sntp 校时问题

1、背景

由于项目中的数据对时间比较敏感，目前常常出现校时问题，导致时间偏差，出现曲线数据丢失和曲线数据重复等问题。

因此对sntp源码进行深入分析。

要了解SNTP，首先需要了解NTP协议。SNTP是NTP的子集，简化了NTP的许多算法和步骤，得到了效率，但时间的精度不如NTP，可是对于民用时间来说足够了，大概最多差距几秒的样子。NTP（Network Time Protocol，网络时间协议）是由RFC 1305定义的时间同步协议，用来在分布式时间服务器和客户端之间进行时间同步。NTP基于UDP报文进行传输，使用的UDP端口号为123。使用NTP的目的是对网络内所有具有时钟的设备进行时钟同步，使网络内所有设备的时钟保持一致，从而使设备能够提供基于统一时间的多种应用。

NTP原理，就是4个时间，假设A时客户端侧，B时服务器

报文离开A时A的时间戳T1，到达B时B的时间戳T2，离开B时B的时间戳T3，到达A时A的时间戳T4，那么在A和B之间来回传输的时间差&T=(T4-T1) – (T3-T2).

那么A和B之间时钟的差=(T2-T1) – &T= （T2-T1）- （(T4-T1)-(T3-T2)）/2 = ((T2-T1)+(T3-T4))/2

1.1 参考文档

SNTP简介_PUSONG568的博客-CSDN博客_sntp端口要了解SNTP，首先需要了解NTP协议。SNTP是NTP的子集，简化了NTP的许多算法和步骤，得到了效率，但时间的精度不如NTP，可是对于民用时间来说足够了，大概最多差距几秒的样子。 NTP（Network Time Protocol，网络时间协议）是由RFC 1305定义的时间同步协议，用来在分布式时间服务器和客户端之间进行时间同步。NTP基于UDP报文进行传输，使用的UDP端口号为123。 使…https://blog.csdn.net/PUSONG568/article/details/80845458

2、SNTP源码分析

一旦时间同步，ESP32将使用内置定时器来执行记时。

RTC时钟用于芯片处于深度睡眠模式时保持准确的时间。

High-resolution定时器用于在ESP32运行时提供微妙精度的时间。

2.1 设置操作模式

/ The operating mode */ static u8_t sntp_opmode; / The UDP pcb used by the SNTP client */ static struct udp_pcb *sntp_pcb; ..... / * @ingroup sntp * Sets the operating mode. * @param operating_mode one of the available operating modes */ void sntp_setoperatingmode(u8_t operating_mode) { LWIP_ASSERT("Invalid operating mode", operating_mode <= SNTP_OPMODE_LISTENONLY); LWIP_ASSERT("Operating mode must not be set while SNTP client is running", sntp_pcb == NULL); sntp_opmode = operating_mode; }

可选的模式就两种:轮询SNTP_OPMODE_POLL和只监听SNTP_OPMODE_LISTENONLY。

2.2 设置服务器hostname

/ * Initialize one of the NTP servers by name * * @param numdns the index of the NTP server to set must be < SNTP_MAX_SERVERS * @param dnsserver DNS name of the NTP server to set, to be resolved at contact time */ void sntp_setservername(u8_t idx, char *server) { if (idx < SNTP_MAX_SERVERS) { sntp_servers[idx].name = server; } }

 支持设置多个NTP服务器时间同步源。

2.3 初始化SNTP模块，并发出请求

/ * @ingroup sntp * Initialize this module. * Send out request instantly or after SNTP_STARTUP_DELAY(_FUNC). */ void sntp_init(void) { #ifdef SNTP_SERVER_ADDRESS #if SNTP_SERVER_DNS sntp_setservername(0, SNTP_SERVER_ADDRESS); #else #error SNTP_SERVER_ADDRESS string not supported SNTP_SERVER_DNS==0 #endif #endif /* SNTP_SERVER_ADDRESS */ if (sntp_pcb == NULL) { sntp_pcb = udp_new_ip_type(IPADDR_TYPE_ANY); LWIP_ASSERT("Failed to allocate udp pcb for sntp client", sntp_pcb != NULL); if (sntp_pcb != NULL) { udp_recv(sntp_pcb, sntp_recv, NULL); if (sntp_opmode == SNTP_OPMODE_POLL) { SNTP_RESET_RETRY_TIMEOUT(); #if SNTP_STARTUP_DELAY sys_timeout((u32_t)SNTP_STARTUP_DELAY_FUNC, sntp_request, NULL); #else sntp_request(NULL); #endif } else if (sntp_opmode == SNTP_OPMODE_LISTENONLY) { ip_set_option(sntp_pcb, SOF_BROADCAST); udp_bind(sntp_pcb, IP_ANY_TYPE, SNTP_PORT); } } } }

上述源码中有创建udp client和设置udp的接收回调函数为sntp_recv，该回调函数具体实现见之后的说明。

2.4 接收回调函数

/ UDP recv callback for the sntp pcb */ static void sntp_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr, u16_t port) { u8_t mode; u8_t stratum; u32_t receive_timestamp[SNTP_RECEIVE_TIME_SIZE]; err_t err; LWIP_UNUSED_ARG(arg); LWIP_UNUSED_ARG(pcb); /* packet received: stop retry timeout */ sys_untimeout(sntp_try_next_server, NULL); sys_untimeout(sntp_request, NULL); err = ERR_ARG; #if SNTP_CHECK_RESPONSE >= 1 /* check server address and port */ if (((sntp_opmode != SNTP_OPMODE_POLL) || ip_addr_cmp(addr, &sntp_last_server_address)) && (port == SNTP_PORT)) #else /* SNTP_CHECK_RESPONSE >= 1 */ LWIP_UNUSED_ARG(addr); LWIP_UNUSED_ARG(port); #endif /* SNTP_CHECK_RESPONSE >= 1 */ { /* process the response */ if (p->tot_len == SNTP_MSG_LEN) { pbuf_copy_partial(p, &mode, 1, SNTP_OFFSET_LI_VN_MODE); mode &= SNTP_MODE_MASK; /* if this is a SNTP response... */ if (((sntp_opmode == SNTP_OPMODE_POLL) && (mode == SNTP_MODE_SERVER)) || ((sntp_opmode == SNTP_OPMODE_LISTENONLY) && (mode == SNTP_MODE_BROADCAST))) { pbuf_copy_partial(p, &stratum, 1, SNTP_OFFSET_STRATUM); if (stratum == SNTP_STRATUM_KOD) { /* Kiss-of-death packet. Use another server or increase UPDATE_DELAY. */ err = SNTP_ERR_KOD; LWIP_DEBUGF(SNTP_DEBUG_STATE, ("sntp_recv: Received Kiss-of-Death\n")); } else { #if SNTP_CHECK_RESPONSE >= 2 /* check originate_timetamp against sntp_last_timestamp_sent */ u32_t originate_timestamp[2]; pbuf_copy_partial(p, &originate_timestamp, 8, SNTP_OFFSET_ORIGINATE_TIME); if ((originate_timestamp[0] != sntp_last_timestamp_sent[0]) || (originate_timestamp[1] != sntp_last_timestamp_sent[1])) { LWIP_DEBUGF(SNTP_DEBUG_WARN, ("sntp_recv: Invalid originate timestamp in response\n")); } else #endif /* SNTP_CHECK_RESPONSE >= 2 */ /* @todo: add code for SNTP_CHECK_RESPONSE >= 3 and >= 4 here */ { /* correct answer */ err = ERR_OK; pbuf_copy_partial(p, &receive_timestamp, SNTP_RECEIVE_TIME_SIZE * 4, SNTP_OFFSET_TRANSMIT_TIME); } } } else { LWIP_DEBUGF(SNTP_DEBUG_WARN, ("sntp_recv: Invalid mode in response: %" U16_F "\n", (u16_t)mode)); /* wait for correct response */ err = ERR_TIMEOUT; } } else { LWIP_DEBUGF(SNTP_DEBUG_WARN, ("sntp_recv: Invalid packet length: %" U16_F "\n", p->tot_len)); } } #if SNTP_CHECK_RESPONSE >= 1 else { /* packet from wrong remote address or port, wait for correct response */ err = ERR_TIMEOUT; } #endif /* SNTP_CHECK_RESPONSE >= 1 */ pbuf_free(p); if (err == ERR_OK) { sntp_process(receive_timestamp); /* Set up timeout for next request (only if poll response was received)*/ if (sntp_opmode == SNTP_OPMODE_POLL) { /* Correct response, reset retry timeout */ SNTP_RESET_RETRY_TIMEOUT(); sys_timeout((u32_t)SNTP_UPDATE_DELAY, sntp_request, NULL); LWIP_DEBUGF(SNTP_DEBUG_STATE, ("sntp_recv: Scheduled next time request: %" U32_F " ms\n", (u32_t)SNTP_UPDATE_DELAY)); } } else if (err != ERR_TIMEOUT) { /* Errors are only processed in case of an explicit poll response */ if (sntp_opmode == SNTP_OPMODE_POLL) { if (err == SNTP_ERR_KOD) { /* Kiss-of-death packet. Use another server or increase UPDATE_DELAY. */ sntp_try_next_server(NULL); } else { /* another error, try the same server again */ sntp_retry(NULL); } } } }

/ Main packet buffer struct */ struct pbuf { / next pbuf in singly linked pbuf chain */ struct pbuf *next; / pointer to the actual data in the buffer */ void *payload; / * total length of this buffer and all next buffers in chain * belonging to the same packet. * * For non-queue packet chains this is the invariant: * p->tot_len == p->len + (p->next? p->next->tot_len: 0) */ u16_t tot_len; / length of this buffer */ u16_t len; / pbuf_type as u8_t instead of enum to save space */ u8_t /*pbuf_type*/ type; / misc flags */ u8_t flags; / * the reference count always equals the number of pointers * that refer to this pbuf. This can be pointers from an application, * the stack itself, or pbuf->next pointers from a chain. */ u16_t ref; #if ESP_LWIP struct netif *l2_owner; void *l2_buf; #endif };

 这里有对NTP报文的解析

1.LI 闰秒标识器，占用2bits

2.VN版本号，占用3bits,标识NTP的版本号，现为3

3.Mode模式，占用3bit,标识模式，具体如下

检查总长度==SNTP_MSG_LEN(48字节)，解析Mode字段—首字节的低三bits—指示协议模式。

 在单播或多播模式时，客户请求把这个字段设置为3，服务器在响应时把这个字段设置为4.

广播时，服务器把这个字段设置为5。

4.stratum(层)，占用8bits

解析Stratum—->SNTP_STRATUM_KOD

其他则

 pbuf_copy_partial(p, &receive_timestamp, SNTP_RECEIVE_TIME_SIZE * 4, SNTP_OFFSET_TRANSMIT_TIME);

获取Transmit Timestamp，即服务器向客户发时间戳的时间

recv的不是ntp应答报文，如果错误原因是SNTP_ERR_KOD,则尝试下一个时间同步源；

其他错误，则重新尝试一次。

/ Sanity check: * Define this to * - 0 to turn off sanity checks (default; smaller code) * - >= 1 to check address and port of the response packet to ensure the * response comes from the server we sent the request to. * - >= 2 to check returned Originate Timestamp against Transmit Timestamp * sent to the server (to ensure response to older request). * - >= 3 @todo: discard reply if any of the LI, Stratum, or Transmit Timestamp * fields is 0 or the Mode field is not 4 (unicast) or 5 (broadcast). * - >= 4 @todo: to check that the Root Delay and Root Dispersion fields are each * greater than or equal to 0 and less than infinity, where infinity is * currently a cozy number like one second. This check avoids using a * server whose synchronization source has expired for a very long time. */ #if !defined SNTP_CHECK_RESPONSE || defined __DOXYGEN__ #define SNTP_CHECK_RESPONSE 0 #endif

5.Poll测试间隔，占用8bits,标识连续信息之间的最大间隔

6.Precision精度，占用8bits，标识本地时钟精度

7.Root Delay根时延，占用8bits，标识在主参考源之间往返的总共时延

8.Root Dispersion根离散，占用8bits，标识在主参考源有关的名义错误

9.Reference Identifier参考时钟标识符，占用8bits，用来标识特殊的参考源

10.参考时间戳，64bit时间戳，本地时钟被修改的最新时间

11、原始时间戳，客户端发送的时间，64bits

12、接收时间戳，服务端接收到的时间，64bits

13、发送时间戳，服务端发送应答的时间，64bits

14、认证符(可选)

2.5 NTP请求

/ * Send out an sntp request. * * @param arg is unused (only necessary to conform to sys_timeout) */ static void sntp_request(void *arg) { ip_addr_t sntp_server_address; err_t err; LWIP_UNUSED_ARG(arg); /* initialize SNTP server address */ #if SNTP_SERVER_DNS if (sntp_servers[sntp_current_server].name) { /* always resolve the name and rely on dns-internal caching & timeout */ ip_addr_set_zero(&sntp_servers[sntp_current_server].addr); err = dns_gethostbyname(sntp_servers[sntp_current_server].name, &sntp_server_address, sntp_dns_found, NULL); if (err == ERR_INPROGRESS) { /* DNS request sent, wait for sntp_dns_found being called */ LWIP_DEBUGF(SNTP_DEBUG_STATE, ("sntp_request: Waiting for server address to be resolved.\n")); return; } else if (err == ERR_OK) { sntp_servers[sntp_current_server].addr = sntp_server_address; } } else #endif /* SNTP_SERVER_DNS */ { sntp_server_address = sntp_servers[sntp_current_server].addr; err = (ip_addr_isany_val(sntp_server_address)) ? ERR_ARG : ERR_OK; } if (err == ERR_OK) { LWIP_DEBUGF(SNTP_DEBUG_TRACE, ("sntp_request: current server address is %s\n", ipaddr_ntoa(&sntp_server_address))); sntp_send_request(&sntp_server_address); } else { /* address conversion failed, try another server */ LWIP_DEBUGF(SNTP_DEBUG_WARN_STATE, ("sntp_request: Invalid server address, trying next server.\n")); sys_timeout((u32_t)SNTP_RETRY_TIMEOUT, sntp_try_next_server, NULL); } }

2.6 重试或下一个时间同步源

/ * Retry: send a new request (and increase retry timeout). * * @param arg is unused (only necessary to conform to sys_timeout) */ static void sntp_retry(void *arg) { LWIP_UNUSED_ARG(arg); LWIP_DEBUGF(SNTP_DEBUG_STATE, ("sntp_retry: Next request will be sent in %" U32_F " ms\n", sntp_retry_timeout)); /* set up a timer to send a retry and increase the retry delay */ sys_timeout(sntp_retry_timeout, sntp_request, NULL); #if SNTP_RETRY_TIMEOUT_EXP { u32_t new_retry_timeout; /* increase the timeout for next retry */ new_retry_timeout = sntp_retry_timeout << 1; /* limit to maximum timeout and prevent overflow */ if ((new_retry_timeout <= SNTP_RETRY_TIMEOUT_MAX) && (new_retry_timeout > sntp_retry_timeout)) { sntp_retry_timeout = new_retry_timeout; } } #endif /* SNTP_RETRY_TIMEOUT_EXP */ } static void sntp_try_next_server(void *arg) { u8_t old_server, i; LWIP_UNUSED_ARG(arg); old_server = sntp_current_server; for (i = 0; i < SNTP_MAX_SERVERS - 1; i++) { sntp_current_server++; if (sntp_current_server >= SNTP_MAX_SERVERS) { sntp_current_server = 0; } if (!ip_addr_isany(&sntp_servers[sntp_current_server].addr) #if SNTP_SERVER_DNS || (sntp_servers[sntp_current_server].name != NULL) #endif ) { LWIP_DEBUGF(SNTP_DEBUG_STATE, ("sntp_try_next_server: Sending request to server %" U16_F "\n", (u16_t)sntp_current_server)); /* new server: reset retry timeout */ SNTP_RESET_RETRY_TIMEOUT(); /* instantly send a request to the next server */ sntp_request(NULL); return; } } /* no other valid server found */ sntp_current_server = old_server; sntp_retry(NULL); }

4、问题解决

参考SDK4.1内的代码，处理时延和使用adjtime函数加快或减慢时钟来靠近目标时间。adjtime在35Min内调用成功，否则失败。由于这个调整是一个长时间的操作，只有直接设置系统时间才可以结束时钟调整。如需加快，用户需要自己设置结束条件，达到结束条件后，直接调用设置系统时间函数结束调整。

校时源不稳定，导致校时不准。推荐使用商业NTP公共服务器，国内推荐使用阿里/腾讯的。如无特殊需要，24小时校时一次即可。