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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) 2010-2018 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 Control module documentation file.
+ */
+
+/**
+ * @defgroup data_control Data Channel Control module
+ *
+ * This module controls the processing of packets as they pass through the
+ * data channel.
+ *
+ * The Data Channel Control module controls the processing of packets as
+ * they pass through the data channel.  The processing includes packet
+ * compression, fragmentation, and the performing of security operations
+ * on the packets.  This module does not do the processing itself, but
+ * passes the packet to other data channel modules to perform the
+ * appropriate actions.
+ *
+ * Packets can travel in two directions through the data channel.  They
+ * can be going to a remote destination which is reachable through a VPN
+ * tunnel, in which case this module prepares them to be sent out through
+ * a VPN tunnel.  On the other hand, they can have been received through a
+ * VPN tunnel from a remote OpenVPN peer, in which case this module
+ * retrieves the packet in its original form as it was before entering the
+ * VPN tunnel on the remote OpenVPN peer.  How this module processes
+ * packets traveling in the two directions is discussed in more detail
+ * below.
+ *
+ * @par Packets to be sent to a remote OpenVPN peer
+ * This module's main function for processing packets traveling in this
+ * direction is \c encrypt_sign(), which performs the following processing
+ * steps:
+ * - Call the \link compression Data Channel Compression module\endlink to
+ *   perform packet compression if necessary.
+ * - Call the \link fragmentation Data Channel Fragmentation
+ *   module\endlink to perform packet fragmentation if necessary.
+ * - Call the \link data_crypto Data Channel Crypto module\endlink to
+ *   perform the required security operations.
+ *
+ * @par
+ * See the \c encrypt_sign() documentation for details of these
+ * interactions.
+ *
+ * @par
+ * After the above processing is complete, the packet is ready to be sent
+ * to a remote OpenVPN peer as a VPN tunnel packet.  The actual sending of
+ * the packet is handled by the \link external_multiplexer External
+ * Multiplexer\endlink.
+ *
+ * @par Packets received from a remote OpenVPN peer
+ * The function that controls how packets traveling in this direction are
+ * processed is \c process_incoming_link().  That function, however, also
+ * performs some of the tasks required for the \link external_multiplexer
+ * External Multiplexer\endlink and is therefore listed as part of that
+ * module, instead of here.
+ *
+ * @par
+ * After the \c process_incoming_link() function has determined that a
+ * received packet is a data channel packet, it performs the following
+ * processing steps:
+ * - Call the \link data_crypto Data Channel Crypto module\endlink to
+ *   perform the required security operations.
+ * - Call the \link fragmentation Data Channel Fragmentation
+ *   module\endlink to perform packet reassembly if necessary.
+ * - Call the \link compression Data Channel Compression module\endlink to
+ *   perform packet decompression if necessary.
+ *
+ * @par
+ * See the \c process_incoming_link() documentation for details of these
+ * interactions.
+ *
+ * @par
+ * After the above processing is complete, the packet is in its original
+ * form again as it was received by the remote OpenVPN peer.  It can now
+ * be routed further to its final destination.  If that destination is a
+ * locally reachable host, then the \link internal_multiplexer Internal
+ * Multiplexer\endlink will send it there.
+ */