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// $Id$
//
// Copyright (C) 2007
// Fraunhofer Institute for Open Communication Systems (FOKUS)
// Competence Center NETwork research (NET), St. Augustin, GERMANY
//     Stefan Bund <g0dil@berlios.de>
//
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/** \mainpage libPPI : The Packet Processing Infrastructure

    The PPI provides an infrastructure to create packet oriented network processing applications. A
    PPI application is built by combining processing modules in a very flexible manner.

    \image html scenario.png Target Scenario

    The PPI concept is built around some key concepts

    \li The PPI is based on processing \ref ppi_packets. It does not handle stream oriented
        channels.
    \li The PPI is built around reusable \ref ppi_modules. Each module is completely independent.
    \li Each module has an arbitrary number of \ref ppi_connectors, inputs and outputs.
    \li The modules are connected to each other using flexible \ref ppi_connections.
    \li Data flow throughout the network is governed via flexible automatic or manual \ref
        ppi_throttling (throttle notifications).
    \li Modules may register additional external \ref ppi_events (file descriptor events or timers).

    The PPI thereby builds on the facilities provided by the other components of the SENF
    framework. The target scenario above depicts a diffserv capable UDLR/ULE router including
    performance optimizations for TCP traffic (PEP). This router is built by combining several
    modules.

    \see \ref ppi_overview \n
        <a href="../../../../Examples/RateStuffer/doc/html/index.html">PPI Example Application:
        RateStuffer</a> \n
        \ref senf::ppi::module "Modules" \n
        \ref senf::ppi::connector "Connectors and Jacks" \n
        \ref event_group
 */

/** \page ppi_overview PPI Overview and Concepts

    \autotoc

    \section ppi_design Design considerations

    The PPI interface is designed to be as simple as possible. It provides sane defaults for all
    configurable parameters to simplify getting started. It also automates all resource
    management. The throttling infrastructure handles blocking conditions (like input exhaustion)
    automatically.

    \section ppi_packets Packets

    The PPI processes packets and uses the <a
    href="@TOPDIR@/senf/Packets/doc/html/index.html">Packet library</a> to handle them. All packets
    are internally passed around as generic \ref senf::Packet references, however connectors may
    optionally be defined as sending or receiving packets of a specific type only.

    \section ppi_modules Modules

    A module is represented by a class derived from senf::ppi::module::Module. Each module has
    several components:

    \li It may have any number of \ref ppi_connectors (inputs and outputs)
    \li Each module declares flow information which details the route packets take within the
        module. This information does not define how the information is processed, it only tells,
        where data arriving on some input will be directed at (\ref
        senf::ppi::module::Module::route())
    \li The module might take additional parameters.
    \li The module might also register additional \ref ppi_events.

    Generally, modules are divided into several categories:

    \li \ref io_modules receive external data or forward packets out of the PPI
    \li \ref routing_modules forward packets within the network
    \li \ref adapter_modules are used to connect incompatible connectors to each other
    \li Application modules are modules implemented to perform an applications function

    Of these modules, normally only the application modules need to be implemented since the library
    provides an extensive set of reusable modules.

    The following example module declares three \ref ppi_connectors "Connectors": \c payload,
    \c stuffing and \c output. These connectors are defined as \e public data members so they
    can be accessed from the outside. This is important as we will see below.

    \code
      class RateStuffer
          : public senf::ppi::module::Module
      {
          SENF_PPI_MODULE(RateStuffer);

          senf::ppi::IntervalTimer timer_;

      public:
          senf::ppi::connector::ActiveInput<> payload;
          senf::ppi::connector::ActiveInput<> stuffing;
          senf::ppi::connector::ActiveOutput<> output;

          RateStuffer(unsigned packetsPerSecond)
              : timer_(1000u, packetsPerSecond)
          {
              route(payload, output);
              route(stuffing, output);

              registerEvent( timer_, &RateStuffer::tick );
          }

      private:
          void tick()
          {
              if (payload)
                  output(payload());
              else
                  output(stuffing());
          }
      };
    \endcode

    The constructor will declare flow information using senf::ppi::module::Module::route(). Then the
    module registers an interval timer which will fire <tt>packetsPerSecond</tt> times every
    <tt>1000</tt> milliseconds.

    The module processing is very simple: Whenever a timer tick arrives a packet is sent. If the \c
    payload input is ready (see \ref ppi_throttling), a payload packet is sent, otherwise a stuffing
    packet is sent. The module will therefore provide a constant stream of packets at a fixed rate
    on \c output (see the
    <a href="@TOPDIR@/Examples/RateStuffer/doc/html/index.html">RateStuffer</a> example application
    for a slightly different approach)

    An example module to generate the stuffing packets could be

    \code
      class CopyPacketGenerator
          : public senf::ppi::module::Module
      {
          SENF_PPI_MODULE(CopyPacketGenerator);
      public:
          senf::ppi::connector::PassiveOutput<>  output;

          CopyPacketGenerator(Packet template)
              : template_ (template)
          {
              noroute(output);
              output.onRequest(&CopyPacketGenerator::makePacket);
          }

      private:
          Packet template_;

          void makePacket()
          {
              output(template_.clone());
          }
      };
    \endcode

    This module just produces a copy of a given packet whenever output is requested.

    \see senf::ppi::module::Module

    \section ppi_connectors Connectors

    The input and output attachment points of a module are called connectors. Each connector may be
    active or passive. This gives us 4 types of connectors:

    \li senf::ppi::connector::ActiveInput
    \li senf::ppi::connector::ActiveOutput
    \li senf::ppi::connector::PassiveInput
    \li senf::ppi::connector::PassiveOutput

    An \e active connector (input or output) is <em>activated by the module</em> to send data or to
    poll for available packets. This means, the modules processing routine will call the connector
    without being signaled by the framework to read the connector. It just actively fetches a
    Packet.

    A \e passive connector is <em>signaled by the framework</em> to fetch data from the module or to
    pass data into the module. The module must register a callback which will be called, whenever a
    packet is requested from the module or whenever a new packet is made available for the module to
    process.

    To send or receive a packet (either actively or passively) the module just calls the
    connector. It is permissible to generate or process multiple packets in one iteration. However,
    you must ensure yourself that enough packets are available to be read if more than one packet
    shall be read. It is also permissible to not handle a packet at all even if signaled to do
    so. The packet will automatically be queued.

    To provide this flexibility, all input connectors incorporate a packet queue. This queue is
    exposed to the module and allows the module to optionally process packets in batches.

    Connectors take an optional template argument which allows to specify the type of packet this
    connector sends or received. This template arguments defaults to \ref senf::Packet.

    \see \ref senf::ppi::connector

    \section ppi_connections Connections

    \image html ratestuffer.png Simple RateStuffer

    To make use of the modules, they have to be instantiated and connections have to be created
    between its connectors. It is possible to connect any pair of input/output connectors as long as
    one of them is active and the other is passive.

    It is possible to connect two active or passive connectors with each other using a special
    adaptor module (senf::ppi::module::PassiveQueue or senf::ppi::module::ActiveFeeder
    respectively).

    Additionally, the connectors must be type-compatible: Either one (or both) of the connectors
    must be untyped (they accept arbitrary senf::Packet's, the optional tempalte argument is empty),
    or they both accept the same type of packet. This check is performed at runtime.

    To complete our simplified example: Lets connect senf::ppi::module::ActiveSocketReader and
    senf::ppi::module::PassiveSocketWriter to our example module:

    \code
      RateStuffer rateStuffer (10);

      senf::Packet stuffingPacket = senf::DataPacket::create(...);
      CopyPacketGenerator generator (stuffingPacket);

      senf::UDPv4ClientSocketHandle inputSocket (1111);
      senf::ppi::module::ActiveSocketSource<> udpInput (inputSocket);

      senf::UDPv4ClientSocketHandle outputSocket ("2.3.4.5:2222");
      senf::ppi::module::PassiveSocketSink<> udpOutput (outputSocket);

      senf::ppi::module::PassiveQueue adaptor;

      senf::ppi::connect(udpInput, adaptor);
      senf::ppi::connect(adaptor, rateStuffer.payload);
      adaptor.qdisc(ThresholdQueueing(10,5));
      senf::ppi::connect(generator, rateStuffer.stuffing);
      senf::ppi::connect(rateStuffer, udpOutput);

      senf::ppi::run();
    \endcode

    This application will read udp-packets coming in on port 1111 and will forward
    them to port 2222 on host 2.3.4.5 with a fixed rate of 10 packets / second.

    We start out by instantiating the necessary modules. Then the connections between these modules
    are set up by successively connecting each output connector to an input connector. As can be
    seen, the name of the connector can be left of if it is named \c output or \c input
    respectively.

    The buffering on the udpInput <-> rateStuffer adaptor is changed so the queue will begin to
    throttle only if more than 10 packets are in the queue. The connection will be unthrottled as
    soon as there are no more than 5 packets left in the queue (see \ref ppi_throttling).

    \section ppi_throttling Throttling

    Throttling and throttle notifications at it's base is about handling blocking conditions. The
    most straight forward blocking condition is that of a file descriptor not being available for
    reading resp. writing. Other blocking conditions can arise for example when a queue fills up or
    if a module for some application specific reason does not want to handle packets for a period of
    time.

    All this is handled using throttle notifications. We need throttle notifications so a passive
    connector can tell it's connected peer that it cannot service further requests until an
    unthrottle notification is sent. This tells us, that from the view of someone implementing a
    module, throttle notifications will always be received on active connectors and be sent on
    passive connectors.

    This tells us, that the direction of control flow (the throttle notifications) is from passive
    to active connectors and does \e not depend on the direction of data flow (which flows from
    output to input connector). Thinking about this, this makes sense: The module with the active
    connector is the one initiating the data processing (after all, it is the \e active part) and
    needs to be told not to request or send packets on it's connector since the connected passive
    peer cannot handle the request.

    So if a passive connector cannot handle requests, the connector must be \e throttled. Throttling
    the connector will forward a throttle notification to its peer. The peer then handles the
    throttling notification.

    There are two ways, throttle notifications can be handled: By automatic throttling or by
    registering callbacks. The default is <em>automatic throttling</em>.

    <em>Automatic throttling</em> is based on the routing information available to the module. Every
    notification received is forwarded within the module along all known routes from active to
    passive connectors (routes which connect to active or passive connectors are absolutely valid,
    they just are not \e forwarding routes, they are ignored by the throttle
    notifications). Together with automatic event throttling (see \ref ppi_events), this is all that
    is normally needed to handle throttle notifications: By forwarding the notifications we ensure,
    that a module's passive connectors will only be signaled when it's corresponding active
    connectors are not throttled (as defined by the routing information). The module is therefore
    not called until the connector(s) are untrhottled.

    <em>Throttle callbacks</em> can optionaly be registerd (with automatic throttling enabled or
    disabled, see \ref senf::ppi::connector::ActiveConnector) to be called when a throttle
    notification is received. The callback may then handle the notification however it sees fit, for
    example by manually throttling some passive connector (see \ref
    senf::ppi::connector::PassiveConnector).

    To enable/disable automatic throttling, the \ref senf::ppi::module::Module::route() command
    returns a reference to a \ref senf::ppi::Route instance. If this route is \e forwarding route,
    (that is, of the connectors is passive and the other is active), the return value will be
    derived from \ref senf::ppi::ForwardingRoute which provides members to control the throttle
    notification forwarding.

    \see
        senf::ppi::module::Module \n
        senf::ppi::Route

    \section ppi_events Events

    Modules may register additional events. These external events are very important since they
    drive the PPI framework. Events are like external calls into the module network which are sent
    whenever some event happens. Some possible events are
    \li timer events (senf::ppi::IntervalTimer)
    \li read or write events on some file descriptor (senf::ppi::IOEvent)
    \li internal events (senf::ppi::IdleEvent)

    The PPI really is not concerned, how the events are called and what information is needed to
    perform the call. This is handled by the <a
    href="@TOPDIR@/senf/Scheduler/doc/html/index.html">Scheduler</a>, which is wrapped by the event
    classes.

    All events are derived from senf::ppi::EventDescriptor. The base class allows to enable and
    disable the event. Each type of event will take descriptor specific constructor arguments to
    describe the event to be generated. Events are declared as (private) data members of the
    module and are then registered using senf::ppi::module::Module::registerEvent().

    Each event when signaled is described by an instance of the descriptor specific \e
    descriptorType \c ::Event class. This instance will hold the event specific information (like
    scheduled time of the event, file handle state and so on). This information is passed to the
    callback.

    Additionaly, events are valid routing targets. This feature allows events to be disabled and
    enabled by throtling notifications. For the sake of routing, an event may be used like an active
    input or output. Iit is \e active from the PPI's point of view since it is signaled from the
    outside and not by some module. It may be either input or output depending on the operation the
    event controls.

    If we take into account event routing, we can extend the \c RateStuffer constructor accordingly:

    \code
          RateStuffer(unsigned packetsPerSecond)
              : timer_(1000u, packetsPerSecond)
          {
              route(payload, output);
              route(stuffing, output);
              route(timer_, output);   // (*)

              registerEvent( timer_, &RateStuffer::tick );
          }
    \endcode

    We have added the marked route call. This way, the \c timer_ will receive throttling
    notifications from the output: Whenever the output is throttled, the event will be disabled
    until the output is unthrottled again.

    \see senf::ppi::EventDescriptor

    \section ppi_run Running the network

    After the network has been set up, senf::ppi::run() is called to execute it. This call will only
    return after all data has been processed. The PPI knows this, when no events are enabled any
    more. Without events, nothing will happen any more since it is the events which drive the
    PPI. Therefore the PPI surmises, that all data has been processed and returns from
    senf::ppi::run().

    This works very well with automatic throttling. When no data is available to be processed any
    more and no more data can be expected to arrive (for Example since data has been read from a
    file which is now exhausted) all events will be disabled automatically via trhottle
    notifications and so signal that any processing should stop.

    \section ppi_flows Information Flow

    The above description conceptually introduces three different flow levels:

    \li The <em>data flow</em> is, where the packets are flowing. This flow always goes from output
        to input connector.
    \li The <em>execution flow</em> describes the flow of execution from one module to another. This
        flow always proceeds from active to passive connector.
    \li The <em>control flow</em> is the flow of throttling notifications. This flow always proceeds
        \e opposite to the execution flow, from passive to active connector.

    This is the outside view, from without any module. These flows are set up using
    senf::ppi::connect() statements.

    Within a module, the different flow levels are defined differently depending on the type of
    flow:

    \li The <em>data flow</em> is defined by how data is processed. The different event and
        connector callbacks will pass packets around and thereby define the data flow
    \li Likewise, the <em>execution flow</em> is defined parallel to the data flow (however possible
        in opposite direction) by how the handler of one connector calls other connectors.
    \li The <em>control flow</em> is set up using senf::ppi::Module::route statements (as long as
        automatic throttling is used. Manual throttling defines the control flow within the
        respective callbacks).

    In nearly all cases, these flows will be parallel. Therefore it makes sense to define the \c
    route statement as defining the 'conceptual data flow' since this is also how control messages
    should flow (sans the direction, which is defined by the connectors active/passive property).

    \see \ref ppi_implementation
 */

/** \page ppi_implementation Implementation Notes

    \section processing Data Processing

    The processing in the PPI is driven by events. Without events <em>nothing will happen</em>. When
    an event is generated, the called module will probably call one of it's active connectors.

    Calling an active connector will directly call the handler registered at the connected passive
    connector. This way the call and data are handed across the connections until an I/O module will
    finally handle the request (by not calling any other connectors).

    Throttling is handled in the same way: Throttling a passive connector will call a corresponding
    (internal) method of the connected active connector. This method will call registered handlers
    and will analyze the routing information of the module for other (passive) connectors to call
    and throttle. This will again create a call chain which terminates at the I/O modules. An event
    which is called to be throttled will disable the event temporarily. Unthrottling works in the
    same way.

    This simple structure is complicated by the existence of the input queues. This affects both
    data forwarding and throttling:
    \li A data request will only be forwarded, if no data is available in the queue
    \li The connection will only be throttled when the queue is empty
    \li Handlers of passive input connectors must be called repeatedly until either the queue is
        empty or the handler does not take any packets from the queue


    \section ppi_logistics Managing the Data Structures

    The PPI itself is a singleton. This simplifies many of the interfaces (We do not need to pass
    the PPI instance). Should it be necessary to have several PPI systems working in parallel
    (either by registering all events with the same event handler or by utilizing multiple threads),
    we can still extend the API by adding an optional PPI instance argument.

    Every module manages a collection of all it's connectors and every connector has a reference to
    it's containing module. In addition, every connector maintains a collection of all it's routing
    targets.

    All this data is initialized via the routing statements. This is, why \e every connector must
    appear in at least one routing statement: These statements will as a side effect initialize the
    connector with it's containing module.

    Since all access to the PPI via the module is via it's base class, unbound member function
    pointers can be provided as handler arguments: They will automatically be bound to the current
    instance. This simplifies the PPI usage considerably. The same is true for the connectors: Since
    they know the containing module, they can explicitly bind unbound member function pointers to
    the instance.

    \section ppi_random_notes Random implementation notes

    Generation of throttle notifications: Backward throttling notifications are automatically
    generated (if this is not disabled) whenever the input queue is non-empty \e after the event
    handler has finished processing. Forward throttling notifications are not generated
    automatically within the connector. However, the Passive-Passive adaptor will generate
    Forward-throttling notifications whenever the input queue is empty.

    \section ppi_classdiagram Class Diagram

    \diaimage classes.dia
 */

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