#ifndef MTU_H#define MTU_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. *//* * It is a fatal error if mtu is less than * this value for tun device. */#define MIN_TUN_MTU     100/* * Default MTU of network over which tunnel data will pass by UDP. * TODO: DEFAULT_UDP_MTU = 1500 is probably too big... */#define DEFAULT_UDP_MTU 1300/* * Default MTU of tunnel device. */#define DEFAULT_TUN_MTU 1300struct frame {  int mtu;       /* MTU of tun/tap device */  /*   * extra_frame: How many extra bytes might each subsystem (crypto, TLS, or, compression)   * add to frame in worst case?   *   * mtu + extra_frame = MTU of UDP transport   */  int extra_frame;  /*   * extra_buffer: Worst case size added to internal buffer due to functions   * such as compression which can potentially expand the size of uncompressible   * data.   */  int extra_buffer;  /*   * extra_tun: max number of bytes that might be removed from head   * of incoming packet from tun device, or prepended to outgoing   * tun packet.   */  int extra_tun;};/* Routines which read struct frame should use the macros below */#define BUF_SIZE(f)          ((f)->mtu + (f)->extra_frame + (f)->extra_buffer + (f)->extra_tun)#define EXTRA_FRAME(f)       ((f)->extra_frame + (f)->extra_tun)#define MTU_SIZE(f)          ((f)->mtu)#define MTU_EXTRA_SIZE(f)    ((f)->mtu + (f)->extra_frame)/* * These values are used as maximum size constraints * on read() or write() from tun/tap device or UDP port. */#define MAX_RW_SIZE_TUN(f)   ((f)->mtu + (f)->extra_tun)#define MAX_RW_SIZE_UDP(f)   ((f)->mtu + (f)->extra_frame)#endif