【正文】 IP destination address indicates that this IP datagram should be processed by upper layer protocols at this node, so the datagram’s payload (a UDP segment) is thus demultiplexed up to UDP, and the DHCP request message is extracted from the UDP segment. The DHCP server now has the DHCP request message.路由器用接口MAC地址為00:22:6B：45:1F：1B接收包含DHCP請求報(bào)文的廣播以太網(wǎng)幀，然后從以太網(wǎng)幀中提取IP報(bào)文。以太網(wǎng)幀有一個(gè)連接在路由器接口源MAC地址00:22:6B:45:1F:1B和Bob電腦的目的MAC地址00:16:D3:23:68:8A。注意只有最后兩個(gè)DHCP步驟在第四章中提出的四個(gè)步驟實(shí)際上是必要的。然而即使Bob的電腦可以通過DHCP ，但還是無法知道其MAC地址。注意在這個(gè)幀的IP報(bào)文段中有目的IP為DNS Server的IP，目的MAC為網(wǎng)關(guān)路由器的MAC。DNS服務(wù)器形成一個(gè)DNS回復(fù)報(bào)文，其中包含主機(jī)名到地址的映射，封裝之后發(fā)到Bob的電腦上。調(diào)用這些路由轉(zhuǎn)發(fā)表的進(jìn)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。HTTP GET消息寫進(jìn)套接字，和GET消息一起變成TCP報(bào)文段的有效載荷的一部分。26。數(shù)據(jù)報(bào)被分用到OS之前創(chuàng)建的TCP套接字，進(jìn)行匹配。因此Bob的電腦第一次必須創(chuàng)建TCP SYN報(bào)文段到HTTP服務(wù)器，封裝數(shù)據(jù)報(bào)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(jī)。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(bào)文的IP數(shù)據(jù)報(bào)到達(dá)DNS服務(wù)器。12. Bob’s laptop receives the frame containing the ARP reply message and extracts the MAC address of the gateway router (00:22:6B:45:1F:1B) from the ARP reply message.Bob的電腦接收包含ARP請求報(bào)文的幀并從ARP請求報(bào)文中提取網(wǎng)關(guān)路由器的MAC地址。DNS報(bào)文被封裝在UDP數(shù)據(jù)報(bào)中，目的端口號為53.，9. Bob’s laptop then places the datagram containing the DNS query message in an Ethernet frame. This frame will be sent (addressed, at the link layer) to the gateway router in bob’s school’s network. However, even though Bob’s laptop knows the IP address of the school’s gateway router () via the DHCP ACK message in step 5 above, it doesn’t know the gateway router’s MAC addressIn order to obtain the MAC address of the gateway router, Bob’s laptop will need to use the ARP protocolBob的電腦將包含DNS請求報(bào)文的數(shù)據(jù)封裝在以太網(wǎng)幀頭中。Bob的電腦開始發(fā)送帶有目的地址的數(shù)據(jù)報(bào)到默認(rèn)網(wǎng)關(guān)。DHCP服務(wù)器產(chǎn)生一個(gè)ACK報(bào)文其中包含這個(gè)IP地址，也包含DNS服務(wù)器的IP（(），默認(rèn)網(wǎng)關(guān)的IP（）。以太網(wǎng)幀的默認(rèn)目的MAC地址為FF:FF:FF:FF:FF:FF，以至于以太網(wǎng)幀能廣播到連接在交換機(jī)（準(zhǔn)確的說應(yīng)該是DHCP服務(wù)器）的全部設(shè)備上，然后幀的源MAC地址是Bob的電腦：00:16:D3:23:68:8A.3. The broadcast Ethernet frame containing the DHCP request is the first frame sent by Bob’s laptop to the Ethernet switch. The switch broadcasts the ining frame on all outgoing ports, including the port connected to the router. Bob的電腦發(fā)到以太網(wǎng)交換機(jī)的第一幀是包括DHCP請求報(bào)文的廣播幀。 } }}
、流量分析（主要是分析時(shí)延、帶寬和抖動(dòng)率）三、E to C TranslationA Day in the Life of a Web Page Request網(wǎng)頁請求的流程前言：本章我們主要討論鏈路層，我們的網(wǎng)絡(luò)之路從協(xié)議棧起步。 last_delay用于保存上一分組的延遲時(shí)間 last_seqno=0。 } if(highest_uid uid) highest_uid = uid。 trace_type==AGT amp。 pkt_type = $5。 if(len == 3) { node_ = substr(node,2,1)。 更新接收到的最新一個(gè)分組的時(shí)間 i++。 } if(len==1) 如果目的節(jié)點(diǎn)接收到分組 { if(event==r amp。 pkt_type = $7。 } { event = $1。i=highest_uid。 node_==2 amp。amp。 有線trace第12個(gè)字段$12為分組的uid}if(len==3) 如果分析的是無線trace記錄if(event==s amp。 無線trace第6個(gè)字段$6為分組的uidpkt_type=$7。啟動(dòng)nam exit 0}puts Starting Simulation...啟動(dòng)調(diào)度器$ns_ run2．NAM動(dòng)畫顯示、awk文件分析、 文件BEGIN { highest_uid=0。此程序無用。 路由協(xié)議set val(x) 500 。 無線信道類型set val(prop) Propagation/TwoRayGround 。定義tcp窗口值 puts $f0 $now $cwnd 。在tcp連接上建立FTP應(yīng)用程序$ftp attachagent $tcp 。清空跟蹤緩沖區(qū) close $tracefd 。中國地質(zhì)大學(xué)(武漢)計(jì)算機(jī)學(xué)院計(jì)算機(jī)網(wǎng)絡(luò)原理課程設(shè)計(jì)報(bào)告姓 名：學(xué) 號：聯(lián)系方式：郵 件：指導(dǎo)老師：陳 喆二○一二年七月目錄一、 WireShark抓包分析TCP三次握手和四次握手實(shí)驗(yàn)實(shí)驗(yàn)思路………………………………………………………………………………...3實(shí)驗(yàn)重點(diǎn)…………………………………………………………………………………3實(shí)驗(yàn)內(nèi)容…………………………………………………………………………………tcp第一次握手…………………………………………………………………………………tcp第二次握手…………………………………………………………………………………tcp第三次握手…………………………………………………………………………………第一次請求數(shù)據(jù)……………………………………………………………………………….第一次回復(fù)數(shù)據(jù)……………………………………………………………………………….tcp斷開連接第一次握手………………………………………………………………….tcp斷開連接第二次握手………………………………………………………………….tcp斷開連接第三次握手………………………………………………………………….tcp斷開連接第四次握手……………………………………...............................圖形顯示..............................................................................................8 二、 NS2仿真模擬真實(shí)網(wǎng)絡(luò)場景實(shí)驗(yàn)有線場景下慢啟動(dòng)+滑動(dòng)窗口實(shí)驗(yàn)、tcl文件……………………………………………………………………………………………..NAM動(dòng)畫顯示………………………………………………………………………………….graph流量分析………………………………………………………………………………..12有線節(jié)點(diǎn)與無線節(jié)點(diǎn)的混合仿真模擬實(shí)驗(yàn)、tcl文件…………………………………………………………………………………………….1NAM動(dòng)畫顯示…………………………………………………………………………………1awk文件分析…………………………………………………………………………………..1graph流量分析………………………………………………………………………………..19無線節(jié)點(diǎn)移動(dòng)仿真模擬（自己的成果）、tcl文件…………………………………………………………………………………………….NAM動(dòng)畫顯示…………………………………………………………………………………24三、 E to C Translation…………………………………………………………………………………25四、總結(jié)…………………………………………………………………………………………………………….32一、WireShark抓包分析TCP三次握手實(shí)驗(yàn)實(shí)驗(yàn)思路：、訪問學(xué)校主頁服務(wù)器，通過WireShark捕獲通信內(nèi)容、分析TCP連接建立的三次握手和連接釋放的四次握手過程實(shí)驗(yàn)重點(diǎn):、瀏覽網(wǎng)頁，抓取三次我輸?shù)陌?，并根?jù)TCP包頭格式記錄各字段取值情況、TCP包頭格式如下：