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SNTP: Implement round-trip delay compensation

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Introduce a new configuration define SNTP_COMP_ROUNDTRIP to compensate
for network round-trip delays when setting the system clock from SNTP.
Note that this feature requires compiler support for 64-bit arithmetic.
This commit is contained in:
Daniel Elstner 2017-03-25 16:57:51 +01:00 committed by goldsimon
parent cc77b308a4
commit 751ee7a534
2 changed files with 74 additions and 4 deletions
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51
src/apps/sntp/sntp.c
View File

@ -158,6 +158,17 @@
#define SNTP_OFFSET_TIMESTAMPS \
(SNTP_OFFSET_TRANSMIT_TIME + 8 - sizeof(struct sntp_timestamps))
/* Round-trip delay arithmetic helpers */
#if SNTP_COMP_ROUNDTRIP
# if !LWIP_HAVE_INT64
# error "SNTP round-trip delay compensation requires 64-bit arithmetic"
# endif
# define SNTP_SEC_FRAC_TO_S64(s, f) \
((s64_t)(((u64_t)(s) << 32) | (u32_t)(f)))
# define SNTP_TIMESTAMP_TO_S64(t) \
SNTP_SEC_FRAC_TO_S64(lwip_ntohl((t).sec), lwip_ntohl((t).frac))
#endif /* SNTP_COMP_ROUNDTRIP */
/**
* 64-bit NTP timestamp, in network byte order.
*/
@ -170,7 +181,7 @@ struct sntp_time {
* Timestamps to be extracted from the NTP header.
*/
struct sntp_timestamps {
#if SNTP_CHECK_RESPONSE >= 2
#if SNTP_COMP_ROUNDTRIP || SNTP_CHECK_RESPONSE >= 2
struct sntp_time orig;
struct sntp_time recv;
#endif
@ -275,6 +286,38 @@ sntp_process(const struct sntp_timestamps *timestamps)
sec = lwip_ntohl(timestamps->xmit.sec);
frac = lwip_ntohl(timestamps->xmit.frac);
#if SNTP_COMP_ROUNDTRIP
# if SNTP_CHECK_RESPONSE >= 2
if (timestamps->recv.sec != 0 || timestamps->recv.frac != 0)
# endif
{
s32_t dest_sec;
u32_t dest_frac;
u32_t step_sec;
/* Get the destination time stamp, i.e. the current system time */
SNTP_GET_SYSTEM_TIME_NTP(dest_sec, dest_frac);
step_sec = (dest_sec < sec) ? ((u32_t)sec - (u32_t)dest_sec)
: ((u32_t)dest_sec - (u32_t)sec);
/* In order to avoid overflows, skip the compensation if the clock step
* is larger than about 34 years. */
if ((step_sec >> 30) == 0) {
s64_t t1, t2, t3, t4;
t4 = SNTP_SEC_FRAC_TO_S64(dest_sec, dest_frac);
t3 = SNTP_SEC_FRAC_TO_S64(sec, frac);
t1 = SNTP_TIMESTAMP_TO_S64(timestamps->orig);
t2 = SNTP_TIMESTAMP_TO_S64(timestamps->recv);
/* Clock offset calculation according to RFC 4330 */
t4 += ((t2 - t1) + (t3 - t4)) / 2;
sec = (u32_t)((u64_t)t4 >> 32);
frac = (u32_t)((u64_t)t4);
}
}
#endif /* SNTP_COMP_ROUNDTRIP */
SNTP_SET_SYSTEM_TIME_NTP(sec, frac);
LWIP_DEBUGF(SNTP_DEBUG_TRACE, ("sntp_process: %s, %" U32_F " us\n",
sntp_format_time(sec), SNTP_FRAC_TO_US(frac)));
@ -289,7 +332,7 @@ sntp_initialize_request(struct sntp_msg *req)
memset(req, 0, SNTP_MSG_LEN);
req->li_vn_mode = SNTP_LI_NO_WARNING | SNTP_VERSION | SNTP_MODE_CLIENT;
#if SNTP_CHECK_RESPONSE >= 2
#if SNTP_CHECK_RESPONSE >= 2 || SNTP_COMP_ROUNDTRIP
{
u32_t sec, frac;
/* Get the transmit timestamp */
@ -297,12 +340,14 @@ sntp_initialize_request(struct sntp_msg *req)
sec = lwip_htonl(sec);
frac = lwip_htonl(frac);
# if SNTP_CHECK_RESPONSE >= 2
sntp_last_timestamp_sent.sec = sec;
sntp_last_timestamp_sent.frac = frac;
# endif
req->transmit_timestamp[0] = sec;
req->transmit_timestamp[1] = frac;
}
#endif /* SNTP_CHECK_RESPONSE >= 2 */
#endif /* SNTP_CHECK_RESPONSE >= 2 || SNTP_COMP_ROUNDTRIP */
}
/**

27
src/include/lwip/apps/sntp_opts.h
View File

@ -109,6 +109,30 @@
#define SNTP_CHECK_RESPONSE 0
#endif
/** Enable round-trip delay compensation.
* Compensate for the round-trip delay by calculating the clock offset from
* the originate, receive, transmit and destination timestamps, as per RFC.
*
* The calculation requires compiler support for 64-bit integers. Also, either
* SNTP_SET_SYSTEM_TIME_US or SNTP_SET_SYSTEM_TIME_NTP has to be implemented
* for setting the system clock with sub-second precision. Likewise, either
* SNTP_GET_SYSTEM_TIME or SNTP_GET_SYSTEM_TIME_NTP needs to be implemented
* with sub-second precision.
*
* Although not strictly required, it makes sense to combine this option with
* SNTP_CHECK_RESPONSE >= 2 for sanity-checking of the received timestamps.
* Also, in order for the round-trip calculation to work, the the difference
* between the local clock and the NTP server clock must not be larger than
* about 34 years. If that limit is exceeded, the implementation will fall back
* to setting the clock without compensation. In order to ensure that the local
* clock is always within the permitted range for compensation, even at first
* try, it may be necessary to store at least the current year in non-volatile
* memory.
*/
#if !defined SNTP_COMP_ROUNDTRIP || defined __DOXYGEN__
#define SNTP_COMP_ROUNDTRIP 0
#endif
/** According to the RFC, this shall be a random delay
* between 1 and 5 minutes (in milliseconds) to prevent load peaks.
* This can be defined to a random generation function,
@ -142,7 +166,8 @@
#endif
/** SNTP macro to get system time, used with SNTP_CHECK_RESPONSE >= 2
* to send in request and compare in response.
* to send in request and compare in response. Also used for round-trip
* delay compensation if SNTP_COMP_ROUNDTRIP != 0.
* Alternatively, define SNTP_GET_SYSTEM_TIME_NTP(sec, frac) in order to
* work with native NTP timestamps instead.
*/