【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(wǎng)關(guān)路由器接收數(shù)據(jù)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(zhuǎn)發(fā)表，上圖中最左邊的路由器。IP數(shù)據(jù)報被放置在鏈路層幀中通過連接到學校路由器進行發(fā)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(wǎng)絡(luò)上接收數(shù)據(jù)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(zhuǎn)發(fā)表發(fā)送數(shù)據(jù)報到DNS服務(wù)器的合適的接口，轉(zhuǎn)發(fā)表通過域內(nèi)網(wǎng)關(guān)協(xié)議和邊界網(wǎng)關(guān)協(xié)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(shù)據(jù)報到達DNS服務(wù)器。DNS服務(wù)器提取DNS查詢報文，在DNS數(shù)據(jù)庫中查找谷歌域名，找到包含谷歌IP地址的DNS源記錄（假設(shè)該地址被放到DNS服務(wù)器的緩沖區(qū)中）。DNS服務(wù)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(fā)到Bob的電腦上。這個報文將被轉(zhuǎn)發(fā)通過電信網(wǎng)絡(luò)到學校路由器，然后在通過以太網(wǎng)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(wù)器的IP地址。最后，經(jīng)過許多工作之后，Bob的電腦準備連接谷歌服務(wù)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(xiàn)在Bob的電腦谷歌的IP,然后就能通過創(chuàng)建TCP套接字來發(fā)送HTTP GET消息到谷歌服務(wù)器。當Bob創(chuàng)建TCP套接字，TCP連接第一次必須和谷歌服務(wù)器執(zhí)行三次握手。因此Bob的電腦第一次必須創(chuàng)建TCP SYN報文段到HTTP服務(wù)器，封裝數(shù)據(jù)報到幀頭中然后發(fā)送到目的MAC為網(wǎng)關(guān)路由器的接口然后發(fā)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(wǎng)絡(luò)上的路由器，電信網(wǎng)絡(luò)和谷歌所在網(wǎng)絡(luò)的路由器都使用各自的轉(zhuǎn)發(fā)表負責轉(zhuǎn)發(fā)包含TCP SYN的報文到谷歌服務(wù)器。調(diào)用這些路由轉(zhuǎn)發(fā)表的進入接口，支配數(shù)據(jù)包在域內(nèi)和域外轉(zhuǎn)發(fā)使用的是BGP邊界網(wǎng)關(guān)路由協(xié)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(wù)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(wù)器和Bob電腦上的TCP鏈接套接字就被創(chuàng)建成功了。一個TCP SYN ACK 報文產(chǎn)生封裝在數(shù)據(jù)報中發(fā)送到Bob的電腦，最后封裝成鏈路層數(shù)據(jù)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(zhuǎn)發(fā)到Bob的電腦上。數(shù)據(jù)報被分用到OS之前創(chuàng)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(fā)送字節(jié)到谷歌，Bob的瀏覽器創(chuàng)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(shù)據(jù)報中然后發(fā)送傳輸?shù)焦雀?，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(wù)器從TCP套接字中讀GET消息，然后創(chuàng)建HTTP回應(yīng)報文，該報文封裝在HTTP回復報文的網(wǎng)頁主體內(nèi)，然后在發(fā)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(shù)據(jù)報通過層層轉(zhuǎn)發(fā)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(yīng)報文段的主體部分提取網(wǎng)頁的html程序，最后顯示網(wǎng)頁。四、 總結(jié)本次計算機網(wǎng)絡(luò)的課程設(shè)計收獲頗多，總體總結(jié)下來可以有以下幾點收獲和體會。26