root/branches/binary/server/doc/binary-protocol-plan-udp.xml @ 687

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<rfc category="info" docName="draft-stone-memcache-udp-01" ipr="none">

  <front>

    <title abbrev="Memcache Over UDP"> Memcache Binary Protocol: Extensions for UDP </title>

    <author fullname="Aaron Stone" surname="Aaron Stone" role="editor">
      <organization>Six Apart, Ltd.</organization>
      <address>
        <postal>
          <street>548 4th Street</street>
          <city>San Francisco</city>
          <region>CA</region>
          <code>94107</code>
          <country>USA</country>
        </postal>
        <email>aaron@serendipity.palo-alto.ca.us</email>
      </address>
    </author>

    <date day="14" month="December" year="2007" />

    <area>Applications</area>

    <keyword>memcache memcached cache udp</keyword>

    <abstract>
      <t>
      This memo explains extensions to the memcache binary protocol for use in a UDP environment.
      </t>

      <t>
      Memcache is a high performance key-value cache. It is intentionally a
      dumb cache, optimized for speed only. Applications using memcache do
      not rely on it for data -- a persistent database with guaranteed reliability
      is strongly recommended -- but applications can run much faster when
      cached data is available in memcache.
      </t>
    </abstract>
  </front>
      
  <middle>
    <section anchor="introduction" title="Introduction">
      <t>
      Memcache is a high performance key-value cache. It is intentionally a
      dumb cache, optimized for speed only. Applications using memcache do
      not rely on it for data -- a persistent database with guaranteed reliability
      is strongly recommended -- but applications can run much faster when
      cached data is available in memcache.
      </t>
      <t>
      Sites may find that, due to their network architecture or application usage patterns,
      the stateless <xref target="UDP"/> protocol better suits their needs. This document
      provides extensions and descriptions of use of the <xref target="MEMCACHE">memcache protocol</xref>
      in a UDP environment.
      </t>
      <t>
      It is a goal of this document to provide sufficient information in each UDP packet
      as to avoid any requirement for statefulness on the part of the server nor significant
      caching of outstanding packets on the part of the client.
      </t>
      <section anchor="conventions" title="Conventions Used In This Document">
        <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="KEYWORDS"/>.
        </t>
      </section>
    </section>

    <section anchor="values" title="Defined Values">
      <section anchor="value-magic" title="Magic Byte">
        <t>
        The magic bytes remains the same as in <xref target="MEMCACHE"/>.
        </t>
      </section>

      <section anchor="value-status" title="Response Status">
        <t>
        Additional status values:
        <list hangIndent="8" style="hanging">
          <t hangText="0x0004">Value is larger than a single response packet</t>
        </list>
        </t>
      </section>

      <section anchor="value-opcodes" title="Command Opcodes">
        <t>
        Additional opcode values:
        <list hangIndent="8" style="hanging">
          <t hangText="0x0C">Get Range</t>
          <t hangText="0x0D">Set Range</t>
        </list>
        </t>
      </section>

      <section anchor="value-types" title="Data Types">
        <t>
        There are no new data types in this extension.
        </t>
      </section>
    </section>

    <section anchor="commands" title="Commands">

      <section anchor="command-get" title="Get Response">
        <t>
        This section extends the behavior of the Get and GetQ commands as described in 
        <xref target="MEMCACHE" x:sec="command-get"/>.
        </t>

        <t>
        When a Get or GetQ request is made via UDP, and the value of the key for which
        the request was made is larger than can be placed into a single UDP packet (noting
        that the protocol header must also be counted), a Get Range response packet
        MUST be sent instead of the Get response packet. In this instance:
        <list style="numbers">
          <t>The Status field of the response header MUST be 0x0004.</t>
          <t>The Offset field of the GetR response extras MUST be 0.</t>
          <t>The Length field of the GetR response extras, and the data contained in
             the Value field of the packet, SHOULD be the maximum
             allowed length of a UDP packet, less the space required by the header
             and extras; however it MAY be any amount below this maximum.</t>
          <t>The Total value length field of the response extras MUST be the
             actual length of the complete value.</t>
        </list>
        </t>

        <t>
        The client, upon receipt of a Get Range response bearing Status 0x004
        and a Message ID corresponding to its Get request, shall then know that
        it has received only the first portion of the value. The client MAY choose
        to request the remaining portion of the value by sending one or more Get Range requests.
        </t>
      </section>

      <section anchor="command-getr-request" title="Get Range Request">
        <t>
          The Get Range request is primarily intended for use over a UDP transport
          to request byte ranges of the value for a key. In the event that the Data version
          check fails to match that of the key, an error MUST be returned.
        </t>
        <t>
      <figure>
        <preamble>Extra data for get range request:</preamble>
          <artwork>
    Byte/     0       |       1       |       2       |       3       |
       /              |               |               |               |
      |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
      +---------------+---------------+---------------+---------------+
     0| Flags                                                         |
      +---------------+---------------+---------------+---------------+
     4| Data version check                                            |
      |                                                               |
      +---------------+---------------+---------------+---------------+
    12| Offset                                                        |
      +---------------+---------------+---------------+---------------+
    16| Length                                                        |
      +---------------+---------------+---------------+---------------+
    Total 20 bytes
      </artwork></figure>
        </t>
      </section>

      <section anchor="command-getr-response" title="Get Range Response">
        <t>
          The Get Range request is primarily intended for use over a UDP transport
          to indicate the location of the bytes of the value for a key contained in
          a given packet. A client receives enough information in each Get Range
          extras to construct an appropriately sized buffer in its own memory and
          blindly insert the contents of the packet at the given byte offset.
        </t>
        <t>
      <figure>
        <preamble>Extra data for get range response:</preamble>
          <artwork>
    Byte/     0       |       1       |       2       |       3       |
       /              |               |               |               |
      |0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|0 1 2 3 4 5 6 7|
      +---------------+---------------+---------------+---------------+
     0| Flags                                                         |
      +---------------+---------------+---------------+---------------+
     4| Data version check                                            |
      |                                                               |
      +---------------+---------------+---------------+---------------+
    12| Offset                                                        |
      +---------------+---------------+---------------+---------------+
    16| Length                                                        |
      +---------------+---------------+---------------+---------------+
    20| Total value length                                            |
      +---------------+---------------+---------------+---------------+
    Total 24 bytes
      </artwork></figure>
        </t>
      </section>

    </section>

    <section anchor="security" title="Security Considerations">
      <t>
      This document does not introduce any new security considerations
      beyond those discussed in <xref target="MEMCACHE" x:sec="security"/>.
      </t>
    </section>

  </middle>

  <back>
    <references title="Normative References">
      <dwdrfc-ref anchor='UDP' src='http://xml.resource.org/public/rfc/bibxml/reference.RFC.0768.xml'/>
      <dwdrfc-ref anchor='KEYWORDS' src='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml'/>
      <!-- FIXME: Get a draft reference for the base document. -->
      <dwdrfc-ref anchor='MEMCACHE' src='http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml'/>
    </references>
  </back>

</rfc>