/*
  *  OpenVPN -- An application to securely tunnel IP networks
  *             over a single TCP/UDP port, with support for SSL/TLS-based
  *             session authentication and key exchange,
  *             packet encryption, packet authentication, and
  *             packet compression.
  *

  *  Copyright (C) 2010-2018 Fox Crypto B.V. <openvpn@fox-it.com>

  *
  *
  *  This program is free software; you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License version 2
  *  as published by the Free Software Foundation.
  *
  *  This program is distributed in the hope that it will be useful,
  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  *  GNU General Public License for more details.
  *

  *  You should have received a copy of the GNU General Public License along
  *  with this program; if not, write to the Free Software Foundation, Inc.,
  *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.

  */
 
 /**
  * @file
  * Data Channel Fragmentation module documentation file.
  */
 
 /**
  * @defgroup fragmentation Data Channel Fragmentation module
  *
  * The Data Channel Fragmentation module offers fragmentation of data
  * channel packets.
  *
  * @par State structures
  * The Data Channel Fragmentation module stores its internal state in a \c
  * fragment_master structure.  One such structure is present for each VPN
  * tunnel, and is stored in \c context.c2.fragment of the \c context
  * associated with that VPN tunnel.
  *
  * @par
  * The \c fragment_master structure contains one \c fragment_list
  * structure \c fragment_master.incoming.  This is a list of \c fragment
  * structures, each of which can store the parts of one fragmented packet
  * while it is being reassembled.  The \c fragment_master structure also
  * contains one \c buffer called \c fragment_master.outgoing, in which a
  * data channel large packet to be sent to a remote OpenVPN peer can be
  * broken up into parts to be sent one by one.
  *
  * @par Initialization and cleanup
  * Every time a new \c fragment_master is needed, it must be allocated and
  * initialized by the \c fragment_init() function.  Similarly, every time
  * a \c fragment_master is no longer needed, it must be cleaned up using
  * the \c fragment_free() function.  These functions take care of the
  * allocation and freeing of the \c fragment_master structure itself and
  * all internal memory required for the use of that structure.  Note that
  * this behavior is different from that displayed by the \link compression
  * Data Channel Compression module\endlink.
  *
  * @par
  * Because of the one-to-one relationship between \c fragment_master
  * structures and VPN tunnels, the above-mentioned initialization and
  * cleanup functions are called directly from the \c init_instance() and
  * \c close_instance() functions, which control the initialization and
  * cleanup of VPN tunnel instances and their associated \c context
  * structures.
  *
  * @par Packet processing functions
  * This module receives data channel packets from the \link data_control
  * Data Channel Control module\endlink and processes them according to the
  * settings of the packet's VPN tunnel.  The \link data_control Data
  * Channel Control module\endlink uses the following interface functions:
  * - For packets which will be sent to a remote OpenVPN peer: \c
  *   fragment_outgoing() \n This function inspects data channel packets as
  *   they are being made ready to be sent as VPN tunnel packets to a
  *   remote OpenVPN peer.  If a packet's size is larger than its
  *   destination VPN tunnel's maximum transmission unit (MTU), then this
  *   module breaks that packet up into smaller parts, each of which is
  *   smaller than or equal to the VPN tunnel's MTU.  See \c
  *   fragment_outgoing() for details.
  * - For packets which have been received from a remote OpenVPN peer: \c
  *   fragment_incoming() \n This function inspects data channel packets
  *   that have been received from a remote OpenVPN peer through a VPN
  *   tunnel.  It reads the fragmentation header of the packet, and
  *   depending on its value performs the appropriate action.  See \c
  *   fragment_incoming() for details.
  *
  * @par Settings that control this module's activity
  * Whether the Data Channel Fragmentation module is active or not depends
  * on the compile-time \c ENABLE_FRAGMENT preprocessor macro and the
  * runtime flag \c options.fragment, which gets its value from the
  * process's configuration sources, such as the configuration file and
  * commandline %options.
  */