Homework Chapter 2
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《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 <cr><lf> are carriage return and line-feed characters (that is, the italized character string <cr> 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/1.1<cr><lf>Host: <cr><lf>User-Agent: Mozilla/5.0 (Windows;U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) <cr><lf>Accept:ext/xml, application/xml, application/xhtml+xml, text/html;q=0.9,
text/plain;q=0.8,image/png,*/*;q=0.5<cr><lf>Accept-Language:
en-us,en;q=0.5<cr><lf>Accept-Encoding: zip,deflate<cr><lf>Accept-Charset:
ISO-8859-1,utf-8;q=0.7,*;q=0.7<cr><lf>Keep-Alive:
300<cr><lf>Connection:keep-alive<cr><lf><cr><lf>
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?
考虑以下的ASCII字符字符串被Wireshark当浏览器发送一个HTTP GET消息(即。
,这是一个HTTP GET消息的实际内容)。
字符< cr > < >如果是回车和换行符字符(即italized下面的文本字符串< cr >代表单一回车字符包含在这一点上的HTTP头)。
回答下列问题,表明在HTTP GET消息下面你找到答案。
a。
什么是文档的URL请求的浏览器?
b。
浏览器运行HTTP版本是什么?
c。
浏览器请求一个非持久性或持久连接?
d。
什么是浏览器的IP地址的主机运行吗?
e。
什么样的浏览器启动这个消息吗?为什么浏览器类型所需的HTTP请求消息吗?
浏览器类型信息需要由服务器发送同一对象的不同版本,不同类型的浏览器。
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 OK<cr><lf>Date: Tue, 07 Mar 2008 12:39:45GMT<cr><lf>Server: Apache/2.0.52 (Fedora) <cr><lf>Last-Modified: Sat, 10 Dec2005 18:27:46 GMT<cr><lf>ETag:
526c3-f22-a88a4c80”<cr><lf>Accept-Ranges: bytes<cr><lf>Content-Length:
3874<cr><lf>Keep-Alive: timeout=max=100<cr><lf>Connection: Keep-Alive<cr><lf>Content-Type: text/html; charset=ISO-8859-1<cr><lf><cr><lf><!doctype html public “-//w3c//dtd html 4.0 transitional//en”><lf><html><lf><head><lf> <meta http-equiv=”Content-Type”content=”text/html; charset=iso-8859-1”><lf> <metaname=”GENERATOR” content=”Mozilla/4.79 [en] (Windows NT 5.0; U) Netscape]”><lf> <title>CMPSCI 453 / 591 /NTU-ST550A Spring 2005
homepage</title><lf></head><lf><much more document text following here (not shown)>
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?
下面的文本显示来自服务器的应答发送响应HTTP GET消息在上面的问题中。
回答下列问题,显示下面的消息,你找到答案。
一个。
服务器能够成功地找到文档吗?文档回复提供时间是什么?
b。
最后修改的文档是什么时候?
c。
有多少字节的文档返回?
d。
第一个是什么5字节的文档返回?服务器同意一个持久连接吗?
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 βis the arrival rate of objects to the access link.
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.
考虑下面的图,有一个机构网络连接到互联网。
假设对象的平均尺寸是900 kb,平均请求率从机构的浏览器起源服务器每秒15请求。
还假设的时间在互联网上需要从路由器的访问链接转发HTTP请求,直到接收到响应平均
3 s。
模型总平均响应时间平均访问延迟之和(即延迟从互联网路由器到路由器)机构和平均网络延迟。
平均
访问延迟,使用Δ/(1-Δβ),Δ发送对象所需的平均时间的访问链接和β的到达率是对象的访问链接。
发现总平均响应时间。
b。
现在假设一个缓存是安装在机构局域网。
想错过率是0.4。
假设命中率增加到0.95。
发现总响应时间。
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.)
打印头你最近收到的一封电子邮件。
多少接受:头行吗?分析每个头线的消息。
(允许打印头文件的电子邮件。
)
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.
为SMTP读RFC 5321。
MTA代表什么?考虑以下收到垃圾邮件从一个真正的垃圾邮件(修改)。
假设只malacious 这个垃圾邮件的发起者和所有其他主机是诚实的,识别malacious主机生成的这个垃圾邮件。
From - Fri Nov 07 13:41:30 2008
Return-Path:<********************>
Received: from
([128.119.240.3])(8.13.1/8.12.6)for<***********.edu>;Fri,7 Nov 2008 13:27:10 -0500
Received: from asusus-4b96 (localhost [127.0.0.1]) by (Spam Firewall) for <***********.edu>;Fri,7Nov200813:27:07-0500(EST)
Received:fromasusus-4b96([58.88.21.177])for<***********.edu>;Fri,07 Nov 2008 13:27:07 -0500 (EST)
Received: from [58.88.21.177] by ; Sat, 8 Nov 2008 01:27:07 +0700 From:"Jonny"<********************>
To:<***********.edu>
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代表邮件传输代理。
邮件转发由源到MTA然后遵循一系列放在到达接收方的邮件阅读器。
我们看到这个垃圾邮件之前放在链。
一个诚实的MTA应该报告接收消息。
注意,在这封邮件中,“asusus-4b96([58.88.21.177])“不报告接收电子邮件。
当我们假定唯一的发起人是不诚实的,所以“asusus-4b96([58.88.21.177])”必须发起人。
6. Answer the following question, using the WHOIS Tools (such as, SmartWhois, WhereIsIP ,CountryWhois, /) 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, , , , )
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,. , , ).
回答下列问题,使用WHOIS工具(如SmartWhois WhereIsIP,CountryWhois,/)和网路资讯查询。
一。
什么是数据库?
b。
使用各种whois数据库在互联网上获得两个DNS服务器的名称。
显示您使用whois数据库。
(例如,,,,)
c。
在本地主机上使用网路资讯查询发送DNS查询三个DNS服务器:本地DNS服务器和两个DNS服务器(b)部分中你发现。
试着查询类型,NS,MX报告。
总结你的发现。
d。
使用nslookup找到Web服务器有多个IP地址。
BUPT的Web服务器有多个IP地址?
e。
使用后面瞎跑whois数据库,以确定你所使用的IP地址范围的机构(如。
,,)。
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.
假设你可以访问本地的缓存DNS服务器你的部门。
你能提出一个方法大致确定Web服务器(本部门以外的)中最受欢迎的用户在你的部门吗?你能想出一个方式来确定外部网站很可能从你们部门的计算机访问几秒钟前?解释一下。
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.
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?
在我们覆盖的覆盖网络，使用查询洪水，我们在一些细节描述了如何一个新的对等连接的覆盖网络。
在这个问题上，我们要探索发生什么情况时，一个对等网的覆盖网络。
假设每个参与节点保持TCP连接的至少四个不同的节点都。
想点X，其中有五个TCP连接到其他的同行，想离开。
首先考虑一个优雅的离去，是这样的，同行的X显式地关闭应用，从而优雅地关闭其五的TCP连接。
五个以前连接的同行会采取什么行动？
现在假设同伴X突然断开与互联网没有通知其五个邻居是关闭TCP连接。
会发生什么？
a. 在这种情况下，这五个客户都立即知道它们少了一个邻居。
考虑五个客户中的一个，比如A。
假设当X 离开后，A只有三个邻居。
这是A需要同另一个对等点建立TCP 连接。
A要有一个活跃对等点的最新列表；他不断地连接列表中地对等点知道其中一个接收它的TCP 连接请求。
b. 在这种情况下，Bob 不能立即知道X 已经离开了。
只有当他尝试向X 发送信息（query 或ping）时，Bob 才会知道X 已经离开。
当Bob 尝试发送信息时，Bob 的TCP 将会产生数个不成功的连接信息。
这时Bob 的TCP 将会通知Gnutella 客户机X 已经离开。
然后Bob 将会尝试与一个新的对等点建立TCP 连接，以此重建第五个连接.
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?
什么是覆盖网络？它包括路由器吗？覆盖网络中的边是什么？查询的洪水覆盖网络的创建和维护？
11. What is the difference between Network Architecture and Application Architecture?
网络体系结构和应用架构有什么区别？
12. List five non-proprietary Internet applications and the application-layer protocols, transport-layer protocols that they use.
列出五个非专利的互联网应用程序和应用层协议，传输层协议，它们使用。
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?
假设你想尽可能快速地从远程客户端到服务器的事务处理。
你会使用UDP或TCP？为什么？
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?
对于客户端-服务器应用程序的TCP服务器程序，为什么必须在客户端程序执行？对于CS应用UDP，为什么客户端程序在服务器程序执行？。