CS 2200 Project 7


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1 Learning Objective
During this project, you will implement the simplest of Transport Layer Protocols, the Stop-and-Wait Protocol
for the reliable transfer of data through an artificial network. Specifically you will:
• Demonstrate how messages are segmented into packets and how they are reassembled.
• Understand why a checksum is needed and when it is used.
• Understand and implement the Stop-and-Wait Protocol with ACK (Acknowledgments), NACK (Negative
Acknowledgments), and re-transmissions.
For a description of the Stop-and-Wait Protocol, read Section 13.6.1 in your textbook.
2 The Protocol Stack
Here is a diagram that shows where the code in this project fits into the protocol stack:
Application Layer
Transport Layer
Network Layer
Link Layer
Physical Layer
Figure 1: The Protocol Stack
Note that the Network Layer, Link Layer, and Physical Layers are all implemented by your operating system
or networking hardware. For the sake of this project, you will be using the UDP transport protocol to
emulate an unreliable network layer. The application program, client.c, simply makes the appropriate calls
to connect to the remote server, send and receive messages, and disconnect from the remote server. You will
be providing the send and receive features for the client to use in rtp.h.
3 Code Walkthrough
Here, we will briefly describe the code provided for this project. It is important that you study and understand
the code given to you. In the past, we have asked students to write a large portion of this code, but we have
found this to be unreasonable during the last few weeks of school. As a compromise, we have ginve you this
code and simply ask you to complete it. However, YOU ARE RESPONSIBLE FOR UNDERSTANDING
The client program takes two arguments. The first argument is the IP address of the server it should
connect to (such as, the localhost), and the second argument is the port number it should connect
to. You may call the client from the terminal like so:
1 $ ./prj7−client 8080
The client.c program represents the application layer. It uses the services provided by the Transport
Layer (rpt.c). It begins by connecting to the remote server. Look at the socket setup() in rpt.c – it
creates the UDP socket that is used to emulate an unreliable network. The socket setup() function also
initializes and returns a CONN INFO structure. Next, the client program sends a message to the remote server
using the rtp send message() function. The client program also receives messages from the network. If
the message isn’t available or if the entire message has not been received, then the rtp receive message()

function should block until the message has been received from the server. The client program continues to
send and receive messages until it is finished. Lastly, the client program calls rtp disconnect() in order to
terminate the connection with the remote server. Note that there are four distinct types of Transport Layer
1. DATA is a data packet that contains part of a message in its payload.
2. LAST DATA is just like a data packet, but it also says that it is the last packet for the message.
3. ACK is an acknowledgment that a packet was properly received.
4. NACK is a negative acknowledgment that the packet was not properly received.
5. TERM is used to tell the server to close the connection early (this is not specifically needed for the
4 Implementation
1. (20 points) Segmentation of Data When data is sent over a network, the data is chopped up into one
or more parts and each part is sent inside a packet. A packet contains information that describes the
message; it contains information such as the source and destination of the packet, the type of data, and
the data itself. The data being sent over the network is referred to as the “payload”. Look in rtp.h
– what other fields does our network packet carry? Think about why each field is needed. How much
payload can we fit into each packet? Note that the packet structure in this project is greatly simplified.
Open up rpt.c and find the packetize() function. Complete this function. Its purpose is to turn a
message into an array of packets. It should do the following:
1. Allocate an array of packets big enough to carry all of the data.
2. Populate all the fields of the packet including the payload. Remember, the last packet should be a
LAST DATA packet. All other packets should be DATA packets. This is very important! The server
does check for this, and it will disconnect if these fields are not filled in properly. If you do not mark
the last packet is LAST DATA, the server will hang forever!
3. The “count” variable points to an integer. Update this integer, setting it equal to the length of the
array you are returning.
4. Return the array of packets.
Hint: Remember that when you divide the length of the message by the length of the packet you are
performing integer division. It may not always be the case that the message is a multiple of the packet
size. Think of a way to handle this special case.
2. (80 points) Bad things happen in an unreliable network. In the Stop-and-Wait protocol, sending a
message requires the following individual steps:
1. Send one packet at a time.
2. After each packet, wait for an ACK or NACK to be received.
3. If a NACK is received, resend the last packet. Otherwise, send the next packet.
Receiving a message requires the following additional steps:
1. Compute the checksum for each packet payload upon arrival.
2. If the checksum does not match the checksum reported in the packet header, send a NACK. If it does
match, send an ACK.
Note that most Stop-and-Wait Protocols also implement a sequence number to handle lost packets. We
will ignore this issue in order to keep the project simple. You should think about why it may be necessary
to keep track of sequence numbers.

Instructor: BIll Leahy
(a) (20 points) Open rtp.c and find the checksum() function. Complete this function. Simply sum
up the ASCII values of each character in the buffer and return the sum. This is how the server
computes the checksum. The server and the client use this checksum.
(b) (30 points) Open rtp.c and find the rtp receive message() function. This message is for
receiving messages from the data connection. To do this, you will use CONN INFO and the recvfrom()
function. If the packet is a DATA packet, the payload will be added to the current message buffer.
The packet’s payload should only be added to the buffer if the checksum of the data matches the
checksum in the packet header. if the checksum matches, you should send an ACK packet using
sendto(). If the checksum does not match, then you should send a NACK packet in the same manner.
(c) (30 points) Open rtp.c and find the rtp send message() function. This function should handle
packetizing the message and sending the packets one by one using sendto(). You should then accept
the server’s response using recvfrom(). If the response is an ACK, you should send the next packet.
Otherwise, you should resend the packet. Continue to do this until all of the packets have been sent.
5 Running the Project
To compile all of the code, use the following command:
1 $ make
To run the server on linux, use the following command to run the server:
1 $ ./prj7−server −p [port number] [−c corruption rate]
To run the server on mac, use the following command to run the client:
1 $ python prj7−server −p [port number] [−c corruption rate]
Regardless of operating system, make sure that python is bound to python 2. The server will not run on
python 3! Your client should work with an UNMODIFIED version of the server. For example, if you wanted
to run a server on port 8080 with a corruption rate of 99%, you would execute the following command:
1 $ ./prj7−server −p 8080 −c .99
If you wanted to run a client that would send messages to this server, you would then execute the following
command (in a different terminal):
1 $ ./prj7−client 8080
The server will take the client’s messages and then convert them into Pig Latin. The server will be printing
out debug statements in order for you to understand what it is doing.
To make a .tar.gz simply execute the following command:
1 $ make submit
Make sure to submit this archive on T-Square.
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