從一個HTTP請求來讀懂HTTP、TCP協議

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"從一個HTTP請求來看網絡分層原理"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"兩臺主機間會通過非常多網絡設備，不管哪個網絡設備都會發生數據丟失，如果發生數據丟失的話，會發生數據重傳，會出現數據重複(之前丟失的包並不是丟失而是產生了延時)。數據傳輸的介質也可能多樣，如內網裏通過網線進行傳輸，連接到公網的話會通過光纖進行連接，所以要實現不同介質間信號的轉換，還有從光纖到路由器無線脈衝轉換，距離遠的話還有信號衰減問題。所以在網絡傳輸過程中有非常多的問題需要解決，把問題分組分層，不同層次間解決不同問題，不同層次間定義標準化接口讓它們間可以進行數據的通信。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"複雜的網絡"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/2ca881c90ac1021517ea89c524ddd692.png-wh_600x-s_379608925.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/89/898d19e0668f3b2d5ce640b5c293f186.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/2ca881c90ac1021517ea89c524ddd692.png-wh_600x-s_379608925.png","title":null}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"爲了簡化網絡的複雜度，網絡通信的不同方面被分解爲多層次結構，每一層只與緊挨着的上層或者下層進行交互，將網絡分層，這樣就可以修改，甚至替換某一層的軟件，只要層與層之間的接口保持不變，就不會影響到其他層。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"OSI( Open System Interconnection Reference Model): 開放系統互聯參考模型"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"TCP/IP 協議族"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"OSI七層理論體系結構"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s6.51cto.com/oss/202010/20/cce1692da335fa5d2dde2a8a30f93227.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e7/e723e4d74254b14ae715a21e7f3abdd6.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s6.51cto.com/oss/202010/20/cce1692da335fa5d2dde2a8a30f93227.png","title":null}}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"物理層："},{"type":"text","text":"解決兩臺主機的通信問題—A往B發送比特流(0101)，B能接收到這些比特流。定義了物理設備的標準如網線的類型，光纖的接口類型以及傳輸介質的傳輸速率等。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"數據鏈路層：由於物理層上的傳輸的比特流可能會出現錯傳、誤傳等，所以數據鏈路層定義瞭如何格式化數據即將比特流封裝成幀，提供了錯誤檢測。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"網絡層："},{"type":"text","text":"隨着節點的增加，點對點通信是需要經過多個節點的，如何找到目標節點，如何找到最優路徑變成爲了首要需求。所以出現了網絡層，主要目的是將網絡地址翻譯成對應的物理地址，分組傳輸、路由選擇，本層的傳輸單位是數據報(分組)，本層需要注意的TCP/IP協議中的TCP協議。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"傳輸層："},{"type":"text","text":"隨着網絡需要的進一步擴大，通信過程中需要傳輸大量的數據，網絡可能會發生中斷，爲了保證傳輸大量文件時的準確性，需要對發送的數據進行切分，切分成一個個的segment進行發送，考慮如何在接受方拼接切分的segment組成完整的數據，以及發現丟失segment時該如何處理，需要注意的協議TCP、UDP。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"會話層："},{"type":"text","text":"不同機器上的用戶之間建立以及管理會話。用於保證應用程序自動收發包和尋址。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":6,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"表示層："},{"type":"text","text":"信息的語義語法，加密解密，轉換翻譯，壓縮解壓縮。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":7,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"應用層："},{"type":"text","text":"規定雙方必須使用固定長度的消息頭，且消息頭必須記錄消息長度等信息。需要注意的是TCP/IP協議中的HTTP協議。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/7f70bcd8b566495c50709b65918b2708.png-wh_600x-s_3042196636.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/73/733b6578e18e8e4c3f4f1c3ee49d60c3.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/7f70bcd8b566495c50709b65918b2708.png-wh_600x-s_3042196636.png","title":null}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"TCP/IP四層模型"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"是OSI的一種實現，包括應用層、運輸層、網際層和網絡接口層。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/098fa5e287316c1fd52125b9112cb8e4.png-wh_600x-s_3103511319.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/76/76cb34519bd4344cb7fc86c2d0e7bc91.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/098fa5e287316c1fd52125b9112cb8e4.png-wh_600x-s_3103511319.png","title":null}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"一個HTTP請求的分層解析流程"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/eac5b8bcad9162d4ec75ccecf4c29ea8.png-wh_600x-s_3348559328.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/f7/f752f69ead1f8d1e76e9e14bd55193c7.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s2.51cto.com/oss/202010/20/eac5b8bcad9162d4ec75ccecf4c29ea8.png-wh_600x-s_3348559328.png","title":null}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如上圖右邊一個服務器部署了一個靜態頁面，通過nginx部署在公網上，瀏覽器通過域名對它進行訪問，瀏覽器輸入域名點回車後是怎麼工作的呢？"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"http://www.dumain.com "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"服務端只認ip地址，瀏覽器將域名解析出來，看下瀏覽器裏有沒有域名對應DNS的緩存，有的話直接拿到服務端的ip地址，沒有的話去本地的host文件看有沒有配置，沒有配置的話纔會發起一個DNS請求用來獲取服務器ip地址。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"DNS也是臺服務器也有自己的ip地址，這時候應用層會構造一個DNS請求報文，應用層會去調用傳輸層的接口一個socket的API，DNS默認使用UDP實現數據傳輸，即應用層調用傳輸層的API，傳輸層會在DNS請求報文基礎上加一個UDP的請求頭，傳輸層將數據交給網絡層，網絡層同樣在UDP請求報文基礎上加IP的請求頭，網絡層會將IP請求報文交給數據鏈路層，數據鏈路層會將自己的mac頭加上去並把對應的請求報文交給下一個機器的mac地址也會加上去，下一個機器的mac地址通過網絡層ARP協議找到，ARP會發送一些請求看下你對應的ip地址的mac地址是多少，最後通過物理層物理介質傳出去，通常傳到路由器上."}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"路由器是三層設備(從下向上)從物理層開始連接，物理層交給數據鏈路層，數據鏈路層看下地址是不是給我的，是給我的進行解析，不是給我的就丟棄，報文再傳給上面一層網絡層，網絡層把數據傳到下一個路由器的地址是多少，會通過運營商的網絡接口傳到運營商的路由器上，運營商有自己的DNS服務器，如果配置的是運營商自己的DNS服務器的話會直接在這個DNS服務器裏找自己對應的域名拿到對應的ip地址，也就是剛請求DNS報文地址，然後原路返回解析直到應用層拿到剛域名對應的ip地址，這樣就可以進行HTTP請求報文的發送，再調用傳輸層協議是TCP參數，同樣每到一層加頭。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"HTTP"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"什麼是HTTP?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"超文本傳輸協議，是一個基於請求與響應，無狀態的，應用層的協議，常基於TCP/IP協議傳輸數據，互聯網上應用最爲廣泛的一種網絡協議,所有的WWW文件都必須遵守這個標準。設計HTTP的初衷是爲了提供一種發佈和接收HTML頁面的方法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"HTTP特點"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"無狀態：協議對客戶端沒有狀態存儲，對事物處理沒有“記憶”能力，比如訪問一個網站需要反覆進行登錄操作。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"無連接：HTTP/1.1之前，由於無狀態特點，每次請求需要通過TCP三次握手四次揮手，和服務器重新建立連接。比如某個客戶機在短時間多次請求同一個資源，服務器並不能區別是否已經響應過用戶的請求，所以每次需要重新響應請求，需要耗費不必要的時間和流量。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"基於請求和響應：基本的特性，由客戶端發起請求，服務端響應。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"簡單快速、靈活。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"通信使用明文、請求和響應不會對通信方進行確認、無法保護數據的完整性。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"HTTP協議版本已經演化到3.0版本，關於協議版本可以查看 快速掌握HTTP1.0 1.1 2.0 3.0的特點及其區別"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"HTTP報文格式"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"HTTP 協議的請求報文和響應報文的結構基本相同，由三大部分組成:"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"起始行(start line)：描述請求或響應的基本信息"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"頭部字段集合(header)：使用 key-value 形式更詳細地說明報文"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"消息正文(entity)：實際傳輸的數據，它不一定是純文本，可以是圖片、視頻等二進制數據"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其中起始行和頭部的字段併成爲 請求頭 或者 響應頭，統稱爲 Header;消息正文也叫實體，稱爲 body。HTTP 協議規定每次發送的報文必須要有 Header，但是可以沒有 body，也就是說頭信息是必須的，實體信息可以沒有。而且在 header 和 body 之間必須要有一個空行(CRLF)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s4.51cto.com/oss/202010/20/1a566786f4f87b134c732c45cca957cb.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/72/729ca82aac598f042a848f9ba59293f9.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s4.51cto.com/oss/202010/20/1a566786f4f87b134c732c45cca957cb.png","title":null}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"請求行報文格式"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s4.51cto.com/oss/202010/20/289db58a4e8ba25b6460fb0f44648815.png-wh_600x-s_2637844719.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/f7/f71d705041c02d0b1887cd8d5fc19760.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s4.51cto.com/oss/202010/20/289db58a4e8ba25b6460fb0f44648815.png-wh_600x-s_2637844719.png","title":null}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"請求方法:如 GET/HEAD/PUT/POST，表示對資源的操作;"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"請求目標:通常是一個 URI，標記了請求方法要操作的資源;"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"版本號:表示報文使用的 HTTP 協議版本。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"響應報文格式"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s6.51cto.com/oss/202010/20/2a016c6c4e954d755a429cc3edfdaba6.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/94/94c457bf22bcba348313c53c41493ede.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s6.51cto.com/oss/202010/20/2a016c6c4e954d755a429cc3edfdaba6.png","title":null}}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"版本號:表示報文使用的 HTTP 協議版本;"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"狀態碼:一個三位數，用代碼的形式表示處理的結果，比如 200 是成功，500 是服務器錯誤;"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"原因:作爲數字狀態碼補充，是更詳細的解釋文字，幫助人理解原因。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"請求及響應報文格式對比"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s3.51cto.com/oss/202010/20/239df0ef71358728caeb8272490eb0bb.png-wh_600x-s_4035336112.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/7d/7debbcca603827b57b4d432b5abf4d74.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s3.51cto.com/oss/202010/20/239df0ef71358728caeb8272490eb0bb.png-wh_600x-s_4035336112.png","title":null}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"HTTP 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發送FIN數據包，代表A不再發送數據"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"B: 收到請求，開始應答 ，避免了A重新發送FIN重試(應答機制)"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"B: 處理完數據之後關閉，關閉連接,及發送FIN請求"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"A: 收到請求後發送ACK應答，B服務可以釋放連接"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"等待 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定一個報文段應該包含多少個字節。"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"MSS: Max Segment Size, 默認 536byte 實際數據 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "},{"type":"link","attrs":{"href":"https://s6.51cto.com/oss/202010/20/6a32cd810c727c3d7a48ad34605a4927.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/77/77f3549184238d5b62c30b0e3b497acf.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s6.51cto.com/oss/202010/20/6a32cd810c727c3d7a48ad34605a4927.png","title":null}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在網絡傳輸過程中可能會出現以下的一些情況："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"客戶端一段時間沒有收到 ack 消息則重傳"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果緩衝區滿了則可能丟包或延時都需要重傳"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"根據報文 sequeence number 字段重排序，還需要丟棄重複包。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"數據傳輸的可靠性"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"停止等待協議如下： "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s3.51cto.com/oss/202010/20/3d92bc12b5c0a1681c07a7e83e38c0bf.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/77/77f3549184238d5b62c30b0e3b497acf.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s3.51cto.com/oss/202010/20/3d92bc12b5c0a1681c07a7e83e38c0bf.png","title":null}},{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"停止等待協議，效率比較低"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"重傳機制如下："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"ack 報文丟失"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/d5/d5a3acdf585df89055dec51c6e2d8dd3.png","alt":"從一個HTTP請求來讀懂HTTP、TCP協議","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"請求報文丟失"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/f8/f84c5a0f47952b1ae629f5a81879cf3d.png","alt":"從一個HTTP請求來讀懂HTTP、TCP協議","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"滑動窗口協議與累計確認(延時ack)"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如上效率低，所以tcp提出了新的協議-滑動窗口協議與累計確認(延時ack)。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"滑動窗口大小同通過tcp三次握手和對端協商，且受網絡狀況影響。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面是一個一個報文，實際可發一批報文，服務器並不是挨個去確認，上面回一個ack浪費資源，單獨響應一個報文時，tcp本身一個報文至少20個字節再加上ip頭報文20字節，所以一個ack至少40字節。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以延時ack的發送，如下圖確認最後一個報文如5就可以，但這樣也有一個問題如3的報文丟了，這時只能確認1和2連續報文，從3以後的報文全要重傳，已確認的報文在緩衝區丟棄掉。 "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"link","attrs":{"href":"https://s6.51cto.com/oss/202010/20/cf6792bf21872507abdce42c1d5e9ffc.png","title":null}}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e1/e1d94e25d65006fac26794493198094b.png","alt":"","title":"","style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"作者：一角錢技術"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"來源：https://www.toutiao.com/i6885366707725009419/"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}