20151122 - Leapseconds via DNSΒΆ

As I had long expected, ITU ducked out of the leapsecond decision business and designated the question “for further study” - which is their phrase of art for “forget it”.

(Other examples of “for further study”: How to convert Voice and Telefax messages to X.400 emails. How to get the world to adopt X.400 emails. How to send TELEX across ISDN D-channels etc. etc.)

So we’re stuck with leap-seconds for another decade or more.

It would be possible for IERS to issue Bulletin C with longer lead-time than 4-6 months if they want to, but I have heard of no movement in that direction.

Even if they did, the german time-transmitter DCF-77 only announces leap-seconds one hour before it happens, so whatever we do, we’re stuck with the leap-second announcement problem.

Here is a work-around:

If you do a DNS lookup on the name ‘leapsecond.utcd.org’ you get the IPv4 number 244.34.36.97 back, and that encodes the content of Bulletin C 50

To decode it, you need to do a bit of math:

Querying currently published leapsecond announcement:

  IP: 244.34.36.97     Error:  0  Year: 2015  Month 12  dTAI:  36  Delta:  0

That means:

   Information is valid until end of UTC-month 12 of year 2015
   After that month: UTC = TAI - 36 seconds
   Until then:       UTC = TAI - 36 seconds

Unless somebody more qualified, authoritative and trustworthy volunteers, I’m going to keep that service running and updated as long as I can.

Here is the reference implementation source code, which explains how it all works:

/*-
 * Copyright (c) 2015 Poul-Henning Kamp <phk@FreeBSD.org>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *
 * Reference implementation of code to retrieve current leap-second
 * announcement via DNS lookup.
 *
 * Specification:
 * --------------
 *
 * The leap second information is encoded into a IPv4 adress as follows:
 *
 *    3                   2                   1                   0
 *  1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |1 1 1 1|        month        | d |   dTAI      |    CRC-8      |
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 * 'month' Count of months since december 1971
 *  = (year - 1971) * 12 + month - 11
 *
 * 'dTAI'  Number of seconds UTC is behind of TAI
    UTC = TAI - dTAI
 *
 * 'd' what happens to dTAI at the end of the month indicated
 *  0 -> nothing
 *  1 -> subtract one from dTAI
 *  2 -> add one to dTAI
 *  3 -> Illegal
 *
 *
 * Example:
 * --------
 *
 * The IPv4 address "244.23.35.255" encodes Bulletin C 49
 *
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 * |1 1 1 1|0 1 0 0 0 0 0 1 0 1 1|1 0|0 1 0 0 0 1 1|1 1 1 1 1 1 1 1|
 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 *
 * month = 0x20b = 523 = (2015 - 1971) * 12 + 6 - 11 -> June 2015
 *
 * d = 0x2 -> +1
 *
 * dTAI = 0x23 = 35 -> UTC = TAI - 35 sec
 *
 * CRC-8 = 0xff -> Calculated message {month d dTAI}.  See below.
 *
 * Design notes:
 * -------------
 *
 * The first four bits puts the resulting IPv4 address into the "class-E"
 * space which is "reserved for future use", as a defense against lying
 * DNS resolvers.
 *
 * At this point, late 2015, it does not look like class-E will ever be
 * allocated for any use.  Most network stacks treat them as martians
 * (ie: patently invalid), and at current consumption rates, they would
 * be gobbled up far faster than we could upgrade network stacks.
 *
 * Therefore no sane DNS resolver should ever return a class-E addres,
 * unless somebody does really strange things with IPv4 numbers.
 *
 * A second layer of defense against lying DNS resolvers is the CRC8
 * integrity check in the last octet.
 *
 * The field widths should be good until about year 2140.
 *
 * At this point in time the dTAI field is considered unsigned, but
 * should strange and awe inspiring geophysical events unfold,
 * spinning up the rotation of the planet, (while implausibly leaving
 * this protocol still relevant) the field can be redefined as signed.
 *
 *
 * Code notes:
 * -----------
 *
 * The reference implementation below has been written for maximum
 * portability an relies on text-processing rather than attempting
 * to pick struct sockaddr apart to decode the IPv4 number.
 *
 */

#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

/*
 * MSB first CRC8 with polynomium (x^8 +x^5 +x^3 +x^2 +x +1)
 *
 * This is by a small margin the best CRC8 for the message length (28 bits)
 *
 * For much more about CRC's than you'd ever want to know:
 *
 *  http://users.ece.cmu.edu/~koopman/crc/index.html
 *
 * PS:  The CRC seed is not random.
 */

static int
crc8(uint32_t inp, int len)
{
    uint32_t crc = 0x54a9abf8 ^ (inp << (32 - len));
    int i;

    for (i = 0; i < len; i++) {
            if (crc & (1U << 31))
                    crc ^= (0x12fU << 23);
            crc <<= 1;
    }
    return (crc >> 24);
}

/*
 * Decode a numeric IPv4 string ("253.253.100.11").
 *
 * 'year' and 'month' is the announced horizon.
 *
 * 'dtai' is what you subtract from TAI to get UTC until that month ends.
 *
 * 'delta' is what you do to dtai at the end of that month
 */

static int
decode_leapsecond(const char *ip, int *year, int *month, int *dtai, int *delta)
{
    int error;
    unsigned o1, o2, o3, o4;
    uint32_t u, d, mn, o;

    /* Zero returns in case of error ------------------------------*/

    if (year != NULL)
            *year = 0;
    if (month != NULL)
            *month = 0;
    if (dtai != NULL)
            *dtai = 0;
    if (delta != NULL)
            *delta = 0;

    /* Convert to 32 bit integer ----------------------------------*/

    error = sscanf(ip, "%u.%u.%u.%u", &o1, &o2, &o3, &o4);
    if (error != 4)
            return (-1);

    u = o1 << 24;
    u |= o2 << 16;
    u |= o3 << 8;
    u |= o4;

    /* Check & remove class E -------------------------------------*/

    if ((u >> 28) != 0xf)
            return (-1);

    u &= (1 << 28) - 1;

    /* Check & remove CRC8 ----------------------------------------*/

    if (crc8(u, 28) != 0x80)
            return (-2);

    u >>= 8;

    /* Split into fields ------------------------------------------*/

    o = u & 0x7f;
    u >>= 7;

    d = u & 3;
    u >>= 2;

    mn = (u & 0x7ff) + 10;

    /* Error checks -----------------------------------------------*/

    if (d == 3)
            return (-3);

    /* Convert to return values -----------------------------------*/

    if (year != NULL)
            *year = 1971 + (mn / 12);

    if (month != NULL)
            *month = 1 + (mn % 12);

    if (dtai != NULL)
            *dtai = o;

    if (delta != NULL) {
            switch (d) {
            case 0: *delta =  0; break;
            case 1: *delta = -1; break;
            case 2: *delta = +1; break;
            }
    }
    return (0);
}

/*
 * Query leapsecond.utcd.org for current leapsecond information
 */

static int
query_leapsecond(int *year, int *month, int *tai, int *delta, char **ip)
{
    struct addrinfo hints, *res, *res0;
    char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
    int error;

    memset(&hints, 0, sizeof(hints));
    hints.ai_family = PF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    error = getaddrinfo("leapsecond.utcd.org", NULL, &hints, &res0);
    if (error != 0) {
            fprintf(stderr, "Lookup error: %s\n", gai_strerror(error));
            return (-10);
    }
    error = -11;
    for (res = res0; res; res = res->ai_next) {
            error = getnameinfo(res->ai_addr,  res->ai_addrlen,
                hbuf, sizeof(hbuf), sbuf, sizeof(sbuf),
                NI_NUMERICHOST | NI_NUMERICSERV);
            if (error != 0)
                    continue;

            error = decode_leapsecond(hbuf, year, month, tai, delta);
            if (error == 0) {
                    if (ip != NULL)
                            *ip = strdup(hbuf);
                    break;
            }
    }
    return (error);
}

static struct test_vector {
    const char *ip;
    int error;
    int year;
    int month;
    int tai;
    int delta;
} test_vectors[] = {
    { "240.3.9.77",          0, 1971, 12,   9, +1 },
    { "240.15.10.108",       0, 1972,  6,  10, +1 },
    { "242.18.28.160",       0, 1993, 12,  28,  0 },
    { "255.76.200.237",      0, 2135,  1,  72, -1 },
    { "127.240.133.76",     -1,    0,  0,   0,  0 },
    { "255.209.76.40",      -2,    0,  0,   0,  0 },
    { "241.179.152.73",     -3,    0,  0,   0,  0 },
    { NULL,                  0,    0,  0,   0,  0 }
};

int
main(int argc, char **argv)
{
    int error;
    int year, month, tai, delta;
    struct test_vector *tv;
    char *ip;

    (void)argc;
    (void)argv;

    printf("Checking test-vectors:\n\n");
    for (tv = test_vectors; tv->ip != NULL; tv++) {
            error = decode_leapsecond(tv->ip,
                &year, &month, &tai, &delta);
            printf("  IP: %-15s  Error: %2d  Year: %4d  "
                "Month %2d  dTAI: %3d  Delta: %2d\n",
                tv->ip, error, year, month, tai, delta);
            assert(error == tv->error);
            assert(year == tv->year);
            assert(month == tv->month);
            assert(tai == tv->tai);
            assert(delta == tv->delta);
    }
    printf("\nIf you see this, the tests ran OK\n");

    printf("\n");
    printf("Querying currently published leapsecond announcement:\n\n");
    error = query_leapsecond(&year, &month, &tai, &delta, &ip);
    if (error) {
            printf("Failed with error %d\n", error);
            return (0);
    }

    printf("  IP: %-15s  Error: %2d  Year: %4d  "
        "Month %2d  dTAI: %3d  Delta: %2d\n",
        ip, error, year, month, tai, delta);

    printf("\nThat means:\n\n");
    printf("   Information is valid until end of UTC-month %d of year %d\n",
        month, year);
    printf("   After that month: UTC = TAI - %d seconds\n", tai + delta);
    printf("   Until then:       UTC = TAI - %d seconds\n", tai);

    return (0);
}

If you run it, you should get this output:

Checking test-vectors:

  IP: 240.3.9.77       Error:  0  Year: 1971  Month 12  dTAI:   9  Delta:  1
  IP: 240.15.10.108    Error:  0  Year: 1972  Month  6  dTAI:  10  Delta:  1
  IP: 242.18.28.160    Error:  0  Year: 1993  Month 12  dTAI:  28  Delta:  0
  IP: 255.76.200.237   Error:  0  Year: 2135  Month  1  dTAI:  72  Delta: -1
  IP: 127.240.133.76   Error: -1  Year:    0  Month  0  dTAI:   0  Delta:  0
  IP: 255.209.76.40    Error: -2  Year:    0  Month  0  dTAI:   0  Delta:  0
  IP: 241.179.152.73   Error: -3  Year:    0  Month  0  dTAI:   0  Delta:  0

If you see this, the tests ran OK

Querying currently published leapsecond announcement:

  IP: 244.34.36.97     Error:  0  Year: 2015  Month 12  dTAI:  36  Delta:  0

That means:

   Information is valid until end of UTC-month 12 of year 2015
   After that month: UTC = TAI - 36 seconds
   Until then:       UTC = TAI - 36 seconds

phk