IK2215 Programming 
Assignment Introduction
Voravit Tanyingyong
2024-09-05 1Programming assignment overview • Design and implement a reliable protocol for sending/receiving datagrams
• Guaranteed UDP (GUDP)
• Enabling reliable transport over UDP
• Based on the Go-Back-N (GBN) protocol
• Automatic repeat request (ARQ)
• Use acknowledgements and timeouts 
for reliable transmission
• Sliding window flow control 
• Multiple packets in flight
• Asynchronous communication
• Unlike TCP, GUDP is not connection-oriented
(no connection establishment)
2024-09-05 2
GUDP
Application
UDP
IP Network
Transport
ApplicationReliable data transfer and Go-Back-N • Computer Networking: a Top-Down Approach 
• Chapter 3.4 – 3.4.3
• Slides from the authors
• Slides 4 – 8 and 12 – 14
2024-09-05 3Principles of reliable data transfer • One of the most important challenges in networking
• Characteristics of unreliable channel will determine complexity of reliable data transfer protocol (rdt)
2024-09-05 4Reliable data transfer: interfaces
2024-09-05 5
send
side
receive
side
rdt_send(): called from above, 
(e.g., by app.). Passed data to 
deliver to receiver upper layer
udt_send(): called by rdt,
to transfer packet over 
unreliable channel to receiver
rdt_rcv(): called when packet 
arrives on rcv-side of channel
deliver_data(): called by 
rdt to deliver data to upperPipelined protocols
• Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts
• Range of sequence numbers must be increased
• Buffering at sender and/or receiver
• Two generic forms of pipelined protocols: Go-Back-N, selective repeat
2024-09-05 6Pipelining: increased utilization
2024-09-05 7
first packet bit transmitted, t = 0
sender receiver
RTT 
last bit transmitted, t = L / R
first packet bit arrives
last packet bit arrives, send ACK
ACK arrives, send next 
packet, t = RTT + L / R
last bit of 2nd packet arrives, send ACK
last bit of 3rd packet arrives, send ACK
3-packet pipelining increases
utilization by a factor of 3!
 
U 
sender 
= .0024 
30.008 = 0.00081 3L / R 
RTT + L / R 
= 
L: a packet size (8000 bits for 1000 bytes)
R: transmission rate (109 bps for 1 Gbps)
RTT: round-trip-time (~ 30 ms for speed-of-light )
Usender: fraction of time the sender is busy sending bits into the channel
L/R = 8 us (0.008 ms)Pipelined protocols: overview
Go-Back-N (GBN)
• Sender can have up to N unack’ed packets in 
pipeline
• Receiver only sends cumulative ack
• Doesn’t ack packet if there’s a gap
• Sender has timer for oldest unacked packet
• When timer expires, retransmit all unacked
packets
Selective Repeat
• Sender can have up to N unack’ed packets in 
pipeline
• Receiver sends individual ack for each packet
• Sender maintains timer for each unacked packet
• When timer expires, retransmit only that unacked
packet
2024-09-05 8GBN sliding window protocol
• Understand the details of a basic sliding window protocol
• An ACK is an ACK (and not a NACK)
• The receiver sends an ACK only if it receives the next packet in sequence*
• You cannot use an ACK to tell the sender that a packet has been lost (i.e., no NACK)
• No duplicate ACK detection
• The sender increases the window in accordance with the ACK
• Retransmissions are triggered by timeouts (and nothing else)
• Receiving an ACK with unexpected sequence number does not trigger a retransmission
2024-09-05 9
* There can be a problem in this case. We will this dicuss it later on.Sliding window flow control
2024
-09
-05 10
Sender Receiver
ACK
4
P
0
Window (size 3)
0 1 2 3
 
4
5
6
7
P
1P
2
0 1 2 3
 
4
5
6
7
0 1 2 3
 
4
5
6
7
P
3
0 1 2 3
 
4
5
6
7
0 1 2 3
 
4
5
6
7
0 1 2 3
 
4
5
6
7
0 1 2 3
 
4
5
6
7
0 1 2 3
 
4
5
6
7
ACK
2
ACK
3Problem when receiver only ACKs the next sequence
Problem:
• If the receiver sends an ACK only if it receives the next 
packet in sequence, a deadlock occurs when all ACKs 
(= number of window size) were lost
Solution:
• Receiver must send ACK with the expected sequence 
number when it receives a packet with a different 
sequence number than the expected sequence number
• Sender upon receiving an ACK can assume all packets with 
(ACK sequence number - 1) were received successfully
2024-09-05 11
Sender Receiver
P0
E0
ACK1
X
P1
E1
P2
E2
ACK2
X
ACK3
X
Timeout!
E3
P0
E3
P1
E3
P2
Timeout!
Send deadlock!
Window (size 3)Go-Back-N sender • k-bit seq # in pkt header (range of sequence numbers is [0, 2k - 1])
• “window” of up to N, consecutive unack’ed pkts allowed
• ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK”
• On receiving ACK(n): move window forward to begin at n+1
• Timer for oldest in-flight pkt
• Timeout(n): retransmit packet n and all higher seq # pkts in window
• TCP implementation sends the next expected sequence in the ACK, i.e., ACK(n) ack’ed all pkts up to n-1
• GUDP implementation will also send the expected sequence in the ACK
2024-09-05 12GBN sender extended FSM*
2024
-09
-05 13
Wait start_timer
udt_send
(sndpkt[base])
udt_send
(sndpkt[base+1])
…
udt_send
(sndpkt[nextseqnum
-1])
timeout
rdt_send(data)
if (nextseqnum < base+N) {
sndpkt
[nextseqnum] = make_pkt
(nextseqnum,data,chksum
)
udt_send
(sndpkt
[nextseqnum])
if (base == nextseqnum
)
start_timer
nextseqnum++ }
else
refuse_data(data)
base = getacknum
(rcvpkt)+1
If (base == nextseqnum
)
stop_timer
else
start_timer
rdt_rcv
(rcvpkt) && 
notcorrupt
(rcvpkt) 
base=1
nextseqnum=1
rdt_rcv(rcvpkt) 
&& corrupt(rcvpkt) Λ Λ
* See
 the course
 book
 chapter
 3.4.3, figure
 3.20GBN receiver extended FSM*
• ACK-only: always send ACK for correctly-received pkt with highest in-order seq #
• May generate duplicate ACKs
• Need only remember expectedseqnum
• out-of-order pkt: 
• Discard (don’t buffer): no receiver buffering!
• Re-ACK pkt with highest in-order seq #
2024-09-05 14
Wait
udt_send(sndpkt)
default
rdt_rcv(rcvpkt)
&& notcurrupt(rcvpkt)
&& hasseqnum(rcvpkt,expectedseqnum) 
extract(rcvpkt,data)
deliver_data(data)
sndpkt = make_pkt(expectedseqnum,ACK,chksum)
udt_send(sndpkt)
expectedseqnum++
expectedseqnum=1
sndpkt = make_pkt(0,ACK,chksum)
Λ
* See the course book chapter 3.4.3, figure 3.21GUDP implementation in java
• GUDP runs in user space, in the same process as the application
We provide:
• GUDPPacket.java: A class for GUDP protocol declarations with associated methods to access the 
GUDP packet header and payload
• GUDPSocketAPI.java: Well-defined API that you must use for your implementation
• GUDPEndPoint.java: A class for keeping track of remote endpoints
• GUDPSocket.java: A class for GUDP library
• SenderThread.java: A class that monitors send buffers and sends packets when they are in the buffer.
• ReceiverThread.java: A class that receives packets from remote endpoints and puts them in the buffers.
2024-09-05 15
UDP Application UDP GUDP Application
• ARQ
• Sliding window flow control
You are not allowed to modify these files!GUDP header
• Version: version of the GUDP protocol
• Use version 1!
• Type: packet type 
• DATA, BSN, ACK, and FIN
• How to use sequence numbers:
• DATA packets: increases by one for each packet sent
• BSN packets: random
• ACK packets: sequence number of next expected DATA packet
• FIN packets: sequence number of last DATA packet plus one
2024-09-05 16
0 7 8 15 16 ** 
+--------+--------+--------+--------+ 
| Version | Type |
+--------+--------+--------+--------+
| Sequence number |
+--------+--------+--------+--------+GUDPSocketAPI.java – API you must use
• Your code must conform to this API
• Class/method declarations defined for the assignment
• You will write the GUDPSocket class that implements this API
• You may add variables, methods, and inner classes in GUDPSocket.java
2024-09-05 17
import java.net.DatagramPacket;
import java.io.IOException;
public interface GUDPSocketAPI {
public void send(DatagramPacket packet) throws IOException;
public void receive(DatagramPacket packet) throws IOException;
public void finish() throws IOException;
public void close() throws IOException;
}GUDPSocket.java – skeleton code for GUDP library
• The skeleton above is incomplete
• The actual file contains more variables and descriptions of what must be done in each method
2024-09-05 18
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.io.IOException;
public class GUDPSocket implements GUDPSocketAPI {
DatagramSocket datagramSocket;
public GUDPSocket(DatagramSocket socket) {
datagramSocket = socket;
}
public void send(DatagramPacket packet) throws IOException {}
public void receive(DatagramPacket packet) throws IOException {}
public void finish() throws IOException {}
public void close() throws IOException {}
}send()
• Send a packet
• The application put data in the DatagramPacket format
• The destination address/port included in the DatagramPacket
• Non-blocking – returns immediately
• Put application data in GUDP send buffer for future delivery
• The DatagramPacket must be encapsulate in GUDP format before putting in the send buffer
2024-09-05 19
public void send(DatagramPacket packet) throws IOException;receive()
• Receive a packet
• The application fetch the data from GUDP receive buffer, otherwise wait for the data to arrive
• GUDP receives incoming packets from remote endpoints independently from the application
• GUDP receive buffer stores packets in GUDP format, which must be decapsulated before sending it to the 
application
• The application handles packets from different senders (which can be differentiated based on the 
information in the packet)
2024-09-05 20
public void receive(DatagramPacket packet) throws IOException;finish()
• Finish sending
• The application calls this method to inform GUDP that it’s done sending
• GUDP completes the actual sending and return when it is done, otherwise report error/timeout by 
throwing the IOException
• Retransmission may occur due to packet lost or arriving out-of-order
• Clean up data structure that you use to track destination end points
2024-09-05 21
public void finish() throws IOException;close()
• Close the GUDP socket and gracefully terminate sender and receiver threads
• The application calls this method to terminate the GUDP socket
• GUDP cleans up, closes the socket, stop sender and receiver threads, and return.
2024-09-05 22
public void close() throws IOException;GUDP sender side
• Data transfer may happen after the application passed all packets to GUDP
• GUDP can send multiple packets (<= window size) before it receives any ACK
2024-09-05 23
send(packet)
Application GUDP Network
GUDP ACK
GUDP BSN
GUDP DATA
GUDP DATA
GUDP ACK
send(packet)
finish()
finish() return
GUDP ACK
GUDP FIN
GUDP ACK
close()
Application terminates after send completion
wait for GUDP to complete sendingGUDP receiver side
• Receive returns only after GUDP has DATA
• Receiver may keep socket open to receive more DATA
2024-09-05 24
receive(packet)
Application GUDP Network
GUDP DATA
GUDP ACK
GUDP ACK
receive(packet)
GUDP BSN
GUDP DATA
GUDP ACK
receive(packet) return
receive(packet) return
Fetch a packet from receive buffer
Otherwise, wait for an incoming packet
Application remains running to receive new connectionsProtocol control block
• An application can open multiple GUDP sockets
• Each GUDP socket can be used for communication 
with multiple peers
• Two levels
• Multiple GUDP sockets
• Multiple peers per socket
• Need to
• Maintain state for per-socket “peers”
• Have a way to look up peer state
• Maintain queues with outbound/inbound packets
2024-09-05 25
Program
Application
GUDP
Socket Socket
PeersSend
Queue
GUDP Implementation: Send and receive processes
2024-09-05 26
SEND
Application
- Handle destination endpoints
- Wrap app data in GUDP
- Put it in send queue
- Take GUDP from send queue
- Wrap it in UDP and send
- Handle timeout/retransmission
send() Send 
thread
Network
Network
receive 
thread
- Receive and process incoming packets
BSN: Create receive endpoint if not exist
DATA: Put GUDP DATA in the receive queue
ACK: Update send queue
FIN: Mark endpoint for removal
- Send ACKs for the received packets
Receive
Queue
receive()
Pick up GUDP from receive queue
Unwrap GUDP to get app data
Application
RECEIVEMain tasks
Part 1: GUDPSocket.java
• Implement core functionalities, including send(), receive(), finish(), and close() methods
• Assume SenderThread.java and ReceiverThread.java are already implemented as described in the 
comments in the files
Part 2: SenderThread.java
• Implement the send thread that monitors send queues and sends packets based on the GBN protocol 
when there are packets in the queues
• Assume GUDPSocket.java and ReceiverThread.java are implemented as described in the comments in 
the files
• GUDPSocket.java from the teacher, not from your part 1 submission!
• ReceiverThread.java handles ACK for the GBN sender, and it also calls FSMSender after it removes all ACKed
packet from the send buffer
2024-09-05 27Teacher implementation: SenderThread class
if senderList is empty, make it wait
while (runFlag)
synchronize senderList
for each endpoint in sendList
run FSMSender(endPoint)
if endpoint is finished, check if its send queue is empty, remove it from sendList otherwise proceed to next endpoint 
if sendList is empty, notify other threads. Otherwise if all endpoints' send queue are empty, make senderList wait
if (!runFlag), notifies other threads
sleep(50)
while (nextseqnum < base+N) && (nextseqnum < last)
get next GUDP packet from the send queue, pack and send it
if (base == nextseqnum) start_timer
nextseqnum++
2024-09-05 28Teacher implementation: FSMSender method
2024-09-05 29Teacher implementation: ReceiverThread class
When packet arrives, unpack it to GUDP format
if incoming packet is an ACK
synchronize senderList
remove all ACK'ed packets from senderList, update related parameters
move to RCV state and call FSMSender
notify senderList
if incoming packet is a BSN
synchronize receiverList
if new end point, add a new GUDPEndPoint, update all parameters, add it to receiverList, and send ACK
else if end point was finished
reset end point parameters and use the new expectedseqnum based on BSN (seq+1), and send ACK
else send ACK with the expectedseqnum
if incoming packet is a DATA
synchronize receiverList
if seq==expectedseqnum, add GUDP packet to end point receive buffer, update all parameters, and send ACK
else send ACK with the expectedseqnum
if incoming packet is a FIN
synchronize receiverList
if seq==expectedseqnum, update all parameters, set end point as finished, and send ACK
else send ACK with the expectedseqnum
2024-09-05 30Grading overview
The application should be able to:
• Send one or multiple files to one or more destinations
• Receive one or multiple files from one or more sources
• Handle unexpected situations gracefully
• Work with other implementations
• To pass, your submission must ensure that:
• The application can at least send and receive one file on one destination correctly
• GUDP must be used in data transmission (show on the wire correctly)
• Sliding window flow control is working correctly (multiple packets in-flight)
• ARQ mechanism is working correctly (handle packet loss correctly)
• Your scores meet the grading criteria (see details on the next two slides)
• Part 1: Deadline: Mon 23 Sep 17:00 sharp • Make-up deadline: Tue 1 Oct 17:00 sharp
• Part 2: Deadline: Mon 30 Sep 17:00 sharp • Make-up deadline: Tue 8 Oct 17:00 sharp
2024-09-05 31Part 1: Grading criteria
Score at least 3.5 points from the bold items and 5 out of 7 in total
1. Send multiple packets in-flight + check packet content (1)
2. Send and receive files with your code without loss (1)
3. Send one file to other receiver without loss (0.5)
4. Send one file to other receiver with loss (0.5)
5. Receive one file from other sender without loss (0.5)
6. Receive one file from other sender with loss (0.5)
7. Send one file to multiple receivers without loss (0.5)
8. Send one file to multiple receivers with loss (0.5)
9. Send multiple files to other receiver without loss (0.5)
10. Send multiple files to other receiver with loss (0.5)
11. Receive multiple files from other sender without loss (0.5)
12. Receive multiple files from other sender with loss (0.5)
2024-09-05 **
>=3.5 points
>=5 pointsPart 2: Grading criteria
Score at least 2.5 points from the bold items and 3.5 out of 5 in total
1. Send multiple packets in-flight + check packet content (1)
2. Send and receive files with your code without loss (1)
3. Send one file to other receiver without loss (0.5)
4. Send one file to other receiver with loss (0.5)
5. Receive one file from other sender without loss (0.5)
6. Receive one file from other sender with loss (0.5)
7. Send one file to multiple receivers without loss (0.5)
8. Send one file to multiple receivers with loss (0.5)
9. Send multiple files to other receiver without loss (0.5)
10. Send multiple files to other receiver with loss (0.5)
11. Receive multiple files from other sender without loss (0.5)
12. Receive multiple files from other sender with loss (0.5)
2024-09-05 33
>=2.5 points
>=3.5 pointsPlagiarism*
• Plagiarism in practical work and computing code
• “It is important that students ‘do their own work’ when they write computer code, when document an 
experiment, create a design or answer a mathematical problem. If they do not do these activities 
themselves, yet claim the results as their own, this is plagiarism.”
• Students who, with unauthorized aids or otherwise attempt to mislead the exam or when a student's 
performance is otherwise to be assessed, may lead to disciplinary action.
2024-09-05 34
* More information on KTH webpage about Cheating and plagiarismTesting
• We provide sample applications that you can use to run with your GUDP code
• VSFtp.java: A class for a simple file transfer protocol
• VSSend.java: An application for sending files over VSFtp
• VSRecv.java: An application for receiving files over VSFtp
• You are responsible for identifying relevant test cases and performing tests
• Need to complete GUDPSocket.java, SenderThread.java, and ReceiverThread.java
• Think through the protocol carefully and know how it should work exactly
• Think through the dynamic behaviour of the GUDP library
• What happens, and when?
• Define the protocol states and transitions
• <current state, event, action, new state>
• If you have question:
• Discussion forum: Q&A for lab activities
• Q&A sessions for verbal discussion or additional support
2024-09-05 35Test service on http://ik2215.ssvl.kth.se
• Only accessible within KTH network
• From outside KTH network, you must connect via KTH VPN-service
• Part 1: http://ik2215.ssvl.kth.se/prg1
• Part 2: http://ik2215.ssvl.kth.se/prg2
• You must provide:
• Your KTH account i.e., KTH email without the “@KTH.SE” part
• Your submission file 
• Part 1: GUDPSocket.java
• Part 2: SenderThread.java
• The test runs at 00:00 everyday
• Slow: 6-10 minutes per submission
• Results send to the KTH account you provided
2024-09-05 362024-09-05 37
### TEST6: receive one file from other sender with loss (0.5p)
OK: Your code can receive one file when first BSN is lost
OK: Your code can receive one file when first DATA is lost
OK: Your code can receive one file when first FIN is lost
OK: Your code can receive one file when first ACK is lost
OK: Your code can receive one file with random loss
TEST6: OK 0.5p ### TEST7: send one file to multiple receivers without loss (0.5p)
OK: Your code can send one file to multiple receivers
TEST7: OK 0.5p ### TEST8: send one file to multiple receivers with loss (0.5p)
OK: Your code can send one file to multiple receivers
TEST8: OK 0.5p ### TEST9: send multiple files to other receiver without loss (0.5p)
OK: Your code can send multiple files to other receiver
TEST9: OK 0.5p ### TEST10: send multiple files to other receiver with loss (0.5p)
OK: Your code can send multiple files to other receiver
TEST10: OK 0.5p ### TEST11: receive multiple files from other sender without loss (0.5p)
OK: Your code can receive one file from other sender
TEST11: OK 0.5p ### TEST12: receive multiple files from other sender with loss (0.5p)
OK: Your code can receive one file from other sender
TEST12: OK 0.5p
##########
IMPORTANT: You pass only if scores of TEST**6 >=3.5 points and TEST**12 >=5.0 points.
You get the scores only when you pass. Otherwise, you get 0 points
RESULTS: PASS
SCORE: 7.0
##########
OK: Code compiles without error.
### TEST1: Send multiple packets in-flight + check packet content (1.0p)
OK: GUDP version must be 1
OK: First packet is GUDP BSN (type 2)
OK: Sequence number is random and not zero or one
OK: BSN packet contains only GUDP header
OK: GUDP version must be 1
OK: Second packet is GUDP DATA (type 1)
OK: Sequence number should be random and not zero
OK: Second packet has an increment sequence number
OK: data packet seems to contain GUDP header + payload
TEST1: OK 1.0p ### TEST2: send and receive files with your code without loss (1.0p)
OK: Your code can send and receive one file
OK: Your code can send and receive multiple files
TEST2: OK 1.0p ### TEST3: send one file to other receiver without loss (0.5p)
OK: Your code can send one file to other receiver
TEST3: OK 0.5p ### TEST4: send one file to other receiver with loss (0.5p)
OK: Your code can send one file when first BSN is lost
OK: Your code can send one file when first DATA is lost
OK: Your code can send one file when first FIN is lost
OK: Your code can send one file when first ACK is lost
OK: Your code can send one file with random loss
TEST4: OK 0.5p ### TEST5: receive one file from other sender without loss (0.5p)
OK: Your code can receive one file from other sender
TEST5: OK 0.5p
Example test output for Part 12024-09-05 38
### TEST7: send one file to multiple receivers without loss (0.5p)
OK: Your code can send one file to multiple receivers
TEST7: OK 0.5p ### TEST8: send one file to multiple receivers with loss (0.5p)
OK: Your code can send one file to multiple receivers
TEST8: OK 0.5p ### TEST9: send multiple files to other receiver without loss (0.5p)
OK: Your code can send multiple files to other receiver
TEST9: OK 0.5p ### TEST10: send multiple files to other receiver with loss (0.5p)
OK: Your code can send multiple files to other receiver
TEST10: OK 0.5p
TEST11: SKIPPED 0.0p
TEST12: SKIPPED 0.0p
##########
IMPORTANT: You pass only if scores of TEST**6 >=2.5 points and TEST**12 >=3.5 points.
You get the scores only when you pass. Otherwise, you get 0 points
RESULTS: PASS
SCORE: 5.0
##########
OK: Code compiles without error.
### TEST1: Send multiple packets in-flight + check packet content (1.0p)
OK: GUDP version must be 1
OK: First packet is GUDP BSN (type 2)
OK: Sequence number is random and not zero or one
OK: BSN packet contains only GUDP header
OK: GUDP version must be 1
OK: Second packet is GUDP DATA (type 1)
OK: Sequence number should be random and not zero
OK: Second packet has an increment sequence number
OK: data packet seems to contain GUDP header + payload
TEST1: OK 1.0p ### TEST2: send and receive files with your code without loss (1.0p)
OK: Your code can send and receive one file
OK: Your code can send and receive multiple files
TEST2: OK 1.0p ### TEST3: send one file to other receiver without loss (0.5p)
OK: Your code can send one file to other receiver
TEST3: OK 0.5p ### TEST4: send one file to other receiver with loss (0.5p)
OK: Your code can send one file when first BSN is lost
OK: Your code can send one file when first DATA is lost
OK: Your code can send one file when first FIN is lost
OK: Your code can send one file when first ACK is lost
OK: Your code can send one file with random loss
TEST4: OK 0.5p
TEST5: SKIPPED 0.0p
TEST6: SKIPPED 0.0p
Example test output for Part 2Useful resources
• Course book: 8th and 7th edition
• Read Chapter 3.4 through Chapter 3.4.3 Go-Back-N (GBN)
• TCP Operational Overview and the TCP Finite State Machine (FSM)
• Producer-consumer in Java: Baeldung, geeksforgeeks
• Java queue implementations: Oracle, Baeldung, geeksforgeeks, 
• Java documentation for different classes:
• DatagramSocket, DatagramPacket, 
• LinkedList, ArrayDeque
• Java wait() and notify() methods
2024-09-05 39What you need to do • Read through relevant materials thoroughly
• Guaranteed UDP (GUDP) page on Canvas
• This programming introduction slides
• Read through the given source code template for the assignment
• Familiarize with java programming
• Plan early and work incrementally
• Submit your code to the test service daily!
2024-09-05 40

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