path: root/doc/dbus-specification.sgml

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<article id="index">
  <artheader>
    <title>D-BUS Protocol Specification</title>
    <releaseinfo>Version 0.4</releaseinfo>
    <date>17 February 2003</date>
    <authorgroup>
      <author>
	<firstname>Havoc</firstname>
	<surname>Pennington</surname>
	<affiliation>
	  <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>
    </authorgroup>
  </artheader>

  <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
            does not require users to understand any complex concepts such as a
            new type system or elaborate APIs. Libraries implementing D-BUS 
            may choose to abstract messages as "method calls" (see 
            <xref linkend="message-conventions-method">).
          </para>
        </listitem>
      </itemizedlist>
    </para>
    <para>
      The base D-BUS protocol is a peer-to-peer 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 D-BUS 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>
  </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 inteprets 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>

    <sect2 id="message-protocol-header-encoding">
      <title>Header Encoding</title>
      <para>
        Following the mandatory fields, there are zero or more named fields (see
        <xref linkend="message-protocol-header-fields">), and then nul bytes
        padding the header such that its total length in bytes is a multiple of
        8.
      </para>
      <para>
        The header MUST begin with the following mandatory fields in the following
        order:
        <informaltable>
          <tgroup cols=2>
            <thead>
              <row>
                <entry>Size</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>1 byte</entry>
                <entry>Endianness flag; ASCII 'l' for little-endian 
                  or ASCII 'B' for big-endian.</entry>
              </row>
              <row>
                <entry>1 byte</entry>
                <entry>Bitwise OR of flags. Unknown flags
                  MUST be ignored. Currently-defined flags are described below.
                </entry>
              </row>
              <row>
                <entry>1 byte</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 0.
                </entry>
              </row>
              <row>
                <entry>1 byte</entry>
                <entry>A nul byte, reserved for future use.
                  Any value for this byte MUST be accepted.
                </entry>
              </row>
              <row>
                <entry>4 bytes</entry>
                <entry>An unsigned 32-bit integer in the
                  message's byte order, indicating the total length in bytes of
                  the header including named fields and any alignment padding.
                  MUST be a multiple of 8.
                </entry>
              </row>
              <row>
                <entry>4 bytes</entry>
                <entry>An unsigned 32-bit integer in the
                  message's byte order, indicating the total length in bytes of
                  the message body.
                </entry>
              </row>      
              <row>
                <entry>4 bytes</entry>
                <entry>The message's serial number, a signed 32-bit integer in
                  the message's byte order. Applications MUST NOT reuse the same
                  serial number for different messages more often than 32-bit
                  integer wraparound. Serial numbers must be greater than 
                  zero.
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        Flags that can appear in the second byte of the header:
        <informaltable>
          <tgroup cols=2>
            <thead>
              <row>
                <entry>Hex value</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>0x1</entry>
                <entry>This message is an error reply.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
    </sect2>

    <sect2 id="message-protocol-header-fields">
      <title>Header Fields</title>
      <para>
        In addition to the required header information mentioned 
        in <xref linkend="message-protocol-header-encoding">, 
          the header may contain zero or more named 
          header fields. These fields are named to allow 
          future versions of this protocol specification to 
          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>
        Header field names MUST consist of 4 non-nul bytes.  The field name is
        NOT nul terminated; it occupies exactly 4 bytes. Following the name, 
        the field MUST have a type code, and then a properly-aligned value 
        of that type. 
        See <xref linkend="message-protocol-arguments"> for a description 
          of how each type is encoded. If an implementation sees a header 
          field name that it does not understand, it MUST ignore 
          that field.
      </para>

      <para>
        Here are the currently-defined named header fields:
        <informaltable>
          <tgroup cols=3>
            <thead>
              <row>
                <entry>Name</entry>
                <entry>Type</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>name</entry>
                <entry>STRING</entry>
                <entry>The name of the message, such as org.freedesktop.Peer.Ping</entry>
              </row>
              <row>
                <entry>rply</entry>
                <entry>INT32</entry>
                <entry>The serial number of the message this message is a reply
                to. (The serial number is one of the mandatory header fields,
                see <xref linkend="message-protocol-header-encoding">.)</entry>
              </row>
              <row>
                <entry>srvc</entry>
                <entry>STRING</entry>
                <entry>The name of the service this message should be routed to. 
                Only used in combination with the message bus, see 
                <xref linkend="message-bus">.</entry>
              </row>
              <row>
                <entry>sndr</entry>
                <entry>STRING</entry>
                <entry>The name of the base service that sent this message. 
                The message bus fills in this field; the field is 
                only meaningful in combination with the message bus.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      
    </sect2>
    <sect2 id="message-protocol-header-padding">
      <title>Header Alignment Padding</title>
      <para>
        To allow implementations to keep the header and the body in a single
        buffer while keeping data types aligned, the total length of the header
        must be a multiple of 8 bytes.  To achieve this, the header MUST be padded
        with nul bytes to align its total length on an 8-byte boundary. 
        The minimum number of padding bytes MUST be used. Because all possible 
        named fields use at least 8 bytes, implementations can distinguish 
        padding (which must be less than 8 bytes) from additional named fields
        (which must be at least 8 bytes).
      </para>
    </sect2>

    <sect2 id="message-protocol-arguments">
      <title>Message Arguments</title>
      <para>
        The message body is made up of arguments. Each argument
        is a type code, followed by the value of the argument 
        in a type-dependent format.
      </para>
      <para>
        The type codes are as follows:
        <informaltable>
          <tgroup cols=3>
            <thead>
              <row>
                <entry>Type name</entry>
                <entry>Code</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>INVALID</entry>
                <entry>0</entry>
                <entry>Not a valid type code (error if it appears in a message)</entry>
              </row><row>
                <entry>NIL</entry>
                <entry>1</entry>
                <entry>Marks an "unset" or "nonexistent" argument</entry>
              </row><row>
                <entry>INT32</entry>
                <entry>2</entry>
                <entry>32-bit signed integer</entry>
              </row><row>
                <entry>UINT32</entry>
                <entry>3</entry>
                <entry>32-bit unsigned integer</entry>
              </row><row>
                <entry>DOUBLE</entry>
                <entry>4</entry>
                <entry>IEEE 754 double</entry>
              </row><row>
                <entry>STRING</entry>
                <entry>5</entry>
                <entry>UTF-8 string (<emphasis>must</emphasis> be valid UTF-8)</entry>
              </row><row>
                <entry>INT32_ARRAY</entry>
                <entry>6</entry>
                <entry>Array of INT32</entry>
              </row><row>
                <entry>UINT32_ARRAY</entry>
                <entry>7</entry>
                <entry>Array of UINT32</entry>
              </row><row>
                <entry>DOUBLE_ARRAY</entry>
                <entry>8</entry>
                <entry>Array of DOUBLE</entry>
              </row><row>
                <entry>BYTE_ARRAY</entry>
                <entry>9</entry>
                <entry>Array of bytes</entry>
              </row><row>
                <entry>STRING_ARRAY</entry>
                <entry>10</entry>
                <entry>Array of STRING</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        The types are encoded as follows:
        <informaltable>
          <tgroup cols=2>
            <thead>
              <row>
                <entry>Type name</entry>
                <entry>Encoding</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>INVALID</entry>
                <entry>Not applicable; cannot be encoded.</entry>
              </row><row>
                <entry>NIL</entry>
                <entry>No data is encoded; the type code is followed immediately 
                by the type code of the next argument.</entry>
              </row><row>
                <entry>INT32</entry>
                <entry>32-bit signed integer in the message's byte order, aligned to 4-byte boundary.</entry>
              </row><row>
                <entry>UINT32</entry>
                <entry>32-bit unsigned integer in the message's byte order, aligned to 4-byte boundary.</entry>
              </row><row>
                <entry>DOUBLE</entry>
                <entry>64-bit IEEE 754 double in the message's byte order, aligned to 8-byte boundary.</entry>
              </row><row>
                <entry>STRING</entry>
                <entry>UINT32 aligned to 4-byte boundary indicating the string's 
                  length in bytes excluding its terminating nul, followed by 
                  string data of the given length, followed by a terminating nul 
                  byte.
                </entry>
              </row><row>
                <entry>INT32_ARRAY</entry>
                <entry>UINT32 giving the number of values in the array, 
                  followed by the given number of INT32 values.
                </entry>
              </row><row>
                <entry>UINT32_ARRAY</entry>
                <entry>UINT32 giving the number of values in the array, 
                  followed by the given number of UINT32 values.
                </entry>
              </row><row>
                <entry>DOUBLE_ARRAY</entry>
                <entry>UINT32 giving the number of values in the array, 
                  followed by the given number of DOUBLE values aligned
                  to 8-byte boundary.
                </entry>
              </row><row>
                <entry>BYTE_ARRAY</entry>
                <entry>UINT32 giving the number of values in the array, 
                  followed by the given number of one-byte values.
                </entry>
              </row><row>
                <entry>STRING_ARRAY</entry>
                <entry>UINT32 giving the number of values in the array, 
                  followed by the given number of STRING values.
                </entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
    </sect2>
  </sect1>

  <sect1 id="auth-protocol">
    <title>Authentication Protocol</title>
    <para>
      Before the flow of messages begins, two applications 
      must authenticate. A simple text protocol is used 
      for authentication; this protocol is a SASL profile, 
      and maps fairly directly from the SASL specification.
    </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 base 64 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</para></listitem>
	  <listitem><para>DATA &lt;data in base 64 encoding&gt;</para></listitem>
	  <listitem><para>ERROR</para></listitem>
	</itemizedlist>
      </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 SHOULD respond with a REJECTED
        command listing the mechanisms it understands.
      </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 SHOULD send a REJECTED command listing the mechanisms
        it does support.
      </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, the
        server should reject authentication by sending REJECTED.
      </para>
      <para>
        If authentication succeeds after exchanging DATA commands, 
        an OK command should 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 base64-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>
    </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 MUST continue as if the
        command causing the ERROR had never been received.
      </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 BsAY3g4gBNo=
            S: OK
            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 bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            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 BsAY3g4gBNo=
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of server doesn't support initial auth mechanism</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of wrong password or the like followed by successful retry</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: REJECTED
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
        <figure>
	  <title>Example of skey cancelled and restarted</title>
	  <programlisting>
            C: AUTH MAGIC_COOKIE BsAY3g4gBNo=
            S: REJECTED KERBEROS_V4 SKEY
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: CANCEL
            S: REJECTED
            C: AUTH SKEY bW9yZ2Fu
            S: DATA OTUgUWE1ODMwOA==
            C: DATA Rk9VUiBNQU5OIFNPT04gRklSIFZBUlkgTUFTSA==
            S: OK
            C: BEGIN
          </programlisting>
	</figure>
      </para>
    </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.
      [FIXME how do you escape colon, comma, and semicolon in the values of the key=value pairs?]
    </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>
      [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>
    <para>
      [FIXME we need to specify in detail each transport and its possible arguments]
      Currently, a transport over local UNIX sockets exists, a debug
      transport that only works in-process and therefore can be used
      for for unit testing also exists. It is possible that other
      transports are added, such as a TCP/IP transport, and a
      transport that works over X11.
    </para>
  </sect1>

  <sect1 id="message-conventions">
    <title>Message Conventions</title>
    <para>
      This section documents conventions that are not essential to D-BUS
      functionality, but should generally be followed in order to simplify
      programmer's lives.
    </para>
    <sect2 id="message-conventions-naming">
      <title>Message Naming</title>
      <para>
        Messages are normally named in the form 
        "org.freedesktop.Peer.Ping", which has three 
        distinct components:
        <variablelist>
          <varlistentry>
            <term>Namespace e.g. <literal>org.freedesktop</literal></term>
            <listitem>
              <para>
                Message names have a Java-style namespace: a reversed domain
                name. The components of the domain are normally lowercase.
              </para>
            </listitem>
          </varlistentry>
          <varlistentry>
            <term>Package or object e.g. <literal>Peer</literal></term>
            <listitem>
              <para>
                The next part of the message name can be thought of as the name
                of a singleton object, or as the name of a package of related
                messages.  More than one dot-separated component might be used
                here. (Note that D-BUS does not define any idea of object
                instances or object references.)  The package or object name is
                capitalized LikeThis.
              </para>
            </listitem>
          </varlistentry>
          <varlistentry>
            <term>Method or operation e.g. <literal>Ping</literal></term>
            <listitem>
              <para>
                The final part of the message name is the most specific, and
                should be a verb indicating an operation to be performed on the
                object.  The method or operation name is capitalized LikeThis.
              </para>
            </listitem>
          </varlistentry>
        </variablelist>
      </para>
      <para>
        A reply to a message conventionally has the same name as the message
        being replied to. When following method call conventions (see <xref
                                                                         linkend="message-conventions-method">), this convention is mandatory, 
          because a message with multiple possible replies can't be mapped
          to method call semantics without special-case code.
      </para>
    </sect2>
    <sect2 id="message-conventions-method">
      <title>Method Call Mapping</title>
      <para>
        Some implementations of D-BUS may present an API that translates object
        method calls into D-BUS messages. This document does not specify in
        detail how such an API should look or work. However, it does specify how
        message-based protocols should be designed to be friendly to such an
        API.
      </para>
      <para>
        Remember that D-BUS does not have object references or object instances.
        So when one application sends the message
        <literal>org.freedesktop.Peer.Ping</literal>, it sends it to another
        application, not to any kind of sub-portion of that application.
        However, a convenience API used within the recipient application may
        route all messages that start with
        <literal>org.freedesktop.Peer</literal> to a particular object instance,
        and may invoke the <literal>Ping()</literal> method on said instance in
        order to handle the message. This is a convenience API based on 
        method calls.
      </para>
      <para>
        A "method call" consists of a message and, optionally, a reply to that
        message. The name of the "method" is the last component of the message,
        for example, <literal>org.freedesktop.Peer.Ping</literal> would map to
        the method <literal>Ping()</literal> on some object.
      </para>
      <para>
        Arguments to a method may be considered "in" (processed by the
        recipient of the message), or "out" (returned to the sender of the
        message in the reply). "inout" arguments are both sent and received,
        i.e. the caller passes in a value which is modified. An "inout" argument 
        is equivalent to an "in" argument, followed by an "out" argument.
      </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>
        The standard reply message MUST have the same name as the message being 
        replied to, and MUST set the "rply" header field to the serial 
        number of the message being replied to.
      </para>
      <para>
        If an error occurs, an error reply may be sent in place of the 
        standard reply. Error replies can be identified by a special 
        header flag, see <xref linkend="message-protocol-header-encoding">.
          Error replies have a name which reflects the type of 
          error that occurred. Error replies would generally 
          be mapped to exceptions in a programming language.
      </para>
    </sect2>
  </sect1>

  <sect1 id="standard-messages">
    <title>Standard Peer-to-Peer Messages</title>
    <para>
      In the following message definitions, "method call notation" is presented
      in addition to simply listing the message names and arguments. The special
      type name ANY means any type other than NIL, and the special type name
      ANY_OR_NIL means any valid type.
      [FIXME the messages here are just made up to illustrate the 
      format for defining them]
    </para>
    <sect2 id="standard-messages-ping">
      <title><literal>org.freedesktop.Peer.Ping</literal></title>
      <para>        
        As a method:
        <programlisting>
          void Ping ()
        </programlisting>
      </para>
      <para>
        On receipt of the message <literal>org.freedesktop.Peer.Ping</literal>,
        an application should reply with
        <literal>org.freedesktop.Peer.Ping</literal>. Neither the 
        message nor its reply have any arguments.
        [FIXME the messages here are just made up to illustrate the 
        format for defining them]
      </para>
    </sect2>
    <sect2 id="standard-messages-get-props">
      <title><literal>org.freedesktop.Props.Get</literal></title>
      <para>
        As a method:
        <programlisting>
          ANY_OR_NIL Get (in STRING property_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 property to get</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>ANY_OR_NIL</entry>
                <entry>The value of the property. The type depends on the property.</entry>
              </row>
            </tbody>
          </tgroup>
        </informaltable>
      </para>
      <para>
        
        [FIXME the messages here are just made up to illustrate the 
        format for defining them]
      </para>
    </sect2>
  </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 send and receive messages from 
        the message bus, as in the peer-to-peer case.
      </para>
      <para>
        The message bus keeps track of a set of
        <firstterm>services</firstterm>. A service is simply a name, such 
        as <literal>com.yoyodyne.Screensaver</literal>, which can be 
        <firstterm>owned</firstterm> by one of the connected applications. 
        The message bus itself always owns the special service 
        <literal>org.freedesktop.DBus</literal>.
      </para>
      <para>
        Messages may have a <literal>srvc</literal> field (see <xref
                                                                  linkend="message-protocol-header-fields">).  When the message bus
          receives a message, if the <literal>srvc</literal> field is absent, the
          message is taken to be a standard peer-to-peer message and interpreted
          by the message bus itself. For example, sending
          an <literal>org.freedesktop.Peer.Ping</literal> message with no 
          <literal>srvc</literal> will cause the message bus itself to reply 
          to the ping immediately; the message bus would never make 
          this message visible to other applications.
      </para>
      <para>
        If the <literal>srvc</literal> field is present, then it indicates a
        request for the message bus to route the message. In the usual case,
        messages are routed to the owner of the named service.
        Messages may also be <firstterm>broadcast</firstterm>
        by sending them to the special service 
        <literal>org.freedesktop.Broadcast</literal>. Broadcast messages are
        sent to all applications with <firstterm>message matching
          rules</firstterm> that match the message.
      </para>
      <para>
        Continuing the <literal>org.freedesktop.Peer.Ping</literal> example, if
        the ping message were sent with a <literal>srvc</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. If
        <literal>org.freedesktop.Peer.Ping</literal> were sent to
        <literal>org.freedesktop.Broadcast</literal>, then multiple applications
        might receive the ping, and all would normally reply to it.
      </para>
    </sect2>

    <sect2 id="message-bus-services">
      <title>Message Bus Services</title>
      <para>
        A service is a name that identifies a certain application. Each
        application connected to the message bus has at least one service name
        assigned at connection time and returned in response to the
        <literal>org.freedesktop.DBus.Hello</literal> message.
        This automatically-assigned service name is called 
        the application's <firstterm>base service</firstterm>. 
        Base service names are unique and MUST never be reused for two different 
        applications.
      </para>
      <para>
        [FIXME I think we should define the format of the base service name, 
        and specify that a regular service name can never be in that 
        format; this allows us to categorically prevent "spoofing" - for 
        example perhaps the base service name starts with a certain 
        character that no real service name can start with]
      </para>
      <para>
        An application can request additional service names to be associated
        with it using the
        <literal>org.freedesktop.DBus.AcquireService</literal>
        message. [FIXME what service names are allowed; ASCII or unicode; 
        length limit; etc.]
      </para>
      <para>
        [FIXME this needs more detail, and should move the service-related message 
        descriptions up into this section perhaps]
        Service ownership handling can be specified in the flags part
        of the <literal>org.freedesktop.DBus.AcquireService</literal>
        message. If an application specifies the
        DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT flag, then all applications
        trying to acquire the service will be put in a queue. When the
        primary owner disconnects from the bus or removes ownership
        from the service, the next application in the queue will be the
        primary owner. If the DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT
        flag is not specified, then the primary owner will lose
        ownership whenever another application requests ownership of the
        service.
      </para>
    </sect2>
    <sect2 id="message-bus-routing">
      <title>Message Bus Message Routing</title>
      <para>
        When a message is received by the message bus, the message's 
        <literal>sndr</literal> header field MUST be set to the base service of
        the application which sent the message.  If the service already has
        a <literal>sndr</literal> field, the pre-existing field is replaced.
        This rule means that a replies are always sent to the base service name,
        i.e. to the same application that sent the message being replied to.
      </para>
      <para>
        [FIXME go into detail about broadcast, multicast, unicast, etc.]
      </para>
    </sect2>
    <sect2 id="message-bus-activation">
      <title>Message Bus Service Activation</title>
      <para>
        <firstterm>Activation</firstterm> means to locate a service 
        owner for a service that is currently unowned. For now, it 
        means to launch an executable that will take ownership of 
        a particular service.
      </para>
      <para>
        To find an executable corresponding to a particular service, the bus
        daemon looks for <firstterm>service description files</firstterm>.
        Service description files define a mapping from service 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.

        <figure>
	  <title>Example service description file</title>
	  <programlisting>
            # Sample service description file
            [D-BUS Service]
            Name=org.gnome.ConfigurationDatabase
            Exec=gconfd-2
          </programlisting>
	</figure>
      </para>
      <para>
        When an application requests a service to be activated, the bus
        daemon tries to find it in the list of activation entries. 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]
      </para>
      <para>
        The executable launched will have the environment variable
        <literal>DBUS_BUS_ADDRESS</literal> set to the address of the
        message bus so it can connect and register the appropriate services.
      </para>
    </sect2>

    <sect2 id="message-bus-types">
      <title>Standard 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.  [specify
          how to find the address of the login session message bus via
          environment variable and/or X 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.
          [specify how to find the address of the system message bus]
        </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 service <literal>org.freedesktop.DBus</literal>
        responds to a number of messages, allowing applications to 
        interact with the message bus.
      </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>Name of the service assigned to the application</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 service. If an application tries to send
          a message to another application, or a message to the message bus
          service that isn't the
          <literal>org.freedesktop.DBus.Hello</literal> message, it will
          be disconnected from the bus.
        </para>
        <para>
          The reply message contains the name of the application's base service.
        </para>
      </sect3>
      <sect3 id="bus-messages-list-services">
        <title><literal>org.freedesktop.DBus.ListServices</literal></title>
        <para>
          As a method:
          <programlisting>
            STRING_ARRAY ListServices ()
          </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_ARRAY</entry>
                  <entry>Array of strings where each string is the name of a service</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Returns a list of all existing services registered with the message bus. 
        </para>
      </sect3>
      <sect3 id="bus-messages-service-exists">
        <title><literal>org.freedesktop.DBus.ServiceExists</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 ServiceExists (in STRING service_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 service</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, 1 if the service exists and 0 otherwise</entry>
                </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Checks if a service with a specified name exists.
        </para>
      </sect3>

      <sect3 id="bus-messages-acquire-service">
        <title><literal>org.freedesktop.DBus.AcquireService</literal></title>
        <para>
          As a method:
          <programlisting>
            UINT32 AcquireService (in STRING service_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 service</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>
          Tries to become owner of a specific service. The flags
          specified can be the following values logically ORed together:

          <informaltable>
            <tgroup cols=3>
              <thead>
                <row>
                  <entry>Identifier</entry>
                  <entry>Value</entry>
                  <entry>Description</entry>
                </row>
              </thead>
              <tbody>
	        <row>
		  <entry>DBUS_SERVICE_FLAGS_PROHIBIT_REPLACEMENT</entry>
		  <entry>0x1</entry>
		  <entry>
                    If the application succeeds in being the owner of the specified service,
                    then ownership of the service can't be transferred until the service
                    disconnects. If this flag is not set, then any application trying to become
                    the owner of the service will succeed and the previous owner will be
                    sent a <literal>org.freedesktop.DBus.ServiceLost</literal> message.
                  </entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_FLAGS_REPLACE_EXISTING</entry>
		  <entry>0x2</entry>
		  <entry>Only become the owner of the service if there is no current owner.</entry>
	        </row>
	      </tbody>
	    </tgroup>
	  </informaltable>

          [FIXME if it's one of the following values, why are the values
          done as flags instead of just 0, 1, 2, 3, 4]
          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_SERVICE_REPLY_PRIMARY_OWNER</entry>
		  <entry>0x1</entry>
		  <entry>The application is now the primary owner of the service.</entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_REPLY_IN_QUEUE</entry>
		  <entry>0x2</entry>
		  <entry>The service already has an owner which do not want to give up ownership and therefore the application has been put in a queue.</entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_REPLY_SERVICE_EXISTS</entry>
		  <entry>0x4</entry>
		  <entry>The service does already have a primary owner, and DBUS_SERVICE_FLAG_REPLACE_EXISTING was not specified when trying to acquire the service.</entry>
	        </row>
	        <row>
		  <entry>DBUS_SERVICE_REPLY_ALREADY_OWNER</entry>
		  <entry>0x8</entry>
		  <entry>The application trying to request ownership of the service is already the owner of it.</entry>
	        </row>
	      </tbody>
	    </tgroup>
	  </informaltable>
        </para>
      </sect3>
      <sect3 id="bus-messages-service-acquired">
        <title><literal>org.freedesktop.DBus.ServiceAcquired</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceAcquired (in STRING service_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 service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is sent to a specific application when it becomes the
          primary owner of a service.
        </para>
      </sect3>
      <sect3 id="bus-messages-service-lost">
        <title><literal>org.freedesktop.DBus.ServiceLost</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceLost (in STRING service_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 service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is sent to a specific application when it loses primary
          ownership of a service.

          [FIXME instead of ServiceLost/ServiceCreated going only to 
          a specific app, why not just OwnerChanged that covers both 
          lost and created and changed owner and deleted]
        </para>
      </sect3>

      <sect3 id="bus-messages-service-created">
        <title><literal>org.freedesktop.DBus.ServiceCreated</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceCreated (in STRING service_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 service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is broadcast to all applications when a service has been
          successfully registered on the message bus.
        </para>
      </sect3>

      <sect3 id="bus-messages-service-deleted">
        <title><literal>org.freedesktop.DBus.ServiceDeleted</literal></title>
        <para>
          As a method:
          <programlisting>
            ServiceDeleted (in STRING service_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 service</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          This message is broadcast to all applications when a service has been
          deleted from the message bus.
        </para>
      </sect3>

      <sect3 id="bus-messages-activate-service">
        <title><literal>org.freedesktop.DBus.ActivateService</literal></title>
        <para>
          As a method:
          <programlisting>
            void ActivateService (in STRING service_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 activate</entry>
                </row>
	        <row>
		  <entry>1</entry>
		  <entry>UINT32</entry>
		  <entry>Flags (currently not used)</entry>
	        </row>
              </tbody>
            </tgroup>
          </informaltable>
        </para>
        <para>
          Tries to launch the executable associated with a service. For more information, see <xref linkend="message-bus-activation">.

            [FIXME need semantics in much more detail here; for example, 
            if I activate a service then send it a message, is the message 
            queued for the new service or is there a race]
        </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-activation"><glossterm>Activation</glossterm>
      <glossdef>
        <para>
          The process of creating an owner for a particular service, 
          typically by launching an executable.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-base-service"><glossterm>Base Service</glossterm>
      <glossdef>
        <para>
          The special service automatically assigned to an application by the 
          message bus. This service may never change owner, and the service 
          name will be unique (never reused during the lifetime of the 
          message bus).
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-broadcast"><glossterm>Broadcast</glossterm>
      <glossdef>
        <para>
          A message sent to the special <literal>org.freedesktop.Broadcast</literal>
          service; the message bus will forward the broadcast message 
          to all applications that have expressed interest in it.
        </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 broadcasts messages between a group of applications
          connected to the message bus. It also manages 
          <firstterm>services</firstterm>.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-service"><glossterm>Service</glossterm>
      <glossdef>
        <para>
          A service is simply a named application that other 
          applications can refer to. For example, the 
          hypothetical <literal>com.yoyodyne.Screensaver</literal>
          service might accept messages that affect 
          a screensaver from Yoyodyne Corporation.
          An application is said to <firstterm>own</firstterm> 
          a service if the message bus has associated the 
          application with the service name.
        </para>
      </glossdef>
    </glossentry>

    <glossentry id="term-service-description-files"><glossterm>Service Description Files</glossterm>
      <glossdef>
        <para>
          ".service files" tell the bus how to activate a particular service.
          See <xref linkend="term-activation">
        </para>
      </glossdef>
    </glossentry>

  </glossary>
</article>