Assignment#2 MMIS 653 Online (Fall 2018)
Total points: 60
Due date: 10/14/2018 11:59PM
1, (2 points) Consider a TCP connection between Host A and Host B. Suppose that the
TCP segments traveling from Host A to Host B have source port number 10526 and
destination port number 80. What are the source and destination port numbers for
segments traveling from Host B to Host A?
2, (5 points) Suppose a process in Host C has a UDP socket with port number 7890,
Suppose both Host A and Host B each send a UDP segment to Host C with destination
port number 7890. Will both of these segments be directed to the same socket at Host C?
If so, how will the process at Host C know that these two segments originated from two
different hosts?
3, (5 points) Suppose that a Web server runs in Host C on port 80. Suppose this Web
server uses persistent connections, and is currently receiving requests from two different
Hosts, A and B. Are all of the requests being sent through the same socket at Host C? If
they are being passing through different sockets, do both of the sockets have port 80?
Discuss and explain.
4, True or false? Please explain your answers. (10 points)
a, Host A is sending Host B a large file over a TCP connection. Assume Host B has no
data to send Host A. Host B will not send acknowledgements to Host A because Host B
cannot piggyback the acknowledgements on data.
b, The size of the TCP RcvWindow never changes throughout the duration of the
connection.
c, Suppose Host A is sending Host B a large file over a TCP connection. The number of
unacknowledged bytes that A sends to B cannot exceed the size of the receive buffer in
Host B.
d, Suppose Host A is sending a large file to Host B over a TCP connection. If the
sequence number for a segment of this connection is m, then the sequence number for the
subsequent segment must be m+1.
e, The TCP segment has a field in its header for RcvWindow.
5, (8 points) Suppose Host A sends two TCP segments back to back to Host B over a
TCP connection. The first segment has sequence number 408; the second has sequence
number 808.
a, How much data is in the first segment?
b, Suppose that the first segment is lost but the second segment arrives at B. In the
acknowledgement that Host B sends back to Host A, what will be the acknowledgement
number?
6, (8 points) UDP and TCP use 1s complement for their checksums. Suppose you have
the following three 8-bit bytes: 01011111, 01110000, 11001001. What is the 1s
complement of the sum of these 8-bit bytes? (Note that although UDP and TCP use 16-
bit words in computing the checksum, for this problem you are being asked to consider 8-
bit sums.) Show all work. Why is it that UDP takes the 1s complement of the sum; that is.
Why not just use the sum? With the 1s complement schema, how does the receiver detect
errors? Is it possible that a 1-bit error will go undetected? How about 2-bit error?
7, (12 points) Consider the following plot of TCP window size as a function of time.
Assume TCP Reno is the protocol experiencing the behavior shown above, answer the
following questions. In all cases, you should provide a short discussion justifying your
answer.
a, Is TCP Congestion avoidance operating in interval [6, 16]? Why or why not?
b, Is TCP congestion avoidance operating in interval [23, 26]? Why or why not?
c, After the 16th transmission round, is segment loss detected by a triple duplicate ACK or
by a timeout? Why?
d, After the 22nd transmission round, is segment loss detected by a triple duplicate ACK
or by a timeout? Why?
e, What is the value of Threshold at the 25th transmission round?
f, What is the value of Threshold at the 6th transmission round?
8. (10 points) Socket Programming Assignment: Web Server (page 181)
In this lab, you will learn the basics of socket programming for TCP connections in
Python: how to create a socket, bind it to a specific address and port, as well as send and
receive a HTTP packet. You will also learn some basics of HTTP header format.
You will develop a web server that handles one HTTP request at a time. Your web server
should accept and parse the HTTP request, get the requested file from the server’s file
system, create an HTTP response message consisting of the requested file preceded by
header lines, and then send the response directly to the client. If the requested file is not
present in the server, the server should send an HTTP “404 Not Found” message back to
the client.
Code Below you will find the skeleton code for the Web server. You are to complete the
skeleton code. The places where you need to fill in code are marked with #Fill in start
and #Fill in end. Each place may require one or more lines of code.
Running the Server Put an HTML file (e.g., HelloWorld.html) in the same directory that the server is in. Run
the server program. Determine the IP address of the host that is running the server (e.g.,
128.238.251.26). From another host or the same host if you run the server on the local
manchine, open a browser and provide the corresponding URL. For example:
‘HelloWorld.html’ is the name of the file you placed in the server directory. Note also the
use of the port number after the colon. You need to replace this port number with
whatever port you have used in the server code. In the above example, we have used the
port number 6789. The browser should then display the contents of HelloWorld.html. If
you omit “:6789”, the browser will assume port 80 and you will get the web page from
the server only if your server is listening at port 80.
You can also run the server program on your own PC and access the webserver from the
same PC using the IP address for local host (127.0.0.1). For example:
Then try to get a file that is not present at the server. You should get a “404 Not Found”
message.
What to Hand in You will hand in the complete server code (a .py file) along with the screen shots of your
client browser, verifying that you actually receive the contents of the HTML file from the
server.
Skeleton Python Code for the Web Server (Use the code in the skeleton.py file as the
skeleton of your .py file and insert lines of code needed between #Fill in state and #Fill
in end. Please note that Python programs get structured through indentation, copy and
paste may change the indentation of the following lines. You are suggested to use Python
3.x.
