nature of the authentication will differ from
   transport to transport.  For some transports, security features are
   often not deployed.  In case a session description has not been
   obtained in a trusted manner, the endpoint should exercise care
   because, among other attacks, the media sessions received may not be
   the intended ones, the destination where media is sent to may not be
   the expected one, any of the parameters of the session may be
   incorrect.

   Incorrect reference or media clock parameters may cause devices or
   streams to synchronize to unintended clock sources.  Normally this
   simply results in failure to establish a session or failure to
   synchronize 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).




Williams, et al.       Expires September 26, 2014              [Page 22]

Internet-Draft         RTP Clock Source Signalling            March 2014


8.  IANA Considerations

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

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

8.1.  Reference Clock SDP Parameter

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

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

     Attribute name:     ts-refclk

     Long form:          Timestamp reference clock source

     Type of name:       att-field

     Type of attribute:  Session, media and source level

     Subject to charset: No

     Purpose:            See section 4 of this document

     Reference:          This document

     Values:             See section 8.3 of this document


                                 Figure 10

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






Williams, et al.       Expires September 26, 2014              [Page 23]

Internet-Draft         RTP Clock Source Signalling            March 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

   The attribute has an extensible parameter field and 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 sub-registry called the Timestamp
   Reference Clock Source Parameters Registry, subordinate to the IANA
   SDP Parameters registry.  Each entry in the Timestamp Reference Clock
   Source Parameters Registry contains:

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

   Long name:  Descriptive name for the timestamp reference clock source

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

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



Williams, et al.       Expires September 26, 2014              [Page 24]

Internet-Draft         RTP Clock Source Signalling            March 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 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 Position System         | This document, section |
   |         |                                | 4                      |
   | gal     | Galileo                        | This document, section |
   |         |                                | 4                      |
   | glonass | Global Navigation Satellite    | This document, section |
   |         | System                         | 4                      |
   | 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 sub-registry called the Media Clock
   Source Parameters registry, subordinate to the IANA SDP Parameters
   registry.  Each entry in the Media Clock Source Parameters Registry
   contains:

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

   Long name:  Descriptive name for the media clock source type

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

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

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





Williams, et al.       Expires September 26, 2014              [Page 25]

Internet-Draft         RTP Clock Source Signalling            March 2014


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

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 which resulted in important improvements to
   this document.


10.  References

10.1.  Normative References

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

   [IEEE1588-2008]



Williams, et al.       Expires September 26, 2014              [Page 26]

Internet-Draft         RTP Clock Source Signalling            March 2014


              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>.

   [IEEE1722]
              Institute of Electrical and Electronics Engineers, "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>.

   [IEEE802.1AS-2011]
              Institute of Electrical and Electronics Engineers, "Timing
              and Synchronization for Time-Sensitive Applications in
              Bridged Local Area Networks", <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.

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

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

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

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

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

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

   [RFC7022]  Begen, A., Perkins, C., Wing, D., and E. Rescorla,
              "Guidelines for Choosing RTP Control Protocol (RTCP)



Williams, et al.       Expires September 26, 2014              [Page 27]

Internet-Draft         RTP Clock Source Signalling            March 2014


              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",
              <http://www.aes.org/standards/>.

   [I-D.ietf-avtcore-idms]
              Brandenburg, R., Stokking, H., Deventer, O., Boronat, F.,
              Montagud, M., and K. Gross, "Inter-destination Media
              Synchronization using the RTP Control Protocol (RTCP)",
              draft-ietf-avtcore-idms-13 (work in progress),
              August 2013.

   [I-D.ietf-avtcore-leap-second]
              Gross, K. and R. Brandenburg, "RTP and Leap Seconds",
              draft-ietf-avtcore-leap-second-07 (work in progress),
              December 2013.

   [IEEE802.1BA-2011]
              Institute of Electrical and Electronics Engineers, "Audio
              Video Bridging (AVB) Systems", <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",
              September 2011.

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

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

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

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

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



Williams, et al.       Expires September 26, 2014              [Page 28]

Internet-Draft         RTP Clock Source Signalling            March 2014


              June 2002.

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

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


Authors' Addresses

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

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


   Kevin Gross
   AVA Networks
   Boulder, CO
   US

   Email: kevin.gross@avanw.com
   URI:   http://www.avanw.com/


   Ray van Brandenburg
   TNO
   Brassersplein 2
   Delft  2612CT
   the Netherlands

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








Williams, et al.       Expires September 26, 2014              [Page 29]

Internet-Draft         RTP Clock Source Signalling            March 2014


   Hans Stokking
   TNO
   Brassersplein 2
   Delft  2612CT
   the Netherlands

   Email: hans.stokking@tno.nl












































Williams, et al.       Expires September 26, 2014              [Page 30]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/