CS3210 Lab 4: Go-back-N Protocol


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The objective of this project is to implement the Go-back-N reliable transmission protocol and measure the round-trip delays. Cumulative ACKs, as described in class, will be used.

The project requires two separate programs, running at the same time, on two different hosts: a sender program that generates and transmits packets; and a receiver, that accepts the packets, and transmits the acknowledgments to the sender. Note that the receiver does not send any data packet; it only sends acknowledgment packets. Communication between the sender and receiver is through UDP sockets. The receiver is set up as a UDP server, and the sender is set up as a UDP client.

1 Sender

The main loop of the sender has these main steps:

1. Generate a packet of length PACKET LENGTH (command-line parameter) bytes. The first byte(s) of the packet contains the sequence number. Packet generation rate is given by the command-line parameter PACKET GEN RATE, in packets per second. You may use a thread that generates packets periodically (based on the above rate) and stores them in a buffer used by the sender’s protocol.

The maximum size of this sender transmission buffer is given by the command-line parameter, MAX BUFFER SIZE (number of packets, not bytes). A newly generated packet will be dropped if the Buffer is full. A sequence number is assigned ONLY if the packet is added to the buffer.

2. Transmit the packet based on the Window Size conditions. Start the timeout timer for this packet’s sequence number. The timeout is set to 100 ms for the first 10 packets and then 2 ∗ RT Tave (in milliseconds) for all other packets.

3. Process the next packet (when available) and transmit it if the sender window is not exhausted, i.e. the total number of unacknowledged packets is at most WINDOW SIZE. 1

4. If an ACK packet arrives, process it, update local state variables and cancel timers corresponding to acknowledged packets. Note that cumulative ACKs are assumed. For each packet received, calculate the Round-trip-Time (RTT) for the packet and update the average RTT (RT Tave) for the packets acknowledged so far.

5. If a timer expires, re-transmit all packets from the first unacknowledged packet. The sender terminates after MAX PACKETS (a command-line parameter) have been successfully ACKNOWLEDGED (OR) if the maximum retransmission attempts for any sequence number exceeds 5.

Summary of Command Line Options:

The command line options provided to the sender are listed below: • -d – Turn ON Debug Mode (OFF if -d flag not present) • -s string – Receiver Name or IP address. • -p integer – Receiver’s Port Number • -l integer – PACKET LENGTH, in bytes • -r integer – PACKET GEN RATE, in packets per second • -n integer – MAX PACKETS • -w integer – WINDOW SIZE • -f integer – MAX BUFFER SIZE Output: The sender will operate in TWO modes: DEBUG and NODEBUG.

The default operation is NODEBUG mode. A command-line flag of -d will turn on DEBUG mode. For both modes, on termination, the sender will print the following information to the screen:



3. Retransmission Ratio: Ratio of Total Number of Transmissions (including Retransmissions) to Number of Packets Acknowledged.

4. Average RTT Value for ALL Acknowledged Packets In DEBUG mode, the Sender will also print the following information for EACH packet when its ACK is received: Seq #: Time Generated: xx:yy RTT: zz Number of Attempts: aa where time is in milliseconds:microseconds format. 2

2 Receiver

The receiver is always waiting to read a packet from the UDP socket it is listening to. Whenever a packet is delivered to the receiver:

1. For each packet received, the receiver randomly decides that the given packet is corrupted and decides to drop the packet; note that you can use rand, rand48, etc. The probability of packet drop is specified as a command-line parameter, denoted RANDOM DROP PROB. This step is used to simulate random network errors.

2. If the packet is NOT dropped, the receiver reads the packet and extracts the sequence number. If the sequence number matches the NEXT EXPECTED sequence number, it transmits an ACK to the sender, and updates local state variables.

Note that Cumulative ACKs are used by the receiver. The ACK packet format will only contain the appropriate sequence number. The ACK packets are NOT dropped and are always assumed to be delivered to the sender.

Thus, RANDOM DROP PROB value is not used by the sender. The receiver terminates after receiving and acknowledging MAX PACKETS (a command-line parameter). Summary of Command Line Options: The command line options provided to the receiver are listed below: • -d – Turn ON Debug Mode (OFF if -d flag not present) • -p integer – Receiver’s Port Number • -n integer – MAX PACKETS • -e float – Packet Error Rate (RANDOM DROP PROB) Output: The receiver will operate in TWO modes: DEBUG and NODEBUG.

The default operation is NODEBUG mode. A command-line flag of -d will turn on DEBUG mode. In DEBUG mode, the Receiver will also print the following information for EACH packet when it is successfully received: Seq #: Time Received: xx:yy Packet dropped: false where time is in milliseconds:microseconds format.

3 Sample Session Assume that you have created the files SenderGBN.c and ReceiverGBN.c and the corresponding executables in your directory. 3 machine1% ./ReceiverGBN -p 12345 -n 400 -e 0.00001 & m2% ./SenderGBN -s machine1 -c1 -p 12345 -l 512 -r 10 -n 400 -w 3 -b 10 Output: PktRate = 10, Drop Prob = 0.00001, Length = 512, Retrans. Ratio = 1.16, Avg RTT: 100:34 4 What to Submit The platform for this project will be Linux and C/C++/Java. Create a tar-gz file with name: Lab4- RollNo.tgz (e.g. Lab4-CS20B099.tgz).

The directory should contain the following files: • All Source Files: Each file MUST contain a header with the following information: // NAME: // Roll Number: // Course: CS3205 Jan. 2023 semester // Lab number: 2 // Date of submission: // I confirm that the source file is entirely written by me without // resorting to any dishonest means. // Website(s) that I used for basic socket programming code are: // URL(s): • Makefile and Script File Typing command ‘make’ or your script program, at the UNIX command prompt, should generate all the required executables. • A Script file obtained by running UNIX command script which will record the way you have finally tested your program.

• a README file containing what port number to use, and instructions to compile, run and test your program. • a COMMENTS file which describes your experience with the project, suggestions for change, and anything else you may wish to say regarding this project. This is your opportunity for feedback, and will be very helpful. • A technical report (in PDF format) that discusses the results obtained by running the programs on any two machines.

Report your observations in 1-2 paragraphs. Discuss what happens when the random packet drop probability increases. The experiments are to be conducted for: packets per second; for values of PACKET LENGTH ∈ {128, 1024} bytes and RANDOM DROP PROB ∈ {10−8 , 10−4}. Thus, there are a total of 4 possible combinations. Prepare TWO tables, one per packet drop probability. 4 5 Help

1. Ask questions EARLY and start your work NOW. Take advantage of the help of the TAs and the instructor. 2. Submissions PAST the extended deadline SHOULD NOT be mailed to the TAs. Only submissions approved by the instructor or uploaded to Moodle within the deadline will be graded. 3. Demonstration of code execution to the TAs MUST be done using the student’s code uploaded on Moodle. 4. NO sharing of code between students, submission of downloaded code (from the Internet, Campus LAN, or anywhere else) is allowed. The first instance of code copying will result in ZERO marks for the Lab component of the Course Grade.

The second instance of code copying will result in a ’U’ Course Grade. Students may also be reported to the Campus Disciplinary Committee, which can impose additional penalties. 5. Please protect your Moodle account password. Do not share it with ANYONE, including your team member. Do not share your academic disk drive space on the Campus LAN. 6. Implement the solutions, step by step. Trying to write the entire program in one setting may lead to frustration and possibly failure. 6 Grading • Go-back-N working correctly: 80 points • Report and Viva Voce: 20 points • NO README, Typescript or COMMENTS file: -10 points • NO File header (with Roll Number etc): -5 points 5