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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 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:
- * - \c P_CONTROL_HARD_RESET_CLIENT_V1 -- %Key method 1, initial %key
- * from client, forget previous state.
- * - \c P_CONTROL_HARD_RESET_SERVER_V1 -- %Key method 1, initial %key
- * from server, forget previous state.
- * - \c P_CONTROL_HARD_RESET_CLIENT_V2 -- %Key method 2, initial %key
- * from client, forget previous state.
- * - \c P_CONTROL_HARD_RESET_SERVER_V2 -- %Key method 2, initial %key
- * from server, forget previous state.
- * - \c 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.
- * - \c P_CONTROL_V1 -- Control channel packet (usually TLS
- * ciphertext).
- * - \c P_ACK_V1 -- Acknowledgement for control channel packets
- * received.
- * - Data channel messages:
- * - \c P_DATA_V1 -- Data channel packet containing 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 and
- *
- * 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()
- * OpenSSL 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:
- * - 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
- *
- * @subsection network_protocol_data_ciphertext Structure of ciphertext data channel messages
- *
- * The P_DATA_* payload represents encrypted, encapsulated tunnel packets
- * which tend to be either IP packets or Ethernet frames. This is
- * essentially the "payload" of the VPN.
- *
- * Data channel packets in ciphertext form consist of the following parts:
- * - HMAC of ciphertext IV + ciphertext (if not disabled by \c --auth
- * none).
- * - Ciphertext IV (size is cipher-dependent, if not disabled by \c
- * --no-iv).
- * - Tunnel packet ciphertext.
- *
- * @subsection network_protocol_data_plaintext Structure of plaintext data channel messages
- *
- * Data channel packets in plaintext form consist of the following parts:
- * - packet-id (4 or 8 bytes, if not disabled by --no-replay).
- * - In TLS mode, 4 bytes are used because the implementation can
- * force a TLS renegotation before \c 2^32 packets are sent.
- * - In pre-shared %key mode, 8 bytes are used (sequence number and \c
- * time_t value) to allow long-term %key usage without packet-id
- * collisions.
- * - User plaintext (n bytes).
- */