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/*
* 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 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 (see the file COPYING included with this
* distribution); if not, write to the Free Software Foundation, Inc.,
* 59 Temple Place, Suite 330, Boston, MA 02111-1307 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.
*/
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