第一、HTTP請求的過程介紹
一個HTTP請求，涉及多個階段
1、DNS解析域名
2、請求從Clinet路由至Server，Clinet與Server建立TCP連接
3、如果使用了HTTPS，還涉及SSL連接的建立
4、server開始準備數據
開始邏輯計算、調後端接口、查數據庫緩存等
5、server開始傳遞數據
數據準備完成，開始給client傳數據
6、數據傳輸完畢
7、整個過程可能還涉及多次重定向
第二、關於CURL的介紹
CURL是利用URL語法在命令行方式下工作的開源數據傳輸工具。
支持：DICT, FILE, FTP, FTPS, Gopher, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, POP3, POP3S, RTMP, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, Telnet and TFTP. curl supports SSL certificates, HTTP POST, HTTP PUT, FTP uploading, HTTP form based upload, proxies, HTTP/2, cookies, user+password authentication (Basic, Plain, Digest, CRAM-MD5, NTLM, Negotiate and Kerberos), file transfer resume, proxy tunneling 等
最新版的curl穩定版爲7.55.1（截止20170817）
源代碼：https://github.com/curl/curl
第三：用CURL檢測Clinet側發起的HTTP請求各階段時間，簡要說明
1、TCP建立連接的耗時：CONNECT-NAMELOOKUP
2、建立TCP連接到server返回client第一個字節的時間：
STARTTRANSFER-CONNECT
3、SERVER處理數據的時間：
可以用STARTTRANSFER - PRETRANSFER計算得到
4、CLIENT接收數據的耗時（開始接收至接收完成）：
TOTAL-STARTTRANSFER
第四、例子：
curl -o /dev/null -s -w time_namelookup:"\t"%{time_namelookup}"\n"time_connect:"\t\t"%{time_connect}"\n"time_appconnect:"\t"%{time_appconnect}"\n"time_pretransfer:"\t"%{time_pretransfer}"\n"time_starttransfer:"\t"%{time_starttransfer}"\n"time_total:"\t\t"%{time_total}"\n"time_redirect:"\t\t"%{time_redirect}"\n" https://www.yqb.com/
1、DNS解析耗時：0.008s
2、TCP建立連接的耗時：0.059-0.008=0.051s
3、SSL握手完成耗時：0.228-0.059=0.169s
169ms，多了一層SSL還是很耗時的
4、server處理數據的時間：0.287-0.228=0.059
59ms，說明服務器處理很快
5、總體的耗時：0.228s
6、整個過程沒有redirect，所以redirect的耗時爲0
再舉一例：
服務器處理數據的耗時：2.305-0.014=2.291s
這就說明server端的性能是值得研究的。
對於server端而言，有需要分析它的耗時：
防火牆->負載均衡->應用->緩存和DB
需要深入去分析這個時間消耗在哪個環節，有針對性的優化。
第五、詳細說明
NAMELOOKUP：從開始計算，域名解析完成的耗時
CURLINFO_NAMELOOKUP_TIME. The time it took from the start until the name resolving was completed.
CONNECT：從開始計算，TCP建立完成的耗時
CURLINFO_CONNECT_TIME. The time it took from the start until the connect to the remote host (or proxy) was completed.
APPCONNECT：從開始計算，應用層（SSL，在TCP之上的應用層）連接/握手完成的耗時
CURLINFO_APPCONNECT_TIME. The time it took from the start until the SSL connect/handshake with the remote host was completed. (Added in in 7.19.0)
PRETRANSFER：從開始計算，準備開始傳輸數據的耗時
CURLINFO_PRETRANSFER_TIME. The time it took from the start until the file transfer is just about to begin. This includes all pre-transfer commands and negotiations that are specific to the particular protocol(s) involved.
STARTTRANSFER：從開始計算，開始傳輸數據的耗時（libcurl接收到第一個字節）
CURLINFO_STARTTRANSFER_TIME. The time it took from the start until the first byte is received by libcurl.
TOTAL：總的耗時
CURLINFO_TOTAL_TIME. Total time of the previous request.
REDIRECT：整個過程重定向的耗時，如果整個過程沒有重定向，這個時間爲0
CURLINFO_REDIRECT_TIME. The time it took for all redirection steps include name lookup, connect, pretransfer and transfer before final transaction was started. So, this is zero if no redirection took place.
另：python也有一個pycurl模塊，大家可以嘗試。
參考：
https://curl.haxx.se/libcurl/c/curl_easy_getinfo.html