1.There is no difference. Throughout this text, the words “host” and “end system” are
used interchangeably. End systems include PCs, workstations, Web servers, mail servers, PDAs, Internet-connected game consoles, etc.
2.
3.HFC bandwidth is shared among the users. On the downstream channel, all packets
emanate from a single source, namely, the head end. Thus, there are no collisions in the downstream channel.
1.3.11 与分组交换网络相比，电路交换网路有哪些优点？在电路交换网络中，TDM比FDM有哪些优点？
答：电路交换在连接期间能保持恒定的传输速率，而分组交换则可能因分组多而变的拥塞。而且电路交换在通信所提供的资源会被预留下来。TDM适合数字信号传输，而FDM适合模拟信号传输，因为现在通信网络多是传输数字信号，所以TDM比FDM好。
23.Routers process network, link and physical layers (layers 1 through 3). (This is a little
bit of a white lie, as modern routers sometimes act as firewalls or caching
components, and process Transport layer as well.) Link layer switches process link and physical layers (layers 1 through2). Hosts process all five layers.
4.
a)d prop= m / s seconds.
b)d trans= L / R seconds.
c)d end-to-end=(m / s + L / R)seconds.
d)The bit is just leaving Host A.
e)The first bit is in the link and has not reached Host B.
f)The first bit has reached Host B.
g)Want
m = L
s =
120 (2.5 ⨯108)= 536 km.
56 ⨯10 3
R
第二章：
1.No. In a P2P file-sharing application, the peer that is receiving a file is typically the
client and the peer that is sending the file is typically the server.
2.The process which initiates the communication is the client; the process that waits
to be contacted is the server.
2.2.11 为什么HTTP,FTP,SMTP,POP3都运行在TCP而不是UDP之上？
答：因为这些都要求可靠的数据传输，所以都运行在TCP上。
16.The message is first sent from Alice’s host to her mail server over HTTP. Alice’s
mail server then sends the message to Bob’s mail server over SMTP. Bob then transfers the message from his mail server to his host over POP3.
假定Alice 使用一个基于web的电子邮件账户（如Hotmail或gmail）向Bob发报文，而Bob使用pop3访问他的邮件服务器来获取自己的邮件。讨论报文是怎样从Alice主机到达Bob主机的。列出在两台主机间移动该报文时所使用的各种应用层协议。
答：从Alice的代理到Alice的邮件服务器是HTTP协议，而从Alice的邮件服务器到Bo d的邮件服务器是SMTP协议，从Bob的邮件服务器到Bob的代理是POP3协议。
R25没找到
26.For the TCP application, as soon as the client is executed, it attempts to initiate a TCP
connection with the server. If the TCP server is not running, then the clie nt will fail to make a connection. For the UDP application, the client does not initiate connections (or attempt to communicate with the UDP server) immediately upon execution
2.7.28 对于2.7节所描述的运行在TCP之上的客户机/服务器应用程序，服务器程序为什么必须先于客户机程序运行？对于2.8节所描述的运行在UDP之上的客户机/服务器应用程序，客户机为什么可以先于服务器程序运行？
答：因为运行在TCP之上的客户机/服务器应用程序在客户机运行后，它会尝试向服务器进程发起连接，而此时如果服务器进程没有运行，客户机进程就可能认为服务器不可达，从而返回错误信息；而运行在UDP之上的客户机/服务器应用程序在客户机进程运行之后不会向服务器进程发起连接，而是等待用户输入，因此客户程序可以先于服务器程序运行。
R28没有
Problem 3
Application layer protocols: DNS and HTTP
Transport layer protocols: UDP for DNS; TCP for HTTP
Problem 5
a)The status code of 200 and the phrase OK indicate that the server was able to
locate the document successfully. The reply was provided on Tuesday, 07 Mar 2008 12:39:45 Greenwich Mean Time.
b)The document index.html was last modified on Saturday 10 Dec 2005 18:27:46
GMT.
c)There are 3874 bytes in the document being returned.
d)The first five bytes of the returned document are :
persistent connection, as indicated by the Connection: Keep-Alive field
P5p24没有
第三章：
a. 主机A通过一条TCP连接向主机B发送一个大文件。假设主机B没有数据发往主机A。因为主机B不能随数据捎带确认信息，所以主机B将不向主机A发送确认。
b. 在连接的整个过程中，TCP的RcvWindow的长度不会变化。
c. 假设主机A通过一条TCP连接向主机B发送一个大文件。主机A发送的未被确认的字节数不会超过接收缓存的大小。
