Computer Network 2015fall
Homework 2
1. Why is it said that FTP sends control information “out-of-band”?
Ans: FTP uses two parallel TCP connections, one connection for sending control
information (such as a request to transfer a file) and another connection for actually
transferring the file. Because the control information is not sent over the same
connection that the file is sent over, FTP sends control information out of band.
2. In Section 2.7, the UDP server described needed only one socket, whereas the
TCP server needed two sockets. Why? If the TCP server were to support n
simultaneous connections, each from a different client host, how many sockets
would the TCP server need? P116 27 题
Ans: With the UDP server, there is no welcoming socket, and all data from different
clients enters the server through this one socket. With the TCP server, there is a
welcoming socket, and each time a client initiates a connection to the server, a new socket
is created. Thus, to support n simultaneous connections, the server would need n+1
sockets.
3. True or False.
a) Suppose a user requests a Web page that consists of some text and two images. For this
page, the client will send one request message and recieve three response messages.
False.
b) Two distinct Web pages (e.g., www.mit.edu/research.html and www.mit.edu/students.html)
can be sent over the same persistent connection.
True.
c) With non-persistent connections between browser and origin server, it is possible for a
single TCP segment to carry two distinct HTTP request messages.
False.
d) The Date: header in the HTTP response message indicates when the object in the response
was last modified.
False.
4. What's iterated query and recursive query?
iterated query:
 contacted server replies with name of server to contact
 “I don’t know this name, but ask this server”
recursive query:
 puts burden of name resolution on contacted name server
 heavy load?
5. Suppose within your web browser you click on a link to obtain a web page. Suppose that
the IP address for the associated URL is not cached in your local host, so that a DNS look up
is necessary to obtain the IP address. Suppose that n DNS servers are visited before your
host receives the IP address from DNS; the successive visits incur a RTT of RTT1, ..., RTTn.
Further suppose that web page associated with the link contains exactly one object, a small
amount of HTML text. Let RTT0 denote the RTT between the local host and the server
containing the object. Assuming zero transmission time of the object, how much time elapses
from when the client clicks on the link until the client receives the object.
The total amount of time to get the IP address is
RTT1 + RTT2 + …+ RTTn
Once the IP address is known, RTT0 elapses to set up the TCP connection and another
RTT0 elapses to request and receive the small object. The total response time is
RTT1 + RTT2 + …+ RTTn + 2 * RTT0.
6. Referring to question 4, suppose the page contains three very small objects. Neglecting
transmission times, how much time elapses with
(a) nonpersistent HTTP with no parallel TCP connections,
Ans : RTT1 + RTT2 + … + RTTn + RTT0 * 6 +2 * RTT0
(b)nonpersistent HTTP with parallel connections,
Ans : RTT1 + RTT2 + … + RTTn + RTT0 * 2 +2 * RTT0
(c) persistent HTTP with pipelining.
Ans : RTT1 + RTT2 + … + RTTn + RTT0 +2 * RTT0