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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) 2010-2026 Sentyron B.V. <openvpn@sentyron.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, see <https://www.gnu.org/licenses/>.
*/
/**
* @file
* Data Channel Compression module documentation file.
*/
/**
* @defgroup compression Data Channel Compression module
*
* This module offers compression of data channel packets.
*
* @par State structures
* The Data Channel Compression module stores its internal state in a \c
* lzo_compress_workspace structure. This state includes flags which
* control the module's behavior and preallocated working memory. One
* such structure is present for each VPN tunnel, and is stored in the \c
* context.c2.lzo_compwork of the \c context associated with that VPN
* tunnel.
*
* @par Initialization and cleanup
* Every time a new \c lzo_compress_workspace is needed, it must be
* initialized using the \c lzo_compress_init() function. Similarly,
* every time a \c lzo_compress_workspace is no longer needed, it must be
* cleaned up using the \c lzo_compress_uninit() function. These
* functions take care of the allocation and freeing of internal working
* memory, but not of the \c lzo_compress_workspace structures themselves.
*
* @par
* Because of the one-to-one relationship between \c
* lzo_compress_workspace 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
* lzo_compress()
* - For packets which have been received from a remote OpenVPN peer: \c
* lzo_decompress()
*
* @par Settings that control this module's activity
* Whether or not the Data Channel Compression module is active depends on
* the compile-time \c ENABLE_LZO preprocessor macro and the runtime flags
* stored in \c lzo_compress_workspace.flags of the associated VPN tunnel.
* The latter are initialized from \c options.lzo, which gets its value
* from the process's configuration sources, such as its configuration
* file or command line %options.
*
* @par Adaptive compression
* The compression module supports adaptive compression. If this feature
* is enabled, the compression routines monitor their own performance and
* turn compression on or off depending on whether it is leading to
* significantly reduced payload size.
*
* @par Compression algorithms
* This module uses the Lempel-Ziv-Oberhumer (LZO) compression algorithms.
* These offer lossless compression and are designed for high-performance
* decompression. This module uses the external \c lzo library's
* implementation of the algorithms.
*
* @par
* For more information on the LZO library, see:\n
* https://www.oberhumer.com/opensource/lzo/
*/
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