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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 Network protocol overview documentation file.
+ */
+
+/**
+ * @page network_protocol OpenVPN's network protocol
+ *
+ * Description of packet structure in OpenVPN's network protocol.
+ *
+ * This document describes the structure of packets exchanged between
+ * OpenVPN peers.  It is based on the protocol description in the \c ssl.h
+ * file.
+ *
+ * @section network_protocol_external Outer structure of packets exchanged between OpenVPN peers
+ *
+ * VPN tunnel packets are transported between OpenVPN peers using the UDP
+ * or TCP protocols.  Their structure is described below.
+ *
+ * @subsection network_protocol_external_structure External packet structure
+ *
+ *  - packet length (16 bits, unsigned) [TCP-mode only]: always sent as
+ *    plain text.  Since TCP is a stream protocol, this packet length
+ *    defines the packetization of the stream.
+ *  - packet opcode and key_id (8 bits) [TLS-mode only]:
+ *     - package message type (high 5 bits)
+ *     - key_id (low 3 bits): the key_id refers to an already negotiated
+ *       TLS session.  OpenVPN seamlessly renegotiates the TLS session by
+ *       using a new key_id for the new session.  Overlap (controlled by
+ *       user definable parameters) between old and new TLS sessions is
+ *       allowed, providing a seamless transition during tunnel operation.
+ *  - payload (n bytes)
+ *
+ * @subsection network_protocol_external_types Message types
+ *
+ * The type of a VPN tunnel packet is indicated by its opcode.  The
+ * following describes the various opcodes available.
+ *
+ *  - Control channel messages:
+ *     - \ref P_CONTROL_HARD_RESET_CLIENT_V1 -- %Key method 1, initial %key
+ *       from client, forget previous state.
+ *     - \ref P_CONTROL_HARD_RESET_SERVER_V1 -- %Key method 1, initial %key
+ *       from server, forget previous state.
+ *     - \ref P_CONTROL_HARD_RESET_CLIENT_V2 -- %Key method 2, initial %key
+ *       from client, forget previous state.
+ *     - \ref P_CONTROL_HARD_RESET_SERVER_V2 -- %Key method 2, initial %key
+ *       from server, forget previous state.
+ *     - \ref P_CONTROL_SOFT_RESET_V1 -- New %key, with a graceful
+ *       transition from old to new %key in the sense that a transition
+ *       window exists where both the old or new key_id can be used.
+ *     - \ref P_CONTROL_V1 -- Control channel packet (usually TLS
+ *       ciphertext).
+ *     - \ref P_ACK_V1 -- Acknowledgement for control channel packets
+ *       received.
+ *  - Data channel messages:
+ *     - \ref P_DATA_V1 -- Data channel packet containing data channel
+ *       ciphertext.
+ *     - \ref P_DATA_V2 -- Data channel packet containing peer-id and data
+ *       channel ciphertext.
+ *
+ * @subsection network_protocol_external_key_id Session IDs and Key IDs
+ *
+ * OpenVPN uses two different forms of packet identifiers:
+ *  - The first form is 64 bits and is used for all control channel
+ *    messages.  This form is referred to as a \c session_id.
+ *  - Data channel messages on the other hand use a shortened form of 3
+ *    bits for efficiency reasons since the vast majority of OpenVPN
+ *    packets in an active tunnel will be data channel messages.  This
+ *    form is referred to as a \c key_id.
+ *
+ * The control and data channels use independent packet-id sequences,
+ * because the data channel is an unreliable channel while the control
+ * channel is a %reliable channel.  Each use their own independent HMAC
+ * keys.
+ *
+ * @subsection network_protocol_external_reliable Control channel reliability layer
+ *
+ * Control channel messages (\c P_CONTROL_* and \c P_ACK_* message types)
+ * are TLS ciphertext packets which have been encapsulated inside of a
+ * reliability layer.  The reliability layer is implemented as a
+ * straightforward acknowledge and retransmit model.
+ *
+ * Acknowledgments of received messages can be encoded in either the
+ * dedicated \c P_ACK_* record or they can be prepended to a \c
+ * P_CONTROL_* message.
+ *
+ * See the \link reliable Reliability Layer\endlink module for a detailed
+ * description.
+ *
+ * @section network_protocol_control Structure of control channel messages
+ *
+ * @subsection network_protocol_control_ciphertext Structure of ciphertext control channel messages
+ *
+ * Control channel packets in ciphertext form consist of the following
+ * parts:
+ *
+ *  - local \c session_id (random 64 bit value to identify TLS session).
+ *  - HMAC signature of entire encapsulation header for HMAC firewall
+ *    [only if \c --tls-auth is specified] (usually 16 or 20 bytes).
+ *  - packet-id for replay protection (4 or 8 bytes, includes sequence
+ *    number and optional \c time_t timestamp).
+ *  - acknowledgment packet-id array length (1 byte).
+ *  - acknowledgment packet-id array (if length > 0).
+ *  - acknowledgment remote session-id (if length > 0).
+ *  - packet-id of this message (4 bytes).
+ *  - TLS payload ciphertext (n bytes) (only for \c P_CONTROL_V1).
+ *
+ * Note that when \c --tls-auth is used, all message types are protected
+ * with an HMAC signature, even the initial packets of the TLS handshake.
+ * This makes it easy for OpenVPN to throw away bogus packets quickly,
+ * without wasting resources on attempting a TLS handshake which will
+ * ultimately fail.
+ *
+ * @subsection network_protocol_control_key_methods Control channel key methods
+ *
+ * Once the TLS session has been initialized and authenticated, the TLS
+ * channel is used to exchange random %key material for bidirectional
+ * cipher and HMAC keys which will be used to secure data channel packets.
+ * OpenVPN currently implements two %key methods.  %Key method 1 directly
+ * derives keys using random bits obtained from the \c rand_bytes() function.
+ * %Key method 2 mixes random %key material from both sides of the connection
+ * using the TLS PRF mixing function.  %Key method 2 is the preferred method and
+ * is the default for OpenVPN 2.0+.
+ *
+ * The @ref key_generation "Data channel key generation" related page
+ * describes the %key methods in more detail.
+ *
+ * @subsection network_protocol_control_plaintext Structure of plaintext control channel messages
+ *
+ *  - %Key method 1 (support removed in OpenVPN 2.5):
+ *     - Cipher %key length in bytes (1 byte).
+ *     - Cipher %key (n bytes).
+ *     - HMAC %key length in bytes (1 byte).
+ *     - HMAC %key (n bytes).
+ *     - %Options string (n bytes, null terminated, client/server %options
+ *       string should match).
+ *  - %Key method 2:
+ *     - Literal 0 (4 bytes).
+ *     - %Key method (1 byte).
+ *     - \c key_source structure (\c key_source.pre_master only defined
+ *       for client -> server).
+ *     - %Options string length, including null (2 bytes).
+ *     - %Options string (n bytes, null terminated, client/server %options
+ *       string must match).
+ *     - [The username/password data below is optional, record can end at
+ *       this point.]
+ *     - Username string length, including null (2 bytes).
+ *     - Username string (n bytes, null terminated).
+ *     - Password string length, including null (2 bytes).
+ *     - Password string (n bytes, null terminated).
+ *
+ * @section network_protocol_data Structure of data channel messages
+ *
+ * The P_DATA_* payload represents encapsulated tunnel packets which tend to be
+ * either IP packets or Ethernet frames. This is essentially the "payload" of
+ * the VPN. Data channel packets consist of a data channel header, and a
+ * payload. There are two possible formats:
+ *
+ * @par P_DATA_V1
+ * P_DATA_V1 packets have a 1-byte header, carrying the \ref P_DATA_V1 \c opcode
+ * and \c key_id, followed by the payload:\n
+ * <tt> [ 5-bit opcode | 3-bit key_id ] [ payload ] </tt>
+ *
+ * @par P_DATA_V2
+ * P_DATA_V2 packets have the same 1-byte opcode/key_id, but carrying the \ref
+ * P_DATA_V2 opcode, followed by a 3-byte peer-id, which uniquely identifies
+ * the peer:\n
+ * <tt> [ 5-bit opcode | 3-bit key_id ] [ 24-bit peer-id ] [ payload ] </tt>
+ *
+ * See @ref data_crypto for details on the data channel payload format.
+ *
+ */