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    <title>GDP GCL Metadata</title>
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    <h1>GDP GCL Metadata</h1>
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    <h4>Eric Allman, Ubiquitous Swarm Lab, U.C. Berkeley<br>
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      Draft Document Version 0.10, 2015-01-23<br>
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    </h4>
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    <p>The GDP supports a limited amount of write-once per-GCL metadata.&nbsp;
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      It can be added when a GCL is created and is thenceforth immutable.<br>
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    </p>
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    <p>Each piece of GCL metadata consists of a name, a data length, and
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      (optionally) some data.&nbsp; Any typing information is implied by the
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      name.<br>
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    </p>
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    <p>Names are exactly four bytes.&nbsp; All names with the high-order octet
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      set to zero are reserved for use by the system.&nbsp; It is recommended
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      that application defined names be exactly four ASCII characters, e.g.,
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      'ABCD' (which is equal to 0x41424344).&nbsp; Note that numeric constants
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      should be used, since the valid C constant 'ABCD' might be represented as
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      0x44434241 on some architectures; it is the numeric constant which defines
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      the name.&nbsp; The name that is all zeros is reserved to indicate "end of
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      list".&nbsp; <i>[[Registration of codes is To Be Done.]]</i><br>
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    </p>
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    <p>Lengths are three bytes packed into a four byte integer, representing the
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      number of octets in the data.&nbsp; The top eight bits are reserved for
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      future use.&nbsp; They must be zero when sent and ignored on receipt.<br>
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    </p>
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    <p>Data is a series of opaque bytes.&nbsp; It might be encrypted, at the
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      discretion of the application.<br>
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    </p>
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    <h2>Sending and Accessing Metadata</h2>
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    <p>The metadata interface is as follows:<br>
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    </p>
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    <p><samp>&nbsp;&nbsp;&nbsp; gdp_gclmd_t&nbsp;&nbsp;&nbsp;&nbsp;
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        *gdp_gclmd_new(void);</samp></p>
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    <p><samp>&nbsp;&nbsp;&nbsp; void&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; gdp_gclmd_free(gdp_gclmd_t *gmd);</samp></p>
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    <p><samp>&nbsp;&nbsp;&nbsp; EP_STAT&nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp;&nbsp; gdp_gclmd_add(gdp_gclmd_t *gmd,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; gdp_gclmd_id_t id,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; size_t len,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; const void *data);<br>
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      </samp></p>
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    <p><samp>&nbsp;&nbsp;&nbsp; EP_STAT&nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp;&nbsp; gdp_gclmd_get(gdp_gclmd_t *gmd,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; int index,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; gdpd_gclmd_id_t *id,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; size_t *len,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; const void **data);<br>
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      </samp></p>
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    <p>These routines allocate and free a data structure (which will be
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      dynamically expanded if needed), add a new metadata field with the
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      indicated name, length, and value (the data will be copied), and return a
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      field at a given index (the data is <em>not</em> copied).<br>
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    </p>
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    <p>To create a new GCL with metadata an application should create a new data
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      structure using <code>gdp_gclmd_new</code>, populate it using <code>gdp_gclmd_add</code>,
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      send it as a parameter to <code>gdp_gcl_create</code>, and then free the
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      metadata using <code>gdp_gclmd_free</code>.<br>
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    </p>
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    <p>To access metadata in an existing GCL an application needs to fetch the
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      metadata:<br>
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    </p>
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    <p><samp>&nbsp;&nbsp;&nbsp; EP_STAT&nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp;&nbsp; gdp_gcl_getmetadata(gdp_gcl_t *gcl,<br>
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;
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        &nbsp;&nbsp;&nbsp; gdp_gclmd_t **gmdp); </samp></p>
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    <p>This fetches the metadata into memory (if it is not already there) and
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      passes a metadata structure back.&nbsp; This structure can then be
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      interrogated using <code>gdp_gclmd_get</code>.&nbsp; <i>[[Who owns the
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        metadata, i.e., who has to do gdp_gclmd_free?]]</i><br>
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      <samp></samp></p>
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    <h2>Representation</h2>
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    <p>There are potentially three different representations for metadata:
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      in-memory, on-the-wire, and on-disk.<br>
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    </p>
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    <h3>In-Memory<br>
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    </h3>
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    <p>The internal representation is opaque as far as the client program is
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      concerned, with all interaction done by function calls in deference to
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      alternate language bindings.&nbsp; The API is described above.&nbsp; <i>[[Details
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        To Be Done.]]</i><br>
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    </p>
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    <h3>On-The-Wire</h3>
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    <p>When sent over the network, all integers are sent in network byte order
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      (big-endian) as is standard for network protocols.&nbsp; The
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      representation for each metadatum is a four-octet integer name.&nbsp; If
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      that is zero then no more metadata fields are present and no more data is
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      sent.&nbsp; If non-zero, the name is followed by a four-octet length which
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      is immediately followed by that number of bytes of data.&nbsp; Note that
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      the integers are not guaranteed to be word aligned.<br>
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    </p>
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    <h3>On Disk</h3>
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    <p><i>[[To be written &mdash; really need to document everything, not just
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        metadata, probably in another document.]]</i><br>
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    </p>
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    <h3> </h3>
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