經典面試題：在瀏覽器地址欄輸入一個 URL 後回車，背後發生了什麼

{"type":"doc","content":[{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"🎓 盡人事，聽天命。博主東南大學研究生在讀，熱愛健身和籃球，正在爲兩年後的秋招準備中，樂於分享技術相關的所見所得，關注公衆號 @ ","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"飛天小牛肉","attrs":{}},{"type":"text","text":"，第一時間獲取文章更新，成長的路上我們一起進步","attrs":{}}]},{"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":"🎁 本文已收錄於 ","attrs":{}},{"type":"link","attrs":{"href":"https://gitee.com/veal98/CS-Wiki","title":null},"content":[{"type":"text","text":"CS-Wiki（Gitee 官方推薦項目，現已 0.9k star)","attrs":{}}],"marks":[{"type":"strong"}]},{"type":"text","text":"，致力打造完善的後端知識體系，在技術的路上少走彎路，歡迎各位小夥伴前來交流學習","attrs":{}}]}],"attrs":{}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"0. 前言","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在瀏覽器的地址欄輸入一個 URL 後回車，背後到底發生了什麼才能使得一個界面完美的展現在我們眼前？","attrs":{}}]},{"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":"今天講解的這道題目，由於其涉及大量網絡協議，可以非常直觀的看出諸位小夥伴對計算機網絡體系的整體把握程度，所以自然成爲了各大公司的面試常客。","attrs":{}}]},{"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":"在瀏覽中輸入 URL 並且獲取響應的過程，其實就是瀏覽器和該 URL 對應的服務器的網絡通信過程。比如我們輸入 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"www.baidu.com","attrs":{}}],"attrs":{}},{"type":"text","text":"，那麼會返回一個百度搜索的界面，這其實就是瀏覽器和百度服務器之間的網絡通信過程。瀏覽器就是客戶端，用於發出請求，而百度的服務器就是服務端，用於接收並響應請求。","attrs":{}}]},{"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":"下面我們就來詳細講解這個龐大的網絡通信過程。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"1. 解析 URL","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不知道有沒有同學會混淆域名和 URL 的概念，可以這樣理解，URL 就是我們輸入的網址，而網址裏面含有域名。舉個例子：","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"www.baidu.com/veal98","attrs":{}}],"attrs":{}},{"type":"text","text":" 是一個網址，而 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"www.baidu.com","attrs":{}}],"attrs":{}},{"type":"text","text":" 就是服務器的域名。","attrs":{}}]},{"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":"URL 各元素的組成如下（當然，下述請求文件的路徑名可以省略）：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/d7/d7e918bd20dfa1a80ed8f974eae9677d.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"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":"這個 URL 請求的目標服務器上的文件路徑就是：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9a/9a384ada423976f6856f7c95ca37d5b9.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"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":"那麼首先，瀏覽器做的第一步就是解析 URL 得到裏面的參數，將域名和需要請求的資源分離開來，從而瞭解需要請求的是哪個服務器，請求的是服務器上什麼資源等等。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2. 瀏覽器封裝 HTTP 請求報文","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對 ","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"URL","attrs":{}}],"attrs":{}},{"type":"text","text":" 進行解析之後，瀏覽器確定了目標服務器和文件名，接下來就需要根據這些消息","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"封裝","attrs":{}},{"type":"text","text":"成一個 HTTP 請求報文發送出去。舉個 HTTP 請求報文的例子：","attrs":{}}]},{"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":"關於 HTTP 協議詳細可見 ","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/98FtlAy0mAtf6tGplQMDqA","title":null},"content":[{"type":"text","text":"HTTP 協議的前世今生","attrs":{}}],"marks":[{"type":"strong"}]},{"type":"text","text":" 這篇文章，這裏不再贅述","attrs":{}}]}],"attrs":{}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b1/b1f41e3a73078f72483d7e2e30a59109.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"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":"解釋一下","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"封裝","attrs":{}},{"type":"text","text":"，這是一個貫穿整個計算機網絡的概念。就是說","attrs":{}},{"type":"text","marks":[{"type":"underline","attrs":{}}],"text":"發送端在層與層之間傳輸數據時，每經過一層必定會被打上一個該層所屬的首部信息。反之，接收端在層與層之間傳輸數據時，每經過一層就會把該層對應的首部信息消去","attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/59/59e259a65d2b45e5d2f9840b955db32a.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"3. DNS 域名解析獲取 IP 地址","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"封裝好 HTTP 請求報文後，在正式還有一項準備工作沒有做，那就是獲取目標服務器的 IP 地址。","attrs":{}}]},{"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":"雖然解析得到了域名，理論瀏覽器已經知道目標服務器是誰了。但是實際上，域名並不是目標服務器真正意義上的地址，互聯網上每一臺計算機都被全世界唯一 IP 地址標識着，但是 IP 地址並不方便記憶，所以才設計出了域名。","attrs":{}}]},{"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":"那麼就需要解析域名獲取目標服務器的 IP 地址。不然空有一個方便記憶的域名咋知道這個請求到底發送到哪裏去呢。由域名轉換得到 IP 地址就是 DNS 協議做的事情，如下：","attrs":{}}]},{"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":"關於 DNS 詳細的內容各位可以回顧 ","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/AfVqL7lEsbRE-YLOPZ4gDQ","title":null},"content":[{"type":"text","text":"超詳細 DNS 協議解析","attrs":{}}],"marks":[{"type":"strong"}]},{"type":"text","text":" 這篇文章，比如什麼是域名，域名服務器，遞歸查詢和迭代查詢等等，寫的已經足夠詳細，此處只列出 DNS 的解析過程。","attrs":{}}]}],"attrs":{}},{"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":"1）首先搜索","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"瀏覽器的 DNS 緩存","attrs":{}},{"type":"text","text":"，緩存中維護着一張域名與 IP 地址的對應表；","attrs":{}}]},{"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":"2）若沒有命中，則繼續搜索","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"操作系統的 DNS 緩存","attrs":{}},{"type":"text","text":"；","attrs":{}}]},{"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":"3）若仍然沒有命中，則操作系統將域名發送至","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"本地域名服務器","attrs":{}},{"type":"text","text":"，本地域名服務器查詢自己的 DNS 緩存，查找成功則返回結果（注意：主機和本地域名服務器之間的查詢方式是","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"遞歸查詢","attrs":{}},{"type":"text","text":"）；","attrs":{}}]},{"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":"4）若本地域名服務器的 DNS 緩存沒有命中，則本地域名服務器向上級域名服務器進行查詢，通過以下方式進行","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"迭代查詢","attrs":{}},{"type":"text","text":"（注意：本地域名服務器和其他域名服務器之間的查詢方式是迭代查詢，防止根域名服務器壓力過大）：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"首先本地域名服務器向","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"根域名服務器","attrs":{}},{"type":"text","text":"發起請求，根域名服務器是最高層次的，它並不會直接指明這個域名對應的 IP 地址，而是返回頂級域名服務器的地址，也就是說給本地域名服務器指明一條道路，讓他去這裏尋找答案","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本地域名服務器拿到這個","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"頂級域名服務器","attrs":{}},{"type":"text","text":"的地址後，就向其發起請求，獲取","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"權限域名服務器","attrs":{}},{"type":"text","text":"的地址","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本地域名服務器根據權限域名服務器的地址向其發起請求，最終得到該域名對應的 IP 地址","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"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":"4）本地域名服務器將得到的 IP 地址返回給操作系統，同時自己將 IP 地址緩存起來","attrs":{}}]},{"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":"5）操作系統將 IP 地址返回給瀏覽器，同時自己也將 IP 地址緩存起來","attrs":{}}]},{"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":"6）至此，瀏覽器就得到了域名對應的 IP 地址，並將 IP 地址緩存起來","attrs":{}}]},{"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":"配合下圖直觀理解：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/9f/9f6405d37759bdf7e8845cf8cef6fcb7.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"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":"需要注意的是，DNS 使用的是 UDP 協議，也就是說上面各種請求的轉發，都是基於 UDP 這個無連接協議的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"4. 建立 TCP 連接","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"獲取到了目標服務器的 IP 地址之後，瀏覽器就知道我等下請求要發給誰了，這個時候就可以開始發送封裝好了的 HTTP 請求報文了，那麼既然需要發送請求，必然就需要 TCP 通過三次握手爲瀏覽器和服務器之間建立可靠的連接，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"保證雙方都具有可靠的接收和發送能力","attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"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":"這裏又是一道經典的面試題：TCP 三次握手和四次揮手，詳細可見 ","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s/u56NcMs68sgi6uDpzJ61yw","title":null},"content":[{"type":"text","text":"關於 TCP 三次握手和四次揮手，滿分回答在此","attrs":{}}],"marks":[{"type":"strong"}]},{"type":"text","text":" 這篇文章。","attrs":{}}]}],"attrs":{}},{"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":"三次握手過程如下圖：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/5e/5eeaa9858321baf7da2c67647e24fc31.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"5. 瀏覽器發送請求","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"TCP 三次握手完成後，瀏覽器與目標服務器之間就建立了一個可靠的虛擬通道，於是瀏覽器就可以發送自己的 HTTP 請求了。","attrs":{}}]},{"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":"需要注意的是，HTTP 請求報文或者響應報文在 TCP 連接通道上進行傳輸的時候，由於這些報文比較大，爲了更容易和準確可靠的傳輸，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"TCP 會將 HTTP 報文按序號分割成若干報文段並加上 TCP 首部，分別進行傳輸。接收方在收到這些報文段後，按照序號以原來的順序重組 HTTP 報文","attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"6. 負責傳輸的 IP 協議","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"實際上，TCP 在三次握手建立連接、四次握手斷開連接、以及連接建立過程中的收發數據（TCP 報文段）等各階段操作時，都是通過 IP 協議進行傳輸的，IP 協議將這些階段的數據添加 IP 首部封裝成 IP 數據報再進行傳輸。","attrs":{}}]},{"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":"IP 數據報的首部存有","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"源 IP 地址","attrs":{}},{"type":"text","text":"和 ","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"目標 IP 地址","attrs":{}},{"type":"text","text":"。所謂源 IP 地址 就是發送方的 IP 地址；目標 IP 地址就是通過 DNS 域名解析得到的目標服務器的 IP 地址。","attrs":{}}]},{"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":"事實上，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"IP 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地址在傳輸過程中是不會變化的","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而數據鏈路層則是根據 MAC 地址在一個接一個的區間中進行傳輸的，每個區間內的出發地址即「源 MAC 地址」，每個區間內的目的地址即「目的 MAC 地址」。顯然，隨着數據的傳輸，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"源 MAC 地址和目的 MAC 地址會不斷的發生變化","attrs":{}}]}],"attrs":{}}],"attrs":{}},{"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":"🍉 這麼說吧，舉個形象點的例子：我們把數據鏈路層當成乘坐高鐵從蘇州到南京，再在南京轉乘到北京，再在北京轉乘到西藏的旅客，那麼網絡層就相當於每個車站的工作人員，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"在數據鏈路層每次轉乘時，網絡層爲其購買了一張標有下一個 MAC 地址的車票","attrs":{}},{"type":"text","text":"。因此，即使旅客（數據鏈路層）不知道其最終目的地也沒有關係，工作人員（網絡層）會給你做出指引。","attrs":{}}]},{"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":"實際上，網絡層做出指引的過程，我們將其稱爲","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"路由控制","attrs":{}},{"type":"text","text":"，其中又涉及到了路由協議比如 OSPF 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地址的映射表","attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"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":"如下圖所示，假定主機 A 向同一鏈路上的主機 B 發送 IP 數據報，已知主機 A 和主機 B 的 IP 地址，它們互不知道對方的 MAC 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地址的映射。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e5/e5653fb861691cb6d74d5164cbe39c3c.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"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":"當然，緩存是有一定期限的，超過這個期限，緩存的內容將被清空。這也使得即使 MAC 地址和 IP 地址的映射關係發生了變化，也依然能夠正確的將數據包發送給目標地址。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"8. 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