《Computer Networks and Internet》Chapter 21. Consider the following string of ASCII characters that w

《Computer Networks and Internet》

Chapter 2

1. Consider the following string of ASCII characters that were captured by Wireshark when the browser sent an HTTP GET message (i.e., this is the actual content of an HTTP GET message). The characters are carriage return and line-feed characters (that is, the italized character string in the text below represents the single carriage-return character that was contained at that point in the HTTP header). Answer the following questions, indicating where in the HTTP GET message below you find the answer.

GET /cs453/index.html HTTP/1.1Host: gaia.cs.umass.eduUser-Agent:

Mozilla/5.0 (Windows;U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) Accept:ext/xml, application/xml, application/xhtml xml, text/html;q=0.9,

text/plain;q=0.8,image/png,*/*;q=0.5Accept-Language:

en-us,en;q=0.5Accept-Encoding: zip,deflateAccept-Charset:

ISO-8859-1,utf-8;q=0.7,*;q=0.7Keep-Alive:

300Connection:keep-alive

a. What is the URL of the document requested by the browser?

b. What version of HTTP is the browser running?

c. Does the browser request a non-persistent or a persistent connection?

d. What is the IP address of the host on which the browser is running?

e. What type of browser initiates this message? Why is the browser type needed in an HTTP request message?

考慮以下的ASCII 字符字符串被Wireshark 當瀏覽器發(fā)送一個HTTP GET消息(即。, 這是一個HTTP GET消息的實際內(nèi)容) 。字符< cr > < >如果是回車和換行符字符(即italized 下面的文本字符串< cr >代表單一回車字符包含在這一點上的HTTP 頭) ?；卮鹣铝袉栴}, 表明在HTTP GET消息下面你找到答案。

a 。什么是文檔的URL 請求的瀏覽器?

b 。瀏覽器運行HTTP 版本是什么?

c 。瀏覽器請求一個非持久性或持久連接?

d 。什么是瀏覽器的IP 地址的主機運行嗎?

e 。什么樣的瀏覽器啟動這個消息嗎? 為什么瀏覽器類型所需的HTTP 請求消息嗎

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瀏覽器類型信息需要由服務(wù)器發(fā)送同一對象的不同版本, 不同類型的瀏覽器。

2. The text below shows the reply sent from the server in response to the HTTP GET message in the question above. Answer the following questions, indicating where in the message below you find the answer.

HTTP/1.1 200 OKDate: Tue, 07 Mar 2008 12:39:45GMTServer: Apache/2.0.52 (Fedora) Last-Modified: Sat, 10 Dec2005 18:27:46 GMTETag:

526c3-f22-a88a4c80”Accept-Ranges: bytesContent-Length:

3874Keep-Alive: timeout=max=100Connection: Keep-AliveContent-Type: text/html; charset=ISO-8859-1

transitional//en”>

a. Was the server able to successfully find the document or not? What time was the document reply provided?

b. When was the document last modified?

c. How many bytes are there in the document being returned?

d. What are the first 5 bytes of the document being returned? Did the server agree to a persistent connection?

下面的文本顯示來自服務(wù)器的應(yīng)答發(fā)送響應(yīng)HTTP GET 消息在上面的問題中。回答下列問題, 顯示下面的消息, 你找到答案。

一個。服務(wù)器能夠成功地找到文檔嗎? 文檔回復(fù)提供時間是什么?

b 。最后修改的文檔是什么時候?

c 。有多少字節(jié)的文檔返回?

d 。第一個是什么5字節(jié)的文檔返回? 服務(wù)器同意一個持久連接嗎?

3. Consider following Figure, for which there is an institutional network connected to the Internet. Suppose that the average object size is 900kb and that the average re quest rate from the institution’s browsers to the origin servers is 15 requests per second. Also suppose that the amount of time it takes from when the router on the Internet side of the access link forwards an HTTP request until it receives the response is 3s on average. Model the total average response time as the sum of the average access delay (that is, the delay from Internet router to institution router) and the average Internet delay. For the average access delay, use Δ/(1–Δβ), where Δ is the average time required to send an object over the access link and β is the arrival rate of objects to the access link.

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Figure 1 Bottleneck between an institutional network and the Internet

a. Find the total average response time.

b. Now suppose a cache is installed in the institutional LAN. Suppose the miss rate is 0.4. Suppose hit rate increases to 0.95. Find the total response time.

考慮下面的圖, 有一個機構(gòu)網(wǎng)絡(luò)連接到互聯(lián)網(wǎng)。假設(shè)對象的平均尺寸是900 kb,平均請求率從機構(gòu)的瀏覽器起源服務(wù)器每秒15請求。還假設(shè)的時間在互聯(lián)網(wǎng)上需要從路由器的訪問鏈接轉(zhuǎn)發(fā)HTTP 請求, 直到接收到響應(yīng)平均3 s 。模型總平均響應(yīng)時間平均訪問延遲之和(即延遲從互聯(lián)網(wǎng)路由器到路由器) 機構(gòu)和平均網(wǎng)絡(luò)延遲。平均訪問延遲, 使用Δ/(1-Δβ),Δ發(fā)送對象所需的平均時間的訪問鏈接和β的到達率是對象的訪問鏈接。

發(fā)現(xiàn)總平均響應(yīng)時間。

b ?，F(xiàn)在假設(shè)一個緩存是安裝在機構(gòu)局域網(wǎng)。想錯過率是0.4。假設(shè)命中率增加到0.95。發(fā)現(xiàn)總響應(yīng)時間。

4. Print out the header of an e-mail message you have recently received. How many Received : header lines are there? Analyze each of the header lines in the message. (Allow printing of an email header file.)

打印頭你最近收到的一封電子郵件。多少接受:頭行嗎? 分析每個頭線的消息。(允許打印頭文件的電子郵件。)

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5. Read RFC 5321 for SMTP. What does MTA stand for? Consider the following received spam email (modified from a real spam email). Assuming only the originator of this spam email is malacious and all other hosts are honest, identify the malacious host that has generated this spam email.

為SMTP 讀RFC 5321。MTA 代表什么? 考慮以下收到垃圾郵件從一個真正的垃圾郵件(修改) 。假設(shè)只malacious 這個垃圾郵件的發(fā)起者和所有其他主機是誠實的, 識別malacious 主機生成的這個垃圾郵件。

From - Fri Nov 07 13:41:30 2008

Return-Path:

Received: from barmail.cs.umass.edu

(barmail.cs.umass.edu [128.119.240.3]) by cs.umass.edu (8.13.1/8.12.6) for ; Fri, 7 Nov 2008 13:27:10 -0500

Received: from asusus-4b96 (localhost [127.0.0.1]) by barmail.cs.umass.edu (Spam Firewall) for ; Fri, 7 Nov 2008 13:27:07 -0500 (EST)

Received: from asusus-4b96 ([58.88.21.177]) by barmail.cs.umass.edu for ; Fri, 07 Nov 2008 13:27:07 -0500 (EST)

Received: from [58.88.21.177] by inbnd55.exchangeddd.com; Sat, 8 Nov 2008 01:27:07 0700 From: "Jonny"

To:

Subject: How to secure your savings

MTA stands for Mail Transfer Agents. A mail is forwarded by a source to a MTA and then it follows a

sequence of MTAs to reach the receiver’s mail reader. We see that this spam email follows a chain of MTAs. An honest MTA should report where it receives the message. Notice that in this email, “asusus-4b96 ([58.88.21.177])” does not report where it receives the email. As we assume that the only the originator is dishonest, so “asusus-4b96 ([58.88.21.177])” must be the originator.

MTA 代表郵件傳輸代理。郵件轉(zhuǎn)發(fā)由源到MTA 然后遵循一系列放在到達接收方的郵件閱讀器。我們看到這個垃圾郵件之前放在鏈。一個誠實的MTA 應(yīng)該報告接收消息。注意, 在這封郵件中,“asusus-4b96([58.88.21.177])“不報告接收電子郵件。當我們假定唯一的發(fā)起人是不誠實的, 所以“asusus-4b96([58.88.21.177])”必須發(fā)起人。

6. Answer the following question, using the WHOIS Tools (such as, SmartWhois, WhereIsIP ,CountryWhois, http://whois.cndns.com/) and nslookup.

a. What is a whois database?

b. Use various whois databases on the Internet to obtain the names of two DNS servers. Indicate which whois databases you used. (such as, www.mit.edu, www.zaobao.com, www.yahoo.com, www.bupt.edu.cn)

c. Use nslookup on your local host to send DNS queries to three DNS servers: your local DNS server and the two DNS servers you found in part (b). Try querying for Type A, NS, and MX reports. Summarize your findings.

d. Use nslookup to find a Web server that has multiple IP addresses. Does the Web server of BUPT have multiple IP addresses?

e. Use the ARIN whois database to determine the IP address range used by your institution (such as,. www.mit.edu, www.google.com, www.bupt.edu.cn).

回答下列問題, 使用WHOIS 工具(如SmartWhois WhereIsIP ,CountryWhois,http://whois.cndns.com/)和網(wǎng)路資訊查詢。

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一。什么是數(shù)據(jù)庫?

b 。使用各種whois 數(shù)據(jù)庫在互聯(lián)網(wǎng)上獲得兩個DNS 服務(wù)器的名稱。顯示您使用whois 數(shù)據(jù)庫。(例如,www.mit.edu,www.mit.edu,www.yahoo.com,www.bupt.edu.cn)

c 。在本地主機上使用網(wǎng)路資訊查詢發(fā)送DNS 查詢?nèi)齻€DNS 服務(wù)器:本地DNS 服務(wù)器和兩個DNS 服務(wù)器(b)部分中你發(fā)現(xiàn)。試著查詢類型,NS,MX 報告?？偨Y(jié)你的發(fā)現(xiàn)。

d 。使用nslookup 找到Web 服務(wù)器有多個IP 地址。BUPT 的Web 服務(wù)器有多個IP 地址?

e 。使用后面瞎跑whois 數(shù)據(jù)庫, 以確定你所使用的IP 地址范圍的機構(gòu)(如。www.mit.edu,www.google.com,www.bupt.edu.cn) 。

7. Suppose you can access the caches in the local DNS servers of your department. Can you propose a way to roughly determine the Web servers (outside your department) that are most popular among the users in your department? Can you come up a way to determine if an external Web site was very likely accessed from a computer in your department a couple of seconds ago? Explain.

假設(shè)你可以訪問本地的緩存DNS 服務(wù)器你的部門。你能提出一個方法大致確定Web 服務(wù)器(本部門以外的) 中最受歡迎的用戶在你的部門嗎? 你能想出一個方式來確定外部網(wǎng)站很可能從你們部門的計算機訪問幾秒鐘前? 解釋一下。

8. Consider distributing a file of F = 15 Gbits to N peers. The server has an upload rate of u s = 30 Mbps, and each peer has a download rate of d i = 2 Mbps and an upload rate of u . For N = 10, 100, and 1,000 and u = 300 Kbps, 700 Kbps, and 2 Mbps, prepare a chart giving the minimum distribution time for each of the combinations of N and u for both client-server distribution and P2P distribution.

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9. In our coverage of an overlay networks using query flooding, we described in some detail how a new peer joins the overlay network. In this problem we want to explore what happens when a peer leaves the overlay network. Suppose every participating peer maintains TCP connections to at least four distinct peers at all times. Suppose Peer X, which has five TCP connections to other peers, wants to leave.

a. First consider the case of a graceful departure, that is, Peer X explicitly closes its application, thereby gracefully closing its five TCP connections. What actions would each of the five formerly connected peers take?

b. Now suppose that Peer X abruptly disconnects from the Internet without notifying its five neighbors that it is closing the TCP connections. What would happen?

在我們覆蓋的覆蓋網(wǎng)絡(luò)，使用查詢洪水，我們在一些細節(jié)描述了如何一個新的對等連接的覆蓋網(wǎng)絡(luò)。在這個問題上，我們要探索發(fā)生什么情況時，一個對等網(wǎng)的覆蓋網(wǎng)絡(luò)。假設(shè)每個參與節(jié)點保持TCP 連接的至少四個不同的節(jié)點都。想點X ，其中有五個TCP 連接到其他的同行，想離開。

首先考慮一個優(yōu)雅的離去，是這樣的，同行的X 顯式地關(guān)閉應(yīng)用，從而優(yōu)雅地關(guān)閉其五的TCP 連接。五個以前連接的同行會采取什么行動？

現(xiàn)在假設(shè)同伴X 突然斷開與互聯(lián)網(wǎng)沒有通知其五個鄰居是關(guān)閉TCP 連接。會發(fā)生什么？

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a. 在這種情況下，這五個客戶都立即知道它們少了一個鄰居?？紤]五個客戶中的一個，比如A 。假設(shè)當 X 離開后，A 只有三個鄰居。這是A 需要同另一個對等點建立 TCP 連接。A 要有一個活躍對等點的最新列表；他不斷地連接列表中地對等點知道其中一個接收它的 TCP 連接請求。

b. 在這種情況下，Bob 不能立即知道 X 已經(jīng)離開了。只有當他嘗試向 X 發(fā)送信息（query 或 ping ）時，Bob 才會知道 X 已經(jīng)離開。當 Bob 嘗試發(fā)送信息時，Bob 的 TCP 將會產(chǎn)生數(shù)個不成功的連接信息。這時 Bob 的 TCP 將會通知 Gnutella 客戶機 X 已經(jīng)離開。然后 Bob 將會嘗試與一個新的對等點建立 TCP 連接，以此重建第五個連接.

10. What is an overlay network? Does it include routers? What are the edges in the overlay network? How is the query-flooding overlay network created and maintained?

什么是覆蓋網(wǎng)絡(luò)？它包括路由器嗎？覆蓋網(wǎng)絡(luò)中的邊是什么？查詢的洪水覆蓋網(wǎng)絡(luò)的創(chuàng)建和維護？

11. What is the difference between Network Architecture and Application Architecture?

網(wǎng)絡(luò)體系結(jié)構(gòu)和應(yīng)用架構(gòu)有什么區(qū)別？

12. List five non-proprietary Internet applications and the application-layer protocols, transport-layer protocols that they use.

列出五個非專利的互聯(lián)網(wǎng)應(yīng)用程序和應(yīng)用層協(xié)議，傳輸層協(xié)議，它們使用。

13. Suppose you wanted to do a transaction from a remote client to a server as fast as possible. Would you use UDP or TCP? Why?

假設(shè)你想盡可能快速地從遠程客戶端到服務(wù)器的事務(wù)處理。你會使用UDP 或TCP

？為什么？

14. For the client-server application over TCP, why must the server program be executed before the client program? For the clientserver application over UDP, why may the client program be executed before the server program?

對于客戶端-服務(wù)器應(yīng)用程序的TCP 服務(wù)器程序，為什么必須在客戶端程序執(zhí)行？對于CS 應(yīng)用UDP ，為什么客戶端程序在服務(wù)器程序執(zhí)行？

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