NTP に関する備忘録

/*
 * NTP packet format.  The mac field is optional.  It isn't really
 * an l_fp either, but for now declaring it that way is convenient.
 * See Appendix A in the specification.
 *
 * Note that all u_fp and l_fp values arrive in network byte order
 * and must be converted (except the mac, which isn't, really).
 */
struct pkt {
        u_char  li_vn_mode;     /* peer leap indicator */
        u_char  stratum;        /* peer stratum */
        u_char  ppoll;          /* peer poll interval */
        s_char  precision;      /* peer clock precision */
        u_fp    rootdelay;      /* roundtrip delay to primary source */
        u_fp    rootdisp;       /* dispersion to primary source*/
        u_int32 refid;          /* reference id */
        l_fp    reftime;        /* last update time */
        l_fp    org;            /* originate time stamp */
        l_fp    rec;            /* receive time stamp */
        l_fp    xmt;            /* transmit time stamp */

#define LEN_PKT_NOMAC   (12 * sizeof(u_int32)) /* min header length */
#define MIN_MAC_LEN     (1 * sizeof(u_int32))   /* crypto_NAK */
#define MAX_MD5_LEN     (5 * sizeof(u_int32))   /* MD5 */
#define MAX_MAC_LEN     (6 * sizeof(u_int32))   /* SHA */

        /*
         * The length of the packet less MAC must be a multiple of 64
         * with an RSA modulus and Diffie-Hellman prime of 256 octets
         * and maximum host name of 128 octets, the maximum autokey
         * command is 152 octets and maximum autokey response is 460
         * octets. A packet can contain no more than one command and one
         * response, so the maximum total extension field length is 864
         * octets. But, to handle humungus certificates, the bank must
         * be broke.
         *
         * The different definitions of the 'exten' field are here for
         * the benefit of applications that want to send a packet from
         * an auto variable in the stack - not using the AUTOKEY version
         * saves 2KB of stack space. The receive buffer should ALWAYS be
         * big enough to hold a full extended packet if the extension
         * fields have to be parsed or skipped.
         */
#ifdef AUTOKEY
        u_int32 exten[(NTP_MAXEXTEN + MAX_MAC_LEN) / sizeof(u_int32)];
#else   /* !AUTOKEY follows */
        u_int32 exten[(MAX_MAC_LEN) / sizeof(u_int32)];
#endif  /* !AUTOKEY */
};

/*
 * NTP uses two fixed point formats.  The first (l_fp) is the "long"
 * format and is 64 bits long with the decimal between bits 31 and 32.
 * This is used for time stamps in the NTP packet header (in network
 * byte order) and for internal computations of offsets (in local host
 * byte order). We use the same structure for both signed and unsigned
 * values, which is a big hack but saves rewriting all the operators
 * twice. Just to confuse this, we also sometimes just carry the
 * fractional part in calculations, in both signed and unsigned forms.
 * Anyway, an l_fp looks like:
 *
 *    0                   1                   2                   3
 *    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *   |                         Integral Part                         |
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *   |                         Fractional Part                       |
 *   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 */
typedef struct {
        union {
                u_int32 Xl_ui;
                int32 Xl_i;
        } Ul_i;
        u_int32 l_uf;
} l_fp;

/* POSIX.1b structure for a time value.  This is like a `struct timeval' but
   has nanoseconds instead of microseconds.  */
struct timespec
{
  __time_t tv_sec;		/* Seconds.  */
#if __WORDSIZE == 64 \
  || (defined __SYSCALL_WORDSIZE && __SYSCALL_WORDSIZE == 64) \
  || __TIMESIZE == 32
  __syscall_slong_t tv_nsec;	/* Nanoseconds.  */
#else
# if __BYTE_ORDER == __BIG_ENDIAN
  int: 32;           /* Padding.  */
  long int tv_nsec;  /* Nanoseconds.  */
# else
  long int tv_nsec;  /* Nanoseconds.  */
  int: 32;           /* Padding.  */
# endif
#endif
};