計算機網絡課程設計-資料下載頁

　　

【正文】 gure . The IP datagram is placed inside a linklayer frame appropriate for the link connecting the school’s router to the leftmost Comcast router and the frame is sent over this link.網關路由器接收數據幀并提取包含DNS查詢報文的IP報文段。路由器查看目的IP ，然后檢查它的轉發表，上圖中最左邊的路由器。IP數據報被放置在鏈路層幀中通過連接到學校路由器進行發送。15. The leftmost router in the Comcast network ? receives the frame, ? extracts the IP datagram, ? examines the datagram’s destination address () and ? determines the outgoing interface on which to forward the datagram towards the DNS server from its forwarding table, which has been filled in by Comcast’s intradomain protocol (such as RIP, OSPF, or ISIS) as well as the Internet’s interdomain protocol, BGP. 最左邊的路由器在電信公司的網絡上接收數據幀，提取IP報文段，檢查報文的目的IP地址，檢查從路由器的轉發表發送數據報到DNS服務器的合適的接口，轉發表通過域內網關協議和邊界網關協議來進行填充。16. Eventually the IP datagram containing the DNS query arrives at the DNS server. The DNS server extracts the DNS query message, looks up the name in its DNS database, and finds the DNS resource record that contains the IP address () for . (assuming that it is currently cached in the DNS server). Recall that this cached data originated in the authoritative DNS server for . The DNS server forms a DNS reply message containing this hostnametoIP address mapping, and places the DNS reply message in a UDP segment, and the segment within an IP datagram addressed to Bob’s laptop (). This datagram will be forwarded back through the Comcast network to the school’s router and from there, via the Ethernet switch to Bob’s laptop. 最后包含DNS查詢報文的IP數據報到達DNS服務器。DNS服務器提取DNS查詢報文，在DNS數據庫中查找谷歌域名，找到包含谷歌IP地址的DNS源記錄（假設該地址被放到DNS服務器的緩沖區中）。DNS服務器形成一個DNS回復報文，其中包含主機名到地址的映射，封裝之后發到Bob的電腦上。這個報文將被轉發通過電信網絡到學校路由器，然后在通過以太網交換機到Bob的電腦。17. Bob’s laptop extracts the IP address of the server from the DNS message. Finally, after a lot of work, Bob’s laptop is now ready to contact the server. Bob的電腦從DNS回復報文中提取谷歌服務器的IP地址。最后，經過許多工作之后，Bob的電腦準備連接谷歌服務器。18. Now that Bob’s laptop has the IP address of , it can create the TCP socket that will be used to send the HTTP GET message to . When Bob creates the TCP socket, the TCP in Bob’s laptop must first perform a threeway handshake with the TCP in . Bob’s laptop thus first create a TCP SYN segment with destination port 80 (for HTTP), places the datagram inside a frame with a destination MAC address of 00:22:6B:45: 1F:1B (the gateway router) and sends the frame to the switch. 現在Bob的電腦谷歌的IP,然后就能通過創建TCP套接字來發送HTTP GET消息到谷歌服務器。當Bob創建TCP套接字，TCP連接第一次必須和谷歌服務器執行三次握手。因此Bob的電腦第一次必須創建TCP SYN報文段到HTTP服務器，封裝數據報到幀頭中然后發送到目的MAC為網關路由器的接口然后發送幀到交換機。19. The routers in the school network, Comcast’s network, and Google’s network forward the datagram containing the TCP SYN towards , using the forwarding table in each router, as in steps 14－16 above. Recall that the router forwarding table entries governing forwarding of packets over the interdomain link between the Comcast and Google networks are determined by the BGP protocol. 學校網絡上的路由器，電信網絡和谷歌所在網絡的路由器都使用各自的轉發表負責轉發包含TCP SYN的報文到谷歌服務器。調用這些路由轉發表的進入接口，支配數據包在域內和域外轉發使用的是BGP邊界網關路由協議。20. Eventually, the datagram containing the TCP SYN arrives at . The TCP SYN message is extracted from the datagram and demultiplexed to the wele socket associated with port 80. A connection socket is created for the TCP connection between the Google HTTP server and Bob’s laptop. A TCP SYNACK segment is generated, placed inside a datagram addressed to Bob’s laptop, and finally placed inside a linklayer frame appropriate for the link connecting to its firsthop router. 最終，包含TCP SYN的報文段到達谷歌服務器。TCP SYN消息被提取出來從該報文段中然后分用到和歡迎套接字綁定的80號端口。這樣，一個在googleHTTP服務器和Bob電腦上的TCP鏈接套接字就被創建成功了。一個TCP SYN ACK 報文產生封裝在數據報中發送到Bob的電腦，最后封裝成鏈路層數據幀鏈接谷歌的第一跳路由器。21. The datagram containing the TCP SYNACK segment is forwarded through the Google, Comcast, and school networks, eventually arriving at the Ethernet card in Bob’s laptop. The datagram is demultiplexed within the operating system to the TCP socket created in step 18, which enters the connected state. 該包含TCP SYN ACK的報文段在通過層層轉發到Bob的電腦上。數據報被分用到OS之前創建的TCP套接字，進行匹配。22. With the socket on Bob’s laptop now (finally) ready to send bytes to , Bob’s browser creates the HTTP GET message containing the URL to be fetched. The HTTP GET message is then written into the socket, with the GET message being the payload of a TCP segment.The TCP segment is placed in a datagram and sent and delivered to as in steps 18－20 above. 有了連接套接字Bob的電腦就可以準備發送字節到谷歌，Bob的瀏覽器創建HTTP GET消息進行訪問，其中使用URL。HTTP GET消息寫進套接字，和GET消息一起變成TCP報文段的有效載荷的一部分。TCP報文段被封裝在數據報中然后發送傳輸到谷歌，進行的是1820部分。23. The HTTP server at reads the HTTP GET message from the TCP socket, creates an HTTP response message, places the requested web page content in the body of the HTTP response message, and sends the message into the TCP socket.谷歌的HTTP服務器從TCP套接字中讀GET消息，然后創建HTTP回應報文，該報文封裝在HTTP回復報文的網頁主體內，然后在發送消息到TCP套接字。24. The datagram containing the HTTP reply message is forwarded through the Google, Comcast, and school networks, and arrives at Bob’s laptop. Bob’s web browser program reads the HTTP response from the socket, extracts the html for the web page from the body of the HTTP response, and finally (finally!) displays the web page!這個包含有HTTP 回復消息的數據報通過層層轉發到Bob的電腦。Bob的web瀏覽器程序讀取HTTP回復報文從套接字，然后從HTTP響應報文段的主體部分提取網頁的html程序，最后顯示網頁。四、 總結本次計算機網絡的課程設計收獲頗多，總體總結下來可以有以下幾點收獲和體會。26