[senf.git] / senf / Packets / Mainpage.dox

// $Id$
//
// 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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// Free Software Foundation, Inc.,
// 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

/** \mainpage The SENF Packet Library

    The SENF Packet library provides facilities to analyze, manipulate and create structured packet
    oriented data (e.g. network packets).

    \autotoc


    \section packet_intro_arch Introduction
    \seechapter \ref packet_arch

    The Packet library consists of several components:

    \li The \ref packet_module manages the packet data and provides the framework for handling the
        chain of packet headers. The visible interface is provided by the Packet class.
    \li \ref packetparser provides the framework for interpreting packet data. It handles
        parsing the packet information into meaningful values.
    \li The \ref protocolbundles provide concrete implementations for interpreting packets of
        some protocol. The Protocol Bundles are built on top of the basic packet library.

    All these components work together to provide a hopefully simple and intuitive interface to
    packet parsing and creation.


    \section packet_intro_usage Tutorial
    \seechapter \ref packet_usage

    This chapter discusses the usage of the packet library from a high level view.


    \section packet_intro_api The packet API

    The packet library API is divided into three areas

    \li the \ref senf::PacketData API for accessing the raw data container
    \li the packet interpreter chain providing \ref packet_module
    \li and \ref packetparser which provides access to protocol specific packet fields.


    \section protocolbundles Supported packet types (protocols)

    Each protocol bundle provides a collection of related concrete packet classes for a group of
    related protocols:

    \li \ref protocolbundle_default : Some basic default protocols: Ethernet, Ip, TCP, UDP
    \li \ref protocolbundle_mpegdvb : MPEG and DVB protocols
    \li \ref protocolbundle_80211 : 802.11 protocols
    \li \ref protocolbundle_80221 : 802.21 protocols

    There are two ways to link with a bundle

    \li If you only work with known packets which you explicitly reference you may just link with
        the corresponding library.
    \li If you need to parse unknown packets and want those to be parsed as complete as possible
        without explicitly referencing the packet type, you will need to link against the combined
        object file built for every bundle. This way, all packets defined in the bundle will be
        included whether they are explicitly referenced or not (and they will all automatically be
        registered).


    \section packet_intro_new Defining new packet types
    \seechapter \ref packet_new

    The packet library provides the framework which allows to define arbitrary packet types. There
    is quite some information needed to completely specify a specific type of packet.

 */

/** \page packet_arch Overall Packet library Architecture

    The packet library handles network packets of a large number of protocols. We work with a packet
    on three levels

    \autotoc


    \section packet_arch_handle The Packet handle

    Whenever we are using a Packet, we are talking about a senf::Packet (or a
    senf::ConcretePacket). This class is a \e handle referencing an internally managed packet data
    structure. So even though we pass senf::Packet instances around by value, they work like
    references. The packet library automatically manages all required memory resources using
    reference counting.

    Different Packet handles may really internally share one Packet data structure if they both
    point to the same packet.


    \section packet_arch_data The Packet as a 'bunch of bytes'

    From the outside, a packet is just a bunch of bytes just as it is read from (or will be
    written to) the wire. At this low-level view, we can access the data in it's raw form but
    have no further information about what kind of packet we have.

    The packet library provides a consistent container interface for this representation.

    \code
    Packet p = ...;

    // Change first byte of packet to 1
    p.data()[0] = 1u;

    // Copy packet data into a vector
    std::vector<char> data (p.data().size());
    std::copy(p.data().begin(), p.data().end(), data.begin());
    \endcode

    This type of access is primarily needed when reading or writing packets (e.g. to/from the
    network).

    \see senf::Packet::data() \n
        senf::PacketData


    \section packet_arch_chain The Interpreter Chain

    On the next level, the packet is divided into a nested list of sub-packets (or headers) called
    interpreters. Each senf::Packet handle internally points to an interpreter or header. This
    allows us to access one and the same packet in different ways.

    Consider an Ethernet Packet with an IP payload holding a UDP packet. We may reference either the
    Ethernet packet as a whole or we may reference the IP or UDP interpreters (sub-packets or
    headers). All handles really refer to the \e same data structure but provide access to a
    different (sub-)range of the data in the packet.

    We can navigate around this chained structure using appropriate members:

    \code
    // eth, ip and udp all reference the same internal packet data albeit at different data ranges
    Packet eth = ...;
    Packet ip = eth.next();
    Packet udp = ip.next();

    eth.next() == ip                   // true
    eth.next().is<IPv4Packet>()        // true
    eth.next().next() == udp           // true
    eth.next().is<UDPPacket>()         // false
    eth.find<UDPPacket>() == udp       // true

    udp.find<EthernetPacket>()         // throws InvalidPacketChainException
    udp.find<EthernetPacket>(senf::nothrow) // An in-valid() senf::Packet which tests as 'false'
    udp.find<UDPPacket> == udp         // true
    udp.first<IPv4Packet>()            // throws InvalidPacketChainException

    udp.prev() == ip                   // true
    udp.prev<EthernetPacket>()         // throws InvalidPacketChainException
    \endcode

    \see \ref packet_module


    \section packet_arch_parser Parsing specific Protocols

    On the next level, the packet library allows us to parse the individual protocols. This gives us
    access to the protocol specific data members of a packet and allows us to access or manipulate a
    packet in a protocol specific way.

    To access this information, we need to use a protocol specific handle, the senf::ConcretePacket
    which takes as a template argument the specific type of packet to be interpreted. This allows us
    to easily interpret or create packets. Here an example on how to create a new Ethernet / IP / UDP
    / Payload packet interpreter chain:

    \code
    // EthernetPacket, IPv4Packet, UDPPacket and DataPacket are typedefs for corresponding
    // ConcretePacket instantiations
    senf::EthernetPacket eth      (senf::EthernetPacket::create());
    senf::IPv4Packet     ip       (senf::IPv4Packet    ::createAfter(eth));
    senf::UDPPacket      udp      (senf::UDPPacket     ::createAfter(ip));
    senf::DataPacket     payload  (senf::DataPacket    ::createAfter(udp,
                                                                     std::string("Hello, world!")));

    udp->source()      = 2000u;
    udp->destination() = 2001u;
    ip->ttl()          = 255u;
    ip->source()       = senf::INet4Address::from_string("192.168.0.1");
    ip->destination()  = senf::INet4Address::from_string("192.168.0.2");
    eth->source()      = senf::MACAddress::from_string("00:11:22:33:44:55");
    eth->destination() = senf::MACAddress::from_string("00:11:22:33:44:66");

    eth.finalizeAll();
    \endcode

    Again, realize, that \a eth, \a ip, \a udp and \a payload share the same internal packet
    data structure (the respective \c data() members all provide access to the same underlying
    container however at different byte ranges): The complete packet can be accessed at
    <tt>eth.data()</tt> whereas <tt>payload.data()</tt> only holds UDP payload (in this case the
    string "Hello, world!").

    \see \ref packetparser \n
        \ref protocolbundles
 */

/** \page packet_usage Using the packet library

    \autotoc

    \section packet_usage_intro Includes

    To use the library, you need to include the appropriate header files. This will probably happen
    automatically when including the specific protocol headers. If needed, you may explicitly use

    \code
    #include "Packets.hh"
    \endcode

    explicitly.

    \warning Never include any other Packets library header directly, only include \c
    Packets.hh or one (or several) protocol headers from the protocol bundles.

    Most every use of the packet library starts with some concrete packet typedef. Some fundamental
    packet types are provided by \ref protocolbundle_default.


    \section packet_usage_create Creating a new packet

    Building on those packet types, this example will build a complex packet: This will be an
    Ethernet packet containing an IPv4 UDP packet. We begin by building the raw packet skeleton:

    \code
    #include "Packets/DefaultBundle/EthernetPacket.hh"
    #include "Packets/DefaultBundle/IPv4Packet.hh"
    #include "Packets/DefaultBundle/UDPPacket.hh"

    senf::EthernetPacket eth      (senf::EthernetPacket::create());
    senf::IPv4Packet     ip       (senf::IPv4Packet    ::createAfter(eth));
    senf::UDPPacket      udp      (senf::UDPPacket     ::createAfter(ip));
    senf::DataPacket     payload  (senf::DataPacket    ::createAfter(udp,
                                                                     std::string("Hello, world!")));
    \endcode

    These commands create what is called an interpreter chain. This chain consists of four
    interpreters. All interpreters reference the same data storage. This data storage is a random
    access sequence which contains the data bytes of the packet.

    \note The data structures allocated are automatically managed using reference counting. In this
        example we have four packet references each referencing the same underlying data
        structure. This data structure will be freed when the last reference to it goes out of
        scope.

    The packet created above already has the correct UDP payload (The string "Hello, world!")
    however all protocol fields are empty. We need to set those protocol fields:

    \code
    udp->source()      = 2000u;
    udp->destination() = 2001u;
    ip->ttl()          = 255u;
    ip->source()       = senf::INet4Address::from_string("192.168.0.1");
    ip->destination()  = senf::INet4Address::from_string("192.168.0.2");
    eth->source()      = senf::MACAddress::from_string("00:11:22:33:44:55");
    eth->destination() = senf::MACAddress::from_string("00:11:22:33:44:66");

    eth.finalizeAll();
    \endcode

    As seen above, packet fields are accessed using the <tt>-></tt> operator whereas other packet
    facilities (like \c finalizeAll()) are directly accessed using the member operator. The field
    values are simply set using appropriately named accessors. As a last step, the \c finalizeAll()
    call will update all calculated fields (fields like next-protocol, header or payload length,
    checksums etc). Now the packet is ready. We may now send it out using a packet socket

    \code
    senf::PacketSocketHandle sock();
    sock.bind( senf::LLSocketAddress("eth0"));
    sock.write(eth.data());
    \endcode


    \section packet_usage_read Reading and parsing packets

    The chain navigation functions are also used to parse a packet. Let's read an Ethernet packet
    from a packet socket handle:

    \code
    senf::PacketSocketHandle sock();
    sock.bind( senf::LLSocketAddress("eth0"));
    senf::EthernetPacket packet (senf::EthernetPacket::create(senf::noinit));
    sock.read(packet.data(),0u);
    \endcode

    This first creates an uninitialized Ethernet packet and then reads into this packet. We can now
    parse this packet. Let's find out, whether this is a UDP packet destined to port 2001:

    \code
    try {
        senf::UDPPacket udp (packet.find<UDPPacket>());
        if (udp->destination() == 2001u) {
            // Voila ...
        }
    } catch (senf::TruncatedPacketException &) {
        std::cerr << "Ooops !! Broken packet received\n";
    } catch (senf::InvalidPacketChainException &) {
        std::cerr << "Not a udp packet\n";
    }
    \endcode

    TruncatedPacketException is thrown by <tt>udp->destination()</tt> if that field cannot be
    accessed (that is it would be beyond the data read which means we have read a truncated
    packet). More generally, whenever a field cannot be accessed because it would be out of bounds
    of the data read, this exception is generated.


    \section packet_usage_container The raw data container

    Every packet is based internally on a raw data container holding the packet data. This container
    is accessed via senf::Packet::data() member.

    This container is a random access container. It can be used like an ordinary STL container and
    supports all the standard container members.

    \code
    Packet p = ...;

    // Insert 5 0x01 bytes
    p.data().insert(p.data().begin()+5, 5, 0x01);

    // Insert data from another container
    p.data().insert(p.data().end(), other.begin(), other.end());

    // Erase a single byte
    p.data().erase(p.data().begin()+5);

    // XOR byte 5 with 0xAA
    p.data()[5] ^= 0xAA;
    \endcode

    A packet consists of a list of interpreters (packet headers or protocols) which all reference
    the same data container at different byte ranges. Each packet consists of the protocol header \e
    plus the packets payload. This means, that the data container ranges of successive packets from
    a single interpreter chain are nested.

    Example: The packet created above (the Ethernet-IP-UDP packet with payload "Hello, world!") has
    4 Interpreters: Ethernet, IPv4, UDP and the UDP payload data. The nested data containers lead to
    the following structure

    \code
    // The ethernet header has a size of 14 bytes
    eth.data().begin() + 14 == ip.data().begin()
    eth.data().end()        == ip.data().end()

    // The IP header has a size of 20 bytes and therefore
    ip.data().begin()  + 20 == udp.data().begin()
    ip.data().end()         == udp.data().end()

    // The UDP header has a size of 8 bytes and thus
    udp.data().begin() +  8 == payload.data().begin()
    udp.data().end()        == payload.data().end()
    \endcode

    This nesting will (and must) always hold: The data range of a subsequent packet will always be
    within the range of it's preceding packet.

    \warning It is forbidden to change the data of a subsequent packet interpreter from the
        preceding packet even if the data container includes this data. If you do so, you may
        corrupt the data structure (especially when changing it's size).

    Every operation on a packet is considered to be \e within this packet and \e without and
    following packet. When inserting or erasing data, the data ranges are all adjusted
    accordingly. So the following are \e not the same even though \c eth.end(), \c ip.end() and \c
    udp.end() are identical.

    \code
    eth.data().insert(eth.data().end(), 5, 0x01);
    assert(    eth.data().end() == ip.data().end() + 5
            && ip.data().end()  == udp.data().end() );

    // Or alternatively: (You could even use eth.data().end() here ... it's the same)
    ip.data().insert(ip.data().end(), 5, 0x01);
    assert(    eth.data().end() == ip.data().end()
            && ip.data().end()  == udp.data().end() + 5 );
    \endcode

    \warning When accessing the packet data via the container interface, you may easily build
        invalid packets since the packet will not be validated against it's protocol.


    \section packet_usage_fields Field access

    When working with concrete protocols, the packet library provides direct access to all the
    protocol information.

    \code
    udp->source()      = 2000u;
    udp->destination() = 2001u;
    ip->ttl()          = 255u;
    ip->source()       = senf::INet4Address::from_string("192.168.0.1");
    ip->destination()  = senf::INet4Address::from_string("192.168.0.2");
    eth->source()      = senf::MACAddress::from_string("00:11:22:33:44:55");
    eth->destination() = senf::MACAddress::from_string("00:11:22:33:44:66");
    \endcode

    The protocol field members above do \e not return references, they return parser instances.
    Protocol fields are accessed via parsers. A parser is a very lightweight class which points into
    the raw packet data and converts between raw data bytes and it's interpreted value: For example
    a senf::UInt16Parser accesses 2 bytes (in network byte order) and converts them to or from a 16
    bit integer. There are a few properties about parsers which need to be understood:

    \li Parsers are created only temporarily when needed. They are created when accessing a protocol
        field and are returned by value.

    \li A parser never contains a value itself, it just references a packets data container.

    \li Parsers can be built using other parsers and may have members which return further parsers.

    The top-level interface to a packets protocol fields is provided by a protocol parser. This
    protocol parser is a composite parser which has members to access the protocol fields (compare
    with the example code above). Some protocol fields may be more complex than a simple value. In
    this case, those accessors may return other composite parsers or collection parsers. Ultimately,
    a value parser will be returned.

    The simple value parsers which return plain values (integer numbers, network addresses etc) can
    be used like those values and can also be assigned corresponding values. More complex parsers
    don't allow simple assignment. However, they can always be copied from another parser <em>of the
    same type</em> using the generalized parser assignment. This type of assignment also works for
    simple parsers and is then identical to a normal assignment.

    \code
    // Copy the complete udp parser from udp packet 2 to packet 1
    udp1.parser() << udp2.parser();
    \endcode

    Additionally, the parsers have a parser specific API which allows to manipulate or query the
    value.

    This is a very abstract description of the parser structure. For a more concrete description, we
    need to differentiate between the different parser types

    \subsection packet_usage_fields_value Simple fields (Value parsers)

    We have already seen value parsers: These are the lowest level building blocks witch parse
    numbers, addresses etc. They return some type of value and can be assigned such a value. More
    formally, they have a \c value_type typedef member which gives the type of value they accept and
    they have an overloaded \c value() member which is used to read or set the value. Some parsers
    have additional functionality: The numeric parser for Example provide conversion and arithmetic
    operators so they can be used like a numeric value.

    If you have a value parser \c valueParser with type \c ValueParser, the following will always be
    valid:
    \code
    // You can read the value and assign it to a variable of the corresponding value_type
    ValueParser::value_type v (valueParser.value());

    // You can assign that value to the parser
    valueParser.value(v);

    // The assignment can also be done using the generic parser assignment
    valueParser << v;
    \endcode


    \subsection packet_usage_fields_composite Composite and protocol parsers

    A composite parser is a parser which just combines several other parsers into a structure: For
    example, the senf::EthernetPacketParser has members \c destination(), \c source() and \c
    type_length(). Those members return parsers again (in this case value parsers) to access the
    protocol fields.

    Composite parsers can be nested; A composite parser may be returned by another composite
    parser. The protocol parser is a composite parser which defines the field for a specific
    protocol header like Ethernet.

    \subsection packet_usage_fields_collection Collection parsers

    Besides simple composites, the packet library has support for more complex collections.

    \li The senf::ArrayParser allows to repeat an arbitrary parser a fixed number of times.
    \li senf::VectorParser and senf::ListParser are two different types of lists with variable
        number of elements
    \li The senf::VariantParser is a discriminated union: It will select one of several parsers
        depending on the value of a discriminant.


    \subsubsection packet_usage_collection_vector Vector and List Parsers

    Remember, that a parser does \e not contain any data: It only points into the raw data
    container. This is also true for the collection parsers. VectorParser and ListParser provide an
    interface which looks like an STL container to access a sequence of elements.

    We will use an \c MLDv2QueryPacket as an example (see <a
    href="http://tools.ietf.org/html/rfc3810#section-5">RFC 3810</a>). Here an excerpt of the
    relevant fields:

    <table class="fields">
    <tr><td>nrOfSources</td><td>Integer</td><td>Number of multicast sources in this packet</td></tr>
    <tr><td>sources</td><td>Vector of IPv6 Addresses</td><td>Multicast sources</td></tr>
    </table>

    To demonstrate nested collections, we use the \c MLDv2ReportPacket as an example. The relevant
    fields of this packet are;

    <table class="fields">
    <tr><td>nrOfRecords</td><td>Integer</td><td>Number of multicast address records</td></tr>
    <tr><td>records</td><td>List of Records</td><td>List of multicast groups and sources</td></tr>
    </table>

    Each Record is a composite with the following relevant fields:

    <table class="fields">
    <tr><td>nrOfSources</td><td>Integer</td><td>Number of sources in this record</td></tr>
    <tr><td>sources</td><td>Vector of IPv6 Addresses</td><td>Multicast sources</td></tr>
    </table>

    The first example will iterate over the sources in a \c MLDv2QueryPacket:

    \code
    MLDv2QueryPacket mld = ...;

    // Instantiate a collection wrapper for the source list
    MLDv2QueryPacket::Parser::sources_t::container sources (mld->sources());

    // Iterate over all the addresses in that list
    for (MLDv2QueryPacket::Parser::sources_t::container::iterator i (sources.begin());
         i != sources.end(); ++i)
        std::cout << *i << std::endl;
    \endcode

    Beside other fields, the MLDv2Query consists of a list of source addresses. The \c sources()
    member returns a VectorParser for these addresses. The collection parsers can only be accessed
    completely using a container wrapper. The container wrapper type is available as the \c
    container member of the collection parser, here it is \c
    MLDv2QueryPacket::Parser::sources_t::container.

    Using this wrapper, we can not only read the data, we can also manipulate the source list. Here
    we copy a list of addresses from an \c std::vector into the packet:

    \code
    std::vector<senf::INet6Address> addrs (...);

    sources.resize(addrs.size());
    std::copy(addrs.begin(), addrs.end(), sources.begin())
    \endcode

    Collection parsers may be nested. To access a nested collection parser, a container wrapper must
    be allocated for each level. An MLD Report (which is a composite parser) includes a list of
    multicast address records called \c records(). Each record is again a composite which contains a
    list of sources called \c sources():

    \code
    MLDv2ReportPacket report = ...;

    // Instantiate a collection wrapper for the list of records:
    MLDv2ReportPacket::Parser::records_t::container records (report->records());

    // Iterate over the multicast address records
    for (MLDv2ReportPacket::Parser::records_t::container::iterator i (records.begin());
         i != records.end(); ++i) {
        // Allocate a collection wrapper for the multicast address record
        typedef MLDv2ReportPacket::Parser::records_t::value_type::sources_t Sources;
        Sources::container sources (i->sources());

        // Iterate over the sources in this record
        for (Sources::container::iterator i (sources.begin());
             i != sources.end(); ++i)
            std::cout << *i << std::endl;
    }
    \endcode

    In this example we also see how to find the type of a parser or container wrapper.
    \li Composite parsers have typedefs for each their fields with a \c _t postfix
    \li The vector or list parsers have a \c value_type typedef which gives the type of the
        element.

    By traversing this hierarchical structure, the types of all the fields can be found.

    The container wrapper is only temporary (even though it has a longer lifetime than a
    parser). Any change made to the packet not via the collection wrapper has the potential to
    invalidate the wrapper if it changes the packets size.

    \see
        senf::VectorParser / senf::VectorParser_Container Interface of the vector parser \n
        senf::ListParser / senf::ListParser_Container Interface of the list parser


    \subsubsection packet_usage_collection_variant The Variant Parser

    The senf::VariantParser is a discriminated union of parsers. It is also used for optional fields
    (using senf::VoidPacketParser as one possible variant which is a parser parsing nothing).  A
    senf::VariantParser is not really a collection in the strict sense: It only ever contains one
    element, the \e type of which is determined by the discriminant.

    For Example, we look at the DTCP HELLO Packet as defined in the UDLR Protocol (see <a
    href="http://tools.ietf.org/html/rfc3077">RFC 3077</a>)

    \code
    DTCPHelloPacket hello (...);

    if (hello->ipVersion() == 4) {
        typedef DTCPHelloPacket::Parser::v4fbipList_t FBIPList;
        FBIPList::container fbips (hello->v4fbipList());
        for (FBIPList::container::iterator i (fbips.begin()); i != fbips.end(); ++i)
            std::cout << *i << std::endl;
    }
    else { // if (hello->ipVersion() == 6)
        typedef DTCPHelloPacket::Parser::v6fbipList_t FBIPList;
        FBIPList::container fbips (hello->v6fbipList());
        for (FBIPList::container::iterator i (fbips.begin()); i != fbips.end(); ++i)
            std::cout << *i << std::endl;
    }
    \endcode

    This packet has a field \c ipVersion() which has a value of 4 or 6. Depending on the version,
    the packet contains a list of IPv4 or IPv6 addresses. Only one of the fields \c v4fbipList() and
    \c v6fbipList() is available at a time. Which one is decided by the value of \c
    ipVersion(). Trying to access the wrong one will provoke undefined behavior.

    Here we have used the variants discriminant (the \c ipVersion() field) to select, which field to
    parse. More generically, every variant field should have a corresponding member to test for it's
    existence:
    \code
    if (hello->has_v4fbipList()) {
        ...
    }
    else { // if (hello->has_v6fbipList())
        ...
    }
    \endcode

    A variant can have more than 2 possible types and you can be sure, that exactly one type will be
    accessible at any time.

    It is not possible to change a variant by simply changing the discriminant:
    \code
    // INVALID CODE:
    hello->ipVersion() = 6;
    \endcode
    Instead, for each variant field there is a special member which switches the variant to that
    type. After switching the type, the field will be in it's initialized (that is mostly zero)
    state.
    \code
    std::vector<senf::INet6Address> addrs (...);

    // Initialize the IPv6 list
    hello->init_v6fbipList();

    // Copy values into that list
    DTCPHelloPacket::Parser::v6fbipList_t::container fbips (hello->v6fbipList());
    fbips.resize(addrs.size());
    std::copy(addrs.begin(), addrs.end(), fbips.begin());
    \endcode

    \note Here we have documented the default variant interface as it is preferred. It is possible
        to define variants in a different way giving other names to the special members (\c has_\e
        name or \c init_\e name etc.). This must be documented with the composite or protocol parser
        which defines the variant.

    \section packet_usage_annotation Annotations

    Sometimes we need to store additional data with a packet. Data, which is not part of the packet
    itself but gives us some information about the packet: A timestamp, the interface the packet was
    received on or other processing related information.

    This type of information can be stored using the annotation interface. The following example
    will read packet data and will store the read timestamp as a packet annotation.

    \code
    struct Timestamp {
        senf::ClockService::clock_t value;
    };

    std::ostream & operator<<(std::ostream & os, Timestamp const & tstamp) {
        os << tstamp.value; return os;
    }

    senf::EthernetPacket packet (senf::EthernetPacket::create(senf::noinit));
    sock.read(packet.data(), 0u);
    packet.annotation<Timestamp>().value = senf::ClockService::now();
    \endcode

    In the same way, the annotation can be used later

    \code
    if (senf::ClockService::now() - packet.annotation<Timestamp>().value
            > senf::ClockService::seconds(1)) {
        // this packet is to old
        // ...
    }
    \endcode

    It is very important to define a specific structure (or class or enum) type for each type of
    annotation. \e Never directly store a fundamental type as an annotation: The name of the type is
    used to look up the annotation, so you can store only one annotation for each built-in type. \c
    typedef does not help since \c typedef does not introduce new type names, it only defines an
    alias.

    The annotation type must support the output \c operator<< for description purposes
    (e.g. for the \ref senf::Packet::dump() "Packet::dump()" member).

    Of course, the annotation structure can be arbitrary. However, one very important caveat: If the
    annotation is not a POD type, it needs to inherit from senf::ComplexAnnotation. A type is POD,
    if it is really just a bunch of bytes: No (non-static) members, no constructor or destructor and
    no base classes and all it's members must be POD too. So the following annotation is complex
    since \c std::string is not POD

    \code
    struct ReadInfo : senf::ComplexAnnotation
    {
        std::string interface;
        senf::ClockService::clock_t timestamp;
    };

    // ...

    packet.annotation<ReadInfo>().interface = "eth0";
    packet.annotation<ReadInfo>().timestamp = senf::ClockService::now();

    // Or store a reference to the annotation for easier access

    ReadInfo & info (packet.annotation<ReadInfo>());

    if (info.interface == "eth0") {
        // ...
    }
    \endcode

    Conceptually, all annotations always exist in every packet, there is no way to query, whether a
    packet holds a specific annotation.

    You should use annotations economically: Every annotation type used in your program will
    allocate an annotation slot in \e all packet data structures. So don't use hundreds of different
    annotation types if this is not really necessary: Reuse annotation types where possible or
    aggregate data into larger annotation structures. The best solution is to use annotations only
    for a small number of packet specific informations. If you really need to manage a train-load of
    data together with the packet consider some other way (e.g. place the packet into another class
    which holds that data).

    \see senf::Packet::annotation() \n
        senf::dumpPacketAnnotationRegistry() for annotation debugging and optimization
 */

/** \page packet_new Defining new Packet types

    Each packet is specified by the following two components:

    \li A protocol parser which defines the protocol specific fields
    \li A packet type class which is a policy class defining the packet

    \autotoc

    \see <a href="../../../../HowTos/NewPacket/doc/html/index.html">NewPacket HowTo</a>

    \section packet_new_parser The protocol parser

    The protocol parser is simply a composite parser. It defines all the protocol
    fields. Additionally, the protocol parser may have additional members which will then be
    accessible via the \c -> operator of the packet. Possibilities here are e.g. checksum
    calculation and validation, packet validation as a whole and so on.

    Defining a protocol parser is quite simple:
    \code
    struct EthernetPacketParser : public PacketParserBase
    {
    #   include SENF_FIXED_PARSER()

        SENF_PARSER_FIELD( destination, MACAddressParser    );
        SENF_PARSER_FIELD( source,      MACAddressParser    );
        SENF_PARSER_FIELD( type_length, UInt16Parser );

        SENF_PARSER_FINALIZE(EthernetPacketParser);
    };
    \endcode

    There are a lot of other possibilities to define fields. See \ref packetparsermacros for a
    detailed description of the macro language which is used to define composite parsers.

    \see
        \ref packetparsermacros

    \section packet_new_type The packet type policy class

    This is a class which provides all the information needed to integrate the new packet type into
    the packet library:

    \li It provides the type of the protocol parser to use
    \li It provides information on how the next protocol can be found and where the payload resides
        in this packet
    \li It provides methods to initialize a new packet and get information about the packet size

    All this information is provided via static or typedef members.

    \code
    struct EthernetPacketType
        : public PacketTypeBase,
          public PacketTypeMixin<EthernetPacketType, EtherTypes>
    {
        typedef PacketTypeMixin<EthernetPacketType, EtherTypes> mixin;
        typedef ConcretePacket<EthernetPacketType> packet;
        typedef EthernetPacketParser parser;

        using mixin::nextPacketRange;
        using mixin::initSize;
        using mixin::init;

        static factory_t nextPacketType(packet p);
        static void dump(packet p, std::ostream & os);
        static void finalize(packet p);
    };

    typedef EthernetPacketType::packet EthernetPacket;
    \endcode

    The definition of senf::EthernetPacket is quite straight forward. This template works for most
    simple packet types.

    \see \ref senf::PacketTypeMixin \n
        \ref senf::PacketTypeBase \n
        \ref senf::PacketRegistry
 */

\f
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