ntp實(shí)現(xiàn)原理

我們看一下為啥說(shuō)ntp從實(shí)現(xiàn)時(shí)間同步的角度來(lái)說(shuō)是很簡(jiǎn)單的。在pc上，以windows為例，手動(dòng)關(guān)閉打開(kāi)一下自動(dòng)更新時(shí)間，會(huì)觸發(fā)一次ntp時(shí)間同步。

這個(gè)時(shí)候，我們可以拿到ntp完整的request/response報(bào)文，如下:

ntp client request:

ntp server response:

如上圖，我們可以看到，我們可以拿到同步原理提到的t1, t2, t3；而t4是client接收到server response時(shí)的時(shí)間，那么上述時(shí)間分別是如何來(lái)的呢。

* t1: client發(fā)送request的前一刻，記下當(dāng)前utc時(shí)間，然后塞入到request transmit timestamp中去。但是這個(gè)不能作為真正的t1來(lái)使用，因?yàn)榍懊嫣岬搅耍琻tp是使用的udp，所以有可能會(huì)丟包。因此真正的t1應(yīng)該ntp server收到client request時(shí)，從request報(bào)文中提取到t1，最后再給到client使用。也就是上圖的t1。
* t2: 也就是server收到client request的時(shí)間
* t3: server發(fā)送response的時(shí)間
* t4: client收到server response的時(shí)間
  
  知道這些原理之后，我們就可以很好設(shè)計(jì)出我們的代碼實(shí)現(xiàn)思路，如下：
 

C++                  
//ntp client                  
// send request with t1                  
std::chrono::time_point_cast(          
        std::chrono::system_clock::now());          
std::time_t time_stamp = time_point.time_since_epoch().count();          
request_message_- >SetTransTs(time_stamp);          
int ret = SendMsg(request_message_- >GetData(), MESSAGE_LENGTH);          


// receive response, and get t1, t2, t3          
int ret = RecvMsg(request_message_- >GetData(), MESSAGE_LENGTH);          
auto req_tx_ts_ = response_message_- >GetOriginTs();  // t1          
auto req_rx_ts_ = response_message_- >GetRecvTs();    // t2          
auto resp_tx_ts_ = response_message_- >GetTransTs();  // t3          
auto time_point = std::chrono::time_point_cast(          
        std::chrono::system_clock::now());          
std::time_t time_stamp = time_point.time_since_epoch().count();          
auto resp_rx_ts_us_ = static_cast(time_stamp);  // t4            

auto offset =            
        req_rx_ts_us_ - req_tx_ts_us_ + resp_tx_ts_us_ - resp_rx_ts_us_;            
offset /= 2;            

// adjust the clock            
AdjustClock(offset);

C++                  
// ntp server                  
// receive ntp request message                  
RecvMsg(request_message_- >GetData(), MESSAGE_LENGTH);                  
// get receive request_frame timestamp, get t1, t2                  
auto time_point = std::chrono::time_point_cast(          
        std::chrono::system_clock::now());          
auto req_rx_ts = time_point.time_since_epoch().count();//t2          
auto req_tx_ts = request_message_- >GetTransTs();//t1          

// send ntp response message          
response_message_- >SetOriginTs(req_tx_ts);//t1          
response_message_- >SetRecvTs(req_rx_ts);//t2          
auto time_point = std::chrono::time_point_cast(          
        std::chrono::system_clock::now());          
auto resp_tx_ts = time_point.time_since_epoch().count();          
response_message_- >SetTransTs(resp_tx_ts);//t3          
SendMsg(response_message_- >GetData(), MESSAGE_LENGTH);