File Transfer Lab 3

ECE361 Computer Networks
File Transfer Lab
Objective
The following practical labs provide you with some hands-on experience with socket
programming. You will use UNIX sockets to implement simple client and server
programs which interact with each other to accomplish a file transfer in a connectionless
manner.
Lab Assignment
In this assignment, you need to implement a server that opens a socket and listens for
incoming data transfer at a particular port number. You also need to implement a client
that reads a binary file from the file system and transfers the file to the server. When the
server receives the client’s data, it writes the data to a file.
References
 Related sections in Chapter 2 of the course textbook.
 The network socket programming “Beej’s Guide to Network Programming”
posted on the course website.
 Section 2.4 of the "Communications Networks", by Leon-Garcia and Widjaja, McGraw
Hill, 2nd Ed., 2004.
Section 1
In this section, you will implement simple client/server programs. The client and server
will use a UDP socket for sending and receiving.
Server Program (server.c)
You should implement a server program, called server.c, in C on a UNIX 
system. Its execution command should have the following structure:
 server <UDP listen port
Upon execution, the server should:
1. Open a UDP socket and listen at the specified port number
2. Receive a message from the client
a. If the message is “ftp”, reply a message “yes” to the client.
b. else, reply a message “no” to the client.
Client Program (deliver.c)
You should implement a send program, called deliver.c, in C on a UNIX system. Its
execution command should have the following structure:
 deliver <server address <server port number
After executing the server, the client should:
1. Ask the user to input a message follows the format:
 ftp <file name
2. Check the existence of the file:
a. If exist, send a message “ftp” to the server
b. else, exit
3. Receive a message from the server:
a. If the message is “yes”, print out “A file transfer can start.”
b. else, exit
Question: Can we use string functions on messages?
Section 2
Based on the above client and server, you need to measure the round trip time from the
client to the server.
Question: How long is the measured round trip time?
Section 3
In this section, you will implement a client and a server to transfer a file. Unlike
simply receiving a message and sending it back, you are required to have a specific
packet format and implement acknowledge for the simple file transfer using UDP
socket.
Packet Format: all packets sent between the client and server must have the following
structure:
struct packet {
unsigned int total_frag;
unsigned int frag_no;
unsigned int size;
char* filename;
char filedata[1000];
}
The total_frag field indicates the total number of fragments of the file. Each packet
contains one fragment. The frag_no field indicates the sequence number of the
fragment, starting from 1. The sizefield should be set to the size of the data.
Therefore, it should be in the range of 0 to 1000. All members of the packet should be
sent as a single string, each field separated by a colon. For instance:
total_frag = 3
frag_no = 2
size = 10
filename = “foobar.txt”
filddata = “lo World!\n”
Your packet should look like this:
packet = “3:2:10:foobar.txt:lo World!\n”
Please remember that while the beginning of the packet is in fact just plain text, the
data portion of the packet may in fact contain binary data. This means that you
should not use string manipulation functions available in C for the data field or
for the whole packet. Only the first part of the packet before data is really a string.
The reason you cannot use string functions is because string functions assume that the
data ends with the null character. This character however, may appear within the data of
the packet. If you were to use strcpy on a packet with binary data, some of your data
may get lost and your program will not function correctly. You should test your
program on both binary data (an image file for instance) as well as a text file. In general,
if your program works for binary data, it will work for a text file.
Acknowledgement: You should implement some sort of acknowledgement in order
to guarantee correct receipt of the file. For this assignment, you may use a simple
stop-and-wait style acknowledgement.
The server may use ACK and NACK packets to control data flow from the sender. The
client should open a UDP socket to listen for acknowledgements from the server. You
will have to carefully coordinate between the client and the server in order to guarantee
correct file transfer.
Client Program (deliver.c):
The execution command should have the following structure:
 deliver <server address <server port number
Upon execution, the client program should read data from a file specified by user and
send it to the server using a UDP socket. If a file is larger than 1000 bytes, the file needs
to be fragmented into smaller packets with maximum size 1000 before transmission.
Server Program (server.c):
 server <UDP listen port
Upon receiving the first packet in a sequence (i.e. frag_no = 1), the program should
read the file name from the packet and create a corresponding file stream on the local
file system. Data read from packets should then be written to this file stream. If the EOF
packet is received, the file stream should be closed.
Section 4
One file is segmented into packets for transfer, and acknowledgement guarantees
correct receipt of the file. If one packet from the client is lost, what will happen? If an
ACK/NACK packet is lost, what will happen?
Timeout: You should implement a timer for ACK/NACK packet at the client. After
sending a packet, the client should wait for an ACK in a time period t1. If the ACK
packet didn’t come within t1, the client assumed a packet loss happened and resend it.
Question: For a timeout, how do we select the value of t1?
Makefile
You should also prepare a makefile that generates the executable file deliver
from deliver.c and the executable file server from server.c.
Execution Example
Assuming you have a file named source.jpg on ug201 which you wish to send to
ug202(In this server, port 5000 is used.):
On the host ug202:
Server 5000
On the host ug201:
deliver ug202.eecg.utoronto.ca 5000
ftp <filename
Remember that your two programs need to be in separate folders as the file cannot be
copied onto itself. You can verify correct operation of your code by performing a binary
diffon the source and destination file.
Deliverables:
The following should be available for the lab evaluation:
 The client program (deliver.c)
 The server program (server.c)
 Makefile to compile your program
 Any extra header files or source code necessary for correct operation of your
code.
Submission Procedure:
For electronic submission, one submission per group is required. You have to
create a tar ball (a1.tar.gz) with all the files needed to compile and run your
programs.
The following command can be used to tar your files:
tar –czvf a1.tar.gz <project directory
where the project directory contains your source code, headers, and Makefiles.
Use the following command on the eecg UNIX system to submit your code:
submitece361s 1 a1.tar.gz
You can perform the electronic submission any number of times before the actual
deadline. A resubmission of a file with the same name simply overwrites the old version.
To see a list of what you have submitted, use the command:
submitece361s –l 1