One document matched: draft-williams-avtext-avbsync-00.xml

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<rfc category="std" docName="draft-williams-avtext-avbsync" ipr="trust200902">
  <front>
    <title abbrev="AVB/RTP Synchronisation">IEEE 1588/802.1AS Synchronisation
    for RTP Streams</title>

    <author fullname="Aidan Williams" initials="A.M." surname="Williams">
      <organization>Audinate</organization>

      <address>
        <postal>
          <street>Level 1, 458 Wattle St</street>

          <city>Ultimo</city>

          <code>2007</code>

          <region>NSW</region>

          <country>Australia</country>
        </postal>

        <phone>+61 2 8090 1000</phone>

        <facsimile>+61 2 8090 1001</facsimile>

        <email>aidan.williams@audinate.com</email>
      </address>
    </author>

    <date month="February" year="2011" />

    <area>Real-time Applications and Infrastructure</area>

    <workgroup>Audio/Video Transport Extensions</workgroup>

    <keyword>Sample</keyword>

    <keyword>Draft</keyword>

    <abstract>
      <t>Specification of an RTP header extension for carrying in-band
      synchronization metadata provided by the IEEE1588/802.1AS Precision Time
      Protocols.</t>
    </abstract>

    <note title="Requirements Language">
      <t>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 <xref
      target="RFC2119">RFC 2119</xref>.</t>
    </note>
  </front>

  <middle>
    <section title="Introduction">
      <t>Synchronisation between RTP flows and between devices rendering RTP
      flows is currently facilitated by means of NTP format timestamps taken
      with respect to a shared reference clock. In professional AV
      applications, the NTP clock synchronisation protocol does not meet the
      time alignment and synchronisation speed requirements of many
      systems.</t>

      <t>Like NTP, the IEEE1588 family of clock synchronisation protocols
      provide a shared reference clock in an network - typically a LAN.
      IEEE1588 provides sub-microsecond synchronisation between devices on a
      LAN and typically locks within seconds at startup rather than minutes.
      With support from Ethernet switches, IEEE1588 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.</t>

      <t>When using IEEE1588 clock synchronisation, networked AV systems can
      achieve sub 1 microsecond time alignment accuracy when rendering
      rendering of AV signals and can support latencies less than 1ms through
      a gigabit LAN.</t>

      <t>Two main flavours of IEEE1588 are in use today:</t>

      <t><list style="symbols">
          <t>IEEE 1588-2008: 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 often called IEEE1588v2 or PTPv2.</t>

          <t>IEEE 802.1AS: "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.</t>
        </list>By using an IEEE 1588 derived reference clock, synchronisation
      of RTP streams and devices in LANs can be considerably improved.</t>
    </section>

    <section anchor="timestamp_formats" title="Timestamp formats">
      <t>A global IEEE 1588/802.1AS timestamp is 80 bits in total, divided
      into two parts:</t>

      <t><list style="empty">
          <t>AS_sec: 48 bits seconds since epoch</t>

          <t>AS_nsec: 32 bits nanoseconds</t>
        </list>A shorter 32 bit timestamp is defined for use in streaming
      media protocols in the following way:<list style="empty">
          <t>as_timestamp = (AS_sec * 10^9 + AS_nsec) modulo 2^32</t>
        </list></t>

      <t>The shorter as_timestamp field covers just over 4 seconds of
      time.</t>
    </section>

    <section title="Header Extension">
      <figure title="IEEE 1588/802.1AS Synchronisation Header Extension">
        <preamble></preamble>

        <artwork><![CDATA[      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |V=2|P|1|  CC   |M|     PT      |       sequence number         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+R
     |                           timestamp                           |T
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+P
     |           synchronisation source (SSRC) identifier            |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |       0xBE    |    0xDE       |           length=2            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+E
     |  ID-A | L=6   | subtype |             reserved                |x
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+t
     |                         as_timestamp                          |n
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                         payload data                          |
     |                             ....                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
      </figure>

      <t>The fields are defined as follows:</t>

      <t><list style="empty">
          <t>subtype: IEEE 1733 RTCP subtype field, see <xref
          target="IEEE1733_RTCP"></xref>.</t>

          <t>as_timestamp: a 32 bit IEEE 1588/802.1AS timestamp as defined in
          <xref target="IEEE1733_RTCP"></xref>.</t>

          <t>reserved: as this specification evolves, additional fields are
          expected to be included in this header.</t>
        </list></t>
    </section>

    <section anchor="IEEE1733_RTCP" title="IEEE 1733 / RTCP">
      <t>IEEEE 1733 defines an RTCP header extension which also includes
      802.1AS timestamp information. The IEEE 1733 RTCP extension signals
      infromation in addition to timestamps, including:</t>

      <t><list style="hanging">
          <t hangText="subtype">an RTCP subtype</t>

          <t hangText="gmIdentity">an 80 bit field uniquely identifying the
          current 802.1AS grand master clock for the network</t>

          <t hangText="stream_id">a 64 bit number identifying the 802.1Qav
          stream associated with this RTP flow</t>

          <t hangText="as_timestamp">the <xref target="timestamp_formats">32
          bit 802.1AS timestamp</xref> associated with the RTP timestamp
          carried in this packet</t>

          <t hangText="rtp_timestamp">the RTP timestamp of a media packet</t>
        </list></t>
    </section>

    <section anchor="IANA" title="IANA Considerations">
      <t>TBD: A URN will be required to signal the presence of this header
      extension, such as:</t>

      <t><list style="empty">
          <t>urn:ietf:params:rtp-hdrext:avb-sync</t>
        </list></t>
    </section>

    <section anchor="Acknowledgements" title="Acknowledgements">
      <t></t>
    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include="reference.RFC.2119"?>
    </references>

    <references title="Informative References">
      <reference anchor="InfRef">
        <front>
          <title></title>

          <author>
            <organization></organization>
          </author>

          <date year="2004" />
        </front>
      </reference>
    </references>

    <section title="An Appendix">
      <t></t>
    </section>
  </back>
</rfc>