path: root/doc/dbus-specification.xml

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<article id="index">
  <articleinfo>
    <title>D-Bus Specification</title>
    <releaseinfo>Version 0.12</releaseinfo>
    <date>7 November 2006</date>
    <authorgroup>
      <author>
	<firstname>Havoc</firstname>
	<surname>Pennington</surname>
	<affiliation>
	  <orgname>Red Hat, Inc.</orgname>
	  <address>
	    <email>hp@pobox.com</email>
	  </address>
	</affiliation>
      </author>
      <author>
	<firstname>Anders</firstname>
	<surname>Carlsson</surname>
	<affiliation>
	  <orgname>CodeFactory AB</orgname>
	  <address>
            <email>andersca@codefactory.se</email>
          </address>
	</affiliation>
      </author>
      <author>
	<firstname>Alexander</firstname>
	<surname>Larsson</surname>
	<affiliation>
	  <orgname>Red Hat, Inc.</orgname>
	  <address>
            <email>alexl@redhat.com</email>
          </address>
	</affiliation>
      </author>
    </authorgroup>
  </articleinfo>

  <sect1 id="introduction">
    <title>Introduction</title>
    <para>
      D-Bus is a system for low-latency, low-overhead, easy to use
      interprocess communication (IPC). In more detail:
      <itemizedlist>
        <listitem>
          <para>
            D-Bus is <emphasis>low-latency</emphasis> because it is designed 
            to avoid round trips and allow asynchronous operation, much like 
            the X protocol.
          </para>
        </listitem>
        <listitem>
          <para>
            D-Bus is <emphasis>low-overhead</emphasis> because it uses a
            binary protocol, and does not have to convert to and from a text
            format such as XML. Because D-Bus is intended for potentially
            high-resolution same-machine IPC, not primarily for Internet IPC,
            this is an interesting optimization.
          </para>
        </listitem>
        <listitem>
          <para>
            D-Bus is <emphasis>easy to use</emphasis> because it works in terms
            of <firstterm>messages</firstterm> rather than byte streams, and
            automatically handles a lot of the hard IPC issues. Also, the D-Bus
            library is designed to be wrapped in a way that lets developers use
            their framework's existing object/type system, rather than learning
            a new one specifically for IPC.
          </para>
        </listitem>
      </itemizedlist>
    </para>

    <para>
      The base D-Bus protocol is a one-to-one (peer-to-peer or client-server)
      protocol, specified in <xref linkend="message-protocol"/>. That is, it is
      a system for one application to talk to a single other
      application. However, the primary intended application of the protocol is the
      D-Bus <firstterm>message bus</firstterm>, specified in <xref
      linkend="message-bus"/>. The message bus is a special application that
      accepts connections from multiple other applications, and forwards
      messages among them.
    </para>

    <para>
      Uses of D-Bus include notification of system changes (notification of when
      a camera is plugged in to a computer, or a new version of some software
      has been installed), or desktop interoperability, for example a file
      monitoring service or a configuration service.
    </para>

    <para>
      D-Bus is designed for two specific use cases:
      <itemizedlist>
        <listitem>
          <para>
            A "system bus" for notifications from the system to user sessions,
            and to allow the system to request input from user sessions.
          </para>
        </listitem>
        <listitem>
          <para>
            A "session bus" used to implement desktop environments such as 
            GNOME and KDE.
          </para>
        </listitem>
      </itemizedlist>
      D-Bus is not intended to be a generic IPC system for any possible 
      application, and intentionally omits many features found in other 
      IPC systems for this reason.
    </para>

    <para>
      At the same time, the bus daemons offer a number of features not found in
      other IPC systems, such as single-owner "bus names" (similar to X
      selections), on-demand startup of services, and security policies.
      In many ways, these features are the primary motivation for developing 
      D-Bus; other systems would have sufficed if IPC were the only goal.
    </para>

    <para>
      D-Bus may turn out to be useful in unanticipated applications, but future
      versions of this spec and the reference implementation probably will not
      incorporate features that interfere with the core use cases.
    </para>

    <para>
      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. However, the
      document could use a serious audit to be sure it makes sense to do
      so. Also, they are not capitalized.
    </para>

    <sect2 id="stability">
      <title>Protocol and Specification Stability</title>
      <para>
        The D-Bus protocol is frozen (only compatible extensions are allowed) as
        of November 8, 2006.  However, this specification could still use a fair
        bit of work to make interoperable reimplementation possible without
        reference to the D-Bus reference implementation. Thus, this
        specification is not marked 1.0. To mark it 1.0, we'd like to see
        someone invest significant effort in clarifying the specification
        language, and growing the specification to cover more aspects of the
        reference implementation's behavior.
      </para>
      <para>
        Until this work is complete, any attempt to reimplement D-Bus will 
        probably require looking at the reference implementation and/or asking
        questions on the D-Bus mailing list about intended behavior. 
        Questions on the list are very welcome.
      </para>
      <para>
        Nonetheless, this document should be a useful starting point and is 
        to our knowledge accurate, though incomplete.
      </para>
    </sect2>
    
  </sect1>

  <sect1 id="message-protocol">
    <title>Message Protocol</title>

    <para>
      A <firstterm>message</firstterm> consists of a
      <firstterm>header</firstterm> and a <firstterm>body</firstterm>. If you
      think of a message as a package, the header is the address, and the body
      contains the package contents. The message delivery system uses the header
      information to figure out where to send the message and how to interpret
      it; the recipient interprets the body of the message.
    </para>
    
    <para>
      The body of the message is made up of zero or more
      <firstterm>arguments</firstterm>, which are typed values, such as an
      integer or a byte array.
    </para>

    <para>
      Both header and body use the same type system and format for 
      serializing data. Each type of value has a wire format. 
      Converting a value from some other representation into the wire
      format is called <firstterm>marshaling</firstterm> and converting
      it back from the wire format is <firstterm>unmarshaling</firstterm>.
    </para>

    <sect2 id="message-protocol-signatures">
      <title>Type Signatures</title>

      <para>
        The D-Bus protocol does not include type tags in the marshaled data; a
        block of marshaled values must have a known <firstterm>type
        signature</firstterm>.  The type signature is made up of <firstterm>type
        codes</firstterm>. A type code is an ASCII character representing the
        type of a value. Because ASCII characters are used, the type signature
        will always form a valid ASCII string. A simple string compare 
        determines whether two type signatures are equivalent.
      </para>

      <para>
        As a simple example, the type code for 32-bit integer (<literal>INT32</literal>) is
        the ASCII character 'i'. So the signature for a block of values 
        containing a single <literal>INT32</literal> would be:
        <programlisting>
          "i"
        </programlisting>
        A block of values containing two <literal>INT32</literal> would have this signature:
        <programlisting>
          "ii"
        </programlisting>        
      </para>

      <para>
        All <firstterm>basic</firstterm> types work like 
        <literal>INT32</literal> in this example. To marshal and unmarshal 
        basic types, you simply read one value from the data
        block corresponding to each type code in the signature.
        In addition to basic types, there are four <firstterm>container</firstterm> 
        types: <literal>STRUCT</literal>, <literal>ARRAY</literal>, <literal>VARIANT</literal>, 
        and <literal>DICT_ENTRY</literal>.
      </para>

      <para>
        <literal>STRUCT</literal> has a type code, ASCII character 'r', but this type 
        code does not appear in signatures. Instead, ASCII characters
        '(' and ')' are used to mark the beginning and end of the struct.
        So for example, a struct containing two integers would have this 
        signature:
        <programlisting>
          "(ii)"
        </programlisting>
        Structs can be nested, so for example a struct containing 
        an integer and another struct:
        <programlisting>
          "(i(ii))"
        </programlisting>
        The value block storing that struct would contain three integers; the
        type signature allows you to distinguish "(i(ii))" from "((ii)i)" or
        "(iii)" or "iii".
      </para>

      <para>
        The <literal>STRUCT</literal> type code 'r' is not currently used in the D-Bus protocol,
        but is useful in code that implements the protocol. This type code 
        is specified to allow such code to interoperate in non-protocol contexts.
      </para>

      <para>
        Empty structures are not allowed; there must be at least one
        type code between the parentheses.
      </para>

      <para>
        <literal>ARRAY</literal> has ASCII character 'a' as type code. The array type code must be
        followed by a <firstterm>single complete type</firstterm>. The single
        complete type following the array is the type of each array element. So
        the simple example is:
        <programlisting>
          "ai"
        </programlisting>
        which is an array of 32-bit integers. But an array can be of any type, 
        such as this array-of-struct-with-two-int32-fields:
        <programlisting>
          "a(ii)"
        </programlisting>
        Or this array of array of integer:
        <programlisting>
          "aai"
        </programlisting>
      </para>

      <para>
        The phrase <firstterm>single complete type</firstterm> deserves some 
        definition. A single complete type is a basic type code, a variant type code, 
        an array with its element type, or a struct with its fields. 
        So the following signatures are not single complete types:
        <programlisting>
          "aa"
        </programlisting>
        <programlisting>
          "(ii"
        </programlisting>
        <programlisting>
          "ii)"
        </programlisting>
        And the following signatures contain multiple complete types:
        <programlisting>
          "ii"
        </programlisting>
        <programlisting>
          "aiai"
        </programlisting>
        <programlisting>
          "(ii)(ii)"
        </programlisting>
        Note however that a single complete type may <emphasis>contain</emphasis>
        multiple other single complete types.
      </para>

      <para>
        <literal>VARIANT</literal> has ASCII character 'v' as its type code. A marshaled value of
        type <literal>VARIANT</literal> will have the signature of a single complete type as part
        of the <emphasis>value</emphasis>.  This signature will be followed by a
        marshaled value of that type.
      </para>

      <para>
        A <literal>DICT_ENTRY</literal> works exactly like a struct, but rather
        than parentheses it uses curly braces, and it has more restrictions.
        The restrictions are: it occurs only as an array element type; it has
        exactly two single complete types inside the curly braces; the first
        single complete type (the "key") must be a basic type rather than a
        container type. Implementations must not accept dict entries outside of
        arrays, must not accept dict entries with zero, one, or more than two
        fields, and must not accept dict entries with non-basic-typed keys. A
        dict entry is always a key-value pair.
      </para>
      
      <para>
        The first field in the <literal>DICT_ENTRY</literal> is always the key.
        A message is considered corrupt if the same key occurs twice in the same
        array of <literal>DICT_ENTRY</literal>. However, for performance reasons
        implementations are not required to reject dicts with duplicate keys.
      </para>

      <para>
        In most languages, an array of dict entry would be represented as a 
        map, hash table, or dict object.
      </para>

      <para>
        The following table summarizes the D-Bus types.
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional Name</entry>
                <entry>Code</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>INVALID</literal></entry>
                <entry>0 (ASCII NUL)</entry>
                <entry>Not a valid type code, used to terminate signatures</entry>
              </row><row>
		<entry><literal>BYTE</literal></entry>
		<entry>121 (ASCII 'y')</entry>
		<entry>8-bit unsigned integer</entry>
              </row><row>
		<entry><literal>BOOLEAN</literal></entry>
		<entry>98 (ASCII 'b')</entry>
		<entry>Boolean value, 0 is <literal>FALSE</literal> and 1 is <literal>TRUE</literal>. Everything else is invalid.</entry>
	      </row><row>
                <entry><literal>INT16</literal></entry>
                <entry>110 (ASCII 'n')</entry>
                <entry>16-bit signed integer</entry>
              </row><row>
                <entry><literal>UINT16</literal></entry>
                <entry>113 (ASCII 'q')</entry>
                <entry>16-bit unsigned integer</entry>
	      </row><row>
                <entry><literal>INT32</literal></entry>
                <entry>105 (ASCII 'i')</entry>
                <entry>32-bit signed integer</entry>
              </row><row>
                <entry><literal>UINT32</literal></entry>
                <entry>117 (ASCII 'u')</entry>
                <entry>32-bit unsigned integer</entry>
	      </row><row>
                <entry><literal>INT64</literal></entry>
                <entry>120 (ASCII 'x')</entry>
                <entry>64-bit signed integer</entry>
              </row><row>
                <entry><literal>UINT64</literal></entry>
                <entry>116 (ASCII 't')</entry>
                <entry>64-bit unsigned integer</entry>
              </row><row>
                <entry><literal>DOUBLE</literal></entry>
                <entry>100 (ASCII 'd')</entry>
                <entry>IEEE 754 double</entry>
              </row><row>
                <entry><literal>STRING</literal></entry>
                <entry>115 (ASCII 's')</entry>
                <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8). Must be nul terminated and contain no other nul bytes.</entry>
              </row><row>
                <entry><literal>OBJECT_PATH</literal></entry>
                <entry>111 (ASCII 'o')</entry>
                <entry>Name of an object instance</entry>
              </row><row>
                <entry><literal>SIGNATURE</literal></entry>
                <entry>103 (ASCII 'g')</entry>
                <entry>A type signature</entry>
              </row><row>
                <entry><literal>ARRAY</literal></entry>
                <entry>97 (ASCII 'a')</entry>
                <entry>Array</entry>
              </row><row>
                <entry><literal>STRUCT</literal></entry>
                <entry>114 (ASCII 'r'), 40 (ASCII '('), 41 (ASCII ')')</entry>
                <entry>Struct</entry>
              </row><row>
                <entry><literal>VARIANT</literal></entry>
                <entry>118 (ASCII 'v') </entry>
                <entry>Variant type (the type of the value is part of the value itself)</entry>
              </row><row>
                <entry><literal>DICT_ENTRY</literal></entry>
                <entry>101 (ASCII 'e'), 123 (ASCII '{'), 125 (ASCII '}') </entry>
                <entry>Entry in a dict or map (array of key-value pairs)</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

    </sect2>

    <sect2 id="message-protocol-marshaling">
      <title>Marshaling (Wire Format)</title>

      <para>
        Given a type signature, a block of bytes can be converted into typed
        values. This section describes the format of the block of bytes.  Byte
        order and alignment issues are handled uniformly for all D-Bus types.
      </para>

      <para>
        A block of bytes has an associated byte order. The byte order 
        has to be discovered in some way; for D-Bus messages, the 
        byte order is part of the message header as described in 
        <xref linkend="message-protocol-messages"/>. For now, assume 
        that the byte order is known to be either little endian or big 
          endian.
      </para>

      <para>
        Each value in a block of bytes is aligned "naturally," for example
        4-byte values are aligned to a 4-byte boundary, and 8-byte values to an
        8-byte boundary. To properly align a value, <firstterm>alignment
        padding</firstterm> may be necessary. The alignment padding must always
        be the minimum required padding to properly align the following value;
        and it must always be made up of nul bytes. The alignment padding must
        not be left uninitialized (it can't contain garbage), and more padding
        than required must not be used.
      </para>

      <para>
        Given all this, the types are marshaled on the wire as follows:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional Name</entry>
                <entry>Encoding</entry>
                <entry>Alignment</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>INVALID</literal></entry>
                <entry>Not applicable; cannot be marshaled.</entry>
                <entry>N/A</entry>
              </row><row>
                <entry><literal>BYTE</literal></entry>
                <entry>A single 8-bit byte.</entry>
                <entry>1</entry>
              </row><row>
                <entry><literal>BOOLEAN</literal></entry>
                <entry>As for <literal>UINT32</literal>, but only 0 and 1 are valid values.</entry>
                <entry>4</entry>
              </row><row>
                <entry><literal>INT16</literal></entry>
                <entry>16-bit signed integer in the message's byte order.</entry>
                <entry>2</entry>
              </row><row>
                <entry><literal>UINT16</literal></entry>
                <entry>16-bit unsigned integer in the message's byte order.</entry>
                <entry>2</entry>
              </row><row>
                <entry><literal>INT32</literal></entry>
                <entry>32-bit signed integer in the message's byte order.</entry>
                <entry>4</entry>
              </row><row>
                <entry><literal>UINT32</literal></entry>
                <entry>32-bit unsigned integer in the message's byte order.</entry>
                <entry>4</entry>
              </row><row>
                <entry><literal>INT64</literal></entry>
                <entry>64-bit signed integer in the message's byte order.</entry>
                <entry>8</entry>
              </row><row>
                <entry><literal>UINT64</literal></entry>
                <entry>64-bit unsigned integer in the message's byte order.</entry>
                <entry>8</entry>
              </row><row>
                <entry><literal>DOUBLE</literal></entry>
                <entry>64-bit IEEE 754 double in the message's byte order.</entry>
                <entry>8</entry>
              </row><row>
                <entry><literal>STRING</literal></entry>
                <entry>A <literal>UINT32</literal> indicating the string's 
                  length in bytes excluding its terminating nul, followed by 
                  non-nul string data of the given length, followed by a terminating nul 
                  byte.
                </entry>
                <entry>
                  4 (for the length)
                </entry>
              </row><row>
                <entry><literal>OBJECT_PATH</literal></entry>
                <entry>Exactly the same as <literal>STRING</literal> except the 
                  content must be a valid object path (see below).
                </entry>
                <entry>
                  4 (for the length)
                </entry>
              </row><row>
                <entry><literal>SIGNATURE</literal></entry>
                <entry>The same as <literal>STRING</literal> except the length is a single 
                  byte (thus signatures have a maximum length of 255)
                  and the content must be a valid signature (see below).
                </entry>
                <entry>
                  1
                </entry>
              </row><row>
                <entry><literal>ARRAY</literal></entry>
                <entry>
                  A <literal>UINT32</literal> giving the length of the array data in bytes, followed by 
                  alignment padding to the alignment boundary of the array element type, 
                  followed by each array element. The array length is from the 
                  end of the alignment padding to the end of the last element,
                  i.e. it does not include the padding after the length,
                  or any padding after the last element.
                  Arrays have a maximum length defined to be 2 to the 26th power or
                  67108864. Implementations must not send or accept arrays exceeding this
                  length.
                </entry>
                <entry>
                  4 (for the length)
                </entry>
              </row><row>
                <entry><literal>STRUCT</literal></entry>
                <entry>
                  A struct must start on an 8-byte boundary regardless of the
                  type of the struct fields. The struct value consists of each
                  field marshaled in sequence starting from that 8-byte
                  alignment boundary.
                </entry>
                <entry>
                  8
                </entry>
	      </row><row>
                <entry><literal>VARIANT</literal></entry>
                <entry>
                  A variant type has a marshaled <literal>SIGNATURE</literal>
                  followed by a marshaled value with the type
                  given in the signature.
                  Unlike a message signature, the variant signature 
                  can contain only a single complete type.
                  So "i" is OK, "ii" is not.
                </entry>
                <entry>
                  1 (alignment of the signature)
                </entry>
	      </row><row>
                <entry><literal>DICT_ENTRY</literal></entry>
                <entry>
                  Identical to STRUCT.
                </entry>
                <entry>
                  8
                </entry>
	      </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      
      <sect3 id="message-protocol-marshaling-object-path">
        <title>Valid Object Paths</title>
        
        <para>
          An object path is a name used to refer to an object instance.
          Conceptually, each participant in a D-Bus message exchange may have
          any number of object instances (think of C++ or Java objects) and each
          such instance will have a path. Like a filesystem, the object
          instances in an application form a hierarchical tree.
        </para>
        
        <para>
          The following rules define a valid object path. Implementations must 
          not send or accept messages with invalid object paths.
          <itemizedlist>
            <listitem>
              <para>
                The path may be of any length.
              </para>
            </listitem>
            <listitem>
              <para>
                The path must begin with an ASCII '/' (integer 47) character, 
                and must consist of elements separated by slash characters.
              </para>
            </listitem>
            <listitem>
              <para>
                Each element must only contain the ASCII characters 
                "[A-Z][a-z][0-9]_"
              </para>
            </listitem>
            <listitem>
              <para>
                No element may be the empty string.
              </para>
            </listitem>
            <listitem>
              <para>
                Multiple '/' characters cannot occur in sequence.
              </para>
            </listitem>
            <listitem>
              <para>
                A trailing '/' character is not allowed unless the 
                path is the root path (a single '/' character).
              </para>
            </listitem>
          </itemizedlist>
        </para>

      </sect3>

      
      <sect3 id="message-protocol-marshaling-signature">
        <title>Valid Signatures</title>
        <para>
          An implementation must not send or accept invalid signatures.
          Valid signatures will conform to the following rules:
          <itemizedlist>
            <listitem>
              <para>
                The signature ends with a nul byte.
              </para>
            </listitem>
            <listitem>
              <para>
                The signature is a list of single complete types. 
                Arrays must have element types, and structs must 
                have both open and close parentheses.
              </para>
            </listitem>
            <listitem>
              <para>
                Only type codes and open and close parentheses are 
                allowed in the signature. The <literal>STRUCT</literal> type code
                is not allowed in signatures, because parentheses
                are used instead.
              </para>
            </listitem>
            <listitem>
              <para>
                The maximum depth of container type nesting is 32 array type
                codes and 32 open parentheses. This implies that the maximum
                total depth of recursion is 64, for an "array of array of array
                of ... struct of struct of struct of ..."  where there are 32
                array and 32 struct.
              </para>
            </listitem>
            <listitem>
              <para>
                The maximum length of a signature is 255.
              </para>
            </listitem>
            <listitem>
              <para>
                Signatures must be nul-terminated.
              </para>
            </listitem>
          </itemizedlist>
        </para>
      </sect3>
      
    </sect2>

    <sect2 id="message-protocol-messages">
      <title>Message Format</title>

      <para>
        A message consists of a header and a body. The header is a block of
        values with a fixed signature and meaning.  The body is a separate block
        of values, with a signature specified in the header.
      </para>

      <para>
        The length of the header must be a multiple of 8, allowing the body to
        begin on an 8-byte boundary when storing the entire message in a single
        buffer. If the header does not naturally end on an 8-byte boundary 
        up to 7 bytes of nul-initialized alignment padding must be added.
      </para>

      <para>
        The message body need not end on an 8-byte boundary.
      </para>

      <para>
        The maximum length of a message, including header, header alignment padding, 
        and body is 2 to the 27th power or 134217728. Implementations must not 
        send or accept messages exceeding this size.
      </para>
      
      <para>
        The signature of the header is:
        <programlisting>
          "yyyyuua(yv)"
        </programlisting>
        Written out more readably, this is:
        <programlisting>
          BYTE, BYTE, BYTE, BYTE, UINT32, UINT32, ARRAY of STRUCT of (BYTE,VARIANT)
        </programlisting>
      </para>

      <para>
        These values have the following meanings:
        <informaltable>
          <tgroup cols="2">
            <thead>
              <row>
                <entry>Value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>1st <literal>BYTE</literal></entry>
                <entry>Endianness flag; ASCII 'l' for little-endian 
                  or ASCII 'B' for big-endian. Both header and body are 
                in this endianness.</entry>
              </row>
              <row>
                <entry>2nd <literal>BYTE</literal></entry>
                <entry><firstterm>Message type</firstterm>. Unknown types must be ignored. 
                  Currently-defined types are described below.
                </entry>
              </row>
              <row>
                <entry>3rd <literal>BYTE</literal></entry>
                <entry>Bitwise OR of flags. Unknown flags
                  must be ignored. Currently-defined flags are described below.
                </entry>
              </row>
              <row>
                <entry>4th <literal>BYTE</literal></entry>
                <entry>Major protocol version of the sending application.  If
                the major protocol version of the receiving application does not
                match, the applications will not be able to communicate and the
                D-Bus connection must be disconnected. The major protocol
                version for this version of the specification is 1.
                </entry>
              </row>
              <row>
                <entry>1st <literal>UINT32</literal></entry>
                <entry>Length in bytes of the message body, starting 
                  from the end of the header. The header ends after 
                  its alignment padding to an 8-boundary.
                </entry>
              </row>
              <row>
                <entry>2nd <literal>UINT32</literal></entry>
                <entry>The serial of this message, used as a cookie 
                  by the sender to identify the reply corresponding
                  to this request.
                </entry>
              </row>      
              <row>
                <entry><literal>ARRAY</literal> of <literal>STRUCT</literal> of (<literal>BYTE</literal>,<literal>VARIANT</literal>)</entry>
                <entry>An array of zero or more <firstterm>header
                  fields</firstterm> where the byte is the field code, and the
                  variant is the field value. The message type determines 
                  which fields are required.
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        <firstterm>Message types</firstterm> that can appear in the second byte
        of the header are:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional name</entry>
                <entry>Decimal value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>INVALID</literal></entry>
                <entry>0</entry>
                <entry>This is an invalid type.</entry>
              </row>
              <row>
                <entry><literal>METHOD_CALL</literal></entry>
                <entry>1</entry>
                <entry>Method call.</entry>
              </row>
              <row>
                <entry><literal>METHOD_RETURN</literal></entry>
                <entry>2</entry>
                <entry>Method reply with returned data.</entry>
              </row>
              <row>
                <entry><literal>ERROR</literal></entry>
                <entry>3</entry>
                <entry>Error reply. If the first argument exists and is a
                string, it is an error message.</entry>
              </row>
              <row>
                <entry><literal>SIGNAL</literal></entry>
                <entry>4</entry>
                <entry>Signal emission.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        Flags that can appear in the third byte of the header:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Conventional name</entry>
                <entry>Hex value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry><literal>NO_REPLY_EXPECTED</literal></entry>
                <entry>0x1</entry>
                <entry>This message does not expect method return replies or
                error replies; the reply can be omitted as an
                optimization. However, it is compliant with this specification
                to return the reply despite this flag and the only harm 
                  from doing so is extra network traffic.
                </entry>
              </row>
              <row>
                <entry><literal>NO_AUTO_START</literal></entry>
                <entry>0x2</entry>
                <entry>The bus must not launch an owner
                  for the destination name in response to this message.
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>

      <sect3 id="message-protocol-header-fields">
        <title>Header Fields</title>

        <para>
          The array at the end of the header contains <firstterm>header
          fields</firstterm>, where each field is a 1-byte field code followed
          by a field value. A header must contain the required header fields for
          its message type, and zero or more of any optional header
          fields. Future versions of this protocol specification may add new
          fields. Implementations must ignore fields they do not
          understand. Implementations must not invent their own header fields;
          only changes to this specification may introduce new header fields.
        </para>

        <para>
          Again, if an implementation sees a header field code that it does not
          expect, it must ignore that field, as it will be part of a new
          (but compatible) version of this specification. This also applies 
          to known header fields appearing in unexpected messages, for 
          example: if a signal has a reply serial it must be ignored
          even though it has no meaning as of this version of the spec.
        </para>

        <para>
          However, implementations must not send or accept known header fields
          with the wrong type stored in the field value. So for example a
          message with an <literal>INTERFACE</literal> field of type
          <literal>UINT32</literal> would be considered corrupt.
        </para>

        <para>
          Here are the currently-defined header fields:
          <informaltable>
            <tgroup cols="5">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Decimal Code</entry>
                  <entry>Type</entry>
                  <entry>Required In</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry><literal>INVALID</literal></entry>
                  <entry>0</entry>
                  <entry>N/A</entry>
                  <entry>not allowed</entry>
                  <entry>Not a valid field name (error if it appears in a message)</entry>
                </row>
                <row>
                  <entry><literal>PATH</literal></entry>
                  <entry>1</entry>
                  <entry><literal>OBJECT_PATH</literal></entry>
                  <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
                  <entry>The object to send a call to,
                    or the object a signal is emitted from.
                    The special path
                    <literal>/org/freedesktop/DBus/Local</literal> is reserved;
                    implementations should not send messages with this path,
                    and the reference implementation of the bus daemon will
                    disconnect any application that attempts to do so.
                  </entry>
                </row>
                <row>
                  <entry><literal>INTERFACE</literal></entry>
                  <entry>2</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry><literal>SIGNAL</literal></entry>
                  <entry>
                    The interface to invoke a method call on, or 
                    that a signal is emitted from. Optional for 
                    method calls, required for signals.
                    The special interface
                    <literal>org.freedesktop.DBus.Local</literal> is reserved;
                    implementations should not send messages with this
                    interface, and the reference implementation of the bus
                    daemon will disconnect any application that attempts to
                    do so.
                  </entry>
                </row>
                <row>
                  <entry><literal>MEMBER</literal></entry>
                  <entry>3</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry><literal>METHOD_CALL</literal>, <literal>SIGNAL</literal></entry>
                  <entry>The member, either the method name or signal name.</entry>
                </row>
                <row>
                  <entry><literal>ERROR_NAME</literal></entry>
                  <entry>4</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry><literal>ERROR</literal></entry>
                  <entry>The name of the error that occurred, for errors</entry>
                </row>
                <row>
                  <entry><literal>REPLY_SERIAL</literal></entry>
                  <entry>5</entry>
                  <entry><literal>UINT32</literal></entry>
                  <entry><literal>ERROR</literal>, <literal>METHOD_RETURN</literal></entry>
                  <entry>The serial number of the message this message is a reply
                    to. (The serial number is the second <literal>UINT32</literal> in the header.)</entry>
                </row>
                <row>
                  <entry><literal>DESTINATION</literal></entry>
                  <entry>6</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry>optional</entry>
                  <entry>The name of the connection this message is intended for.
                    Only used in combination with the message bus, see 
                    <xref linkend="message-bus"/>.</entry>
                </row>
                <row>
                  <entry><literal>SENDER</literal></entry>
                  <entry>7</entry>
                  <entry><literal>STRING</literal></entry>
                  <entry>optional</entry>
                  <entry>Unique name of the sending connection.
                    The message bus fills in this field so it is reliable; the field is
                    only meaningful in combination with the message bus.</entry>
                </row>
                <row>
                  <entry><literal>SIGNATURE</literal></entry>
                  <entry>8</entry>
                  <entry><literal>SIGNATURE</literal></entry>
                  <entry>optional</entry>
                  <entry>The signature of the message body.
                  If omitted, it is assumed to be the 
                  empty signature "" (i.e. the body must be 0-length).</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-protocol-names">
      <title>Valid Names</title>
      <para>
        The various names in D-Bus messages have some restrictions.
      </para>
      <para>
        There is a <firstterm>maximum name length</firstterm> 
        of 255 which applies to bus names, interfaces, and members. 
      </para>
      <sect3 id="message-protocol-names-interface">
        <title>Interface names</title>
        <para>
          Interfaces have names with type <literal>STRING</literal>, meaning that 
          they must be valid UTF-8. However, there are also some 
          additional restrictions that apply to interface names 
          specifically:
          <itemizedlist>
            <listitem><para>Interface names are composed of 1 or more elements separated by
                a period ('.') character. All elements must contain at least 
                one character.
                </para>
            </listitem>
            <listitem><para>Each element must only contain the ASCII characters 
                "[A-Z][a-z][0-9]_" and must not begin with a digit.
                </para>
            </listitem>

	    <listitem><para>Interface names must contain at least one '.' (period)
              character (and thus at least two elements).
              </para></listitem>

	    <listitem><para>Interface names must not begin with a '.' (period) character.</para></listitem>
	    <listitem><para>Interface names must not exceed the maximum name length.</para></listitem>
          </itemizedlist>
        </para>
      </sect3>
      <sect3 id="message-protocol-names-bus">
        <title>Bus names</title>
        <para>
          Connections have one or more bus names associated with them.
          A connection has exactly one bus name that is a unique connection
          name. The unique connection name remains with the connection for
          its entire lifetime.
          A bus name is of type <literal>STRING</literal>,
          meaning that it must be valid UTF-8. However, there are also
          some additional restrictions that apply to bus names 
          specifically:
          <itemizedlist>
            <listitem><para>Bus names that start with a colon (':')
                character are unique connection names.
                </para>
            </listitem>
            <listitem><para>Bus names are composed of 1 or more elements separated by
                a period ('.') character. All elements must contain at least 
                one character.
                </para>
            </listitem>
            <listitem><para>Each element must only contain the ASCII characters 
                "[A-Z][a-z][0-9]_-". Only elements that are part of a unique
                connection name may begin with a digit, elements in
                other bus names must not begin with a digit.
                </para>
            </listitem>

	    <listitem><para>Bus names must contain at least one '.' (period)
              character (and thus at least two elements).
              </para></listitem>

	    <listitem><para>Bus names must not begin with a '.' (period) character.</para></listitem>
	    <listitem><para>Bus names must not exceed the maximum name length.</para></listitem>
          </itemizedlist>
        </para>
        <para>
          Note that the hyphen ('-') character is allowed in bus names but
          not in interface names.
        </para>
      </sect3>
      <sect3 id="message-protocol-names-member">
        <title>Member names</title>
        <para>
          Member (i.e. method or signal) names:
          <itemizedlist>
	    <listitem><para>Must only contain the ASCII characters
                "[A-Z][a-z][0-9]_" and may not begin with a
                digit.</para></listitem>
	    <listitem><para>Must not contain the '.' (period) character.</para></listitem>
	    <listitem><para>Must not exceed the maximum name length.</para></listitem>
	    <listitem><para>Must be at least 1 byte in length.</para></listitem>
          </itemizedlist>
        </para>
      </sect3>
      <sect3 id="message-protocol-names-error">
        <title>Error names</title>
        <para>
          Error names have the same restrictions as interface names.
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-protocol-types">
      <title>Message Types</title>
      <para>
        Each of the message types (<literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>, <literal>ERROR</literal>, and
        <literal>SIGNAL</literal>) has its own expected usage conventions and header fields.
        This section describes these conventions.
      </para>
      <sect3 id="message-protocol-types-method">
        <title>Method Calls</title>
        <para>
          Some messages invoke an operation on a remote object.  These are
          called method call messages and have the type tag <literal>METHOD_CALL</literal>. Such
          messages map naturally to methods on objects in a typical program.
        </para>
        <para>
          A method call message is required to have a <literal>MEMBER</literal> header field
          indicating the name of the method. Optionally, the message has an
          <literal>INTERFACE</literal> field giving the interface the method is a part of. In the
          absence of an <literal>INTERFACE</literal> field, if two interfaces on the same object have
          a method with the same name, it is undefined which of the two methods
          will be invoked. Implementations may also choose to return an error in
          this ambiguous case. However, if a method name is unique
          implementations must not require an interface field.
        </para>
        <para>
          Method call messages also include a <literal>PATH</literal> field
          indicating the object to invoke the method on. If the call is passing
          through a message bus, the message will also have a
          <literal>DESTINATION</literal> field giving the name of the connection
          to receive the message.
        </para>
        <para>
          When an application handles a method call message, it is required to
          return a reply. The reply is identified by a <literal>REPLY_SERIAL</literal> header field
          indicating the serial number of the <literal>METHOD_CALL</literal> being replied to. The
          reply can have one of two types; either <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>.
        </para>
        <para>
          If the reply has type <literal>METHOD_RETURN</literal>, the arguments to the reply message 
          are the return value(s) or "out parameters" of the method call. 
          If the reply has type <literal>ERROR</literal>, then an "exception" has been thrown, 
          and the call fails; no return value will be provided. It makes 
          no sense to send multiple replies to the same method call.
        </para>
        <para>
          Even if a method call has no return values, a <literal>METHOD_RETURN</literal> 
          reply is required, so the caller will know the method 
          was successfully processed.
        </para>
        <para>
          The <literal>METHOD_RETURN</literal> or <literal>ERROR</literal> reply message must have the <literal>REPLY_SERIAL</literal> 
          header field.
        </para>
        <para>
          If a <literal>METHOD_CALL</literal> message has the flag <literal>NO_REPLY_EXPECTED</literal>, 
          then as an optimization the application receiving the method 
          call may choose to omit the reply message (regardless of 
          whether the reply would have been <literal>METHOD_RETURN</literal> or <literal>ERROR</literal>). 
          However, it is also acceptable to ignore the <literal>NO_REPLY_EXPECTED</literal>
          flag and reply anyway.
        </para>
        <para>
          Unless a message has the flag <literal>NO_AUTO_START</literal>, if the
          destination name does not exist then a program to own the destination
          name will be started before the message is delivered.  The message
          will be held until the new program is successfully started or has
          failed to start; in case of failure, an error will be returned. This
          flag is only relevant in the context of a message bus, it is ignored
          during one-to-one communication with no intermediate bus.
        </para>
        <sect4 id="message-protocol-types-method-apis">
          <title>Mapping method calls to native APIs</title>
          <para>
            APIs for D-Bus may map method calls to a method call in a specific
            programming language, such as C++, or may map a method call written
            in an IDL to a D-Bus message.
          </para>
          <para>
            In APIs of this nature, arguments to a method are often termed "in"
            (which implies sent in the <literal>METHOD_CALL</literal>), or "out" (which implies
            returned in the <literal>METHOD_RETURN</literal>). Some APIs such as CORBA also have
            "inout" arguments, which are both sent and received, i.e. the caller
            passes in a value which is modified. Mapped to D-Bus, an "inout"
            argument is equivalent to an "in" argument, followed by an "out"
            argument. You can't pass things "by reference" over the wire, so
            "inout" is purely an illusion of the in-process API.
          </para>
          <para>
            Given a method with zero or one return values, followed by zero or more
            arguments, where each argument may be "in", "out", or "inout", the
            caller constructs a message by appending each "in" or "inout" argument,
            in order. "out" arguments are not represented in the caller's message.
          </para>
          <para>
            The recipient constructs a reply by appending first the return value 
            if any, then each "out" or "inout" argument, in order. 
            "in" arguments are not represented in the reply message.
          </para>
          <para>
            Error replies are normally mapped to exceptions in languages that have
            exceptions.
          </para>
          <para>
            In converting from native APIs to D-Bus, it is perhaps nice to 
            map D-Bus naming conventions ("FooBar") to native conventions 
            such as "fooBar" or "foo_bar" automatically. This is OK 
            as long as you can say that the native API is one that 
            was specifically written for D-Bus. It makes the most sense
            when writing object implementations that will be exported 
            over the bus. Object proxies used to invoke remote D-Bus 
            objects probably need the ability to call any D-Bus method,
            and thus a magic name mapping like this could be a problem.
          </para>
          <para>
            This specification doesn't require anything of native API bindings;
            the preceding is only a suggested convention for consistency 
            among bindings.
          </para>
        </sect4>
      </sect3>

      <sect3 id="message-protocol-types-signal">
        <title>Signal Emission</title>
        <para>
          Unlike method calls, signal emissions have no replies. 
          A signal emission is simply a single message of type <literal>SIGNAL</literal>.
          It must have three header fields: <literal>PATH</literal> giving the object 
          the signal was emitted from, plus <literal>INTERFACE</literal> and <literal>MEMBER</literal> giving
          the fully-qualified name of the signal. The <literal>INTERFACE</literal> header is required
          for signals, though it is optional for method calls.
        </para>
      </sect3>

      <sect3 id="message-protocol-types-errors">
        <title>Errors</title>
        <para>
          Messages of type <literal>ERROR</literal> are most commonly replies 
          to a <literal>METHOD_CALL</literal>, but may be returned in reply 
          to any kind of message. The message bus for example
          will return an <literal>ERROR</literal> in reply to a signal emission if 
          the bus does not have enough memory to send the signal.
        </para>
        <para>
          An <literal>ERROR</literal> may have any arguments, but if the first 
          argument is a <literal>STRING</literal>, it must be an error message.
          The error message may be logged or shown to the user
          in some way.
        </para>
      </sect3>

      <sect3 id="message-protocol-types-notation">
        <title>Notation in this document</title>
        <para>
          This document uses a simple pseudo-IDL to describe particular method 
          calls and signals. Here is an example of a method call:
          <programlisting>
            org.freedesktop.DBus.StartServiceByName (in STRING name, in UINT32 flags,
                                                     out UINT32 resultcode)
          </programlisting>
          This means <literal>INTERFACE</literal> = org.freedesktop.DBus, <literal>MEMBER</literal> = StartServiceByName, 
          <literal>METHOD_CALL</literal> arguments are <literal>STRING</literal> and <literal>UINT32</literal>, <literal>METHOD_RETURN</literal> argument
          is <literal>UINT32</literal>. Remember that the <literal>MEMBER</literal> field can't contain any '.' (period)
          characters so it's known that the last part of the name in
          the "IDL" is the member name.
        </para>
        <para>
          In C++ that might end up looking like this:
          <programlisting>
            unsigned int org::freedesktop::DBus::StartServiceByName (const char  *name,
                                                                     unsigned int flags);
          </programlisting>
          or equally valid, the return value could be done as an argument:
          <programlisting>
            void org::freedesktop::DBus::StartServiceByName (const char   *name, 
                                                             unsigned int  flags,
                                                             unsigned int *resultcode);
          </programlisting>
          It's really up to the API designer how they want to make 
          this look. You could design an API where the namespace wasn't used 
          in C++, using STL or Qt, using varargs, or whatever you wanted.
        </para>
        <para>
          Signals are written as follows:
          <programlisting>
            org.freedesktop.DBus.NameLost (STRING name)
          </programlisting>
          Signals don't specify "in" vs. "out" because only 
          a single direction is possible.
        </para>
        <para>
          It isn't especially encouraged to use this lame pseudo-IDL in actual
          API implementations; you might use the native notation for the
          language you're using, or you might use COM or CORBA IDL, for example.
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-protocol-handling-invalid">
      <title>Invalid Protocol and Spec Extensions</title>
      
      <para>
        For security reasons, the D-Bus protocol should be strictly parsed and
        validated, with the exception of defined extension points. Any invalid
        protocol or spec violations should result in immediately dropping the
        connection without notice to the other end. Exceptions should be
        carefully considered, e.g. an exception may be warranted for a
        well-understood idiosyncrasy of a widely-deployed implementation.  In
        cases where the other end of a connection is 100% trusted and known to
        be friendly, skipping validation for performance reasons could also make
        sense in certain cases.
      </para>

      <para>
        Generally speaking violations of the "must" requirements in this spec 
        should be considered possible attempts to exploit security, and violations 
        of the "should" suggestions should be considered legitimate (though perhaps
        they should generate an error in some cases).
      </para>

      <para>
        The following extension points are built in to D-Bus on purpose and must
        not be treated as invalid protocol. The extension points are intended
        for use by future versions of this spec, they are not intended for third
        parties.  At the moment, the only way a third party could extend D-Bus
        without breaking interoperability would be to introduce a way to negotiate new
        feature support as part of the auth protocol, using EXTENSION_-prefixed
        commands. There is not yet a standard way to negotiate features.
        <itemizedlist>
          <listitem>
            <para>
              In the authentication protocol (see <xref linkend="auth-protocol"/>) unknown 
                commands result in an ERROR rather than a disconnect. This enables 
                future extensions to the protocol. Commands starting with EXTENSION_ are 
                reserved for third parties.
            </para>
          </listitem>
          <listitem>
            <para>
              The authentication protocol supports pluggable auth mechanisms.
            </para>
          </listitem>
          <listitem>
            <para>
              The address format (see <xref linkend="addresses"/>) supports new
              kinds of transport.
            </para>
          </listitem>
          <listitem>
            <para>
              Messages with an unknown type (something other than
              <literal>METHOD_CALL</literal>, <literal>METHOD_RETURN</literal>,
              <literal>ERROR</literal>, <literal>SIGNAL</literal>) are ignored. 
              Unknown-type messages must still be well-formed in the same way 
              as the known messages, however. They still have the normal 
              header and body.
            </para>
          </listitem>
          <listitem>
            <para>
              Header fields with an unknown or unexpected field code must be ignored, 
              though again they must still be well-formed.
            </para>
          </listitem>
          <listitem>
            <para>
              New standard interfaces (with new methods and signals) can of course be added.
            </para>
          </listitem>
        </itemizedlist>
      </para>

    </sect2>

  </sect1>

  <sect1 id="auth-protocol">
    <title>Authentication Protocol</title>
    <para>
      Before the flow of messages begins, two applications must
      authenticate. A simple plain-text protocol is used for
      authentication; this protocol is a SASL profile, and maps fairly
      directly from the SASL specification. The message encoding is
      NOT used here, only plain text messages.
    </para>
    <para>
      In examples, "C:" and "S:" indicate lines sent by the client and
      server respectively.
    </para>
    <sect2 id="auth-protocol-overview">
      <title>Protocol Overview</title>
      <para>
        The protocol is a line-based protocol, where each line ends with
        \r\n. Each line begins with an all-caps ASCII command name containing
        only the character range [A-Z_], a space, then any arguments for the
        command, then the \r\n ending the line. The protocol is
        case-sensitive. All bytes must be in the ASCII character set.

        Commands from the client to the server are as follows:

        <itemizedlist>
	  <listitem><para>AUTH [mechanism] [initial-response]</para></listitem>
	  <listitem><para>CANCEL</para></listitem>
	  <listitem><para>BEGIN</para></listitem>
	  <listitem><para>DATA &lt;data in hex encoding&gt;</para></listitem>
	  <listitem><para>ERROR [human-readable error explanation]</para></listitem>
	</itemizedlist>

        From server to client are as follows:

        <itemizedlist>
	  <listitem><para>REJECTED &lt;space-separated list of mechanism names&gt;</para></listitem>
	  <listitem><para>OK &lt;GUID in hex&gt;</para></listitem>
	  <listitem><para>DATA &lt;data in hex encoding&gt;</para></listitem>
	  <listitem><para>ERROR</para></listitem>
	</itemizedlist>
      </para>
      <para>
        Unofficial extensions to the command set must begin with the letters 
        "EXTENSION_", to avoid conflicts with future official commands.
        For example, "EXTENSION_COM_MYDOMAIN_DO_STUFF".
      </para>
    </sect2>
    <sect2 id="auth-nul-byte">
      <title>Special credentials-passing nul byte</title>
      <para>
        Immediately after connecting to the server, the client must send a
        single nul byte. This byte may be accompanied by credentials
        information on some operating systems that use sendmsg() with
        SCM_CREDS or SCM_CREDENTIALS to pass credentials over UNIX domain
        sockets. However, the nul byte must be sent even on other kinds of
        socket, and even on operating systems that do not require a byte to be
        sent in order to transmit credentials. The text protocol described in
        this document begins after the single nul byte. If the first byte
        received from the client is not a nul byte, the server may disconnect 
        that client.
      </para>
      <para>
        A nul byte in any context other than the initial byte is an error; 
        the protocol is ASCII-only.
      </para>
      <para>
        The credentials sent along with the nul byte may be used with the 
        SASL mechanism EXTERNAL.
      </para>
    </sect2>
    <sect2 id="auth-command-auth">
      <title>AUTH command</title>
      <para>
        If an AUTH command has no arguments, it is a request to list
        available mechanisms. The server must respond with a REJECTED
        command listing the mechanisms it understands, or with an error.
      </para>
      <para>
        If an AUTH command specifies a mechanism, and the server supports
        said mechanism, the server should begin exchanging SASL
        challenge-response data with the client using DATA commands.
      </para>
      <para>
        If the server does not support the mechanism given in the AUTH
        command, it must send either a REJECTED command listing the mechanisms
        it does support, or an error.
      </para>
      <para>
        If the [initial-response] argument is provided, it is intended for use
        with mechanisms that have no initial challenge (or an empty initial
        challenge), as if it were the argument to an initial DATA command. If
        the selected mechanism has an initial challenge and [initial-response]
        was provided, the server should reject authentication by sending
        REJECTED.
      </para>
      <para>
        If authentication succeeds after exchanging DATA commands, 
        an OK command must be sent to the client.
      </para>
      <para>
        The first octet received by the client after the \r\n of the OK
        command must be the first octet of the authenticated/encrypted 
        stream of D-Bus messages.
      </para>
      <para>
        The first octet received by the server after the \r\n of the BEGIN
        command from the client must be the first octet of the
        authenticated/encrypted stream of D-Bus messages.
      </para>
    </sect2>
    <sect2 id="auth-command-cancel">
      <title>CANCEL Command</title>
      <para>
        At any time up to sending the BEGIN command, the client may send a
        CANCEL command. On receiving the CANCEL command, the server must
        send a REJECTED command and abort the current authentication
        exchange.
      </para>
    </sect2>
    <sect2 id="auth-command-data">
      <title>DATA Command</title>
      <para>
        The DATA command may come from either client or server, and simply 
        contains a hex-encoded block of data to be interpreted 
        according to the SASL mechanism in use.
      </para>
      <para>
        Some SASL mechanisms support sending an "empty string"; 
        FIXME we need some way to do this.
      </para>
    </sect2>
    <sect2 id="auth-command-begin">
      <title>BEGIN Command</title>
      <para>
        The BEGIN command acknowledges that the client has received an 
        OK command from the server, and that the stream of messages
        is about to begin. 
      </para>
      <para>
        The first octet received by the server after the \r\n of the BEGIN
        command from the client must be the first octet of the
        authenticated/encrypted stream of D-Bus messages.
      </para>
    </sect2>
    <sect2 id="auth-command-rejected">
      <title>REJECTED Command</title>
      <para>
        The REJECTED command indicates that the current authentication
        exchange has failed, and further exchange of DATA is inappropriate.
        The client would normally try another mechanism, or try providing
        different responses to challenges.
      </para><para>
        Optionally, the REJECTED command has a space-separated list of
        available auth mechanisms as arguments. If a server ever provides
        a list of supported mechanisms, it must provide the same list 
        each time it sends a REJECTED message. Clients are free to 
        ignore all lists received after the first.
      </para>
    </sect2>
    <sect2 id="auth-command-ok">
      <title>OK Command</title>
      <para>
        The OK command indicates that the client has been authenticated,
        and that further communication will be a stream of D-Bus messages
        (optionally encrypted, as negotiated) rather than this protocol.
      </para>
      <para>
        The first octet received by the client after the \r\n of the OK
        command must be the first octet of the authenticated/encrypted 
        stream of D-Bus messages.
      </para>
      <para>
        The client must respond to the OK command by sending a BEGIN
        command, followed by its stream of messages, or by disconnecting.
        The server must not accept additional commands using this protocol 
        after the OK command has been sent.
      </para>
      <para>
        The OK command has one argument, which is the GUID of the server.
        See <xref linkend="addresses"/> for more on server GUIDs.
      </para>
    </sect2>
    <sect2 id="auth-command-error">
      <title>ERROR Command</title>
      <para>
        The ERROR command indicates that either server or client did not
        know a command, does not accept the given command in the current
        context, or did not understand the arguments to the command. This
        allows the protocol to be extended; a client or server can send a
        command present or permitted only in new protocol versions, and if
        an ERROR is received instead of an appropriate response, fall back
        to using some other technique.
      </para>
      <para>
        If an ERROR is sent, the server or client that sent the
        error must continue as if the command causing the ERROR had never been
        received. However, the the server or client receiving the error 
        should try something other than whatever caused the error; 
        if only canceling/rejecting the authentication.
      </para>
      <para>
        If the D-Bus protocol changes incompatibly at some future time,
        applications implementing the new protocol would probably be able to
        check for support of the new protocol by sending a new command and
        receiving an ERROR from applications that don't understand it. Thus the
        ERROR feature of the auth protocol is an escape hatch that lets us
        negotiate extensions or changes to the D-Bus protocol in the future.
      </para>
    </sect2>
    <sect2 id="auth-examples">
      <title>Authentication examples</title>
      
      <para>
        <figure>
	  <title>Example of successful magic cookie authentication</title>
	  <programlisting>
            (MAGIC_COOKIE is a made up mechanism)

            C: AUTH MAGIC_COOKIE 3138363935333137393635383634
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of finding out mechanisms then picking one</title>
	  <programlisting>
            C: AUTH
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of client sends unknown command then falls back to regular auth</title>
	  <programlisting>
            C: FOOBAR
            S: ERROR
            C: AUTH MAGIC_COOKIE 3736343435313230333039
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of server doesn't support initial auth mechanism</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE 3736343435313230333039
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of wrong password or the like followed by successful retry</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE 3736343435313230333039
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: REJECTED
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of skey cancelled and restarted</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE 3736343435313230333039
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: CANCEL
            S: REJECTED
            C: AUTH SKEY 7ab83f32ee
            S: DATA 8799cabb2ea93e
            C: DATA 8ac876e8f68ee9809bfa876e6f9876g8fa8e76e98f
            S: OK 1234deadbeef
            C: BEGIN
          </programlisting>
	</figure>
      </para>
    </sect2>
    <sect2 id="auth-states">
      <title>Authentication state diagrams</title>
      
      <para>
        This section documents the auth protocol in terms of 
        a state machine for the client and the server. This is 
        probably the most robust way to implement the protocol.
      </para>

      <sect3 id="auth-states-client">
        <title>Client states</title>
        
        <para>
          To more precisely describe the interaction between the
          protocol state machine and the authentication mechanisms the
          following notation is used: MECH(CHALL) means that the
          server challenge CHALL was fed to the mechanism MECH, which
          returns one of

          <itemizedlist>
            <listitem>
              <para>
                CONTINUE(RESP) means continue the auth conversation
                and send RESP as the response to the server;
              </para>
            </listitem>

            <listitem>
              <para>
                OK(RESP) means that after sending RESP to the server
                the client side of the auth conversation is finished
                and the server should return "OK";
              </para>
            </listitem>

            <listitem>
              <para>
                ERROR means that CHALL was invalid and could not be
                processed.
              </para>
            </listitem>
          </itemizedlist>
          
          Both RESP and CHALL may be empty.
        </para>
        
        <para>
          The Client starts by getting an initial response from the
          default mechanism and sends AUTH MECH RESP, or AUTH MECH if
          the mechanism did not provide an initial response.  If the
          mechanism returns CONTINUE, the client starts in state
          <emphasis>WaitingForData</emphasis>, if the mechanism
          returns OK the client starts in state
          <emphasis>WaitingForOK</emphasis>.
        </para>
        
        <para>
          The client should keep track of available mechanisms and
          which it mechanisms it has already attempted. This list is
          used to decide which AUTH command to send. When the list is
          exhausted, the client should give up and close the
          connection.
        </para>

        <formalpara>
          <title><emphasis>WaitingForData</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive DATA CHALL
                  <simplelist>
                    <member>
                      MECH(CHALL) returns CONTINUE(RESP) &rarr; send
                      DATA RESP, goto
                      <emphasis>WaitingForData</emphasis>
                    </member>

                    <member>
                      MECH(CHALL) returns OK(RESP) &rarr; send DATA
                      RESP, goto <emphasis>WaitingForOK</emphasis>
                    </member>

                    <member>
                      MECH(CHALL) returns ERROR &rarr; send ERROR
                      [msg], goto <emphasis>WaitingForData</emphasis>
                    </member>
                  </simplelist>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive REJECTED [mechs] &rarr;
                  send AUTH [next mech], goto
                  WaitingForData or <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive ERROR &rarr; send
                  CANCEL, goto
                  <emphasis>WaitingForReject</emphasis>
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive OK &rarr; send
                  BEGIN, terminate auth
                  conversation, authenticated
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive anything else &rarr; send
                  ERROR, goto
                  <emphasis>WaitingForData</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

        <formalpara>
          <title><emphasis>WaitingForOK</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive OK &rarr; send BEGIN, terminate auth
                  conversation, <emphasis>authenticated</emphasis>
                </para>
              </listitem>
              <listitem>
                <para>
                  Receive REJECT [mechs] &rarr; send AUTH [next mech],
                  goto <emphasis>WaitingForData</emphasis> or
                  <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive DATA &rarr; send CANCEL, goto
                  <emphasis>WaitingForReject</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send CANCEL, goto
                  <emphasis>WaitingForReject</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send ERROR, goto
                  <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

        <formalpara>
          <title><emphasis>WaitingForReject</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive REJECT [mechs] &rarr; send AUTH [next mech],
                  goto <emphasis>WaitingForData</emphasis> or
                  <emphasis>WaitingForOK</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; terminate auth
                  conversation, disconnect
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

      </sect3>

      <sect3 id="auth-states-server">
        <title>Server states</title>
 
        <para>
          For the server MECH(RESP) means that the client response
          RESP was fed to the the mechanism MECH, which returns one of

          <itemizedlist>
            <listitem>
              <para>
                CONTINUE(CHALL) means continue the auth conversation and
                send CHALL as the challenge to the client;
              </para>
            </listitem>

            <listitem>
              <para>
                OK means that the client has been successfully
                authenticated;
              </para>
            </listitem>

            <listitem>
              <para>
                REJECT means that the client failed to authenticate or
                there was an error in RESP.
              </para>
            </listitem>
          </itemizedlist>

          The server starts out in state
          <emphasis>WaitingForAuth</emphasis>.  If the client is
          rejected too many times the server must disconnect the
          client.
        </para>

        <formalpara>
          <title><emphasis>WaitingForAuth</emphasis></title>
          <para>
            <itemizedlist>

              <listitem>
                <para>
                  Receive AUTH &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive AUTH MECH RESP

                  <simplelist>
                    <member>
                      MECH not valid mechanism &rarr; send REJECTED
                      [mechs], goto
                      <emphasis>WaitingForAuth</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns CONTINUE(CHALL) &rarr; send
                      DATA CHALL, goto
                      <emphasis>WaitingForData</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns OK &rarr; send OK, goto
                      <emphasis>WaitingForBegin</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns REJECT &rarr; send REJECTED
                      [mechs], goto
                      <emphasis>WaitingForAuth</emphasis>
                    </member>
                  </simplelist>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive BEGIN &rarr; terminate
                  auth conversation, disconnect
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send
                  ERROR, goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

       
        <formalpara>
          <title><emphasis>WaitingForData</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive DATA RESP
                  <simplelist>
                    <member>
                      MECH(RESP) returns CONTINUE(CHALL) &rarr; send
                      DATA CHALL, goto
                      <emphasis>WaitingForData</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns OK &rarr; send OK, goto
                      <emphasis>WaitingForBegin</emphasis>
                    </member>

                    <member>
                      MECH(RESP) returns REJECT &rarr; send REJECTED
                      [mechs], goto
                      <emphasis>WaitingForAuth</emphasis>
                    </member>
                  </simplelist>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive BEGIN &rarr; terminate auth conversation,
                  disconnect
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive CANCEL &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send ERROR, goto
                  <emphasis>WaitingForData</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

        <formalpara>
          <title><emphasis>WaitingForBegin</emphasis></title>
          <para>
            <itemizedlist>
              <listitem>
                <para>
                  Receive BEGIN &rarr; terminate auth conversation,
                  client authenticated
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive CANCEL &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive ERROR &rarr; send REJECTED [mechs], goto
                  <emphasis>WaitingForAuth</emphasis>
                </para>
              </listitem>

              <listitem>
                <para>
                  Receive anything else &rarr; send ERROR, goto
                  <emphasis>WaitingForBegin</emphasis>
                </para>
              </listitem>
            </itemizedlist>
          </para>
        </formalpara>

      </sect3>
      
    </sect2>
    <sect2 id="auth-mechanisms">
      <title>Authentication mechanisms</title>
      <para>
        This section describes some new authentication mechanisms.
        D-Bus also allows any standard SASL mechanism of course.
      </para>
      <sect3 id="auth-mechanisms-sha">
        <title>DBUS_COOKIE_SHA1</title>
        <para>
          The DBUS_COOKIE_SHA1 mechanism is designed to establish that a client
          has the ability to read a private file owned by the user being
          authenticated. If the client can prove that it has access to a secret
          cookie stored in this file, then the client is authenticated. 
          Thus the security of DBUS_COOKIE_SHA1 depends on a secure home 
          directory.
        </para>
        <para>
          Authentication proceeds as follows:
          <itemizedlist>
            <listitem>
              <para>
                The client sends the username it would like to authenticate 
                as.
              </para>
            </listitem>
            <listitem>
              <para>
                The server sends the name of its "cookie context" (see below); a
                space character; the integer ID of the secret cookie the client
                must demonstrate knowledge of; a space character; then a
                hex-encoded randomly-generated challenge string.
              </para>
            </listitem>
            <listitem>
              <para>
                The client locates the cookie, and generates its own hex-encoded
                randomly-generated challenge string.  The client then
                concatenates the server's hex-encoded challenge, a ":"
                character, its own hex-encoded challenge, another ":" character,
                and the hex-encoded cookie.  It computes the SHA-1 hash of this
                composite string.  It sends back to the server the client's
                hex-encoded challenge string, a space character, and the SHA-1
                hash.
              </para>
            </listitem>
            <listitem>
              <para>
                The server generates the same concatenated string used by the
                client and computes its SHA-1 hash. It compares the hash with
                the hash received from the client; if the two hashes match, the
                client is authenticated.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Each server has a "cookie context," which is a name that identifies a
          set of cookies that apply to that server. A sample context might be
          "org_freedesktop_session_bus". Context names must be valid ASCII,
          nonzero length, and may not contain the characters slash ("/"),
          backslash ("\"), space (" "), newline ("\n"), carriage return ("\r"),
          tab ("\t"), or period ("."). There is a default context,
          "org_freedesktop_general" that's used by servers that do not specify
          otherwise.
        </para>
        <para>
          Cookies are stored in a user's home directory, in the directory
          <filename>~/.dbus-keyrings/</filename>. This directory must 
          not be readable or writable by other users. If it is, 
          clients and servers must ignore it. The directory 
          contains cookie files named after the cookie context.
        </para>
        <para>
          A cookie file contains one cookie per line. Each line 
          has three space-separated fields:
          <itemizedlist>
            <listitem>
              <para>
                The cookie ID number, which must be a non-negative integer and
                may not be used twice in the same file.
              </para>
            </listitem>
            <listitem>
              <para>
                The cookie's creation time, in UNIX seconds-since-the-epoch
                format.
              </para>
            </listitem>
            <listitem>
              <para>
                The cookie itself, a hex-encoded random block of bytes. The cookie
                may be of any length, though obviously security increases 
                as the length increases.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Only server processes modify the cookie file.
          They must do so with this procedure:
          <itemizedlist>
            <listitem>
              <para>
                Create a lockfile name by appending ".lock" to the name of the
                cookie file.  The server should attempt to create this file
                using <literal>O_CREAT | O_EXCL</literal>.  If file creation
                fails, the lock fails. Servers should retry for a reasonable
                period of time, then they may choose to delete an existing lock
                to keep users from having to manually delete a stale
                lock. <footnote><para>Lockfiles are used instead of real file
                locking <literal>fcntl()</literal> because real locking
                implementations are still flaky on network
                filesystems.</para></footnote>
              </para>
            </listitem>
            <listitem>
              <para>
                Once the lockfile has been created, the server loads the cookie
                file. It should then delete any cookies that are old (the
                timeout can be fairly short), or more than a reasonable
                time in the future (so that cookies never accidentally 
                become permanent, if the clock was set far into the future 
                at some point). If no recent keys remain, the 
                server may generate a new key.
              </para>
            </listitem>
            <listitem>
              <para>
                The pruned and possibly added-to cookie file 
                must be resaved atomically (using a temporary 
                file which is rename()'d).
              </para>
            </listitem>
            <listitem>
              <para>
                The lock must be dropped by deleting the lockfile.
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Clients need not lock the file in order to load it, 
          because servers are required to save the file atomically.          
        </para>
      </sect3>
    </sect2>
  </sect1>
  <sect1 id="addresses">
    <title>Server Addresses</title>
    <para>
      Server addresses consist of a transport name followed by a colon, and
      then an optional, comma-separated list of keys and values in the form key=value.
      Each value is escaped.
    </para>
    <para>
      For example: 
      <programlisting>unix:path=/tmp/dbus-test</programlisting>
      Which is the address to a unix socket with the path /tmp/dbus-test.
    </para>
    <para>
      Value escaping is similar to URI escaping but simpler.
      <itemizedlist>
        <listitem>
          <para>
            The set of optionally-escaped bytes is:
            <literal>[0-9A-Za-z_-/.\]</literal>. To escape, each
            <emphasis>byte</emphasis> (note, not character) which is not in the
            set of optionally-escaped bytes must be replaced with an ASCII
            percent (<literal>%</literal>) and the value of the byte in hex.
            The hex value must always be two digits, even if the first digit is
            zero. The optionally-escaped bytes may be escaped if desired.
          </para>
        </listitem>
        <listitem>
          <para>
            To unescape, append each byte in the value; if a byte is an ASCII
            percent (<literal>%</literal>) character then append the following
            hex value instead. It is an error if a <literal>%</literal> byte
            does not have two hex digits following. It is an error if a
            non-optionally-escaped byte is seen unescaped.
          </para>
        </listitem>
      </itemizedlist>
      The set of optionally-escaped bytes is intended to preserve address 
      readability and convenience.
    </para>

    <para>
      A server may specify a key-value pair with the key <literal>guid</literal>
      and the value a hex-encoded 16-byte sequence. <xref linkend="uuids"/>
      describes the format of the <literal>guid</literal> field.  If present,
      this UUID may be used to distinguish one server address from another. A
      server should use a different UUID for each address it listens on. For
      example, if a message bus daemon offers both UNIX domain socket and TCP
      connections, but treats clients the same regardless of how they connect,
      those two connections are equivalent post-connection but should have
      distinct UUIDs to distinguish the kinds of connection.
    </para>
    
    <para>
      The intent of the address UUID feature is to allow a client to avoid
      opening multiple identical connections to the same server, by allowing the
      client to check whether an address corresponds to an already-existing
      connection.  Comparing two addresses is insufficient, because addresses
      can be recycled by distinct servers, and equivalent addresses may look
      different if simply compared as strings (for example, the host in a TCP
      address can be given as an IP address or as a hostname).
    </para>

    <para>
      Note that the address key is <literal>guid</literal> even though the 
      rest of the API and documentation says "UUID," for historical reasons.
    </para>

    <para>
      [FIXME clarify if attempting to connect to each is a requirement 
      or just a suggestion]
      When connecting to a server, multiple server addresses can be
      separated by a semi-colon. The library will then try to connect
      to the first address and if that fails, it'll try to connect to
      the next one specified, and so forth. For example
      <programlisting>unix:path=/tmp/dbus-test;unix:path=/tmp/dbus-test2</programlisting>
    </para>

  </sect1>
  
  <sect1 id="transports">
    <title>Transports</title>
    <para>
      [FIXME we need to specify in detail each transport and its possible arguments]
    
      Current transports include: unix domain sockets (including 
      abstract namespace on linux), TCP/IP, and a debug/testing transport using 
      in-process pipes. Future possible transports include one that 
      tunnels over X11 protocol.
    </para>
  
    <sect2 id="transports-unix-domain-sockets">
      <title>Unix Domain Sockets</title>
      <para>
        Unix domain sockets can be either paths in the file system or on Linux 
	kernels, they can be abstract which are similar to paths but
	do not show up in the file system.  
      </para>

      <para>
        When a socket is opened by the D-Bus library it truncates the path 
	name right before the first trailing Nul byte.  This is true for both
	normal paths and abstract paths.  Note that this is a departure from
	previous versions of D-Bus that would create sockets with a fixed 
	length path name.  Names which were shorter than the fixed length
	would be padded by Nul bytes.
      </para>
    </sect2>
  </sect1>

  <sect1 id="naming-conventions">
    <title>Naming Conventions</title>
    
    <para>
      D-Bus namespaces are all lowercase and correspond to reversed domain
      names, as with Java. e.g. "org.freedesktop"
    </para>
    <para>
      Interface, signal, method, and property names are "WindowsStyleCaps", note
      that the first letter is capitalized, unlike Java.
    </para>
    <para>
      Object paths are normally all lowercase with underscores used rather than
      hyphens.
    </para>
  </sect1>

  <sect1 id="uuids">
    <title>UUIDs</title>
    <para>
      A working D-Bus implementation uses universally-unique IDs in two places.
      First, each server address has a UUID identifying the address, 
      as described in <xref linkend="addresses"/>. Second, each operating
      system kernel instance running a D-Bus client or server has a UUID
      identifying that kernel, retrieved by invoking the method
      org.freedesktop.DBus.Peer.GetMachineId() (see <xref
      linkend="standard-interfaces-peer"/>).
    </para>
    <para>
      The term "UUID" in this document is intended literally, i.e. an
      identifier that is universally unique. It is not intended to refer to
      RFC4122, and in fact the D-Bus UUID is not compatible with that RFC.
    </para>
    <para>
      The UUID must contain 128 bits of data and be hex-encoded.  The
      hex-encoded string may not contain hyphens or other non-hex-digit
      characters, and it must be exactly 32 characters long.  To generate a
      UUID, the current reference implementation concatenates 96 bits of random
      data followed by the 32-bit time in seconds since the UNIX epoch (in big
      endian byte order).
    </para>
    <para>
      It would also be acceptable and probably better to simply generate 128
      bits of random data, as long as the random number generator is of high
      quality. The timestamp could conceivably help if the random bits are not
      very random. With a quality random number generator, collisions are
      extremely unlikely even with only 96 bits, so it's somewhat academic.
    </para>
    <para>
      Implementations should, however, stick to random data for the first 96 bits
      of the UUID.
    </para>
  </sect1>
    
  <sect1 id="standard-interfaces">
    <title>Standard Interfaces</title>
    <para>
      See <xref linkend="message-protocol-types-notation"/> for details on 
       the notation used in this section. There are some standard interfaces
      that may be useful across various D-Bus applications.
    </para>
    <sect2 id="standard-interfaces-peer">
      <title><literal>org.freedesktop.DBus.Peer</literal></title>
      <para>
        The <literal>org.freedesktop.DBus.Peer</literal> interface 
        has two methods:
        <programlisting>
          org.freedesktop.DBus.Peer.Ping ()
          org.freedesktop.DBus.Peer.GetMachineId (out STRING machine_uuid)
        </programlisting>
      </para>
      <para>
        On receipt of the <literal>METHOD_CALL</literal> message
        <literal>org.freedesktop.DBus.Peer.Ping</literal>, an application should do
        nothing other than reply with a <literal>METHOD_RETURN</literal> as
        usual.  It does not matter which object path a ping is sent to.  The
        reference implementation handles this method automatically.
      </para>
      <para>
        On receipt of the <literal>METHOD_CALL</literal> message
        <literal>org.freedesktop.DBus.Peer.GetMachineId</literal>, an application should 
        reply with a <literal>METHOD_RETURN</literal> containing a hex-encoded 
        UUID representing the identity of the machine the process is running on.
        This UUID must be the same for all processes on a single system at least
        until that system next reboots. It should be the same across reboots 
        if possible, but this is not always possible to implement and is not 
        guaranteed.
        It does not matter which object path a GetMachineId is sent to.  The
        reference implementation handles this method automatically.
      </para>
      <para>
        The UUID is intended to be per-instance-of-the-operating-system, so may represent
        a virtual machine running on a hypervisor, rather than a physical machine.
        Basically if two processes see the same UUID, they should also see the same
        shared memory, UNIX domain sockets, process IDs, and other features that require 
        a running OS kernel in common between the processes.
      </para>
      <para>
        The UUID is often used where other programs might use a hostname. Hostnames 
        can change without rebooting, however, or just be "localhost" - so the UUID
        is more robust.
      </para>
      <para>
        <xref linkend="uuids"/> explains the format of the UUID.
      </para>
    </sect2>

    <sect2 id="standard-interfaces-introspectable">
      <title><literal>org.freedesktop.DBus.Introspectable</literal></title>
      <para>
        This interface has one method:
        <programlisting>
          org.freedesktop.DBus.Introspectable.Introspect (out STRING xml_data)
        </programlisting>
      </para>
      <para>
        Objects instances may implement
        <literal>Introspect</literal> which returns an XML description of
        the object, including its interfaces (with signals and methods), objects
        below it in the object path tree, and its properties.
      </para>
      <para>
        <xref linkend="introspection-format"/> describes the format of this XML string.
      </para>
    </sect2>
    <sect2 id="standard-interfaces-properties">
      <title><literal>org.freedesktop.DBus.Properties</literal></title>
      <para>
        Many native APIs will have a concept of object <firstterm>properties</firstterm> 
        or <firstterm>attributes</firstterm>. These can be exposed via the 
        <literal>org.freedesktop.DBus.Properties</literal> interface.
      </para>
      <para>
        <programlisting>
              org.freedesktop.DBus.Properties.Get (in STRING interface_name,
                                                   in STRING property_name,
                                                   out VARIANT value);
              org.freedesktop.DBus.Properties.Set (in STRING interface_name,
                                                   in STRING property_name,
                                                   in VARIANT value);
              org.freedesktop.DBus.Properties.GetAll (in STRING interface_name,
                                                      out DICT&lt;STRING,VARIANT&gt; props);
        </programlisting>
      </para>
      <para>
        The available properties and whether they are writable can be determined
        by calling <literal>org.freedesktop.DBus.Introspectable.Introspect</literal>,
        see <xref linkend="standard-interfaces-introspectable"/>.
      </para>
      <para>
        An empty string may be provided for the interface name; in this case, 
        if there are multiple properties on an object with the same name, 
        the results are undefined (picking one by according to an arbitrary 
        deterministic rule, or returning an error, are the reasonable 
        possibilities).
      </para>
    </sect2>
  </sect1>

  <sect1 id="introspection-format">
    <title>Introspection Data Format</title>
    <para>
      As described in <xref linkend="standard-interfaces-introspectable"/>, 
      objects may be introspected at runtime, returning an XML string 
      that describes the object. The same XML format may be used in 
      other contexts as well, for example as an "IDL" for generating 
      static language bindings.
    </para>
    <para>
      Here is an example of introspection data:
      <programlisting>
        &lt;!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
         "http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd"&gt;
        &lt;node name="/org/freedesktop/sample_object"&gt;
          &lt;interface name="org.freedesktop.SampleInterface"&gt;
            &lt;method name="Frobate"&gt;
              &lt;arg name="foo" type="i" direction="in"/&gt;
              &lt;arg name="bar" type="s" direction="out"/&gt;
              &lt;arg name="baz" type="a{us}" direction="out"/&gt;
              &lt;annotation name="org.freedesktop.DBus.Deprecated" value="true"/&gt;
            &lt;/method&gt;
            &lt;method name="Bazify"&gt;
              &lt;arg name="bar" type="(iiu)" direction="in"/&gt;
              &lt;arg name="bar" type="v" direction="out"/&gt;
            &lt;/method&gt;
            &lt;method name="Mogrify"&gt;
              &lt;arg name="bar" type="(iiav)" direction="in"/&gt;
            &lt;/method&gt;
            &lt;signal name="Changed"&gt;
              &lt;arg name="new_value" type="b"/&gt;
            &lt;/signal&gt;
            &lt;property name="Bar" type="y" access="readwrite"/&gt;
          &lt;/interface&gt;
          &lt;node name="child_of_sample_object"/&gt;
          &lt;node name="another_child_of_sample_object"/&gt;
       &lt;/node&gt;
      </programlisting>
    </para>
    <para>
      A more formal DTD and spec needs writing, but here are some quick notes.
      <itemizedlist>
        <listitem>
          <para>
            Only the root &lt;node&gt; element can omit the node name, as it's
            known to be the object that was introspected.  If the root
            &lt;node&gt; does have a name attribute, it must be an absolute
            object path. If child &lt;node&gt; have object paths, they must be
            relative.
          </para>
        </listitem>
        <listitem>
          <para>
            If a child &lt;node&gt; has any sub-elements, then they 
            must represent a complete introspection of the child.
            If a child &lt;node&gt; is empty, then it may or may 
            not have sub-elements; the child must be introspected
            in order to find out. The intent is that if an object 
            knows that its children are "fast" to introspect
            it can go ahead and return their information, but 
            otherwise it can omit it.
          </para>
        </listitem>
        <listitem>
          <para>
            The direction element on &lt;arg&gt; may be omitted, 
            in which case it defaults to "in" for method calls 
            and "out" for signals. Signals only allow "out" 
            so while direction may be specified, it's pointless.
          </para>
        </listitem>
        <listitem>
          <para>
            The possible directions are "in" and "out", 
            unlike CORBA there is no "inout"
          </para>
        </listitem>
        <listitem>
          <para>
            The possible property access flags are 
            "readwrite", "read", and "write"
          </para>
        </listitem>
        <listitem>
          <para>
            Multiple interfaces can of course be listed for 
            one &lt;node&gt;.
          </para>
        </listitem>
        <listitem>
          <para>
            The "name" attribute on arguments is optional.
          </para>
        </listitem>
      </itemizedlist>
    </para>
    <para>
        Method, interface, property, and signal elements may have
        "annotations", which are generic key/value pairs of metadata.
	They are similar conceptually to Java's annotations and C# attributes.
        Well-known annotations:
     </para>
     <informaltable>
       <tgroup cols="3">
	 <thead>
	   <row>
	     <entry>Name</entry>
	     <entry>Values (separated by ,)</entry>
	     <entry>Description</entry>
	   </row>
	 </thead>
	 <tbody>
	   <row>
	     <entry>org.freedesktop.DBus.Deprecated</entry>
	     <entry>true,false</entry>
	     <entry>Whether or not the entity is deprecated; defaults to false</entry>
	   </row>
	   <row>
	     <entry>org.freedesktop.DBus.GLib.CSymbol</entry>
	     <entry>(string)</entry>
	     <entry>The C symbol; may be used for methods and interfaces</entry>
	   </row>
	   <row>
	     <entry>org.freedesktop.DBus.Method.NoReply</entry>
	     <entry>true,false</entry>
	     <entry>If set, don't expect a reply to the method call; defaults to false.</entry>
	   </row>
	 </tbody>
       </tgroup>
     </informaltable>
  </sect1>
  <sect1 id="message-bus">
    <title>Message Bus Specification</title>
    <sect2 id="message-bus-overview">
      <title>Message Bus Overview</title>
      <para>
        The message bus accepts connections from one or more applications. 
        Once connected, applications can exchange messages with other 
        applications that are also connected to the bus.
      </para>
      <para>
        In order to route messages among connections, the message bus keeps a
        mapping from names to connections. Each connection has one
        unique-for-the-lifetime-of-the-bus name automatically assigned.
        Applications may request additional names for a connection. Additional
        names are usually "well-known names" such as
        "org.freedesktop.TextEditor". When a name is bound to a connection,
        that connection is said to <firstterm>own</firstterm> the name.
      </para>
      <para>
        The bus itself owns a special name, <literal>org.freedesktop.DBus</literal>. 
        This name routes messages to the bus, allowing applications to make 
        administrative requests. For example, applications can ask the bus 
        to assign a name to a connection.
      </para>
      <para>
        Each name may have <firstterm>queued owners</firstterm>.  When an
        application requests a name for a connection and the name is already in
        use, the bus will optionally add the connection to a queue waiting for 
        the name. If the current owner of the name disconnects or releases
        the name, the next connection in the queue will become the new owner.
      </para>

      <para>
        This feature causes the right thing to happen if you start two text
        editors for example; the first one may request "org.freedesktop.TextEditor", 
        and the second will be queued as a possible owner of that name. When 
        the first exits, the second will take over.
      </para>

      <para>
        Messages may have a <literal>DESTINATION</literal> field (see <xref
        linkend="message-protocol-header-fields"/>).  If the
        <literal>DESTINATION</literal> field is present, it specifies a message
        recipient by name. Method calls and replies normally specify this field.
      </para>

      <para>
        Signals normally do not specify a destination; they are sent to all
        applications with <firstterm>message matching rules</firstterm> that
        match the message.
      </para>

      <para>
        When the message bus receives a method call, if the
        <literal>DESTINATION</literal> field is absent, the call is taken to be
        a standard one-to-one message and interpreted by the message bus
        itself. For example, sending an
        <literal>org.freedesktop.DBus.Peer.Ping</literal> message with no
        <literal>DESTINATION</literal> will cause the message bus itself to
        reply to the ping immediately; the message bus will not make this
        message visible to other applications.
      </para>

      <para>
        Continuing the <literal>org.freedesktop.DBus.Peer.Ping</literal> example, if
        the ping message were sent with a <literal>DESTINATION</literal> name of
        <literal>com.yoyodyne.Screensaver</literal>, then the ping would be
        forwarded, and the Yoyodyne Corporation screensaver application would be
        expected to reply to the ping.
      </para>
    </sect2>

    <sect2 id="message-bus-names">
      <title>Message Bus Names</title>
      <para>
        Each connection has at least one name, assigned at connection time and
        returned in response to the
        <literal>org.freedesktop.DBus.Hello</literal> method call.  This
        automatically-assigned name is called the connection's <firstterm>unique
        name</firstterm>.  Unique names are never reused for two different
        connections to the same bus.
      </para>
      <para>
        Ownership of a unique name is a prerequisite for interaction with 
        the message bus. It logically follows that the unique name is always 
        the first name that an application comes to own, and the last 
        one that it loses ownership of.
      </para>
      <para>
        Unique connection names must begin with the character ':' (ASCII colon
        character); bus names that are not unique names must not begin
        with this character. (The bus must reject any attempt by an application
        to manually request a name beginning with ':'.) This restriction
        categorically prevents "spoofing"; messages sent to a unique name
        will always go to the expected connection.
      </para>
      <para>
        When a connection is closed, all the names that it owns are deleted (or
        transferred to the next connection in the queue if any).
      </para>
      <para>
        A connection can request additional names to be associated with it using
        the <literal>org.freedesktop.DBus.RequestName</literal> message. <xref
        linkend="message-protocol-names-bus"/> describes the format of a valid
        name. These names can be released again using the
        <literal>org.freedesktop.DBus.ReleaseName</literal> message.
      </para>

      <sect3 id="bus-messages-request-name">
        <title><literal>org.freedesktop.DBus.RequestName</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 RequestName (in STRING name, in UINT32 flags)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to request</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>UINT32</entry>
                  <entry>Return value</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This method call should be sent to
          <literal>org.freedesktop.DBus</literal> and asks the message bus to
          assign the given name to the method caller. Each name maintains a
          queue of possible owners, where the head of the queue is the primary
          or current owner of the name. Each potential owner in the queue
          maintains the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and
          DBUS_NAME_FLAG_DO_NOT_QUEUE settings from its latest RequestName
          call.  When RequestName is invoked the following occurs:
          <itemizedlist>
            <listitem>
              <para>
                If the method caller is currently the primary owner of the name,
                the DBUS_NAME_FLAG_ALLOW_REPLACEMENT and DBUS_NAME_FLAG_DO_NOT_QUEUE
                values are updated with the values from the new RequestName call, 
                and nothing further happens.
              </para>
            </listitem>

            <listitem>
              <para>
                If the current primary owner (head of the queue) has
                DBUS_NAME_FLAG_ALLOW_REPLACEMENT set, and the RequestName
                invocation has the DBUS_NAME_FLAG_REPLACE_EXISTING flag, then
                the caller of RequestName replaces the current primary owner at
                the head of the queue and the current primary owner moves to the
                second position in the queue. If the caller of RequestName was 
                in the queue previously its flags are updated with the values from 
                the new RequestName in addition to moving it to the head of the queue.
              </para>
            </listitem>

            <listitem>
              <para>
                If replacement is not possible, and the method caller is
                currently in the queue but not the primary owner, its flags are
                updated with the values from the new RequestName call.
              </para>
            </listitem>

            <listitem>
              <para>
                If replacement is not possible, and the method caller is
                currently not in the queue, the method caller is appended to the
                queue.
              </para>
            </listitem>

            <listitem>
              <para>
                If any connection in the queue has DBUS_NAME_FLAG_DO_NOT_QUEUE
                set and is not the primary owner, it is removed from the
                queue. This can apply to the previous primary owner (if it
                was replaced) or the method caller (if it updated the
                DBUS_NAME_FLAG_DO_NOT_QUEUE flag while still stuck in the
                queue, or if it was just added to the queue with that flag set).
              </para>
            </listitem>
          </itemizedlist>
        </para>
        <para>
          Note that DBUS_NAME_FLAG_REPLACE_EXISTING results in "jumping the
          queue," even if another application already in the queue had specified
          DBUS_NAME_FLAG_REPLACE_EXISTING.  This comes up if a primary owner
          that does not allow replacement goes away, and the next primary owner
          does allow replacement. In this case, queued items that specified
          DBUS_NAME_FLAG_REPLACE_EXISTING <emphasis>do not</emphasis>
          automatically replace the new primary owner. In other words,
          DBUS_NAME_FLAG_REPLACE_EXISTING is not saved, it is only used at the
          time RequestName is called. This is deliberate to avoid an infinite loop
          anytime two applications are both DBUS_NAME_FLAG_ALLOW_REPLACEMENT 
          and DBUS_NAME_FLAG_REPLACE_EXISTING.
        </para>
        <para>
          The flags argument contains any of the following values logically ORed
          together:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
	        <row>
		  <entry>DBUS_NAME_FLAG_ALLOW_REPLACEMENT</entry>
		  <entry>0x1</entry>
		  <entry>

                    If an application A specifies this flag and succeeds in
                    becoming the owner of the name, and another application B
                    later calls RequestName with the
                    DBUS_NAME_FLAG_REPLACE_EXISTING flag, then application A
                    will lose ownership and receive a
                    <literal>org.freedesktop.DBus.NameLost</literal> signal, and
                    application B will become the new owner. If DBUS_NAME_FLAG_ALLOW_REPLACEMENT
                    is not specified by application A, or DBUS_NAME_FLAG_REPLACE_EXISTING
                    is not specified by application B, then application B will not replace
                    application A as the owner.

                  </entry>
	        </row>
	        <row>
		  <entry>DBUS_NAME_FLAG_REPLACE_EXISTING</entry>
		  <entry>0x2</entry>
		  <entry>

                    Try to replace the current owner if there is one. If this
                    flag is not set the application will only become the owner of
                    the name if there is no current owner. If this flag is set,
                    the application will replace the current owner if
                    the current owner specified DBUS_NAME_FLAG_ALLOW_REPLACEMENT.

                  </entry>
	        </row>
	        <row>
		  <entry>DBUS_NAME_FLAG_DO_NOT_QUEUE</entry>
		  <entry>0x4</entry>
		  <entry>

                    Without this flag, if an application requests a name that is
                    already owned, the application will be placed in a queue to
                    own the name when the current owner gives it up. If this
                    flag is given, the application will not be placed in the
                    queue, the request for the name will simply fail.  This flag
                    also affects behavior when an application is replaced as
                    name owner; by default the application moves back into the
                    waiting queue, unless this flag was provided when the application
                    became the name owner.

                  </entry>
	        </row>
	      </tbody>
	    </tgroup>
	  </informaltable>

          The return code can be one of the following values:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
	        <row>
                  <entry>DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER</entry>
		  <entry>1</entry> <entry>The caller is now the primary owner of
		  the name, replacing any previous owner. Either the name had no
		  owner before, or the caller specified
		  DBUS_NAME_FLAG_REPLACE_EXISTING and the current owner specified
                  DBUS_NAME_FLAG_ALLOW_REPLACEMENT.</entry>
	        </row>
	        <row>
		  <entry>DBUS_REQUEST_NAME_REPLY_IN_QUEUE</entry>
		  <entry>2</entry>

		  <entry>The name already had an owner,
                    DBUS_NAME_FLAG_DO_NOT_QUEUE was not specified, and either
                    the current owner did not specify
                    DBUS_NAME_FLAG_ALLOW_REPLACEMENT or the requesting
                    application did not specify DBUS_NAME_FLAG_REPLACE_EXISTING.
                    </entry>
	        </row>
	        <row>
		  <entry>DBUS_REQUEST_NAME_REPLY_EXISTS</entry> <entry>3</entry>
		  <entry>The name already has an owner,
		  DBUS_NAME_FLAG_DO_NOT_QUEUE was specified, and either
		  DBUS_NAME_FLAG_ALLOW_REPLACEMENT was not specified by the
		  current owner, or DBUS_NAME_FLAG_REPLACE_EXISTING was not
		  specified by the requesting application.</entry>
	        </row>
	        <row>
		  <entry>DBUS_REQUEST_NAME_REPLY_ALREADY_OWNER</entry>
		  <entry>4</entry>
		  <entry>The application trying to request ownership of a name is already the owner of it.</entry>
	        </row>
	      </tbody>
	    </tgroup>
	  </informaltable>
        </para>
       </sect3>

       <sect3 id="bus-messages-release-name">
        <title><literal>org.freedesktop.DBus.ReleaseName</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 ReleaseName (in STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to release</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>UINT32</entry>
                  <entry>Return value</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This method call should be sent to
          <literal>org.freedesktop.DBus</literal> and asks the message bus to
          release the method caller's claim to the given name. If the caller is
          the primary owner, a new primary owner will be selected from the
          queue if any other owners are waiting. If the caller is waiting in
          the queue for the name, the caller will removed from the queue and
          will not be made an owner of the name if it later becomes available.
          If there are no other owners in the queue for the name, it will be
          removed from the bus entirely.

          The return code can be one of the following values:

          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Conventional Name</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
	        <row>
                  <entry>DBUS_RELEASE_NAME_REPLY_RELEASED</entry>
                  <entry>1</entry> <entry>The caller has released his claim on
                  the given name. Either the caller was the primary owner of
                  the name, and the name is now unused or taken by somebody
                  waiting in the queue for the name, or the caller was waiting
                  in the queue for the name and has now been removed from the
                  queue.</entry>
	        </row>
	        <row>
		  <entry>DBUS_RELEASE_NAME_REPLY_NON_EXISTENT</entry>
		  <entry>2</entry>
		  <entry>The given name does not exist on this bus.</entry>
	        </row>
	        <row>
		  <entry>DBUS_RELEASE_NAME_REPLY_NOT_OWNER</entry>
		  <entry>3</entry>
		  <entry>The caller was not the primary owner of this name,
                  and was also not waiting in the queue to own this name.</entry>
	        </row>
	      </tbody>
	    </tgroup>
	  </informaltable>
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-bus-routing">
      <title>Message Bus Message Routing</title>
      <para>
        FIXME 
      </para>
      <sect3 id="message-bus-routing-match-rules">
        <title>Match Rules</title>
        <para>
	  An important part of the message bus routing protocol is match  
	  rules. Match rules describe what messages can be sent to a client
          based on the contents of the message.  When a message is routed
          through the bus it is compared to clients' match rules.  If any
          of the rules match, the message is dispatched to the client.
          If none of the rules match the message never leaves the bus.  This
          is an effective way to control traffic over the bus and to make sure
          only relevant message need to be processed by the client.
        </para>
        <para>
          Match rules are added using the AddMatch bus method 
          (see xref linkend="bus-messages-add-match"/>).  Rules are 
          specified as a string of comma separated key/value pairs. 
          Excluding a key from the rule indicates a wildcard match.  
          For instance excluding the the member from a match rule but 
          adding a sender would let all messages from that sender through.
          An example of a complete rule would be 
          "type='signal',sender='org.freedesktop.DBus',interface='org.freedesktop.DBus',member='Foo',path='/bar/foo',destination=':452345.34',arg2='bar'"
        </para>
        <para>
          The following table describes the keys that can be used to create 
          a match rule:
          The following table summarizes the D-Bus types.
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Key</entry>
                  <entry>Possible Values</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry><literal>type</literal></entry>
                  <entry>'signal', 'method_call', 'method_return', 'error'</entry>
                  <entry>Match on the message type.  An example of a type match is type='signal'</entry>
                </row>
                <row>
                  <entry><literal>sender</literal></entry>
                  <entry>A bus or unique name (see <xref linkend="term-bus-name"/>
                  and <xref linkend="term-unique-name"/> respectively)
                  </entry>
                  <entry>Match messages sent by a particular sender.  An example of a sender match
                  is sender='org.freedesktop.Hal'</entry>
                </row>
                <row>
                  <entry><literal>interface</literal></entry>
                  <entry>An interface name (see <xref linkend="message-protocol-names-interface"/>)</entry>
                  <entry>Match messages sent over or to a particular interface.  An example of an
                  interface match is interface='org.freedesktop.Hal.Manager'.
                  If a message omits the interface header, it must not match any rule 
                  that specifies this key.</entry>
                </row>
                <row>
                  <entry><literal>member</literal></entry>
                  <entry>Any valid method or signal name</entry>
                  <entry>Matches messages which have the give method or signal name. An example of
                  a member match is member='NameOwnerChanged'</entry>
                </row>
                <row>
                  <entry><literal>path</literal></entry>
                  <entry>An object path (see <xref linkend="message-protocol-marshaling-object-path"/>)</entry>
                  <entry>Matches messages which are sent from or to the given object. An example of a
                  path match is path='/org/freedesktop/Hal/Manager'</entry>
                </row>
                <row>
                  <entry><literal>destination</literal></entry>
                  <entry>A unique name (see <xref linkend="term-unique-name"/>)</entry>
                  <entry>Matches messages which are being sent to the given unique name. An
                  example of a destination match is destination=':1.0'</entry>
                </row>
                <row>
                  <entry><literal>arg[0, 1, 2, 3, ...]</literal></entry>
                  <entry>Any string</entry>
                  <entry>Arg matches are special and are used for further restricting the 
                  match based on the arguments in the body of a message.  As of this time
                  only string arguments can be matched.  An example of an argument match 
                  would be arg3='Foo'. Only argument indexes from 0 to 63 should be 
                  accepted.</entry>
                </row>
                <row>
                  <entry><literal>arg[0, 1, 2, 3, ...]path</literal></entry>
                  <entry>Any string</entry>
                  <entry>Argument path matches provide a specialised form of wildcard
                  matching for path-like namespaces. As with normal argument matches,
                  if the argument is exactly equal to the string given in the match
                  rule then the rule is satisfied. Additionally, there is also a
                  match when either the string given in the match rule or the
                  appropriate message argument ends with '/' and is a prefix of the
                  other. An example argument path match is arg0path='/aa/bb/'. This
                  would match messages with first arguments of '/', '/aa/',
                  '/aa/bb/', '/aa/bb/cc/' and '/aa/bb/cc'. It would not match
                  messages with first arguments of '/aa/b', '/aa' or even '/aa/bb'.</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
      </sect3>
    </sect2>
    <sect2 id="message-bus-starting-services">
      <title>Message Bus Starting Services</title>
      <para>
        The message bus can start applications on behalf of other applications.
        In CORBA terms, this would be called <firstterm>activation</firstterm>.
        An application that can be started in this way is called a
        <firstterm>service</firstterm>.
      </para>
      <para>
        With D-Bus, starting a service is normally done by name. That is,
        applications ask the message bus to start some program that will own a
        well-known name, such as <literal>org.freedesktop.TextEditor</literal>.
        This implies a contract documented along with the name 
        <literal>org.freedesktop.TextEditor</literal> for which objects 
        the owner of that name will provide, and what interfaces those 
        objects will have.
      </para>
      <para>
        To find an executable corresponding to a particular name, the bus daemon
        looks for <firstterm>service description files</firstterm>.  Service
        description files define a mapping from names to executables. Different
        kinds of message bus will look for these files in different places, see
        <xref linkend="message-bus-types"/>.
      </para>
      <para>
        [FIXME the file format should be much better specified than "similar to
        .desktop entries" esp. since desktop entries are already
        badly-specified. ;-)] Service description files have the ".service" file
        extension. The message bus will only load service description files
        ending with .service; all other files will be ignored.  The file format
        is similar to that of <ulink
        url="http://www.freedesktop.org/standards/desktop-entry-spec/desktop-entry-spec.html">desktop
        entries</ulink>. All service description files must be in UTF-8
        encoding. To ensure that there will be no name collisions, service files
        must be namespaced using the same mechanism as messages and service
        names.

        <figure>
	  <title>Example service description file</title>
	  <programlisting>
            # Sample service description file
            [D-BUS Service]
            Names=org.freedesktop.ConfigurationDatabase;org.gnome.GConf;
            Exec=/usr/libexec/gconfd-2
          </programlisting>
	</figure>
      </para>
      <para>
        When an application asks to start a service by name, the bus daemon tries to
        find a service that will own that name. It then tries to spawn the
        executable associated with it. If this fails, it will report an
        error. [FIXME what happens if two .service files offer the same service;
        what kind of error is reported, should we have a way for the client to
        choose one?]
      </para>
      <para>
        The executable launched will have the environment variable
        <literal>DBUS_STARTER_ADDRESS</literal> set to the address of the
        message bus so it can connect and request the appropriate names.
      </para>
      <para>
        The executable being launched may want to know whether the message bus
        starting it is one of the well-known message buses (see <xref
        linkend="message-bus-types"/>). To facilitate this, the bus must also set
        the <literal>DBUS_STARTER_BUS_TYPE</literal> environment variable if it is one
        of the well-known buses. The currently-defined values for this variable
        are <literal>system</literal> for the systemwide message bus,
        and <literal>session</literal> for the per-login-session message
        bus. The new executable must still connect to the address given
        in <literal>DBUS_STARTER_ADDRESS</literal>, but may assume that the
        resulting connection is to the well-known bus.
      </para>
      <para>
        [FIXME there should be a timeout somewhere, either specified
        in the .service file, by the client, or just a global value
        and if the client being activated fails to connect within that
        timeout, an error should be sent back.]
      </para>

      <sect3 id="message-bus-starting-services-scope">
        <title>Message Bus Service Scope</title>
        <para>
          The "scope" of a service is its "per-", such as per-session,
          per-machine, per-home-directory, or per-display. The reference
          implementation doesn't yet support starting services in a different
          scope from the message bus itself. So e.g. if you start a service
          on the session bus its scope is per-session.
        </para>
        <para>
          We could add an optional scope to a bus name. For example, for
          per-(display,session pair), we could have a unique ID for each display
          generated automatically at login and set on screen 0 by executing a
          special "set display ID" binary. The ID would be stored in a
          <literal>_DBUS_DISPLAY_ID</literal> property and would be a string of
          random bytes. This ID would then be used to scope names.
          Starting/locating a service could be done by ID-name pair rather than
          only by name.
        </para>
        <para>
          Contrast this with a per-display scope. To achieve that, we would 
          want a single bus spanning all sessions using a given display.
          So we might set a <literal>_DBUS_DISPLAY_BUS_ADDRESS</literal> 
          property on screen 0 of the display, pointing to this bus.
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-bus-types">
      <title>Well-known Message Bus Instances</title>
      <para>
        Two standard message bus instances are defined here, along with how 
        to locate them and where their service files live.
      </para>
      <sect3 id="message-bus-types-login">
        <title>Login session message bus</title>
        <para>
          Each time a user logs in, a <firstterm>login session message
            bus</firstterm> may be started. All applications in the user's login
          session may interact with one another using this message bus.
        </para>
        <para>
          The address of the login session message bus is given 
          in the <literal>DBUS_SESSION_BUS_ADDRESS</literal> environment 
          variable. If that variable is not set, applications may 
          also try to read the address from the X Window System root 
          window property <literal>_DBUS_SESSION_BUS_ADDRESS</literal>.
          The root window property must have type <literal>STRING</literal>.
          The environment variable should have precedence over the 
          root window property.
        </para>
        <para>
          [FIXME specify location of .service files, probably using 
          DESKTOP_DIRS etc. from basedir specification, though login session 
          bus is not really desktop-specific]
        </para>
      </sect3>
      <sect3 id="message-bus-types-system">
        <title>System message bus</title>
        <para>
          A computer may have a <firstterm>system message bus</firstterm>,
          accessible to all applications on the system. This message bus may be
          used to broadcast system events, such as adding new hardware devices, 
          changes in the printer queue, and so forth.
        </para>
        <para>
          The address of the system message bus is given 
          in the <literal>DBUS_SYSTEM_BUS_ADDRESS</literal> environment 
          variable. If that variable is not set, applications should try 
          to connect to the well-known address
          <literal>unix:path=/var/run/dbus/system_bus_socket</literal>.
          <footnote>
            <para>
              The D-Bus reference implementation actually honors the 
              <literal>$(localstatedir)</literal> configure option 
              for this address, on both client and server side.
            </para>
          </footnote>
        </para>
        <para>
          [FIXME specify location of system bus .service files]
        </para>
      </sect3>
    </sect2>

    <sect2 id="message-bus-messages">
      <title>Message Bus Messages</title>
      <para>
        The special message bus name <literal>org.freedesktop.DBus</literal>
        responds to a number of additional messages.
      </para>

      <sect3 id="bus-messages-hello">
        <title><literal>org.freedesktop.DBus.Hello</literal></title>
        <para>
          As a method:
          <programlisting>
            STRING Hello ()
          </programlisting>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Unique name assigned to the connection</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Before an application is able to send messages to other applications
          it must send the <literal>org.freedesktop.DBus.Hello</literal> message
          to the message bus to obtain a unique name. If an application without
          a unique name tries to send a message to another application, or a
          message to the message bus itself that isn't the
          <literal>org.freedesktop.DBus.Hello</literal> message, it will be
          disconnected from the bus.
        </para>
        <para>
          There is no corresponding "disconnect" request; if a client wishes to
          disconnect from the bus, it simply closes the socket (or other 
          communication channel).
        </para>
      </sect3>
      <sect3 id="bus-messages-list-names">
        <title><literal>org.freedesktop.DBus.ListNames</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of STRING ListNames ()
          </programlisting>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of STRING</entry>
                  <entry>Array of strings where each string is a bus name</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Returns a list of all currently-owned names on the bus.
        </para>
      </sect3>
      <sect3 id="bus-messages-list-activatable-names">
        <title><literal>org.freedesktop.DBus.ListActivatableNames</literal></title>
        <para>
          As a method:
          <programlisting>
            ARRAY of STRING ListActivatableNames ()
          </programlisting>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>ARRAY of STRING</entry>
                  <entry>Array of strings where each string is a bus name</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Returns a list of all names that can be activated on the bus.
        </para>
      </sect3>
      <sect3 id="bus-messages-name-exists">
        <title><literal>org.freedesktop.DBus.NameHasOwner</literal></title>
        <para>
          As a method:
          <programlisting>
            BOOLEAN NameHasOwner (in STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to check</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
          Reply arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>BOOLEAN</entry>
                  <entry>Return value, true if the name exists</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Checks if the specified name exists (currently has an owner).
        </para>
      </sect3>

      <sect3 id="bus-messages-name-owner-changed">
        <title><literal>org.freedesktop.DBus.NameOwnerChanged</literal></title>
        <para>
          This is a signal:
          <programlisting>
            NameOwnerChanged (STRING name, STRING old_owner, STRING new_owner)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name with a new owner</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>STRING</entry>
		  <entry>Old owner or empty string if none</entry>
	        </row>
	        <row>
		  <entry>2</entry>
		  <entry>STRING</entry>
		  <entry>New owner or empty string if none</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This signal indicates that the owner of a name has changed.
          It's also the signal to use to detect the appearance of 
          new names on the bus.
        </para>
      </sect3>
      <sect3 id="bus-messages-name-lost">
        <title><literal>org.freedesktop.DBus.NameLost</literal></title>
        <para>
          This is a signal:
          <programlisting>
            NameLost (STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name which was lost</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This signal is sent to a specific application when it loses
          ownership of a name.
        </para>
      </sect3>

      <sect3 id="bus-messages-name-acquired">
        <title><literal>org.freedesktop.DBus.NameAcquired</literal></title>
        <para>
          This is a signal:
          <programlisting>
            NameAcquired (STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name which was acquired</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This signal is sent to a specific application when it gains
          ownership of a name.
        </para>
      </sect3>

      <sect3 id="bus-messages-start-service-by-name">
        <title><literal>org.freedesktop.DBus.StartServiceByName</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 StartServiceByName (in STRING name, in UINT32 flags)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name of the service to start</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags (currently not used)</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>UINT32</entry>
                <entry>Return value</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
          Tries to launch the executable associated with a name. For more information, see <xref linkend="message-bus-starting-services"/>.

        </para>
        <para>
          The return value can be one of the following values:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Identifier</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
	        <row>
                  <entry>DBUS_START_REPLY_SUCCESS</entry>
                  <entry>1</entry>
                  <entry>The service was successfully started.</entry>
                </row>
                <row>
                  <entry>DBUS_START_REPLY_ALREADY_RUNNING</entry>
                  <entry>2</entry>
                  <entry>A connection already owns the given name.</entry>
                </row>
              </tbody>
             </tgroup>
           </informaltable>
        </para>

      </sect3>

      <sect3 id="bus-messages-get-name-owner">
        <title><literal>org.freedesktop.DBus.GetNameOwner</literal></title>
        <para>
          As a method:
          <programlisting>
            STRING GetNameOwner (in STRING name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name to get the owner of</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>STRING</entry>
                <entry>Return value, a unique connection name</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Returns the unique connection name of the primary owner of the name
        given. If the requested name doesn't have an owner, returns a
        <literal>org.freedesktop.DBus.Error.NameHasNoOwner</literal> error.
       </para>
      </sect3>

      <sect3 id="bus-messages-get-connection-unix-user">
        <title><literal>org.freedesktop.DBus.GetConnectionUnixUser</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 GetConnectionUnixUser (in STRING connection_name)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Name of the connection to query</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>UINT32</entry>
                <entry>unix user id</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Returns the unix uid of the process connected to the server. If unable to
	determine it, a <literal>org.freedesktop.DBus.Error.Failed</literal>
	error is returned.
       </para>
      </sect3>

      <sect3 id="bus-messages-add-match">
        <title><literal>org.freedesktop.DBus.AddMatch</literal></title>
        <para>
          As a method:
          <programlisting>
            AddMatch (in STRING rule)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Match rule to add to the connection</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Adds a match rule to match messages going through the message bus (see <xref linkend='message-bus-routing-match-rules'/>). 
	If the bus does not have enough resources the <literal>org.freedesktop.DBus.Error.OOM</literal>
	error is returned.
       </para>
      </sect3>
      <sect3 id="bus-messages-remove-match">
        <title><literal>org.freedesktop.DBus.RemoveMatch</literal></title>
        <para>
          As a method:
          <programlisting>
            RemoveMatch (in STRING rule)
          </programlisting>
          Message arguments:
          <informaltable>
            <tgroup cols="3">
              <thead>
                <row>
                  <entry>Argument</entry>
                  <entry>Type</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
                <row>
                  <entry>0</entry>
                  <entry>STRING</entry>
                  <entry>Match rule to remove from the connection</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        Removes the first rule that matches (see <xref linkend='message-bus-routing-match-rules'/>). 
	If the rule is not found the <literal>org.freedesktop.DBus.Error.MatchRuleNotFound</literal>
	error is returned.
       </para>
      </sect3>

      <sect3 id="bus-messages-get-id">
        <title><literal>org.freedesktop.DBus.GetId</literal></title>
        <para>
          As a method:
          <programlisting>
            GetId (out STRING id)
          </programlisting>
        Reply arguments:
        <informaltable>
          <tgroup cols="3">
            <thead>
              <row>
                <entry>Argument</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0</entry>
                <entry>STRING</entry>
                <entry>Unique ID identifying the bus daemon</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
        Gets the unique ID of the bus. The unique ID here is shared among all addresses the 
        bus daemon is listening on (TCP, UNIX domain socket, etc.) and its format is described in 
        <xref linkend="uuids"/>. Each address the bus is listening on also has its own unique 
        ID, as described in <xref linkend="addresses"/>. The per-bus and per-address IDs are not related.
        There is also a per-machine ID, described in <xref linkend="standard-interfaces-peer"/> and returned
        by org.freedesktop.DBus.Peer.GetMachineId().
        For a desktop session bus, the bus ID can be used as a way to uniquely identify a user's session.
        </para>
      </sect3>

    </sect2>

  </sect1>
<!--
  <appendix id="implementation-notes">
    <title>Implementation notes</title>
    <sect1 id="implementation-notes-subsection">
      <title></title>
      <para>
      </para>
    </sect1>
  </appendix>
-->

  <glossary><title>Glossary</title>
    <para>
      This glossary defines some of the terms used in this specification.
    </para>

    <glossentry id="term-bus-name"><glossterm>Bus Name</glossterm>
      <glossdef>
        <para>
          The message bus maintains an association between names and
          connections. (Normally, there's one connection per application.)  A
          bus name is simply an identifier used to locate connections. For
          example, the hypothetical <literal>com.yoyodyne.Screensaver</literal>
          name might be used to send a message to a screensaver from Yoyodyne
          Corporation.  An application is said to <firstterm>own</firstterm> a
          name if the message bus has associated the application's connection
          with the name.  Names may also have <firstterm>queued
          owners</firstterm> (see <xref linkend="term-queued-owner"/>).
            The bus assigns a unique name to each connection, 
            see <xref linkend="term-unique-name"/>. Other names 
              can be thought of as "well-known names" and are 
              used to find applications that offer specific functionality.
        </para>
      </glossdef>
    </glossentry>
      
    <glossentry id="term-message"><glossterm>Message</glossterm>
      <glossdef>
        <para>
          A message is the atomic unit of communication via the D-Bus
          protocol. It consists of a <firstterm>header</firstterm> and a
          <firstterm>body</firstterm>; the body is made up of
          <firstterm>arguments</firstterm>.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-message-bus"><glossterm>Message Bus</glossterm>
      <glossdef>
        <para>
          The message bus is a special application that forwards 
          or routes messages between a group of applications
          connected to the message bus. It also manages 
          <firstterm>names</firstterm> used for routing
          messages.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-name"><glossterm>Name</glossterm>
      <glossdef>
        <para>
          See <xref linkend="term-bus-name"/>. "Name" may 
            also be used to refer to some of the other names
            in D-Bus, such as interface names.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="namespace"><glossterm>Namespace</glossterm>
      <glossdef>
	<para>
          Used to prevent collisions when defining new interfaces or bus
	  names. The convention used is the same one Java uses for defining
	  classes: a reversed domain name.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-object"><glossterm>Object</glossterm>
      <glossdef>
        <para>
          Each application contains <firstterm>objects</firstterm>, which have
          <firstterm>interfaces</firstterm> and
          <firstterm>methods</firstterm>. Objects are referred to by a name,
          called a <firstterm>path</firstterm>.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="one-to-one"><glossterm>One-to-One</glossterm>
      <glossdef>
	<para>
          An application talking directly to another application, without going
          through a message bus. One-to-one connections may be "peer to peer" or
          "client to server." The D-Bus protocol has no concept of client
          vs. server after a connection has authenticated; the flow of messages
          is symmetrical (full duplex).
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-path"><glossterm>Path</glossterm>
      <glossdef>
        <para>
          Object references (object names) in D-Bus are organized into a
          filesystem-style hierarchy, so each object is named by a path. As in
          LDAP, there's no difference between "files" and "directories"; a path
          can refer to an object, while still having child objects below it.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-queued-owner"><glossterm>Queued Name Owner</glossterm>
      <glossdef>
        <para>
          Each bus name has a primary owner; messages sent to the name go to the
          primary owner. However, certain names also maintain a queue of
          secondary owners "waiting in the wings." If the primary owner releases
          the name, then the first secondary owner in the queue automatically
          becomes the new owner of the name.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-service"><glossterm>Service</glossterm>
      <glossdef>
        <para>
          A service is an executable that can be launched by the bus daemon.
          Services normally guarantee some particular features, for example they
          may guarantee that they will request a specific name such as
          "org.freedesktop.Screensaver", have a singleton object
          "/org/freedesktop/Application", and that object will implement the
          interface "org.freedesktop.ScreensaverControl".
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
      <glossdef>
        <para>
          ".service files" tell the bus about service applications that can be
          launched (see <xref linkend="term-service"/>). Most importantly they
          provide a mapping from bus names to services that will request those
            names when they start up.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-unique-name"><glossterm>Unique Connection Name</glossterm>
      <glossdef>
        <para>
          The special name automatically assigned to each connection by the
          message bus. This name will never change owner, and will be unique
          (never reused during the lifetime of the message bus).
          It will begin with a ':' character.
        </para>
      </glossdef>
    </glossentry>

  </glossary>
</article>