Lab 5: Stop and Wait for an Unreliable Channel


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COEN 146: Computer Networks
Lab 5: Stop and Wait for an Unreliable Channel
1. To build a Stop and Wait reliable protocol on top of UDP to provide a reliable transport service
TFv2 – Stop and Wait for an Unreliable Channel
In Labs 3 and 4, you have developed a TCP and a UDP client/server to transfer file. Let us name it as TFv1. To
ensure a reliable transfer over UDP, you need to implement a reliable Stop and Wait protocol. This version of file
transfer is then TFv2.
TFv2 will be built on top of UDP, and it is supposed to provide a reliable transport service to TFv1. Messages are
sent one at a time, and each message needs to be acknowledged when received, before a new message can be
sent. TFv2 implements basically the protocol rdt2.2 presented in the text book (depicted as well at the end of this
TFv2 consists of a client and a server. Communication is unidirectional, i.e., data flows from the client to the
server. The server starts first and waits for messages. The client starts the communication. Messages have seq
number 0 or 1. Before sending each message, a 1-byte checksum is calculated and added to the header. After
sending each message, the client waits for a corresponding ACK. When it arrives, if it is not the corresponding
ACK (or if the checksum does not match), the message is sent again. If it is the corresponding ACK, the client
changes state and returns to the application, which can now send one more message. This means TFv2 blocks
on writes until an ACK is received.
The server, after receiving a message, checks its checksum. If the message is correct and has the right seq
number, the server sends an ACK0 or ACK1 message (according to the seq number) to the client, changes state
accordingly, and deliver data to the application.
The protocol should deal properly with duplicate data messages and duplicate ACK messages. Follow the FSM
in the lecture notes and the textbook!
TFv2 message contains the header and the application data. No reordering is necessary, since TFv2 is sending
the exact message given by the application, one by one.
The checksum must be calculated for messages from the server and client. To calculate the checksum, calculate
the XOR off all the bytes (header or header + data) when member cksum (see below) is zero. To verify your
protocol, use the result of a random function to decide whether to send the right checksum or just zero. This will
fake the error effect.
seq_ack int (32 bits) // SEQ for data and ACK for Acknowledgement
len int (32 bits) // Length of the data in byes (zero for ACKS)
cksum int (32 bits) // Checksum calculated (by byte)
data char (10 bytes)
 Member seq_ack is used as SEQ, and the data is in member data.
 Each packet may have 10 or less bytes of data, and the sender only sends the necessary bytes.
 After transmitting the file, a packet with no data (len = 0) is sent to notify the receiver that the file is complete.
 Member seq_ack is used as ACK, and data is empty (len = 0).
COEN 146 – Lab assignment 5 1/3

Requirements to complete the lab
Show the TA correct execution of the programs you wrote and upload source code to Camino.
Be sure to retain copies (machine and/or printed) of your source code. You will want these for study purposes and
to resolve any grading questions (should they arise)
Please start each program with a descriptive block that includes minimally the following information:
* Name: <your name>
* Date:
* Title: Lab5 – ….
* Description: This program … <you should
* complete an appropriate description here.>
rdt2.2 sender – FSM
COEN 146 – Lab assignment 5 2/3
rdt2.2 receiver – FSM
COEN 146 – Lab assignment 5 3/3

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