/*
  *  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) 2002-2018 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 uncompressible 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 */