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) 2002-2021 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, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#ifndef MTU_H
#define MTU_H

#include "buffer.h"

/*
 *
 * Packet maninipulation 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

/*
 * MTU Defaults for TAP devices
 */
#define TAP_MTU_EXTRA_DEFAULT  32

/*
 * Default MSSFIX value, used for reducing TCP MTU size
 */
#define MSSFIX_DEFAULT     1450

/*
 * Alignment of payload data such as IP packet or
 * ethernet frame.
 */
#define PAYLOAD_ALIGN 4


/**************************************************************************/
struct frame {
    int link_mtu;               
    int link_mtu_dynamic;       
    int extra_frame;            
    int extra_buffer;           
    int extra_tun;              
    int extra_link;             
    /*
     * Alignment control
     */
#define FRAME_HEADROOM_MARKER_DECRYPT     (1<<0)
#define FRAME_HEADROOM_MARKER_FRAGMENT    (1<<1)
#define FRAME_HEADROOM_MARKER_READ_LINK   (1<<2)
#define FRAME_HEADROOM_MARKER_READ_STREAM (1<<3)
    unsigned int align_flags;
    int align_adjust;
};

/* Forward declarations, to prevent includes */
struct options;

/* Routines which read struct frame should use the macros below */

/*
 * Overhead added to packet payload due to encapsulation
 */
#define EXTRA_FRAME(f)           ((f)->extra_frame)

/*
 * Delta between tun payload size and final TCP/UDP datagram size
 * (not including extra_link additions)
 */
#define TUN_LINK_DELTA(f)        ((f)->extra_frame + (f)->extra_tun)

/*
 * This is the size to "ifconfig" the tun or tap device.
 */
#define TUN_MTU_SIZE(f)          ((f)->link_mtu - TUN_LINK_DELTA(f))
#define TUN_MTU_SIZE_DYNAMIC(f)  ((f)->link_mtu_dynamic - TUN_LINK_DELTA(f))

/*
 * This is the maximum packet size that we need to be able to
 * read from or write to a tun or tap device.  For example,
 * a tap device ifconfiged to an MTU of 1200 might actually want
 * to return a packet size of 1214 on a read().
 */
#define PAYLOAD_SIZE(f)          ((f)->link_mtu - (f)->extra_frame)
#define PAYLOAD_SIZE_DYNAMIC(f)  ((f)->link_mtu_dynamic - (f)->extra_frame)

/*
 * Max size of a payload packet after encryption, compression, etc.
 * overhead is added.
 */
#define EXPANDED_SIZE(f)         ((f)->link_mtu)
#define EXPANDED_SIZE_DYNAMIC(f) ((f)->link_mtu_dynamic)
#define EXPANDED_SIZE_MIN(f)     (TUN_MTU_MIN + TUN_LINK_DELTA(f))

/*
 * These values are used as maximum size constraints
 * on read() or write() from TUN/TAP device or TCP/UDP port.
 */
#define MAX_RW_SIZE_TUN(f)       (PAYLOAD_SIZE(f))
#define MAX_RW_SIZE_LINK(f)      (EXPANDED_SIZE(f) + (f)->extra_link)

/*
 * Control buffer headroom allocations to allow for efficient prepending.
 */
#define FRAME_HEADROOM_BASE(f)     (TUN_LINK_DELTA(f) + (f)->extra_buffer + (f)->extra_link)
#define FRAME_HEADROOM(f)          frame_headroom(f, 0)
#define FRAME_HEADROOM_ADJ(f, fm)  frame_headroom(f, fm)

/*
 * Max size of a buffer used to build a packet for output to
 * the TCP/UDP port.
 */
#define BUF_SIZE(f)              (TUN_MTU_SIZE(f) + FRAME_HEADROOM_BASE(f) * 2)

/*
 * Function prototypes.
 */

void frame_finalize(struct frame *frame,
                    bool link_mtu_defined,
                    int link_mtu,
                    bool tun_mtu_defined,
                    int tun_mtu);

void frame_subtract_extra(struct frame *frame, const struct frame *src);

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);

/*
 * frame_set_mtu_dynamic and flags
 */

#define SET_MTU_TUN         (1<<0) /* use tun/tap rather than link sizing */
#define SET_MTU_UPPER_BOUND (1<<1) /* only decrease dynamic MTU */

void frame_set_mtu_dynamic(struct frame *frame, int mtu, unsigned int flags);

/*
 * allocate a buffer for socket or tun layer
 */
void alloc_buf_sock_tun(struct buffer *buf,
                        const struct frame *frame,
                        const bool tuntap_buffer,
                        const unsigned int align_mask);

void frame_init_mssfix(struct frame *frame, const struct options *options);

/*
 * 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);

const char *format_extended_socket_error(int fd, int *mtu, struct gc_arena *gc);

#endif

/*
 * Calculate a starting offset into a buffer object, dealing with
 * headroom and alignment issues.
 */
static inline int
frame_headroom(const struct frame *f, const unsigned int flag_mask)
{
    const int offset = FRAME_HEADROOM_BASE(f);
    const int adjust = (flag_mask & f->align_flags) ? f->align_adjust : 0;
    const int delta = ((PAYLOAD_ALIGN << 24) - (offset + adjust)) & (PAYLOAD_ALIGN - 1);
    return offset + delta;
}

/*
 * frame member adjustment functions
 */

static inline void
frame_add_to_link_mtu(struct frame *frame, const int increment)
{
    frame->link_mtu += increment;
}

static inline void
frame_add_to_extra_frame(struct frame *frame, const unsigned int increment)
{
    frame->extra_frame += increment;
}

static inline void
frame_remove_from_extra_frame(struct frame *frame, const unsigned int decrement)
{
    frame->extra_frame -= decrement;
}

static inline void
frame_add_to_extra_tun(struct frame *frame, const int increment)
{
    frame->extra_tun += increment;
}

static inline void
frame_add_to_extra_link(struct frame *frame, const int increment)
{
    frame->extra_link += increment;
}

static inline void
frame_add_to_extra_buffer(struct frame *frame, const int increment)
{
    frame->extra_buffer += increment;
}

static inline void
frame_align_to_extra_frame(struct frame *frame)
{
    frame->align_adjust = frame->extra_frame + frame->extra_link;
}

static inline void
frame_or_align_flags(struct frame *frame, const unsigned int flag_mask)
{
    frame->align_flags |= flag_mask;
}

static inline bool
frame_defined(const struct frame *frame)
{
    return frame->link_mtu > 0;
}

#endif /* ifndef MTU_H */