作者：tacy lee
寫本文的目的就是想搞清除http1.1和1.0的具體區別，看過很多別人寫的關于他們的概念，可能是沒動手去測試，總是模模糊糊
Persistent HTTP connections
我們知道，早先的WEB網站都是由一些文字頁面組成，一個頁面可能就是一個簡單的http請求，里面也不會嵌套css，gif類似的資源文件，所以在1.0標準實現的時候沒有去考慮連接的效率（用完就丟棄，【1】），現在的頁面很多都非常復雜，在里面嵌套二三十個資源文件是很正常的事情,創建二三十個tcp連接去下載這些資源，對于tcp連接的利用率非常低。
另外一方面，tcp連接打開之后，首先采用slow start算法【2】，開始先傳輸幾個小的packet探測網絡情況來確定傳輸速率，而且頁面上嵌入的資源文件往往都比較小，也就是說如果采用http 1.0,就意味著很多的資源傳輸都是在slow start下完成的，效率也很低。
Persistent HTTP connections就是讓連接重復利用，不需要為每個資源文件都建立一個tcp連接。也不存在slow start問題。
http規范里面對于持久連接的數量建議為2（firefox和iexplorer缺省都是采用該值），增加該值會增加服務器的壓力（需要創建更多的連接），而且對于網絡狀況不好的情況只會導致更大的阻塞，所以不建議修改該值【3】
Clients that use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD NOT maintain more than 2 connections with any server or proxy. A proxy SHOULD use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion
* firefox 的Persistent HTTP connections設置
network.http.max-persistent-connections-per-server
* iexplorer的Persistent HTTP connections設置
http://support.microsoft.com/?kbid=282402
Pipelining
在HTTP 1.0的時候，客戶端的請求都是等到前一個請求完成之后，再發送另一個請求，一個packet里面只能請求一個資源。Pipelining可以讓你在一個packet里面請求多個資源，減少了packet的數量，具體請參考下面示例。
* firefox Pipelining設置
network.http.pipelining （缺省為false，推薦打開）
network.http.pipelining.maxrequests (缺省為4，最大為8，建議設置為8）
* iexplorer設置
ie沒有實現pipelining
* 沒有實現Pipelining的瀏覽器發送的packet
No. Time Source Destination Protocol Info
52 0.576526 192.168.1.61 203.81.29.137 HTTP GET /images/index_03.gif HTTP/1.1
Frame 52 (620 bytes on wire, 620 bytes captured)
Ethernet II, Src: QuantaCo_c3:16:11 (00:16:36:c3:16:11), Dst: Intel_5f:99:3a (00:0e:0c:5f:99:3a)
Internet Protocol, Src: 192.168.1.61 (192.168.1.61), Dst: 203.81.XXX.XXX (203.81.XXX.XXX)
Transmission Control Protocol, Src Port: 2320 (2320), Dst Port: http (80), Seq: 1122, Ack: 8456, Len: 566
Hypertext Transfer Protocol
GET /images/index_03.gif HTTP/1.1\r\n
Host: www.primeton.com\r\n
User-Agent: Mozilla/5.0 (Windows;U;Windows NT 5.1; en-US; rv:1.8.1.6)Gecko/20070725 Firefox/2.0.0.6\r\n
Accept: image/png,*/*;q=0.5\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7\r\n
Keep-Alive: 300\r\n
Connection: keep-alive\r\n
Referer: http://www.primeton.com/\r\n
Cookie: __utmz=33497343.1189057682.16.1.utmccn=(direct)|utmcsr=(direct)|utmcmd=(none); __utma=3349734
3.241348620.1172460177.1189057682.1189130143.17; __utmb=33497343; __utmc=33497343\r\n
\r\n
* 實現了Pipelining的瀏覽器發送的packet，可以看到，這個包里面請求了兩個資源（index_03.gif和SH.jpg)
No. Time Source Destination Protocol Info
39 0.558943 192.168.1.61 203.81.29.137 HTTP GET /images/index_03.gif HTTP/1.1
Frame 39 (1514 bytes on wire, 1514 bytes captured)
Ethernet II, Src: QuantaCo_c3:16:11 (00:16:36:c3:16:11), Dst: Intel_5f:99:3a (00:0e:0c:5f:99:3a)
Internet Protocol, Src: 192.168.1.61 (192.168.1.61), Dst: 203.81.XXX.XXX (203.81.XXX.XXX)
Transmission Control Protocol, Src Port: 2309 (2309), Dst Port: http (80), Seq: 1158, Ack: 24226, Len: 1460
Hypertext Transfer Protocol
GET /images/index_03.gif HTTP/1.1\r\n
Host: www.primeton.com\r\n
User-Agent: Mozilla/5.0 (Windows;U;Windows NT 5.1; en-US; rv:1.8.1.6)Gecko/20070725 Firefox/2.0.0.6\r\n
Accept: image/png,*/*;q=0.5\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7\r\n
Keep-Alive: 300\r\n
Connection: keep-alive\r\n
Referer: http://www.primeton.com/\r\n
Cookie: __utmz=33497343.1189057682.16.1.utmccn=(direct)|utmcsr=(direct)|utmcmd=(none); __utma=3349734
3.241348620.1172460177.1189057682.1189130143.17; __utmb=33497343; __utmc=33497343\r\n
\r\n
Hypertext Transfer Protocol
GET /images/SH.jpg HTTP/1.1\r\n
Host: www.primeton.com\r\n
User-Agent: Mozilla/5.0 (Windows;U;Windows NT 5.1; en-US; rv:1.8.1.6)Gecko/20070725 Firefox/2.0.0.6\r\n
Accept: image/png,*/*;q=0.5\r\n
Accept-Language: en-us,en;q=0.5\r\n
Accept-Encoding: gzip,deflate\r\n
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7\r\n
Keep-Alive: 300\r\n
Connection: keep-alive\r\n
Referer: http://www.primeton.com/\r\n
Cookie: __utmz=33497343.1189057682.16.1.utmccn=(direct)|utmcsr=(direct)|utmcmd=(none); __utma=334973
43.241348620.1172460177.1189057682.1189130143.17; __utmb=33497343; __utmc=33497343\r\n
\r\n
chunked
http/1.0不支持chunked packet（當sender無法確定傳輸數據大小的時候使用）,keep-alive對于http/1.0同樣有效，但是不支持chuncked packet
====== 參考資料： ======
* 【1】：Nielsen, H.F., Gettys, J., Baird-Smith, A., Prud'hommeaux, E., Lie, H., and C. Lilley. [[http://www.w3.org/Protocols/HTTP/Performance/Pipeline.html|Network Performance Effects of HTTP/1.1, CSS1, and PNG]], Proceedings of ACM SIGCOMM '97, Cannes France, September 1997.[jg642]
* 【2】：Spero, S., [[http://www.w3.org/Protocols/HTTP/1.0/HTTPPerformance.html|Analysis of HTTP Performance Problems]], July 1994.
* 【3】：[[http://www.w3.org/Protocols/rfc2616|Hypertext Transfer Protocol -- HTTP/1.1]]