src/openvpn/mtu.h - openvpn - Git at Google

 /*
  *  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


 /**************************************************************************/
 /**
  * Packet geometry parameters.
  */
 struct frame {
     int link_mtu;               /**< Maximum packet size to be sent over
                                  *   the external network interface. */

     int link_mtu_dynamic;       /**< Dynamic MTU value for the external
                                  *   network interface. */

     int extra_frame;            /**< Maximum number of bytes that all
                                  *   processing steps together could add.
                                  *   @code
                                  *   frame.link_mtu = "socket MTU" - extra_frame;
                                  *   @endcode
                                  */

     int extra_buffer;           /**< Maximum number of bytes that
                                  *   processing steps could expand the
                                  *   internal work buffer.
                                  *
                                  *   This is used by the \link compression
                                  *   Data Channel Compression
                                  *   module\endlink to give enough working
                                  *   space for worst-case expansion of
                                  *   incompressible content. */

     int extra_tun;              /**< Maximum number of bytes in excess of
                                  *   the tun/tap MTU that might be read
                                  *   from or written to the virtual
                                  *   tun/tap network interface. */

     int extra_link;             /**< Maximum number of bytes in excess of
                                  *   external network interface's MTU that
                                  *   might be read from or written to it. */

     /*
      * 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(int 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);

 /** Set the --mssfix option. */
 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_add_to_align_adjust(struct frame *frame, const int increment)
 {
     frame->align_adjust += 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 */
	/*
	* 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


	/**************************************************************************/
	/**
	* Packet geometry parameters.
	*/
	struct frame {
	int link_mtu; /**< Maximum packet size to be sent over
	* the external network interface. */

	int link_mtu_dynamic; /**< Dynamic MTU value for the external
	* network interface. */

	int extra_frame; /**< Maximum number of bytes that all
	* processing steps together could add.
	* @code
	* frame.link_mtu = "socket MTU" - extra_frame;
	* @endcode
	*/

	int extra_buffer; /**< Maximum number of bytes that
	* processing steps could expand the
	* internal work buffer.
	*
	* This is used by the \link compression
	* Data Channel Compression
	* module\endlink to give enough working
	* space for worst-case expansion of
	* incompressible content. */

	int extra_tun; /**< Maximum number of bytes in excess of
	* the tun/tap MTU that might be read
	* from or written to the virtual
	* tun/tap network interface. */

	int extra_link; /**< Maximum number of bytes in excess of
	* external network interface's MTU that
	* might be read from or written to it. */

	/*
	* 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(int 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);

	/** Set the --mssfix option. */
	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_add_to_align_adjust(struct frame *frame, const int increment)
	{
	frame->align_adjust += 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 */