ECE361 COMPUTER NETWORKS
File Transfer


H. TIMORABADI
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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 specific 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

• Section 2.4 and related sections on Berkeley API from Chapter 2 of the
Communication Networks by Alberto Leon-Garcia and Indra Widjaja, McGraw Hill,
2nd Edition, 2004.

• The network socket programming “Beej’s Guide to Network Programming” available
on the course website under the course section.



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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

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 with a message “yes” to the client.

b. else, reply with a message “no” to the client.


Client Program (deliver.c)

You should implement a client program, called “deliver.c”, in C on a UNIX system. The
client program will send a message to the server. Its execution command should have the
following structure:

deliver

After executing the server, the client should:

1. Ask the user to input a message as follows:
ftp
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?


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Section 2

Based on the client and server in section 1, 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 size field 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
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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 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 to guarantee correct file transfer.

Client Program (deliver.c):

The execution command should have the following structure:

deliver

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

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.


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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 of 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

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 diff on
the source and destination file.



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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

where the project directory contains your source code, headers, and Makefiles.

Use the following command on the eecg UNIX system to submit your code:

submitece361f 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:

submitece361f –l 1

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