Network Time Protocol（NTP）是用来使计算机时间同步化的一种协议，它可以使计算机对其服务器或时钟源（如石英钟，GPS等等)做同步化，它可以提供高精准度的时间校正（LAN上与标准间差小于1毫秒，WAN上几十毫秒），且可介由加密确认的方式来防止恶毒的协议攻击。
 
    NTP提供准确时间，首先要有准确的时间来源，这一时间应该是国际标准时间UTC。 NTP获得UTC的时间来源可以是原子钟、天文台、卫星，也可以从Internet上获取。这样就有了准确而可靠的时间源。时间是按NTP服务器的等级传播。按照距离外部UTC 源的远近将所有服务器归入不同的Stratun（层）中。Stratum-1在顶层，有外部UTC接入，而Stratum-2则从Stratum-1获取时间，Stratum-3从Stratum-2获取时间，以此类推，但Stratum层的总数限制在15以内。所有这些服务器在逻辑上形成阶梯式的架构并相互连接，而Stratum-1的时间服务器是整个系统的基础。
 
    进行网络协议实现时最重要的是了解协议数据格式。NTP数据包有48个字节，其中NTP包头16字节，时间戳32个字节。其协议格式如下图所示。
 
 
 
 
    其协议字段的含义如下所示。
 
      LI：跳跃指示器，警告在当月最后一天的最终时刻插入的迫近闺秒（闺秒）。
 
      VN：版本号。
 
      Mode：工作模式。该字段包括以下值：0－预留；1－对称行为；3－客户机；4－服务器；5－广播；6－NTP控制信息。NTP协议具有3种工作模式，分别为主/被动对称模式、客户/服务器模式、广播模式。在主/被动对称模式中，有一对一的连接，双方均可同步对方或被对方同步，先发出申请建立连接的一方工作在主动模式下，另一方工作在被动模式下；客户/服务器模式与主/被动模式基本相同，惟一区别在于客户方可被服务器同步，但服务器不能被客户同步；在广播模式中，有一对多的连接，服务器不论客户工作在何种模式下，都会主动发出时间信息，客户根据此信息调整自己的时间。
 
     Stratum：对本地时钟级别的整体识别。
 
     Poll：有符号整数表示连续信息间的最大间隔。
 
     Precision：有符号整数表示本地时钟精确度。
 
     Root Delay：表示到达主参考源的一次往复的总延迟，它是有15～16位小数部分的符号定点小数。
 
     Root Dispersion：表示一次到达主参考源的标准误差，它是有15～16位小数部分的无符号定点小数。
 
     Reference Identifier：识别特殊参考源。
 
      Originate Timestamp：这是向服务器请求分离客户机的时间，采用64位时标格式。
 
     Receive Timestamp：这是向服务器请求到达客户机的时间，采用64位时标格式。
 
     Transmit Timestamp：这是向客户机答复分离服务器的时间，采用64位时标格式。
 
     Authenticator（Optional）：当实现了NTP认证模式时，主要标识符和信息数字域就包括已定义的信息认证代码（MAC）信息。
 
---------------------------------------------------------------------------------------------------------------------------
 
    以上对NTP协议原理的描述来自网络，感谢网络上的朋友！
 
    以下是我写的模拟NTP协议进行时钟同步的Demo程序。完整的代码可从这里下载： http://download.csdn.net/detail/chexlong/3787414
 
NTP.h
 
#ifndef NTP_H
#define NTP_H

#include <winsock2.h>
#include <stdio.h>
#include <time.h>
#pragma comment(lib, "WS2_32")	// 链接到WS2_32.lib


#define LI 0
#define VN 3
#define MODE 3
#define STRATUM 0
#define POLL 4 
#define PREC -6

#define JAN_1970      0x83aa7e80      /* 2208988800 1970 - 1900 in seconds */
#define NTPFRAC(x) (4294 * (x) + ((1981 * (x))>>11))
#define USEC(x) (((x) >> 12) - 759 * ((((x) >> 10) + 32768) >> 16))

typedef struct ntptime_t 
{
	unsigned int coarse;
	unsigned int fine;
}ntptime;

typedef struct ntp_packet_t
{
	//header
	unsigned int leap_year_indicator:2;
	unsigned int version_number:3;
	unsigned int mode:3;
	unsigned int stratum :8;
	unsigned int poll :8;
	unsigned int precision :8;

	unsigned int root_delay;
	unsigned int root_dispersion;
	unsigned int reference_identifier;

	//时间戳
	ntptime  reference_timestamp; //T4
	ntptime originate_timestamp;  //T1
	ntptime receive_timestamp;    //T2
	ntptime transmit_timestamp;   //T3
}ntp_packet;

typedef union
{
	ntp_packet ntp;
	char c[48];
}NTP_PACKET_T;

void GetNTPTime(ntptime *ntpTime)
{
	SYSTEMTIME sysTime;
	FILETIME fileTime;
	GetSystemTime(&sysTime);
	SystemTimeToFileTime(&sysTime,&fileTime);
	ntpTime->coarse = fileTime.dwHighDateTime;
	ntpTime->fine = fileTime.dwLowDateTime;
}


SOCKET CreateSock(char *psLocalIP, unsigned short usLocalPort, unsigned int iTimeOut)
{
	//创建套节字
	SOCKET sock = ::socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
	if( INVALID_SOCKET == sock )
	{
		printf("Failed socket() %d \n", ::WSAGetLastError());
		return 0;
	}

	if (0 != iTimeOut)
	{
		//设置套接字发送，接收超时时间
		int iRet = setsockopt(sock, SOL_SOCKET, SO_SNDTIMEO, (char*)&iTimeOut, sizeof(iTimeOut));
		iRet = setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, (char*)&iTimeOut, sizeof(iTimeOut));
	}

	//绑定本地地址
	sockaddr_in addrLocal;
	memset(&addrLocal, 0, sizeof(addrLocal));
	addrLocal.sin_family = AF_INET;
	addrLocal.sin_addr.s_addr = htonl(INADDR_ANY);
	addrLocal.sin_port = htons(usLocalPort);

	if ( 0 != bind(sock, (struct sockaddr*)&addrLocal, sizeof(addrLocal)) )
	{
		printf("Failed bind() %d \n", ::WSAGetLastError());
		closesocket(sock);
		return 0;
	}

	return sock;
}

#endif
 
 
 
Base64.h
 
#ifndef BASE64_H
#define BASE64_H

const int pr2six[256]={
	64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,
	64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,62,64,64,64,63,
	52,53,54,55,56,57,58,59,60,61,64,64,64,64,64,64,64,0,1,2,3,4,5,6,7,8,9,
	10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,64,64,64,64,64,64,26,27,
	28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,
	64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,
	64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,
	64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,
	64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,
	64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,
	64,64,64,64,64,64,64,64,64,64,64,64,64
};

char six2pr[64] = {
	'A','B','C','D','E','F','G','H','I','J','K','L','M',
	'N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
	'a','b','c','d','e','f','g','h','i','j','k','l','m',
	'n','o','p','q','r','s','t','u','v','w','x','y','z',
	'0','1','2','3','4','5','6','7','8','9','+','/'
};

void Base64Decode(const char *bufcoded, char * dst,int *nbytes)
{
	int nbytesdecoded;
	const char *bufin = bufcoded;
	unsigned char *bufout = (unsigned char*)dst;
	int nprbytes;

	/* Figure out how many characters are in the input buffer.
	* If this would decode into more bytes than would fit into
	* the output buffer, adjust the number of input bytes downwards.
	*/
	//bufin = bufcoded;
	while(pr2six[(int)*(bufin++)] <= 63){}
	nprbytes = (int)(bufin - bufcoded - 1);
	nbytesdecoded = ((nprbytes+3)/4) * 3;

	bufin = bufcoded;

	while (nprbytes > 0)
	{
		*(bufout++) =
			(unsigned char) (pr2six[(int)*bufin] << 2 | pr2six[(int)bufin[1]] >> 4);
		*(bufout++) =
			(unsigned char) (pr2six[(int)bufin[1]] << 4 | pr2six[(int)bufin[2]] >> 2);
		*(bufout++) =
			(unsigned char) (pr2six[(int)bufin[2]] << 6 | pr2six[(int)bufin[3]]);
		bufin += 4;
		nprbytes -= 4;
	}

	if(nprbytes & 03)
	{
		if(pr2six[(int)bufin[-2]] > 63)
			nbytesdecoded -= 2;
		else
			nbytesdecoded -= 1;
	}
	dst[nbytesdecoded] = '\0';
	if ( nbytes )
	{
		*nbytes = nbytesdecoded;
	}
}

void Base64Encode(const unsigned char *bufin, unsigned int nbytes, char * dst)
{
	unsigned char *outptr = (unsigned char *)dst;
	unsigned int i;

	for (i=0; i<nbytes; i += 3)
	{
		*(outptr++) = six2pr[*bufin >> 2];            /* c1 */
		*(outptr++) = six2pr[((*bufin << 4) & 060) | ((bufin[1] >> 4) & 017)]; /*c2*/
		*(outptr++) = six2pr[((bufin[1] << 2) & 074) | ((bufin[2] >> 6) & 03)];/*c3*/
		*(outptr++) = six2pr[bufin[2] & 077];         /* c4 */

		bufin += 3;
	}

	/* If nbytes was not a multiple of 3, then we have encoded too
	* many characters.  Adjust appropriately.
	*/
	if(i == nbytes+1)
	{
		/* There were only 2 bytes in that last group */
		outptr[-1] = '=';
	} 
	else if(i == nbytes+2)
	{
		/* There was only 1 byte in that last group */
		outptr[-1] = '=';
		outptr[-2] = '=';
	}

	*(outptr++) = '\0';

	//size_t len = outptr - 1 - (unsigned char*)dst;
}


#endif
 

 NTPClient.cpp
 
#include "../Include/NTP.h"
#include "../Include/Base64.h"

//NTP请求包打包
int CreateNtpPacket(NTP_PACKET_T &ntpPacket)
{
	static unsigned long ulNTPId = 0;
	ulNTPId++;
	char szBuf[1024];
	int iDataLen;

	ntpPacket.ntp.leap_year_indicator = LI;
	ntpPacket.ntp.version_number = VN;
	ntpPacket.ntp.mode = MODE;
	ntpPacket.ntp.stratum = STRATUM;
	ntpPacket.ntp.poll = POLL;
	ntpPacket.ntp.precision = (PREC&0xFF);
	ntpPacket.ntp.root_delay = htonl(1<<16);
	ntpPacket.ntp.root_dispersion = htonl(1<<16);
	ntpPacket.ntp.reference_identifier = 0;

	ntpPacket.ntp.reference_timestamp.coarse = 0;
	ntpPacket.ntp.reference_timestamp.fine = 0;
	GetNTPTime(&ntpPacket.ntp.originate_timestamp);
	ntpPacket.ntp.receive_timestamp.coarse = 0;
	ntpPacket.ntp.receive_timestamp.fine = 0;
	ntpPacket.ntp.transmit_timestamp.coarse = 0;
	ntpPacket.ntp.transmit_timestamp.fine = 0;

	return 0;
}

int NtpRequest(SOCKET &sock)
{
	int iRet;
	sock = CreateSock("127.0.0.1", 4567, 5000);
	if ( 0 == sock )
	{
		printf("Failed create sock1 \n");
		return -1;
	}

	//填写远程地址信息
	sockaddr_in addrRemote; 
	addrRemote.sin_family = AF_INET;
	addrRemote.sin_port = htons(5678);
	addrRemote.sin_addr.S_un.S_addr = inet_addr("127.0.0.1");

	while (true)
	{
		char szTimeValue[128];
		char szbuf[128];
		memset(szTimeValue,0,128);
		memset(szbuf,0,128);

		NTP_PACKET_T ntpPacket;
		memset(&ntpPacket,0,sizeof(ntpPacket));
		CreateNtpPacket(ntpPacket);

		//Base64进行编码
		Base64Encode((unsigned char *)ntpPacket.c, sizeof(ntpPacket), szTimeValue);

		//发送NTP请求数据包
		iRet = ::sendto(sock, szTimeValue, strlen(szTimeValue), 0, (sockaddr*)&addrRemote, sizeof(addrRemote));
		if ( SOCKET_ERROR == iRet )
		{
			printf("Failed sendto() sock %d \n", ::WSAGetLastError());
			closesocket(sock);
		}
		else
		{
			printf("Successed sendto sock = %d, data lenth = %d \n", sock, iRet);
		}

		memset(szTimeValue,0,sizeof(szTimeValue));
		int nLen = sizeof(addrRemote);
	
		//接收NTP请求应答消息
		iRet = ::recvfrom(sock, szbuf, 128, 0, (sockaddr*)&addrRemote, &nLen); 
		
		if(iRet > 0)
		{
			szbuf[iRet] = '\0';
			printf(" 接收到数据（%s）：%s \n", ::inet_ntoa(addrRemote.sin_addr), szbuf);

			//Base54解码
			Base64Decode(szbuf, szTimeValue, 0);
			if ( '\0' == szTimeValue[0] )
			{
				printf("time value is empty. \n");
				closesocket(sock);
				::WSACleanup();	
				return -1;
			}
			memset(&ntpPacket,0,sizeof(ntpPacket));
			memcpy(ntpPacket.c,szTimeValue,sizeof(ntpPacket));

			//设置本地系统时钟
			SYSTEMTIME sysTime;
			struct tm *tm1 = _localtime32((__time32_t*)&ntpPacket.ntp.transmit_timestamp.coarse);
			sysTime.wYear = tm1->tm_year+1900;
			sysTime.wMonth = tm1->tm_mon+1;
			sysTime.wDay = tm1->tm_mday;
			sysTime.wHour = tm1->tm_hour;
			sysTime.wMinute = tm1->tm_min;
			sysTime.wSecond = tm1->tm_sec;
			sysTime.wMilliseconds = 0;
			//SetLocalTime(&sysTime);

			printf("NTP ok, %4d-%02d-%02d %02d:%02d:%02d %d \n", sysTime.wYear,sysTime.wMonth,sysTime.wDay,sysTime.wHour,sysTime.wMinute,sysTime.wSecond,sysTime.wMilliseconds);
		}
		else
		{
			printf("Failed recvfrom() sock %d \n", ::WSAGetLastError());
		}

		Sleep(5000);
	}

}

int main(int argc, char* argv[])
{
	int iRet;
	WSADATA wsaData;
	WORD sockVersion = MAKEWORD(2, 2);
	if(::WSAStartup(sockVersion, &wsaData) != 0)
	{
		exit(0);
	}
	
	SOCKET sock;
	iRet = NtpRequest(sock);
	if (0 != iRet)
	{
		printf("ntp request failed. \n");
	}

	system("pause");

	closesocket(sock);
	::WSACleanup();	

	return 0;
}
 

 NTPServer.cpp
 
#include "../Include/Base64.h"
#include "../Include/NTP.h"

int StartNtpServer(SOCKET &sock)
{
	int iRet;
	sock = CreateSock("127.0.0.1", 5678, 0);
	if ( 0 == sock )
	{
		printf("Failed create sock1 \n");
		return -1;
	}

	sockaddr_in addrRemote; 

	char szTimeValue[128];
	char szbuf[128];
	memset(szTimeValue,0,128);
	memset(szbuf,0,128);
	NTP_PACKET_T ntpPacket;

	while (true)
	{
		memset(&addrRemote,0,sizeof(addrRemote));
		int nLen = sizeof(addrRemote);

		//接收来自客户端的NTP请求消息
		iRet = ::recvfrom(sock, szbuf, 128, 0, (sockaddr*)&addrRemote, &nLen); 
		if(iRet > 0)
		{
			szbuf[iRet] = '\0';
			printf(" 接收到数据（%s）：%s \n", ::inet_ntoa(addrRemote.sin_addr), szbuf);
		}

		//base64解码消息
		Base64Decode(szbuf, szTimeValue, 0);
		
		//构造NTP请求应答数据包
		memset(&ntpPacket,0,sizeof(ntpPacket));
		memcpy(ntpPacket.c,szTimeValue,sizeof(ntpPacket));
		ntpPacket.ntp.mode = 4;
		ntpPacket.ntp.transmit_timestamp.coarse = _time32(NULL);
		memset(szTimeValue,0,sizeof(szTimeValue));

		//Base64编码NTP请求应答消息
		Base64Encode((unsigned char *)ntpPacket.c,sizeof(NTP_PACKET_T),szTimeValue);

		//填写远程地址信息
		addrRemote.sin_family = AF_INET;
		addrRemote.sin_port = htons(4567);
		addrRemote.sin_addr.S_un.S_addr = inet_addr("127.0.0.1");

		//发送NTP请求应答消息
		iRet = ::sendto(sock, szTimeValue, strlen(szTimeValue), 0, (sockaddr*)&addrRemote, sizeof(addrRemote));
		if ( SOCKET_ERROR == iRet )
		{
			printf("Failed sendto() sock %d \n", ::WSAGetLastError());
			closesocket(sock);
			return -1;
		}
		else
		{
			printf("Successed sendto sock = %d, data lenth = %d \n", sock, iRet);
		}
	}

	return 0;
}

int main(int argc, char* argv[])
{
	WSADATA wsaData;
	WORD sockVersion = MAKEWORD(2, 2);
	if(::WSAStartup(sockVersion, &wsaData) != 0)
	{
		exit(0);
	}
	
	SOCKET sock;
	int iRet = StartNtpServer(sock);
	if (0 != iRet)
	{
		printf("start ntp server failed. \n");
	}

	system("pause");
	closesocket(sock);
	::WSACleanup();	
	return 0;
}

 

     上述代码代码在VC2005环境中已测试通过，以下是客户端和服务端程序截图：
 
 
客户端
 
 
服务端