mtu.h

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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) 2002-2025 OpenVPN Inc <sales@openvpn.net>
 *
 *  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/>.
 */
 
#ifndef MTU_H
#define MTU_H
 
#include "buffer.h"
 
/*
 *
 * Packet manipulation routes such as encrypt, decrypt, compress, decompress
 * are passed a frame buffer that looks like this:
 *
 *    [extra_frame bytes] [mtu bytes] [extra_frame_bytes] [compression overflow bytes]
 *                         ^
 *                   Pointer passed to function points here so that routine
 *                   can make use of extra_frame bytes before pointer
 *                   to prepend headers, etc.
 *
 *    extra_frame bytes is large enough for all encryption related overhead.
 *
 *    mtu bytes will be the MTU size set in the ifconfig statement that configures
 *      the TUN or TAP device such as:
 *
 *      ifconfig $1 10.1.0.2 pointopoint 10.1.0.1 mtu 1450
 *
 *    Compression overflow bytes is the worst-case size expansion that would be
 *    expected if we tried to compress mtu + extra_frame bytes of incompressible data.
 */
 
/*
 * Standard ethernet MTU
 */
#define ETHERNET_MTU 1500
 
/*
 * It is a fatal error if mtu is less than
 * this value for tun device.
 */
#define TUN_MTU_MIN 100
 
/*
 * Default MTU of network over which tunnel data will pass by TCP/UDP.
 */
#define LINK_MTU_DEFAULT 1500
 
/*
 * Default MTU of tunnel device.
 */
#define TUN_MTU_DEFAULT 1500
 
/*
 * Minimum maximum MTU
 */
#define TUN_MTU_MAX_MIN 1600
 
/*
 * MTU Defaults for TAP devices
 */
#define TAP_MTU_EXTRA_DEFAULT 32
 
/*
 * Default MSSFIX value, used for reducing TCP MTU size
 */
#define MSSFIX_DEFAULT 1492
 
/*
 * Default maximum size of control channel packets
 */
#define TLS_MTU_DEFAULT 1250
 
/*
 * Alignment of payload data such as IP packet or
 * ethernet frame.
 */
#define PAYLOAD_ALIGN 4
 
 
/**************************************************************************/
struct frame
{
    struct
    {
        /* This struct holds all the information about the buffers that are
         * allocated to match this frame */
        int payload_size; 
        int headroom; 
        int tailroom; 
    } buf;
 
    uint16_t mss_fix;      
    int max_fragment_size; 
    int tun_mtu;           
    int tun_max_mtu;       
    int extra_tun;         
};
 
/* Forward declarations, to prevent includes */
struct options;
 
/*
 * Control buffer headroom allocations to allow for efficient prepending.
 */
 
/*
 * Max size of a buffer used to build a packet for output to
 * the TCP/UDP port or to read a packet from a tap/tun device.
 *
 * Most of our code only prepends headers but compression needs the extra bytes
 * *after* the data as compressed data might end up larger than the original
 * data. Also crypto needs an extra block for encryption. Therefore tailroom is
 * larger than the headroom.
 */
#define BUF_SIZE(f) ((f)->buf.headroom + (f)->buf.payload_size + (f)->buf.tailroom)
 
/*
 * Function prototypes.
 */
 
void frame_print(const struct frame *frame, int level, const char *prefix);
 
void set_mtu_discover_type(socket_descriptor_t sd, int mtu_type, sa_family_t proto_af);
 
int translate_mtu_discover_type_name(const char *name);
 
/* forward declaration of key_type */
struct key_type;
 
size_t frame_calculate_payload_size(const struct frame *frame, const struct options *options,
                                    const struct key_type *kt);
 
size_t frame_calculate_payload_overhead(size_t extra_tun, const struct options *options,
                                        const struct key_type *kt);
 
 
size_t frame_calculate_protocol_header_size(const struct key_type *kt,
                                            const struct options *options, bool occ);
 
size_t calc_options_string_link_mtu(const struct options *options, const struct frame *frame);
 
unsigned int calc_packet_id_size_dc(const struct options *options, const struct key_type *kt);
 
/*
 * allocate a buffer for socket or tun layer
 */
void alloc_buf_sock_tun(struct buffer *buf, const struct frame *frame);
 
/*
 * EXTENDED_SOCKET_ERROR_CAPABILITY functions -- print extra error info
 * on socket errors, such as PMTU size.  As of 2003.05.11, only works
 * on Linux 2.4+.
 */
 
#if EXTENDED_SOCKET_ERROR_CAPABILITY
 
void set_sock_extended_error_passing(int sd, sa_family_t proto_af);
 
const char *format_extended_socket_error(int fd, int *mtu, struct gc_arena *gc);
 
#endif
 
#endif /* ifndef MTU_H */