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    <h1>libvirt RPC infrastructure</h1>

    <ul id="toc"></ul>

    <p>
      libvirt includes a basic protocol and code to implement
      an extensible, secure client/server RPC service. This was
      originally designed for communication between the libvirt
      client library and the libvirtd daemon, but the code is
      now isolated to allow reuse in other areas of libvirt code.
      This document provides an overview of the protocol and
      structure / operation of the internal RPC library APIs.
    </p>


    <h2><a name="protocol">RPC protocol</a></h2>

    <p>
      libvirt uses a simple, variable length, packet based RPC protocol.
      All structured data within packets is encoded using the
      <a href="http://en.wikipedia.org/wiki/External_Data_Representation">XDR standard</a>
      as currently defined by <a href="https://tools.ietf.org/html/rfc4506">RFC 4506</a>.
      On any connection running the RPC protocol, there can be multiple
      programs active, each supporting one or more versions. A program
      defines a set of procedures that it supports. The procedures can
      support call+reply method invocation, asynchronous events,
      and generic data streams. Method invocations can be overlapped,
      so waiting for a reply to one will not block the receipt of the
      reply to another outstanding method. The protocol was loosely
      inspired by the design of SunRPC. The definition of the RPC
      protocol is in the file <code>src/rpc/virnetprotocol.x</code>
      in the libvirt source tree.
    </p>

    <h3><a href="protocolframing">Packet framing</a></h3>

    <p>
      On the wire, there is no explicit packet framing marker. Instead
      each packet is preceded by an unsigned 32-bit integer giving
      the total length of the packet in bytes. This length includes
      the 4-bytes of the length word itself. Conceptually the framing
      looks like this:
    </p>

<pre>
|~~~   Packet 1   ~~~|~~~   Packet 2   ~~~|~~~  Packet 3    ~~~|~~~

+-------+------------+-------+------------+-------+------------+...
| n=U32 | (n-4) * U8 | n=U32 | (n-4) * U8 | n=U32 | (n-4) * U8 |
+-------+------------+-------+------------+-------+------------+...

|~ Len ~|~   Data   ~|~ Len ~|~   Data   ~|~ Len ~|~   Data   ~|~

</pre>

    <h3><a href="protocoldata">Packet data</a></h3>

    <p>
      The data in each packet is split into two parts, a short
      fixed length header, followed by a variable length payload.
      So a packet from the illustration above is more correctly
      shown as
    </p>

<pre>

+-------+-------------+---------------....---+
| n=U32 | 6*U32       | (n-(7*4))*U8         |
+-------+-------------+---------------....---+

|~ Len ~|~  Header   ~|~  Payload     ....  ~|
</pre>


    <h3><a href="protocolheader">Packet header</a></h3>
    <p>
      The header contains 6 fields, encoded as signed/unsigned 32-bit
      integers.
    </p>

    <pre>
+---------------+
| program=U32   |
+---------------+
| version=U32   |
+---------------+
| procedure=S32 |
+---------------+
| type=S32      |
+---------------+
| serial=U32    |
+---------------+
| status=S32    |
+---------------+
    </pre>

    <dl>
      <dt><code>program</code></dt>
      <dd>
        This is an arbitrarily chosen number that will uniquely
        identify the "service" running over the stream.
      </dd>
      <dt><code>version</code></dt>
      <dd>
        This is the version number of the program, by convention
        starting from '1'. When an incompatible change is made
        to a program, the version number is incremented. Ideally
        both versions will then be supported on the wire in
        parallel for backwards compatibility.
      </dd>
      <dt><code>procedure</code></dt>
      <dd>
        This is an arbitrarily chosen number that will uniquely
        identify the method call, or event associated with the
        packet. By convention, procedure numbers start from 1
        and are assigned monotonically thereafter.
      </dd>
      <dt><code>type</code></dt>
      <dd>
        <p>
        This can be one of the following enumeration values
        </p>
        <ol>
          <li>call: invocation of a method call</li>
          <li>reply: completion of a method call</li>
          <li>event: an asynchronous event</li>
          <li>stream: control info or data from a stream</li>
        </ol>
      </dd>
      <dt><code>serial</code></dt>
      <dd>
        This is an number that starts from 1 and increases
        each time a method call packet is sent. A reply or
        stream packet will have a serial number matching the
        original method call packet serial. Events always
        have the serial number set to 0.
      </dd>
      <dt><code>status</code></dt>
      <dd>
        <p>
        This can one of the following enumeration values
        </p>
        <ol>
          <li>ok: a normal packet. this is always set for method calls or events.
            For replies it indicates successful completion of the method. For
            streams it indicates confirmation of the end of file on the stream.</li>
          <li>error: for replies this indicates that the method call failed
            and error information is being returned. For streams this indicates
            that not all data was sent and the stream has aborted</li>
          <li>continue: for streams this indicates that further data packets
            will be following</li>
        </ol>
      </dd>
    </dl>

    <h3><a href="protocolpayload">Packet payload</a></h3>

    <p>
      The payload of a packet will vary depending on the <code>type</code>
      and <code>status</code> fields from the header.
    </p>

    <ul>
      <li>type=call: the in parameters for the method call, XDR encoded</li>
      <li>type=call-with-fds: number of file handles, then the in parameters for the method call, XDR encoded, followed by the file handles</li>
      <li>type=reply+status=ok: the return value and/or out parameters for the method call, XDR encoded</li>
      <li>type=reply+status=error: the error information for the method, a virErrorPtr XDR encoded</li>
      <li>type=reply-with-fds+status=ok: number of file handles, the return value and/or out parameters for the method call, XDR encoded, followed by the file handles</li>
      <li>type=reply-with-fds+status=error: number of file handles, the error information for the method, a virErrorPtr XDR encoded, followed by the file handles</li>
      <li>type=event: the parameters for the event, XDR encoded</li>
      <li>type=stream+status=ok: no payload</li>
      <li>type=stream+status=error: the error information for the method, a virErrorPtr XDR encoded</li>
      <li>type=stream+status=continue: the raw bytes of data for the stream. No XDR encoding</li>
    </ul>

    <p>
      With the two packet types that support passing file descriptors, in
      between the header and the payload there will be a 4-byte integer
      specifying the number of file descriptors which are being sent.
      The actual file handles are sent after the payload has been sent.
      Each file handle has a single dummy byte transmitted as a carrier
      for the out of band file descriptor. While the sender should always
      send '\0' as the dummy byte value, the receiver ought to ignore the
      value for the sake of robustness.
    </p>

    <p>
      For the exact payload information for each procedure, consult the XDR protocol
      definition for the program+version in question
    </p>

    <h3><a name="wireexamples">Wire examples</a></h3>

    <p>
      The following diagrams illustrate some example packet exchanges
      between a client and server
    </p>

    <h4><a name="wireexamplescall">Method call</a></h4>

    <p>
      A single method call and successful
      reply, for a program=8, version=1, procedure=3, which 10 bytes worth
      of input args, and 4 bytes worth of return values. The overall input
      packet length is 4 + 24 + 10 == 38, and output packet length 32
    </p>

    <pre>
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. |  --&gt; S  (call)
       +--+-----------------------+-----------+

       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo |  &lt;-- S  (reply)
       +--+-----------------------+--------+
    </pre>

    <h4><a name="wireexamplescallerr">Method call with error</a></h4>

    <p>
      An unsuccessful method call will instead return an error object
    </p>

    <pre>
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. |  --&gt; S   (call)
       +--+-----------------------+-----------+

       +--+-----------------------+--------------------------+
C &lt;--  |48| 8 | 1 | 3 | 2 | 1 | 0 | .o.oOo.o.oOo.o.oOo.o.oOo |  &lt;-- S  (error)
       +--+-----------------------+--------------------------+
    </pre>

    <h4><a name="wireexamplescallup">Method call with upload stream</a></h4>

    <p>
      A method call which also involves uploading some data over
      a stream will result in
    </p>

    <pre>
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. |  --&gt; S  (call)
       +--+-----------------------+-----------+

       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo |  &lt;-- S  (reply)
       +--+-----------------------+--------+

       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       ...
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+
C --&gt;  |24| 8 | 1 | 3 | 3 | 1 | 0 | --&gt; S  (stream finish)
       +--+-----------------------+
       +--+-----------------------+
C &lt;--  |24| 8 | 1 | 3 | 3 | 1 | 0 | &lt;-- S  (stream finish)
       +--+-----------------------+
    </pre>

    <h4><a name="wireexamplescallbi">Method call bidirectional stream</a></h4>

    <p>
      A method call which also involves a bi-directional stream will
      result in
    </p>

    <pre>
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. |  --&gt; S  (call)
       +--+-----------------------+-----------+

       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo |  &lt;-- S  (reply)
       +--+-----------------------+--------+

       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C &lt;--  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  &lt;-- S  (stream data down)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C &lt;--  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  &lt;-- S  (stream data down)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C &lt;--  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  &lt;-- S  (stream data down)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C &lt;--  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  &lt;-- S  (stream data down)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       ..
       +--+-----------------------+-------------....-------+
C --&gt;  |38| 8 | 1 | 3 | 3 | 1 | 2 | .o.oOo.o.oOo....o.oOo. |  --&gt; S  (stream data up)
       +--+-----------------------+-------------....-------+
       +--+-----------------------+
C --&gt;  |24| 8 | 1 | 3 | 3 | 1 | 0 | --&gt; S  (stream finish)
       +--+-----------------------+
       +--+-----------------------+
C &lt;--  |24| 8 | 1 | 3 | 3 | 1 | 0 | &lt;-- S  (stream finish)
       +--+-----------------------+
    </pre>


    <h4><a name="wireexamplescallmany">Method calls overlapping</a></h4>
    <pre>
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 1 | 0 | .o.oOo.o. |  --&gt; S  (call 1)
       +--+-----------------------+-----------+
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 2 | 0 | .o.oOo.o. |  --&gt; S  (call 2)
       +--+-----------------------+-----------+
       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 2 | 0 | .o.oOo |  &lt;-- S  (reply 2)
       +--+-----------------------+--------+
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 3 | 0 | .o.oOo.o. |  --&gt; S  (call 3)
       +--+-----------------------+-----------+
       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 3 | 0 | .o.oOo |  &lt;-- S  (reply 3)
       +--+-----------------------+--------+
       +--+-----------------------+-----------+
C --&gt;  |38| 8 | 1 | 3 | 0 | 4 | 0 | .o.oOo.o. |  --&gt; S  (call 4)
       +--+-----------------------+-----------+
       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo |  &lt;-- S  (reply 1)
       +--+-----------------------+--------+
       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 4 | 0 | .o.oOo |  &lt;-- S  (reply 4)
       +--+-----------------------+--------+
    </pre>

    <h4><a name="wireexamplescallfd">Method call with passed FD</a></h4>

    <p>
      A single method call with 2 passed file descriptors and successful
      reply, for a program=8, version=1, procedure=3, which 10 bytes worth
      of input args, and 4 bytes worth of return values. The number of
      file descriptors is encoded as a 32-bit int. Each file descriptor
      then has a 1 byte dummy payload. The overall input
      packet length is 4 + 24 + 4 + 2 + 10 == 44, and output packet length 32.
    </p>

    <pre>
       +--+-----------------------+---------------+-------+
C --&gt;  |44| 8 | 1 | 3 | 0 | 1 | 0 | 2 | .o.oOo.o. | 0 | 0 |  --&gt; S  (call)
       +--+-----------------------+---------------+-------+

       +--+-----------------------+--------+
C &lt;--  |32| 8 | 1 | 3 | 1 | 1 | 0 | .o.oOo |  &lt;-- S  (reply)
       +--+-----------------------+--------+
    </pre>


    <h2><a name="security">RPC security</a></h2>

    <p>
      There are various things to consider to ensure an implementation
      of the RPC protocol can be satisfactorily secured
    </p>

    <h3><a name="securitytls">Authentication/encryption</a></h3>

    <p>
      The basic RPC protocol does not define or require any specific
      authentication/encryption capabilities. A generic solution to
      providing encryption for the protocol is to run the protocol
      over a TLS encrypted data stream. x509 certificate checks can
      be done to form a crude authentication mechanism. It is also
      possible for an RPC program to negotiate an encryption /
      authentication capability, such as SASL, which may then also
      provide per-packet data encryption. Finally the protocol data
      stream can of course be tunnelled over transports such as SSH.
    </p>

    <h3><a name="securitylimits">Data limits</a></h3>

    <p>
      Although the protocol itself defines many arbitrary sized data values in the
      payloads, to avoid denial of service attack there are a number of size limit
      checks prior to encoding or decoding data. There is a limit on the maximum
      size of a single RPC message, limit on the maximum string length, and limits
      on any other parameter which uses a variable length array. These limits can
      be raised, subject to agreement between client/server, without otherwise
      breaking compatibility of the RPC data on the wire.
    </p>

    <h3><a name="securityvalidate">Data validation</a></h3>

    <p>
      It is important that all data be fully validated before performing
      any actions based on the data. When reading an RPC packet, the
      first four bytes must be read and the max packet size limit validated,
      before any attempt is made to read the variable length packet data.
      After a complete packet has been read, the header must be decoded
      and all 6 fields fully validated, before attempting to dispatch
      the payload. Once dispatched, the payload can be decoded and passed
      on to the appropriate API for execution. The RPC code must not take
      any action based on the payload, since it has no way to validate
      the semantics of the payload data. It must delegate this to the
      execution API (e.g. corresponding libvirt public API).
    </p>

    <h2><a name="internals">RPC internal APIs</a></h2>

    <p>
      The generic internal RPC library code lives in the <code>src/rpc/</code>
      directory of the libvirt source tree. Unless otherwise noted, the
      objects are all threadsafe. The core object types and their
      purposes are:
    </p>

    <h3><a name="apioverview">Overview of RPC objects</a></h3>

    <p>
      The following is a high level overview of the role of each
      of the main RPC objects
    </p>

    <dl>
      <dt><code>virNetSASLContextPtr</code> (virnetsaslcontext.h)</dt>
      <dd>The virNetSASLContext APIs maintain SASL state for a network
        service (server or client). This is primarily used on the server
        to provide a whitelist of allowed SASL usernames for clients.
      </dd>

      <dt><code>virNetSASLSessionPtr</code> (virnetsaslcontext.h)</dt>
      <dd>The virNetSASLSession APIs maintain SASL state for a single
        network connection (socket). This is used to perform the multi-step
        SASL handshake and perform encryption/decryption of data once
        authenticated, via integration with virNetSocket.
      </dd>

      <dt><code>virNetTLSContextPtr</code> (virnettlscontext.h)</dt>
      <dd>The virNetTLSContext APIs maintain TLS state for a network
        service (server or client). This is primarily used on the server
        to provide a whitelist of allowed x509 distinguished names, as
        well as diffie-hellman keys. It can also do validation of
        x509 certificates prior to initiating a connection, in order
        to improve detection of configuration errors.
      </dd>

      <dt><code>virNetTLSSessionPtr</code> (virnettlscontext.h)</dt>
      <dd>The virNetTLSSession APIs maintain TLS state for a single
        network connection (socket). This is used to perform the multi-step
        TLS handshake and perform encryption/decryption of data once
        authenticated, via integration with virNetSocket.
      </dd>

      <dt><code>virNetSocketPtr</code> (virnetsocket.h)</dt>
      <dd>The virNetSocket APIs provide a higher level wrapper around
        the raw BSD sockets and getaddrinfo APIs. They allow for creation
        of both server and client sockets. Data transports supported are
        TCP, UNIX, SSH tunnel or external command tunnel. Internally the
        TCP socket impl uses the getaddrinfo info APIs to ensure correct
        protocol-independent behaviour, thus supporting both IPv4 and IPv6.
        The socket APIs can be associated with a virNetSASLSessionPtr or
        virNetTLSSessionPtr object to allow seamless encryption/decryption
        of all writes and reads. For UNIX sockets it is possible to obtain
        the remote client user ID and process ID. Integration with the
        libvirt event loop also allows use of callbacks for notification
        of various I/O conditions
      </dd>

      <dt><code>virNetMessagePtr</code> (virnetmessage.h)</dt>
      <dd>The virNetMessage APIs provide a wrapper around the libxdr
        API calls, to facilitate processing and creation of RPC
        packets. There are convenience APIs for encoding/encoding the
        packet headers, encoding/decoding the payload using an XDR
        filter, encoding/decoding a raw payload (for streams), and
        encoding a virErrorPtr object. There is also a means to
        add to/serve from a linked-list queue of messages.</dd>

      <dt><code>virNetClientPtr</code> (virnetclient.h)</dt>
      <dd>The virNetClient APIs provide a way to connect to a
        remote server and run one or more RPC protocols over
        the connection. Connections can be made over TCP, UNIX
        sockets, SSH tunnels, or external command tunnels. There
        is support for both TLS and SASL session encryption.
        The client also supports management of multiple data streams
        over each connection. Each client object can be used from
        multiple threads concurrently, with method calls/replies
        being interleaved on the wire as required.
      </dd>

      <dt><code>virNetClientProgramPtr</code> (virnetclientprogram.h)</dt>
      <dd>The virNetClientProgram APIs are used to register a
        program+version with the connection. This then enables
        invocation of method calls, receipt of asynchronous
        events and use of data streams, within that program+version.
        When created a set of callbacks must be supplied to take
        care of dispatching any incoming asynchronous events.
      </dd>

      <dt><code>virNetClientStreamPtr</code> (virnetclientstream.h)</dt>
      <dd>The virNetClientStream APIs are used to control transmission and
        receipt of data over a stream active on a client. Streams provide
        a low latency, unlimited length, bi-directional raw data exchange
        mechanism layered over the RPC connection
      </dd>

      <dt><code>virNetServerPtr</code> (virnetserver.h)</dt>
      <dd>The virNetServer APIs are used to manage a network server. A
        server exposed one or more programs, over one or more services.
        It manages multiple client connections invoking multiple RPC
        calls in parallel, with dispatch across multiple worker threads.
      </dd>

      <dt><code>virNetDaemonPtr</code> (virnetdaemon.h)</dt>
      <dd>The virNetDaemon APIs are used to manage a daemon process. A
        deamon is a process that might expose one or more servers.  It
        handles most process-related details, network-related should
        be part of the underlying server.
      </dd>

      <dt><code>virNetServerMDNSPtr</code> (virnetservermdns.h)</dt>
      <dd>The virNetServerMDNS APIs are used to advertise a server
        across the local network, enabling clients to automatically
        detect the existence of remote services. This is done by
        interfacing with the Avahi mDNS advertisement service.
      </dd>

      <dt><code>virNetServerClientPtr</code> (virnetserverclient.h)</dt>
      <dd>The virNetServerClient APIs are used to manage I/O related
        to a single client network connection. It handles initial
        validation and routing of incoming RPC packets, and transmission
        of outgoing packets.
      </dd>

      <dt><code>virNetServerProgramPtr</code> (virnetserverprogram.h)</dt>
      <dd>The virNetServerProgram APIs are used to provide the implementation
        of a single program/version set. Primarily this includes a set of
        callbacks used to actually invoke the APIs corresponding to
        program procedure numbers. It is responsible for all the serialization
        of payloads to/from XDR.</dd>

      <dt><code>virNetServerServicePtr</code> (virnetserverservice.h)</dt>
      <dd>The virNetServerService APIs are used to connect the server to
        one or more network protocols. A single service may involve multiple
        sockets (ie both IPv4 and IPv6). A service also has an associated
        authentication policy for incoming clients.
      </dd>
    </dl>

    <h3><a name="apiclientdispatch">Client RPC dispatch</a></h3>

    <p>
      The client RPC code must allow for multiple overlapping RPC method
      calls to be invoked, transmission and receipt of data for multiple
      streams and receipt of asynchronous events. Understandably this
      involves coordination of multiple threads.
    </p>

    <p>
      The core requirement in the client dispatch code is that only
      one thread is allowed to be performing I/O on the socket at
      any time. This thread is said to be "holding the buck". When
      any other thread comes along and needs to do I/O it must place
      its packets on a queue and delegate processing of them to the
      thread that has the buck. This thread will send out the method
      call, and if it sees a reply will pass it back to the waiting
      thread. If the other thread's reply hasn't arrived, by the time
      the main thread has got its own reply, then it will transfer
      responsibility for I/O to the thread that has been waiting the
      longest. It is said to be "passing the buck" for I/O.
    </p>

    <p>
      When no thread is performing any RPC method call, or sending
      stream data there is still a need to monitor the socket for
      incoming I/O related to asynchronous events, or stream data
      receipt. For this task, a watch is registered with the event
      loop which triggers whenever the socket is readable. This
      watch is automatically disabled whenever any other thread
      grabs the buck, and re-enabled when the buck is released.
    </p>

    <h4><a name="apiclientdispatchex1">Example with buck passing</a></h4>

    <p>
      In the first example, a second thread issues an API call
      while the first thread holds the buck. The reply to the
      first call arrives first, so the buck is passed to the
      second thread.
    </p>

    <pre>
        Thread-1
           |
           V
       Call API1()
           |
           V
       Grab Buck
           |           Thread-2
           V              |
       Send method1       V
           |          Call API2()
           V              |
        Wait I/O          V
           |&lt;--------Queue method2
           V              |
       Send method2       V
           |          Wait for buck
           V              |
        Wait I/O          |
           |              |
           V              |
       Recv reply1        |
           |              |
           V              |
       Pass the buck-----&gt;|
           |              V
           V           Wait I/O
       Return API1()      |
                          V
                      Recv reply2
                          |
                          V
                     Release the buck
                          |
                          V
                      Return API2()
    </pre>

    <h4><a name="apiclientdispatchex2">Example without buck passing</a></h4>

    <p>
      In this second example, a second thread issues an API call
      which is sent and replied to, before the first thread's
      API call has completed. The first thread thus notifies
      the second that its reply is ready, and there is no need
      to pass the buck
    </p>

    <pre>
        Thread-1
           |
           V
       Call API1()
           |
           V
       Grab Buck
           |           Thread-2
           V              |
       Send method1       V
           |          Call API2()
           V              |
        Wait I/O          V
           |&lt;--------Queue method2
           V              |
       Send method2       V
           |          Wait for buck
           V              |
        Wait I/O          |
           |              |
           V              |
       Recv reply2        |
           |              |
           V              |
      Notify reply2------&gt;|
           |              V
           V          Return API2()
        Wait I/O
           |
           V
       Recv reply1
           |
           V
     Release the buck
           |
           V
       Return API1()
    </pre>

    <h4><a name="apiclientdispatchex3">Example with async events</a></h4>

    <p>
      In this example, only one thread is present and it has to
      deal with some async events arriving. The events are actually
      dispatched to the application from the event loop thread
    </p>

    <pre>
        Thread-1
           |
           V
       Call API1()
           |
           V
       Grab Buck
           |
           V
       Send method1
           |
           V
        Wait I/O
           |          Event thread
           V              ...
       Recv event1         |
           |               V
           V          Wait for timer/fd
       Queue event1        |
           |               V
           V           Timer fires
        Wait I/O           |
           |               V
           V           Emit event1
       Recv reply1         |
           |               V
           V          Wait for timer/fd
       Return API1()       |
                          ...
    </pre>

    <h3><a name="apiserverdispatch">Server RPC dispatch</a></h3>

    <p>
      The RPC server code must support receipt of incoming RPC requests from
      multiple client connections, and parallel processing of all RPC
      requests, even many from a single client. This goal is achieved through
      a combination of event driven I/O, and multiple processing threads.
    </p>

    <p>
      The main libvirt event loop thread is responsible for performing all
      socket I/O. It will read incoming packets from clients and will
      transmit outgoing packets to clients. It will handle the I/O to/from
      streams associated with client API calls. When doing client I/O it
      will also pass the data through any applicable encryption layer
      (through use of the virNetSocket / virNetTLSSession and virNetSASLSession
      integration). What is paramount is that the event loop thread never
      do any task that can take a non-trivial amount of time.
    </p>

    <p>
      When reading packets, the event loop will first read the 4 byte length
      word. This is validated to make sure it does not exceed the maximum
      permissible packet size, and the client is set to allow receipt of the
      rest of the packet data. Once a complete packet has been received, the
      next step is to decode the RPC header. The header is validated to
      ensure the request is sensible, ie the server should not receive a
      method reply from a client. If the client has not yet authenticated,
      a security check is also applied to make sure the procedure is on the
      whitelist of those allowed prior to auth. If the packet is a method
      call, it will be placed on a global processing queue. The event loop
      thread is now done with the packet for the time being.
    </p>

    <p>
      The server has a pool of worker threads, which wait for method call
      packets to be queued. One of them will grab the new method call off
      the queue for processing. The first step is to decode the payload of
      the packet to extract the method call arguments. The worker does not
      attempt to do any semantic validation of the arguments, except to make
      sure the size of any variable length fields is below defined limits.
    </p>

    <p>
      The worker now invokes the libvirt API call that corresponds to the
      procedure number in the packet header. The worker is thus kept busy
      until the API call completes. The implementation of the API call
      is responsible for doing semantic validation of parameters and any
      MAC security checks on the objects affected.
    </p>

    <p>
      Once the API call has completed, the worker thread will take the
      return value and output parameters, or error object and encode
      them into a reply packet. Again it does not attempt to do any
      semantic validation of output data, aside from variable length
      field limit checks. The worker thread puts the reply packet on
      the transmission queue for the client. The worker is now finished
      and goes back to wait for another incoming method call.
    </p>

    <p>
      The main event loop is back in charge and when the client socket
      becomes writable, it will start sending the method reply packet
      back to the client.
    </p>

    <p>
      At any time the libvirt connection object can emit asynchronous
      events. These are handled by callbacks in the main event thread.
      The callback will simply encode the event parameters into a new
      data packet and place the packet on the client transmission
      queue.
    </p>

    <p>
      Incoming and outgoing stream packets are also directly handled
      by the main event thread. When an incoming stream packet is
      received, instead of placing it in the global dispatch queue
      for the worker threads, it is sidetracked into a per-stream
      processing queue. When the stream becomes writable, queued
      incoming stream packets will be processed, passing their data
      payload on the stream. Conversely when the stream becomes
      readable, chunks of data will be read from it, encoded into
      new outgoing packets, and placed on the client's transmit
      queue.
    </p>

    <h4><a name="apiserverdispatchex1">Example with overlapping methods</a></h4>

    <p>
      This example illustrates processing of two incoming methods with
      overlapping execution
    </p>

    <pre>
   Event thread    Worker 1       Worker 2
       |               |              |
       V               V              V
    Wait I/O       Wait Job       Wait Job
       |               |              |
       V               |              |
   Recv method1        |              |
       |               |              |
       V               |              |
   Queue method1       V              |
       |          Serve method1       |
       V               |              |
    Wait I/O           V              |
       |           Call API1()        |
       V               |              |
   Recv method2        |              |
       |               |              |
       V               |              |
   Queue method2       |              V
       |               |         Serve method2
       V               V              |
    Wait I/O      Return API1()       V
       |               |          Call API2()
       |               V              |
       V         Queue reply1         |
   Send reply1         |              |
       |               V              V
       V           Wait Job       Return API2()
    Wait I/O           |              |
       |              ...             V
       V                          Queue reply2
   Send reply2                        |
       |                              V
       V                          Wait Job
    Wait I/O                          |
       |                             ...
      ...
    </pre>

    <h4><a name="apiserverdispatchex2">Example with stream data</a></h4>

    <p>
      This example illustrates processing of stream data
    </p>

    <pre>
   Event thread
       |
       V
    Wait I/O
       |
       V
   Recv stream1
       |
       V
   Queue stream1
       |
       V
    Wait I/O
       |
       V
   Recv stream2
       |
       V
   Queue stream2
       |
       V
    Wait I/O
       |
       V
   Write stream1
       |
       V
   Write stream2
       |
       V
    Wait I/O
       |
      ...
    </pre>

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