[senf.git] / HowTos / NewPacket / Mainpage.dox

// $Id$
//
// Copyright (C) 2008 
// 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 HowTo: Defining and using a new 'libPacket' Packet Type 

    This howto will introduce the facilities needed to define a new packet type. As example, the
    \c GREPacket type is defined. 

    \autotoc


    \section howto_newpacket_start Getting started

    Before starting with the implementation, we look at the specification of the GRE packet. This is
    found in <a href="http://tools.ietf.org/html/rfc2784">RFC 2784</a> in Section 2.1:

    <pre>
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |C|       Reserved0       | Ver |         Protocol Type         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Checksum (optional)      |       Reserved1 (Optional)    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    </pre>

    Using this protocol definition, we can decide the first important question: Whether the packet
    header is fixed size or dynamically sized. As we see above, the header incorporates optional
    fields. Therefore it must be dynamically sized. The RFC further details, that if the \a Checksum
    \a Present bit \a C is set, both \a Checksum and \a Reserved1 are present, otherwise they must
    both be omitted.

    Another information we take from the RFC is, that the \a Protocol \a Type is used to define the
    type of payload which directly follows the GRE header. This value is an <a
    href="http://www.iana.org/assignments/ethernet-numbers">ETHERTYPE</a> value. To allow the packet
    library to automatically parse the GRE payload data, we need to tell the packet library which
    ETHERTYPE represents which packet type. This association already exists in form of the
    senf::EtherTypes registry. Our GRE packet will therefore utilize this registry.

    To summarize, we have gathered the following information:

    \li The GRE packet header is a dynamically sized header.
    \li The GRE packet header utilizes the senf::EtherTypes registry for next-header selection


    \section howto_newpacket_parser Implementing the GRE Parser

    The next step in creating a new packet type is to implement the parser. The parser is
    responsible for turning a bunch of bytes into an interpreted header with specific fields. The
    parser will later be constructed with an iterator (pointer) to the first byte to be interpreted
    as a GRE header and will provide member functions to access the header fields. You can implement
    these members manually but the SENF library provides a large set of helper macros which simplify
    this task considerably.


    \subsection howto_newpacket_parser_skeleton The PacketParser skeleton

    \code
    #include <senf/Packets.hh>

    struct GREPacketParser : public senf::PacketParser
    {
    #   include SENF_PARSER()

        // Define fields

        SENF_PARSER_FINALIZE(GREPacketParser);
    };
    \endcode

    This is the standard skeleton of any parser class: We need to inherit senf::PacketParser and
    start out by including either \ref SENF_PARSER() or \ref SENF_FIXED_PARSER(). Which, depends on
    whether we define a fixed size or a dynamically sized parser. As \c GREPacketParser is dynamically
    sized, we include \ref SENF_PARSER().

    After the fields are defined, we need to call the \ref SENF_PARSER_FINALIZE() macro to close of
    the parser definition. This call takes the name of the parser being defined as it's sole
    argument.

    This is already a valid parser, albeit not a very usable one since it defines no fields. We now
    go back to define the parser fields and begin with the simple part: Those fields which are
    always present.


    \subsection howto_newpacket_parser_simple Simple field definitions

    \code
    SENF_PARSER_BITFIELD  ( checksumPresent, 1, bool     );
    SENF_PARSER_SKIP_BITS (                 12           );
    SENF_PARSER_BITFIELD  ( version,         3, unsigned );
    SENF_PARSER_BITFIELD  ( protocolType,   16, unsigned );
    \endcode
    
    This is a direct transcript of the field definition above. There are quite a number of macros
    which may be used to define fields. All these macros are documented in '\ref
    packetparsermacros'.

    This is a correct \c GREPacket header definition but we can optimize a little bit: Since the \a
    protocolType field is aligned on a byte boundary, instead of defining it as a bitfield, we can
    define it as a UInt16 field:
    
    \code
    SENF_PARSER_BITFIELD  ( checksumPresent,  1, bool     );
    SENF_PARSER_SKIP_BITS (                  12           );
    SENF_PARSER_BITFIELD  ( version,          3, unsigned );

    SENF_PARSER_FIELD     ( protocolType,    senf::UInt16Parser );
    \endcode

    Whereas \ref SENF_PARSER_BITFIELD can define only bit-fields, \ref SENF_PARSER_FIELD can define
    almost arbitrary field types. The type is specified by passing the name of another parser to
    \ref SENF_PARSER_FIELD.

    It is important to understand, that the accessors do \e not return the parsed field value. They
    return another \e parser which is used to further interpret the value. This is the inherent
    recursive nature of the SENF packet parsers. This allows to define wildly complex header formats
    if needed. Of course, at some point we need the real value. This is, what the so called
    <em>value parsers</em> do: They interpret some bytes or bits and return the value of that field
    (not a parser). Examples are the bitfield parsers returned by the accessors generated by
    SENF_PARSER_BITFIELD (like senf::UIntFieldParser) or the senf::UInt16Parser.

    What happens in the above macros? Most of the macros define an accessor for a specific field: \a
    checksumPresent() or \a protocolType(). They also manage a <em>current Offset</em>. This value
    is advanced according to the field size whenever a new field is defined (and since this parser
    is defined as a dynamically sized parser, this offset is not a constant, it is an expression
    which calculates the offset of a field depending on the preceding data).


    \subsection howto_newpacket_parser_variant Defining optional fields: The 'variant' parser

    We now come to the optional fields. Since there are two fields which need to be disabled/enabled
    together, we first need to define an additional sub-parser which combines those two
    fields. After this parser is defined, we can use \ref SENF_PARSER_VARIANT() to add this parser
    as an optional parser to the GRE header.
    
    \code
    struct GREPacketParser_OptFields : public senf::PacketParser
    {
    #   include SENF_FIXED_PARSER()

        SENF_PARSER_FIELD ( checksum, senf::UInt16Parser );
        SENF_PARSER_SKIP  (           2                  );

        SENF_PARSER_FINALIZE(GREPacketParser_OptFields);
    };
    \endcode

    This parser only parses the two optional fields of which the reserved field is just skipped. The
    parser this time is a fixed size parser. We can now use this parser to continue the \c GREPacketParser
    implementation:

    \code
    SENF_PARSER_BITFIELD  ( checksumPresent,  1, bool     );
    SENF_PARSER_SKIP_BITS (                  12           );
    SENF_PARSER_BITFIELD  ( version,          3, unsigned );

    SENF_PARSER_FIELD     ( protocolType,    senf::UInt16Parser );

    SENF_PARSER_VARIANT   ( optionalFields,  checksumPresent,
                                             (senf::VoidPacketParser)
                                             (GREPacketParser_OptFields) );
    \endcode

    For a variant parser, two things need to be specified: A selector and a list of variant
    parsers. The selector is another parser field which is used to decide, which variant to
    choose. In this simple case, the field must be an unsigned integer (more precisely a value
    parser which returns a value which is implicitly convertible to \c unsigned). This value is used
    as index into the list of variant types. So in our case, 0 is associated with
    senf::VoidPacketParser whereas 1 is associated with \c
    GREPacketParser_OptFields. (senf::VoidPacketParser is a special empty parser which is used in a
    Variant to denote cases in which the variant parser should not parse anything)

    This parser will work, it is however not very safe and not very usable. If \a p is a GREPacketParser
    instance, than we access the fields via:
    \code
    p.checksumPresent()                    = true;
    p.version()                            = 4u;
    p.protocolType()                       = 0x86dd;
    p.optionalFields().get<1>().checksum() = 12345u;
    \endcode
    
    There are two problems here:
    \li accessing the checksum field is quite unwieldy
    \li changing the checksumPresent() value will break the parser

    The reason for the second problem lies in the fact, that the variant parser needs to be informed
    whenever the selector (here \a checksumPresent) is changed since the variant parser must ensure,
    that the header data stays consistent. In this example, whenever the checksumPresent field is
    enabled, the variant parser needs to insert additional 4 bytes of data and remove those bytes,
    when the checksumPresent field is disabled. 


    \subsection howto_newpacket_parser_fixvariant Fixing access by providing custom accessor members

    Since we don't want to allow the \a checksumPresent() field to be changed directly, we mark this
    field as read-only:

    \code
    SENF_PARSER_BITFIELD_RO ( checksumPresent,  1, bool     );
    \endcode

    To change the \a checksumPresent() value, the variant parser provides special members to change
    the currently selected variant:

    \code
    p.optionalFields().init<0>();
    p.optionalFields().init<1>();
    \endcode
    
    These statements also change the selector field (in this case \a checksumPresent()) to the
    correct value: The first statements switches to the first variant and therefore in this case
    disables the checksum field. The second statement will switch to the second variant and enable
    the checksum field.

    Again, these statements are relatively complex. So we complete the parser by providing simple
    additional members which wrap these complicated calls. While doing this, we also mark the
    variant as a private field so it is not directly accessible any more (since we now have the
    additional helpers which are used to access the variant, we don't want anyone to mess around
    with it directly).

    \code
    SENF_PARSER_PRIVATE_VARIANT ( optionalFields_, checksumPresent,
                                                     (senf::VoidPacketParser)
                                                     (GREPacketParser_OptFields)    );

    typedef GREPacketParser_OptFields::checksum_t checksum_t;
    checksum_t checksum() const
        { return optionalFields_().get<1>().checksum(); }

    void enableChecksum()  const { optionalFields_().init<1>(); }
    void disableChecksum() const { optionalFields_().init<0>(); }
    \endcode

    Above code has one other twist we need to discuss: the \a checksum_t typedef. This is added as a
    convenience to the user of this parser. The \c SENF_PARSER_* macros which define a field all
    define some additional symbols providing further information about the field. Of these
    additional symbols, the most important is <em>field</em><code>_t</code>, which is the (parser)
    type returned by the field. This helps to work with a parser in more complex situations
    (e.g. when using \ref parsecollection) since it allows to access the parser type without exact
    knowledge of this type (which may become quite complex if templates are involved) as long as the
    field name is known. Since we provide an accessor for the \a checksum field, we also provide the
    \a checksum_t typedef for this accessor.

    The \c GREPacketParser is now simple and safe to use. The only responsibility of the user now is to
    only access \a checksum() if the \a checksumPresent() field is set. Otherwise, the behavior is
    undefined (in debug builds, the parser will terminate the application with an assert).

    
    \subsection howto_newpacket_parser_add Providing additional functionality

    The \c GREPacketParser is now complete. But we can do better: A packet parser is not restricted
    to simply parsing data. Depending on the packet type, additional members can be arbitrarily
    defined. In the case of \c GREPacket, we provide one additional member, \a calculateChecksum()
    which does just that: It calculates the checksum of the GRE packet.

    \code
    #include <senf/Utils/IpChecksum.hh>

    checksum_t::value_type calculateChecksum() const 
    {
        if (!checksumEnabled()) 
            return 0;

        senf::IpChecksum cs;
        cs.feed( i(), i(4) );
        // Skip even number of 0 bytes (the 2 bytes checksum field)
        // cs.feed(0); cs.feed(0);
        cs.feed( i(6), data().end() );

        return cs.sum()
    }
    \endcode

    This code just implements what is defined in the RFC: The checksum covers the complete GRE
    packet including it's header with the checksum field temporarily set to 0. Instead of really
    changing the checksum field we manually pass the correct data to \a cs. 

    We use the special <tt>i(</tt><i>offset</i><tt>)</tt> helper to get iterators \a offset number
    of bytes into the data. This helper has the additional benefit of range-checking the returned
    iterator and thereby safe os from errors due to truncated packets: If the offset is out of
    range, a TruncatedPacketException will be thrown.

    In this code we utilize some additional information provided by senf::PacketParserBase. The \a
    i() member returns an iterator to the first byte the parser is interpreting whereas \a data()
    returns a reference to the packet data container for the packet being parsed. Access to \a
    data() should be restricted as much as possible. It is safe when defining new packet parsers
    (like GREPacketParser). It's usage from sub parsers (like GREPacketParser_OptFields or even
    senf::UInt16Parser) would be much more arcane and should be avoided.


    \subsection howto_newpacket_parser_final The complete GREPacketParser implementation

    So this is now the complete implementation of the \c GREPacketParser:

    \code
    #include <senf/Packets.hh>
    
    struct GREPacketParser_OptFields : public senf::PacketParser
    {
    #   include SENF_FIXED_PARSER()

        SENF_PARSER_FIELD           ( checksum,        senf::UInt16Parser            );
        SENF_PARSER_SKIP            (                   2                            );

        SENF_PARSER_FINALIZE(GREPacketParser_OptFields);
    };

    struct GREPacketParser : public senf::PacketParser
    {
    #   include SENF_PARSER()

        SENF_PARSER_BITFIELD_RO     ( checksumPresent,  1, bool                      );
        SENF_PARSER_SKIP_BITS       (                  12                            );
        SENF_PARSER_BITFIELD        ( version,          3, unsigned                  );

        SENF_PARSER_FIELD           ( protocolType,    senf::UInt16Parser            );

        SENF_PARSER_PRIVATE_VARIANT ( optionalFields_, checksumPresent,
                                                         (senf::VoidPacketParser)
                                                         (GREPacketParser_OptFields) );

        typedef GREPacketParser_OptFields::checksum_t checksum_t;
        checksum_t checksum() const
            { return optionalFields_().get<1>().checksum(); }

        void enableChecksum()  const { optionalFields_().init<1>(); }
        void disableChecksum() const { optionalFields_().init<0>(); }
    
        SENF_PARSER_FINALIZE(GREPacketParser);

        checksum_t::value_type calculateChecksum() const;
    };

    // In the implementation (.cc) file:

    #include <senf/Utils/IpChecksum.hh>
    
    GREPacketParser::checksum_t::value_type GREPacketParser::calculateChecksum() const
    {
        if (!checksumEnabled()) 
            return 0;

        validate(6);
        senf::IpChecksum cs;
        cs.feed( i(), i()+4 );
        // Skip even number of 0 bytes (the 2 bytes checksum field)
        // cs.feed(0); cs.feed(0);
        cs.feed( i()+6, data().end() );

        return cs.sum()
    }
    \endcode


    \section howto_newpacket_type Defining the packet type

    After we have implemented the \c GREPacketParser we need now to build the packet type. This is
    done by providing a special policy class called the 'packet type'. This class encapsulates all
    the information the packet library needs to know about a packet:


    \subsection howto_newpacket_type_skeleton The packet type skeleton

    For every type of packet, the 'packet type' class will look roughly the same. If the packet
    utilizes a registry and is not hopelessly complex, the packet type will almost always look like
    this:
    \code
    #include <senf/Packets.hh>

    struct GREPacketType
        : public senf::PacketTypeBase,
          public senf::PacketTypeMixin<GREPacketType, EtherTypes>
    {
        typedef senf::PacketTypeMixin<GREPacketType, EtherTypes> mixin;
        typedef senf::ConcretePacket<GREPacketType> packet;
        typedef senf::GREPacketParser parser;
    
        using mixin::nextPacketRange;
        using mixin::nextPacketType;
        using mixin::init;
        using mixin::initSize;

        // Define members here
    };
    \endcode
    We note, that \c GREPacketType derives from two classes: senf::PacketTypeBase and
    senf::PacketTypeMixin. senf::PacketTypeBase must be inherited by every packet type class. the
    senf::PacketTypeMixin provides default implementations for some members which are useful for
    most kinds of packets. If a packet type is very complex and these defaults don't work, the mixin
    class can and should be left out.

    Of the typedefs, only \a parser is mandatory. It defines the packet parser to use to interpret
    this type of packet. \a mixin and \a packet are defined to simplify the following
    definitions (More on \a packet and senf::ConcretePacket later).

    The next block of statements imports all the default implementations provided by the mixin
    class:
    
    \li \a nextPacketRange provides information about where the next packet lives within the GRE
        packet.
    \li \a nextPacketType provides the type of the next packet from information in the GRE packet.
    \li \a init is called to initialize a new GRE packet. This call is forwarded to \c
        GREPacketParser::init.
    \li \a initSize is called to find the size of an empty (newly create) GRE packet. This is also
        provided by GREPacketParser.
    
    With these default implementations provided by the mixin, only a few additional members are
    needed to complete the \c GREPacketType: \a nextPacketKey, \a finalize, and \a dump


    \subsection howto_newpacket_type_registry Utilizing the packet registry

    We want the GRE packet to utilize the senf::EtherTypes registry to find the type of packet
    contained in the GRE payload. A registry maps an arbitrary key value to a packet type
    represented by a packet factory instance. The details have already been taken care of by the
    senf::PacketTypeMixin (it provides the \a nextPacketType member). However, to lookup the packet
    in the registry, the mixin needs to know the key value. To this end, we implement \a
    nextPacketKey(), which is very simple:

    \code
    static key_t nextPacketKey(packet p) { return p->protocolType(); }
    \endcode

    All \c GREPacketType members are static. They are passed the packet in question as an
    argument. \a nextPacketKey() just needs to return the value of the correct packet field. And
    since the \c parser type (as defined as a typedef) allows direct access to the packet parser
    using the <tt>-></tt> operator, we can simply access that value.

    The \c key_t return type is a typedef provided by the mixin class it is taken from the type of
    registry, in this case it is senf::EtherTypes::key_t (which is defined as a 16 bit unsigned
    integer value).

    With this information, the packet library can now find out the type of packet needed to parse
    the GRE payload -- as long as the protocolType() is registered with the senf::EtherTypes
    registry. If this is not the case, the packet library will not try to interpret the payload, it
    will return a senf::DataPacket.

    One special case of GRE encapsulation occurs when layer 2 frames and especially ethernet frames
    are carried in the GRE payload. The ETHERTYPE registry normally only contains layer 3 protocols
    (like IP or IPX) however for this special case, the value 0x6558 has been added to the ETHERTYPE
    registry. So we need to add this value to inform the packet library to parse the payload as an
    ethernet packet if the \a protocolType() is 0x6558. This happens in the implementation file (the
    \c .cc file):

    \code
    #include <senf/Packets/DefaultBundle/EthernetPacket.hh>

    SENF_PACKET_REGISTRY_REGISTER( senf::EtherTypes, 0x6558, senf::EthernetPacket );
    \endcode

    This macro registers the value 0x6558 in the senf::EtherTypes registry and associates it with
    the packet type senf::EthernetPacket. This macro declares an anonymous static variable, it
    therefore must always be placed in the implementation file and \e never in an include file.

    Additionally, we want the GRE packet to be parsed when present as an IP payload. Therefore we
    additionally need to register GRE in the senf::IpTypes registry. Looking at the <a
    href="http://www.iana.org/assignments/protocol-numbers">IP protocol numbers</a>, we find that
    GRE has been assigned the value 47:

    \code
    #include <senf/Packets/DefaultBundle/IPv4Packet.hh>

    SENF_PACKET_REGISTRY_REGISTER( senf::IpTypes, 47, GREPacket );
    \endcode

    But wait -- what is \c GREPacket ? This question is answered a few section further down.


    \subsection howto_newpacket_type_invariants Providing packet invariants

    Many packets have some invariants that must hold: The payload size must be equal to some field,
    a checksum must match and so on. When packets are newly created or changed, these invariants
    have to be updated to be correct. This is the responsibility of the \a finalize() member. This
    is also the place, where the \a protocolType() field is assigned.

    \code
    static void finalize(packet p) 
    {
        if (p->checksumPresent())
            p->checksum() << p->calculateChecksum();
        p->protocolType() << key(p.next(senf::nothrow));
    }
    \endcode
    
    \a finalize() first updates the \a checksum() field if present. It then sets the \a
    protocolType() field depending on the \e next packet. The \c key() function is provided by the
    mixin class: It will lookup the \e type of a packet in the registry and return that packets key
    in the registry.

    Here we are using the more generic parser assignment expressed using the \c << operator. This
    operator in the most cases works like an ordinary assignment, however it can also be used to
    assign parsers to each other efficiently and it supports 'optional values' (as provided by <a
    href="http://www.boost.org/libs/optional/doc/optional.html">Boost.Optional</a> and as returned
    by \c key()).


    \subsection howto_newpacket_type_dump Writing out a complete packet: The 'dump()' member

    For diagnostic purposes, every packet should provide a meaningful \a dump() member which writes
    out the complete packet. This member is simple to implement and is often very helpful when
    tracking down problems.
    
    \code
    #include <boost/io/ios_state.hpp>

    static void dump(packet p, std::ostream & os)
    {
        boost::io::ios_all_saver ias(os);
        os << "General Routing Encapsulation:\n"
           << "  checksum present              : " << p->checksumPresent() ? "true" : "false" << "\n"
           << "  version                       : " << p->version() << "\n"
           << "  protocol type                 : 0x" << std::hex << std::setw(4) << std::setfill('0')
                                                     << p->protocolType() << "\n";
        if (p->checksumPresent())
            os << "  checksum                     : 0x" << std::hex << std::setw(4)
                                                        << std::setfill('0') << p->checksum() << "\n"; 
    }
    \endcode
    
    This member is quite straight forward. We should try to adhere to the formating standard shown
    above: The first line should be the type of packet/header being dumped followed by one line for
    each protocol field. The colon's should be aligned at column 33 with the field name indented by
    2 spaces. 

    The \c boost::ios_all_saver is just used to ensure, that the stream formatting state is restored
    correctly at the end of the method. An instance of this type will save the stream state when
    constructed and will restore that state when destructed.

    \subsection howto_newpacket_type_final Final touches

    The \c GREPacket implementation is now almost complete. The only thing missing is the \c
    GREPacket itself. \c GREPacket is just a typedef for a specific senf::ConcretePacket template
    instantiation. Here the complete GREPacket definition:

    \code
    #include <senf/Packets.hh>

    struct GREPacketType
        : public senf::PacketTypeBase,
          public senf::PacketTypeMixin<GREPacketType, EtherTypes>
    {
        typedef senf::PacketTypeMixin<GREPacketType, EtherTypes> mixin;
        typedef senf::ConcretePacket<GREPacketType> packet;
        typedef senf::GREPacketParser parser;
    
        using mixin::nextPacketRange;
        using mixin::nextPacketType;
        using mixin::init;
        using mixin::initSize;

        static key_t nextPacketKey(packet p) { return p->protocolType(); }
    
        static void finalize(packet p) {
            if (p->checksumPresent()) p->checksum() << p->calculateChecksum();
            p->protocolType() << key(p.next(senf::nothrow));
        }
    
        static void dump(packet p, std::ostream & os);
    };

    typedef GREPacketType::packet GREPacket;
    
    // In the implementation (.cc) file:

    #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
    #include <senf/Packets/DefaultBundle/IPv4Packet.hh>

    SENF_PACKET_REGISTRY_REGISTER( senf::EtherTypes, 0x6558, senf::EthernetPacket );
    SENF_PACKET_REGISTRY_REGISTER( senf::IpTypes, 47, GREPacket );

    void GREPacketType::dump(packet p, std::ostream & os)
    {
        boost::io::ios_all_saver ias(os);
        os << "General Routing Encapsulation:\n"
           << "  checksum present              : " << p->checksumPresent() ? "true" : "false" << "\n"
           << "  version                       : " << p->version() << "\n"
           << "  protocol type                 : 0x" << std::hex << std::setw(4) << std::setfill('0')
                                                     << p->protocolType() << "\n";
        if (p->checksumPresent())
            os << "  checksum                     : 0x" << std::hex << std::setw(4)
                                                        << std::setfill('0') << p->checksum() << "\n"; 
    }
    \endcode


    \section howto_newpacket_advanced Going further

    \subsection howto_newpacket_advanced_valid Checking the GRE packet for validity

    We have implemented the \c GREPacket completely by now. There is however still room for
    improvement. Reading the RFC, there are some restrictions which a packet needs to obey to be
    considered valid. If the packet is not valid it cannot be parsed and should be dropped. There
    are two conditions which lead to this: If one of the \a reserved0 fields first 5 bits is set or
    if the version is not 0. We will add a \a valid() check to the parser and utilize this check in
    the packet type.

    So first lets update the parser. We will need to change the fields a little bit so we have
    access to the first 5 bits of \a reserved0. We therefore replace the first three field
    statements with

    \code
    SENF_PARSER_BITFIELD_RO      ( checksumPresent,  1, bool                      );
    SENF_PARSER_PRIVATE_BITFIELD ( reserved0_5bits_, 5, unsigned                  );
    SENF_PARSER_SKIP_BITS        (                   7                            );
    SENF_PARSER_PRIVATE_BITFIELD ( version_,         3, unsigned                  );
    \endcode

    We have added an additional private bitfield \a reserved0_5bits_() and we made the \a version_()
    field private since we do not want the user to change the value. We could have used a read-only
    field in this special case (since the valid value is 0) but generally in a case like this since
    the field will be initialized by the parser (see next section on how). the field should be
    private with an additional public read-only accessor.

    We will now add two additional simple members to the parser

    \code
    typedef version__t version_t;
    version_t::value_type version() const { return version_(); }

    bool valid() const { return version() == 0 && reserved0_5bits_() == 0; }
    \endcode

    I think, both are quite straight forward: \a version() will allow the user to read out the value
    of the version field. However, since it does \e not return a parser but a numeric value, the
    access is read-only. The \a valid() member will just check the restrictions as defined in the RFC.

    Now to the packet type. We want to refrain from parsing the payload if the packet is
    invalid. This is important: If the packet is not valid, we have no idea, whether the payload is
    what we surmise it to be (if any of the \a reserved0_5bits_() are set, the packet is from an
    older GRE RFC and the header is quite a bit longer so the payload will be incorrect).

    So we need to change the logic which is used by the packet library to find the type of the next
    packet. We have two ways to do this: We keep using the default \c nextPacketType()
    implementation as provided by the senf::PacketTypeMixin and have our \a nextPacketKey()
    implementation return a key value which is guaranteed never to be registered in the registry.

    The more flexible possibility is implementing \c nextPacketType() ourselves. In this case, the
    first method would suffice, but we will choose to go the second route to show how to write the
    \c nextPacketType() member. We therefore remove the \c using declaration of \c nextPacketType()
    and also remove the \a nextPacketKey() implementation. Instead we add the following to the
    packet type

    \code
    // disabled: using Min::nextPacketType;

    factory_t nextPacketType(packet p) { return p->valid() ? lookup(p->protocolType()) : no_factory(); }
    \endcode
    
    As we see, this is still quite simple. \c factory_t is provided by senf::PacketTypeBase. For our
    purpose it is an opaque type which somehow enables the packet library to create a new packet of
    a specified packet type. The \c factory_t has a special value, \c no_factory() which stands for
    the absence of any concrete factory. In a boolean context this (and only this) \c factory_t
    value tests \c false.

    The \c lookup() member is provided by the senf::PacketTypeMixin. It looks up the key passed as
    argument in the registry and returns the factory or \c no_factory(), if the key was not found in
    the registry.

    In this case this is all. We can however return the factory for some specific type easily as in
    the following example:

    \code
    factory_t nextPacketType(packet p) { 
        if (p->valid()) {
            if (p->protocolType() == 0x6558)  return senf::EthernetPacket::factory();
            else                              return lookup(p->protocolType());
        }
        else                                  return no_factory();
    }
    \endcode

    Of course, this example is academic since senf::EthernetPacket is correctly registered in the
    senf::EtherTypes registry but you get the picture.


    \subsection howto_newpacket_advanced_init Non-trivial packet initialization

    When we create a new GRE packet using the packet library, the library will initialize the packet
    with all 0 bytes. It just happens, that this is correct for our GRE packet. Lets just for the
    sake of experiment assume, the GRE packet would have to set \a version() to 1 not 0. In this
    case, the default initialization would not suffice. It is however very simple to explicitly
    initialize the packet. The initialization happens within the parser. We just add

    \code
    SENF_PARSER_INIT() { version_() << 1u; }
    \endcode

    to \c GREPacketParser. Here we see, why we have defined \a version_() as a private and not a
    read-only field.


    \section howto_newpacket_final The ultimate GRE packet implementation completed

    So here we now have \c GREPacket finally complete in all it's glory. First the header file \c
    GREPacket.hh:

    \code
    #ifndef GRE_PACKET_HH
    #define GRE_PACKET_HH

    #include <senf/Packets.hh>
    
    struct GREPacketParser_OptFields : public senf::PacketParser
    {
    #   include SENF_FIXED_PARSER()

        SENF_PARSER_FIELD           ( checksum,        senf::UInt16Parser            );
        SENF_PARSER_SKIP            (                   2                            );

        SENF_PARSER_FINALIZE(GREPacketParser_OptFields);
    };

    struct GREPacketParser : public senf::PacketParser
    {
    #   include SENF_PARSER()

        SENF_PARSER_BITFIELD_RO      ( checksumPresent,  1, bool                      );
        SENF_PARSER_PRIVATE_BITFIELD ( reserved0_5bits_, 5, unsigned                  );
        SENF_PARSER_SKIP_BITS        (                   7                            );
        SENF_PARSER_PRIVATE_BITFIELD ( version_,         3, unsigned                  );

        SENF_PARSER_FIELD            ( protocolType,    senf::UInt16Parser            );

        SENF_PARSER_PRIVATE_VARIANT  ( optionalFields_, checksumPresent,
                                                          (senf::VoidPacketParser)
                                                          (GREPacketParser_OptFields) );

        typedef version__t version_t;
        version_t::value_type version() const { return version_(); }

        bool valid() const { return version() == 0 && reserved0_5bits_() == 0; }

        typedef GREPacketParser_OptFields::checksum_t checksum_t;
        checksum_t checksum() const
            { return optionalFields_().get<1>().checksum(); }

        void enableChecksum()  const { optionalFields_().init<1>(); }
        void disableChecksum() const { optionalFields_().init<0>(); }
    
        SENF_PARSER_FINALIZE(GREPacketParser);

        checksum_t::value_type calculateChecksum() const;
    };

    struct GREPacketType
        : public senf::PacketTypeBase,
          public senf::PacketTypeMixin<GREPacketType, EtherTypes>
    {
        typedef senf::PacketTypeMixin<GREPacketType, EtherTypes> mixin;
        typedef senf::ConcretePacket<GREPacketType> packet;
        typedef senf::GREPacketParser parser;
    
        using mixin::nextPacketRange;
        using mixin::init;
        using mixin::initSize;

        factory_t nextPacketType(packet p) 
            { return p->valid() ? lookup(p->protocolType()) : no_factory(); }
    
        static void finalize(packet p) {
            if (p->checksumPresent()) p->checksum() << p->calculateChecksum();
            p->protocolType() << key(p.next(senf::nothrow));
        }
    
        static void dump(packet p, std::ostream & os);
    };

    typedef GREPacketType::packet GREPacket;
    
    #endif
    \endcode

    And the implementation file \c GREPacket.cc:

    \code
    #include "GREPacket.hh"
    #include <senf/Utils/IpChecksum.hh>
    #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
    #include <senf/Packets/DefaultBundle/IPv4Packet.hh>

    SENF_PACKET_REGISTRY_REGISTER( senf::EtherTypes, 0x6558, senf::EthernetPacket );
    SENF_PACKET_REGISTRY_REGISTER( senf::IpTypes, 47, GREPacket );

    GREPacketParser::checksum_t::value_type GREPacketParser::calculateChecksum() const
    {
        if (!checksumEnabled()) 
            return 0;

        validate(6);
        senf::IpChecksum cs;
        cs.feed( i(), i()+4 );
        // Skip even number of 0 bytes (the 2 bytes checksum field)
        // cs.feed(0); cs.feed(0);
        cs.feed( i()+6, data().end() );

        return cs.sum()
    }

    void GREPacketType::dump(packet p, std::ostream & os)
    {
        boost::io::ios_all_saver ias(os);
        os << "General Routing Encapsulation:\n"
           << "  checksum present              : " << p->checksumPresent() ? "true" : "false" << "\n"
           << "  version                       : " << p->version() << "\n"
           << "  protocol type                 : 0x" << std::hex << std::setw(4) << std::setfill('0')
                                                     << p->protocolType() << "\n";
        if (p->checksumPresent())
            os << "  checksum                     : 0x" << std::hex << std::setw(4)
                                                        << std::setfill('0') << p->checksum() << "\n"; 
    }
    \endcode


    \section howto_newpacket_using Using the newly created GRE packet type

    The GRE packet is now fully integrated into the packet library framework. For example, we can
    read GRE packets from a raw INet socket and forward decapsulated Ethernet frames to a packet
    socket.

    \code
    #include <iostream>
    #include <senf/Packets.hh>
    #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
    #include <senf/Socket/Protocols/INet/RawINetProtocol.hh>
    #include <senf/Socket/Protocols/Raw/PacketSocketHandle.hh>
    #include "GREPacket.hh"

    int main(int, char const **)
    {
        senf::RawV6ClientSocketHandle isock (47u); // 47 = Read GRE packets
        senf::PacketSocketHandle osock;

        while (1) {
            try {
                GREPacket gre (GREPacket::create(senf::noinit));
                isock.read(gre.data(),0u);
                if (gre->checksumPresent() && gre->checksum() != gre->calculateChecksum())
                    throw InvalidPacketChainException();
                osock.write(gre.next<EthernetPacket>().data())
            }
            catch (senf::TruncatedPacketException & ex) {
                std::cerr << "Malformed packet dropped\n";
            }
            catch (senf::InvalidPacketChainException & ex) {
                std::cerr << "Invalid GRE packet dropped\n";
            }
        }
    }
    \endcode

    Or we can do the opposite: Read ethernet packets from a \c tap device and send them out GRE
    encapsulated.

    \code
    #include <iostream>
    #include <senf/Packets.hh>
    #include <senf/Packets/DefaultBundle/EthernetPacket.hh>
    #include <senf/Socket/Protocols/INet/RawINetProtocol.hh>
    #include <senf/Socket/Protocols/Raw/TunTapSocketHandle.hh>
    #include "GREPacket.hh"

    int main(int argc, char const ** argv)
    {
        if (argc != 2) {
            std::cerr << "Usage: " << argv[0] << " <tunnel endpoint>\n";
            return 1;
        }

        senf::TapSocketHandle tap ("tap0");
        senf::ConnectedRawV6ClientSocketHandle osock (47u, senf::INet6SocketAddress(argv[1]));
    
        while (1) {
            senf::EthernetPacket eth (senf::EthernetPacket::create(senf::noinit));
            isock.read(eth.data(),0u);
            GREPacket gre (senf::GREPacket::createBefore(eth));
            gre.finalize();
            osock.write(gre.data());
        }
    }
    \endcode

    
    \section howto_newpacket_further Further reading

    Lets start with references to the important API's (Use the 'Show all members' link to get the
    complete API of one of the classes and templates):

    \li senf::ConcretePacket : this is the API provided by the packet handles.
    \li senf::PacketData : this API provides raw data access accessible via the handles 'data'
        member.
    \li senf::PacketParserBase : this is the generic parser API. This API is accessible via the
        packets \c -> operator or via the sub-parsers returned by the field accessors.

    When implementing new packet's, the following information will be helpful:

    \li senf::PacketTypeBase : here you find a description of the members which need to be
        implemented to provide a 'packet type'. Most of these members will normally be provided by
        the mixin helper.
    \li senf::PacketTypeMixin : here you find all about the packet type mixin and how to use it.
    \li \ref packetparser : This section describes the packet parser facility.
    \li \ref packetparsermacros : A complete list and documentation of all the packet parser macros.
    \li There are several lists of available reusable packet parsers: \ref parseint, \ref
        parsecollection. However, this list is not complete as there are other protocol specific
        reusable parsers (without claiming to be exhaustive: senf::INet4AddressParser,
        senf::INet6AddressParser, senf::MACAddressParser)

 */

\f
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