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Assignment for Term 1, 2024
Version 1.0
Due: 11:59am (noon) Thursday, 18 April 2024 (Week 10)
1. Change Log
Version 1.0 released on 7th March 2024.
2. Goal and learning objectives
For this assignment, you are to implement a reliable transport protocol over the UDP protocol. We
will refer to the reliable transport protocol that you will be implementing in this assignment as
Simple Transport Protocol (STP). STP will include most (but not all) of the features that are
described in Sections 3.5.4 - 3.5.6 of the text Computer Networking by Kurose and Ross (7th or 8th
ed.) or equivalent parts from the Week 4/5 lecture notes. Examples of these features include
timeout, ACK, sequence numbers, sliding window, etc. Note that these features are commonly
found in many transport protocols. Therefore, this assignment will give you an opportunity to
implement some of these basic features of a transport protocol. In addition, you may have wondered
why the designer of the TCP/IP protocol stack includes such a feature-less transport protocol as
UDP. You will find in this assignment that you can design your own transport protocol and run it
over UDP. This is the case for some multimedia delivery services on the Internet, where they have
implemented their own proprietary transport protocol over UDP. QUIC, a newly proposed transport
protocol also runs over UDP and implements additional functionalities such as reliability.
Recall that UDP provides point-to-point, unreliable datagram service between a pair of hosts. In this
programming assignment, you will develop a more structured protocol, STP, which ensures
reliable, end-to-end delivery of data in the face of packet loss. STP provides a byte-stream
abstraction like TCP and sends pipelined data segments using a sliding window. However, STP
does not implement congestion control or flow control. Finally, whereas TCP allows fully
bidirectional communication, your implementation of STP will be asymmetric. There will be two
distinct STP endpoints, "sender" and "receiver", respectively. Data packets will only flow in the
"forward" direction from the sender to the receiver, while acknowledgments will only flow in the
"reverse" direction from the receiver back to the sender. To support reliability in a protocol like
STP, state must be maintained at both endpoints. Thus, as in TCP, connection set-up and connection
teardown phases will be an integral part of the protocol. STP should implement a sliding window
protocol wherein multiple segments can be sent by the sender in a pipelined manner. Like TCP,
STP will include some elements of both Go-Back-N (GBN) and Selective Repeat (SR). You will
use your STP protocol to transfer a text file (examples provided on the assignment webpage) from
the sender to the receiver.
The sender program must also emulate the behaviour of an unreliable communication channel
between the sender and receiver. Even though UDP segments can get lost, the likelihood of such
losses is virtually zero in our test environment, where the sender and receiver will be executed on
the same machine. Further, to properly test the implementation of your sender program, we would
like to control the unreliable behaviour of the underlying channel. The sender program should
Updates to the assignment, including any corrections and clarifications, will be posted on the
course website. Please make sure that you check the course website regularly for updates.
2
emulate loss of STP segments in both directions – (i) DATA, SYN, and FIN segments in the
forward direction and (ii) ACK segments in the reverse direction. You may assume that the
underlying channel will never reorder or corrupt STP segments (in both directions).
Note that it is mandatory that you implement STP over UDP. Do not use TCP sockets. You
will not receive any marks for this assignment if you use TCP sockets.
2.1 Learning Objectives
On completing this assignment, you will gain sufficient expertise in the following skills:
1. Detailed understanding of how reliable transport protocols such as TCP function.
2. Socket programming for UDP transport protocol.
3. Protocol and message design.
Non-CSE Student Version: The rationale for this option is that students enrolled in a program that
does not include a computer science component have had very limited exposure to programming
and in particular working on complex programming assignments. A Non-CSE student is a student
who is not enrolled in a CSE program (single or double degree). Examples would include students
enrolled exclusively in a single degree program such as Mechatronics or Aerospace or Actuarial
Studies or Law. Students enrolled in dual degree programs that include a CSE program as one
of the degrees do not qualify. Any student who meets this criterion and wishes to avail of this
option MUST email cs3331@cse.unsw.edu.au to seek approval before 5pm, 29th March (Friday,
Week 7). If approved, we will send you the specification for the non-CSE version of the
assignment. We will assume by default that all students are attempting the CSE version of the
assignment unless they have sought explicit permission. No exceptions.
3. Assignment Specification
STP should be implemented as two separate programs: sender and receiver. You should implement
unidirectional transfer of data from the sender to the receiver. As illustrated in Figure 1, data
segments will flow from sender to receiver while ACK segments will flow from receiver to sender.
The sender and receiver programs will be run from different terminals on the same machine, so you
can use localhost, i.e., 127.0.0.1 as the IP address for the sender and receiver in your program. Let
us reiterate this, STP must be implemented on top of UDP. Do not use TCP sockets. If you use
TCP, you will not receive any marks for your assignment.
You will find it useful to review Sections 3.5.4 - 3.5.6 of the text (or the relevant parts from the
Week 5 lecture notes). It may also be useful to review the basic concepts of reliable data transfer
from Section 3.4 (or relevant parts from the Week 4 lecture notes). Section 3.5 of the textbook
which covers the bulk of the discussion on TCP is available to download on the assignment page.
Figure 1: This depicts the assignment setup. A file is to be transferred from the Sender to the Receiver, both running on the same
machine. Data segments will flow from the sender to receiver, while ACK segments will flow from the receiver to sender.
Data
Ack Sender Receiver
UDP Socket1
sender_port specified as argument
UDP Socket 2
receiver_port specified as argument
3
3.1 File Names
The main code for the sender should be contained in the following files: sender.c, or
Sender.java, or sender.py. You may create additional files such as header files or other
class files and name them as you wish.
The sender should accept the following seven arguments:
1. sender_port: the UDP port number to be used by the sender to send STP segments to the
receiver. The sender will receive ACK segments from the receiver through this same port. We
recommend using a random port number between 49152 to 65535 (dynamic port number
range) for the sender and receiver ports.
2. receiver_port: the UDP port number on which receiver is expecting to receive STP
segments from the sender. The receiver should send ACK segments to the sender through this
same port. We recommend using a random port number in the same range noted above.
3. txt_file_to_send: the name of the text file that must be transferred from sender to
receiver using your reliable transport protocol. You may assume that the file included in the
argument will be available in the current working directory of the sender with the “read”
access permissions set (execute “chmod +r txt_file_to_send” at the terminal in the
directory containing the file, where txt_file_to_send is set to the actual name of the
file).
4. max_win: the maximum window size in bytes for the sender window. This should be an
unsigned integer. Effectively, this is the maximum number of data bytes that the sender can
transmit in a pipelined manner and for which ACKs are outstanding. max_win must be
greater than or equal to 1000 bytes (MSS) and does not include STP headers. When
max_win is set to 1000 bytes, STP will effectively behave as a stop-and-wait protocol,
wherein the sender transmits one data segment at any given time and waits for the
corresponding ACK segment. While testing, we will ensure that max_win is a multiple of
1000 bytes (e.g., 5000 bytes).
5. rto: the value of the retransmission timer in milliseconds. This should be an unsigned
integer.
6. flp: forward loss probability, which is the probability that any segment in the forward
direction (DATA, FIN, SYN) is lost. This should be a float value between 0 and 1 (inclusive).
If flp is 0.1, then the sender will drop approximately 10% of the segments that it intends to
send to the receiver.
7. rlp: reverse loss probability, which is the probability of a segment in the reverse direction
(i.e., ACKs) being lost. This should be a float value between 0 and 1 (inclusive). If rlp is
0.05, then the sender will drop approximately 5% of the ACK segments received.
The sender should be initiated as follows:
If you use Java:
java Sender sender_port receiver_port txt_file_to_send max_win rto flp rlp
If you use C:
./sender sender_port receiver_port txt_file_to_send max_win rto flp rlp
If you use Python:
python3 sender.py sender_port receiver_port txt_file_to_send max_win rto flp rlp
4
During testing, we will ensure that the 7 arguments provided are in the correct format. We will not
test for erroneous arguments, missing arguments, etc. That said, it is good programming practice to
check for such input errors.
The main code for the receiver should be contained in the following files: receiver.c, or
Receiver.java, or receiver.py. You may create additional files such as header files or
other class files and name them as you wish.
The receiver should accept the following four arguments:
1. receiver_port: the UDP port number to be used by the receiver to receive STP segments
from the sender. This argument should match the second argument for the sender.
2. sender_port: the UDP port number to be used by the sender to send STP segments to the
receiver. This argument should match the first argument for the sender.
3. txt_file_received: the name of the text file into which the data sent by the sender
should be stored (this is the file that is being transferred from sender to receiver). You may
assume that the receiver program will have permission to create files in its working directory
(execute “chmod +w .” at the terminal to allow the creation of files in the working
directory), and that a file with this name does not exist in the working directory or may be
overwritten.
4. max_win: the receive window size. This argument should match the fourth argument for the
sender. It is provided to the receiver to ensure that the receiver can initialise a buffer
sufficient to hold all the data that the sender can transmit in a pipelined manner.
The receiver should be initiated as follows:
If you use Java:
java Receiver receiver_port sender_port txt_file_received max_win
If you use C:
./receiver receiver_port sender_port txt_file_received max_win
If you use Python:
python3 receiver.py receiver_port sender_port txt_file_received max_win
During testing, we will ensure that the 4 arguments provided are in the correct format. We will not
test for erroneous arguments, missing arguments, etc. That said, it is good programming practice to
check for such input errors.
The receiver must be initiated before initiating the sender. The two programs will be executed on
the same machine. Pay attention to the order of the port numbers to be specified in the arguments
for the two programs as they are in reverse order (sender port is first for the sender while receiver
port is first for the receiver). If you receive an error that one or both port numbers are in use, then
choose different values from the dynamic port number range (49152 to 65535) and try again.
The sender and receiver should exit after the file transfer is complete and the required information,
as stated in the subsequent sections of this document, is written to the sender and receiver log files.
3.2 Segment Format
STP segments must have 2 *two*-byte fields: "type" and "seqno" headers. Each of these store
unsigned integer values in network byte order, i.e., big-endian.
type (2 bytes) seqno (2 bytes) Data (0 to MSS bytes)
5
The "type" field takes on 4 possible values. DATA = 0, ACK = 1, SYN = 2, FIN = 3.
Unlike TCP, in which multiple types can be set simultaneously, STP segments must be of exactly
one of the types specified above.
The "seqno" field indicates the sequence number of the segment. This field is used in all segments.
For DATA segments, the sequence number increases by the size (in bytes) of each segment
(excluding headers). For ACK segments, the sequence number acts as a cumulative
acknowledgment, and indicates the number of the next byte expected by the receiver. For SYN
segments, the sequence number is the initial sequence number (ISN), which should be a randomly
chosen integer from 0 to 2^16 - 1, which is the maximum sequence number. The sequence number
of the first DATA segment of the connection should thus be ISN+1. For FIN segments, the
sequence number is one larger than the sequence number of the last byte of the last data segment of
the connection. The Maximum Segment Size (MSS) (excluding headers) for STP segment is 1000
bytes. A DATA segment can thus be up to 1004 bytes long. The last DATA segment for the file
being transferred may contain less than 1000 bytes as the file size may not be a multiple of 1000
bytes. All segments excluding DATA segments should only contain the headers and must thus be
exactly 4 bytes long.
The logic for determining the sequence number and ack number in STP is like TCP. However, STP
does not use a separate ack number header field. Rather, the “seqno” field contains the ack number
for the ACK segments.
3.3 State Diagram
The asymmetry between sender and receiver leads to somewhat different state diagrams for the two
endpoints. The state diagram for STP is shown below, which depicts the normal behaviour for both
end points.
Figure 2: State diagram depicting the normal behaviour of STP.
The receiver can be in four possible states: CLOSED, LISTEN, ESTABLISHED and TIME_WAIT.
Initially, it is in the CLOSED state. Upon issuing a passive open, it enters the LISTEN state. Note
that the receiver is the passive host in our protocol and is initiated first, while the sender is initiated
next and actively opens the connection. While in the LISTEN state, the receiver waits for a SYN
segment to arrive on the correct port number. When it does, it responds with an ACK, and moves to
the ESTABLISHED state. Should a duplicate SYN arrive while the receiver is in the
ESTABLISHED state, it reissues the ACK and remains in the ESTABLISHED state. The ACKs
sent by the receiver are cumulative (like TCP). After the sender has reliably transmitted all data
6
(and received acknowledgments), it will send a FIN segment to the receiver. Upon receipt of the
FIN, the receiver moves to the TIME_WAIT state. As in TCP, it remains in TIME_WAIT for two
maximum segment lifetimes (MSLs) before re-entering the CLOSED state. This is to ensure that
the receiver can respond to potentially retransmitted FIN segments from the sender. You may
assume that the MSL is 1 seconds. In other words, the receiver should remain in TIME_WAIT for 2
seconds and then transition to CLOSED.
The sender can be in five possible states: CLOSED, SYN_SENT, ESTABLISHED, CLOSING and
FIN_WAIT. Like the receiver, the sender starts in the CLOSED state. It then issues an active open
by sending a SYN segment (to the receiver's port), thus entering the SYN_SENT state. This SYN
transmission also includes the initial sequence number (ISN) of the conversation. The ISN should
be chosen at random from the valid range of possible sequence numbers (0 to 2^16 – 1). If a
corresponding ACK is not received within rto msec, the sender should retransmit the SYN
segment. In the common case in which the SYN is acknowledged correctly (the ACK must have the
correct sequence number = ISN + 1), the sender enters the ESTABLISHED state and starts
transmitting DATA segments. The sender maintains a single timer (for rto msec) for the oldest
unacknowledged packet and only retransmits this packet if the timer expires. Once all DATA
segments have been transmitted the sender enters the CLOSING state. At this point, the sender must
still ensure that any buffered data arrives at the receiver reliably. Upon verification of successful
transmission, the sender sends a FIN segment with the appropriate sequence number (1 greater than
the sequence number of the last data byte) and enters the FIN_WAIT state. Once the FIN segment
is acknowledged, the sender re-enters the CLOSED state. If an ACK is not received before the
timer (rto msec) expires, the sender should retransmit the FIN segment.
Strictly speaking, you don’t have to implement the CLOSED state at the start for the sender. Your
sender program when executed can immediately send the SYN segment and enter the SYN_SENT
state. Also, when the sender is in the FIN_WAIT state and receives the ACK for the FIN segment,
the program can simply exit. This is because the sender only transmits a single file in one execution
and quits following the reliable file transfer.
Unlike TCP which follows a three-way handshake (SYN, SYN/ACK, ACK) for connection setup
and independent connection closures (FIN, ACK) in each direction, STP follows a two-way
connection setup (SYN, ACK) and one directional connection closure (FIN, ACK) process. The
setup and closure are always initiated by the sender.
If one end point detects behaviour that is unexpected, it should simply print a message to the
terminal indicating that the connection is being reset, and then terminate. For example, if the
receiver receives a DATA segment while it is in the LISTEN state (where it is expecting a SYN
segment). The state transition diagram on the previous page does not capture such erroneous
scenarios. You are free to specify the format of the message that is printed. Note that, we will NOT
be rigorously testing your code for such unexpected behaviour.
3.4 List of features to be implemented by the sender.
You are required to implement the following features in the sender (and equivalent functionality in
the receiver).
1. The sender should first open a UDP socket on sender_port and initiate a two-way
handshake (SYN, ACK) for the connection establishment. The sender sends a SYN segment, and
the receiver responds with an ACK. This is different to the three-way handshake implemented by
TCP. If the ACK is not received before a timeout (rto msec), the sender should retransmit the
SYN.
2. The sender must choose a random initial sequence number (ISN) between 0 and 216 – 1.
7
Remember to perform sequence number arithmetic modulo 216. The sequence numbers should cycle
back to zero after reaching 216 – 1.
3. A one-directional (forward) connection termination (FIN, ACK). The sender will initiate the
connection close once the entire file has been reliably transmitted by sending the FIN segment and
the receiver will respond with an ACK. This is different to the bi-directional close implemented by
TCP. If the ACK is not received before a timeout (rto msec), the sender should retransmit the FIN.
The sender should terminate after connection closure.
4. STP implements a sliding window protocol like TCP, whereby multiple segments can be
transmitted by the sender in a pipelined manner. The sender should maintain a buffer to store all
unacknowledged segments. The total amount of data that the sender can transmit in a pipelined
manner and for which acknowledgments are pending is limited by max_win. Similar to TCP, as
the sender receives ACK segments, the left edge of the window can slide forward, and the sender
can transmit the next DATA segments (if there is pending data to be sent).
5. Each STP segment transmitted by the sender (including DATA, SYN, FIN) must be encapsulated
in a UDP segment and transmitted through the UDP socket.
6. The sender must maintain a single timer for retransmission of data segments (Section 3.5.4 of the
text). The value of the timeout will be supplied as a command-line argument to the sender program
(rto msec). This timer is for the oldest unacknowledged data segment. In the event of a timeout,
only the oldest unacknowledged data segment should be retransmitted (like TCP). The sender
should not retransmit all unacknowledged segments. Remember that you are NOT implementing
Go-Back-N.
7. The sender should implement all the features mentioned in Section 3.5.4 of the text, except for
doubling the timeout interval. You are expected to implement the functionality of the simplified
TCP sender (Figure 3.33 of the text) and fast retransmit (i.e., the sender should retransmit the oldest
unacknowledged data segment on three duplicate ACKs) (pages 247-248).
8. The use of the “seqno” field was outlined in Section 3.2. For DATA segments, the sequence
number increases by the size (in bytes) of each segment (excluding headers). For ACK segments,
the sequence number acts as a cumulative acknowledgment, and indicates the number of the next
byte expected by the receiver. The logic is thus like TCP, except that STP does not use a separate
ACK header field. The ACK segments use the seqno header field to indicate the ACK numbers.
9. The sender will receive each ACK segment from the receiver through the same socket which the
sender uses to transmit data. The ACK segment will be encapsulated in a UDP segment. The sender
must first extract the ACK segment from the UDP segment and then process it as per the operation
of the STP protocol. ACK segments have the same format as DATA segments but do not contain
any data.
10. The sender program should emulate the behaviour of an unreliable communication channel
between the sender and receiver. UDP segments can occasionally experience loss in a network, but
the likelihood is very low when the sender and receiver are executed on the same machine.
Moreover, to properly test the implementation of your sender program, we would like to control the
unreliable behaviour of the underlying channel. The sender emulates loss of STP segments in both
directions which can be controlled through two command line argument: (i) flp: determines the
probability of a DATA, SYN, or FIN segment created by the sender being dropped. In other words,
each such segment created by the sender will be dropped with a probability flp. If the segment is
not dropped, then it will be transmitted through the socket to the receiver. (ii) rlp: determines the
probability of an ACK segment in the reverse direction from the sender being dropped. In other
words, each ACK segment arriving at the sender socket will be dropped with a probability rlp. If
the segment is not dropped, then it will be processed as per the STP protocol.
8
Note about Random Number Generation
You will need to generate random numbers to establish an initial sequence number (ISN) and
implement segment loss. If you have not learnt about the principles behind random number
generators, you need to know that random numbers are in fact generated by a deterministic formula
by a computer program. Therefore, strictly speaking, random number generators are called pseudorandom number generators because the numbers are not truly random. The deterministic formula
for random number generation in Python, Java and C use an input parameter called a seed. If a fixed
seed is used, then the same sequence of random numbers will be produced, each time the program is
executed. This will thus likely generate the same ISN and the same sequence of segment loss in
each execution of the sender. While this may be useful for debugging purposes, it is not a realistic
representation of an unreliable channel. Thus, you must ensure that you do not use a fixed seed in
your submitted program. A simple way to use a different seed for each execution is to base the seed
on the system time.
The following code fragments in Python, Java and C generate random numbers between 0 and 1
with a different seed in each execution.
• In Python, you initialise a random number generator by using random.seed(). By default,
the random number generator uses the current system time. After that you can generate a
random floating point number between (0,1) by using random.random().
• In Java, you initialise a random number generator by using Random random = new
Random(). This constructor sets the seed of the random number generator to a value very
likely to be distinct from any other invocation of this constructor. After that, you can generate a
random floating point number between (0,1) by using float x =
random.nextFloat().
• In C, you initialise a random number generator by using srand(time(NULL)). After that,
you can generate a random floating point number between (0,1) by using float x =
rand()/((float)(RAND_MAX)+1). Note that, RAND_MAX is the maximum value
returned by the rand() function.
11. The sender should maintain a log file named sender_log.txt where it records the
information about each segment that it sends and receives. Information about dropped segments
should also be included. You may assume that the sender program will have permission to create
files in its current working directory. If the file already exists, then it should be overwritten. Start
each entry on a new line. The format should be as follows:
Assignment for Term 1, 2024
Version 1.0
Due: 11:59am (noon) Thursday, 18 April 2024 (Week 10)
1. Change Log
Version 1.0 released on 7th March 2024.
2. Goal and learning objectives
For this assignment, you are to implement a reliable transport protocol over the UDP protocol. We
will refer to the reliable transport protocol that you will be implementing in this assignment as
Simple Transport Protocol (STP). STP will include most (but not all) of the features that are
described in Sections 3.5.4 - 3.5.6 of the text Computer Networking by Kurose and Ross (7th or 8th
ed.) or equivalent parts from the Week 4/5 lecture notes. Examples of these features include
timeout, ACK, sequence numbers, sliding window, etc. Note that these features are commonly
found in many transport protocols. Therefore, this assignment will give you an opportunity to
implement some of these basic features of a transport protocol. In addition, you may have wondered
why the designer of the TCP/IP protocol stack includes such a feature-less transport protocol as
UDP. You will find in this assignment that you can design your own transport protocol and run it
over UDP. This is the case for some multimedia delivery services on the Internet, where they have
implemented their own proprietary transport protocol over UDP. QUIC, a newly proposed transport
protocol also runs over UDP and implements additional functionalities such as reliability.
Recall that UDP provides point-to-point, unreliable datagram service between a pair of hosts. In this
programming assignment, you will develop a more structured protocol, STP, which ensures
reliable, end-to-end delivery of data in the face of packet loss. STP provides a byte-stream
abstraction like TCP and sends pipelined data segments using a sliding window. However, STP
does not implement congestion control or flow control. Finally, whereas TCP allows fully
bidirectional communication, your implementation of STP will be asymmetric. There will be two
distinct STP endpoints, "sender" and "receiver", respectively. Data packets will only flow in the
"forward" direction from the sender to the receiver, while acknowledgments will only flow in the
"reverse" direction from the receiver back to the sender. To support reliability in a protocol like
STP, state must be maintained at both endpoints. Thus, as in TCP, connection set-up and connection
teardown phases will be an integral part of the protocol. STP should implement a sliding window
protocol wherein multiple segments can be sent by the sender in a pipelined manner. Like TCP,
STP will include some elements of both Go-Back-N (GBN) and Selective Repeat (SR). You will
use your STP protocol to transfer a text file (examples provided on the assignment webpage) from
the sender to the receiver.
The sender program must also emulate the behaviour of an unreliable communication channel
between the sender and receiver. Even though UDP segments can get lost, the likelihood of such
losses is virtually zero in our test environment, where the sender and receiver will be executed on
the same machine. Further, to properly test the implementation of your sender program, we would
like to control the unreliable behaviour of the underlying channel. The sender program should
Updates to the assignment, including any corrections and clarifications, will be posted on the
course website. Please make sure that you check the course website regularly for updates.
2
emulate loss of STP segments in both directions – (i) DATA, SYN, and FIN segments in the
forward direction and (ii) ACK segments in the reverse direction. You may assume that the
underlying channel will never reorder or corrupt STP segments (in both directions).
Note that it is mandatory that you implement STP over UDP. Do not use TCP sockets. You
will not receive any marks for this assignment if you use TCP sockets.
2.1 Learning Objectives
On completing this assignment, you will gain sufficient expertise in the following skills:
1. Detailed understanding of how reliable transport protocols such as TCP function.
2. Socket programming for UDP transport protocol.
3. Protocol and message design.
Non-CSE Student Version: The rationale for this option is that students enrolled in a program that
does not include a computer science component have had very limited exposure to programming
and in particular working on complex programming assignments. A Non-CSE student is a student
who is not enrolled in a CSE program (single or double degree). Examples would include students
enrolled exclusively in a single degree program such as Mechatronics or Aerospace or Actuarial
Studies or Law. Students enrolled in dual degree programs that include a CSE program as one
of the degrees do not qualify. Any student who meets this criterion and wishes to avail of this
option MUST email cs3331@cse.unsw.edu.au to seek approval before 5pm, 29th March (Friday,
Week 7). If approved, we will send you the specification for the non-CSE version of the
assignment. We will assume by default that all students are attempting the CSE version of the
assignment unless they have sought explicit permission. No exceptions.
3. Assignment Specification
STP should be implemented as two separate programs: sender and receiver. You should implement
unidirectional transfer of data from the sender to the receiver. As illustrated in Figure 1, data
segments will flow from sender to receiver while ACK segments will flow from receiver to sender.
The sender and receiver programs will be run from different terminals on the same machine, so you
can use localhost, i.e., 127.0.0.1 as the IP address for the sender and receiver in your program. Let
us reiterate this, STP must be implemented on top of UDP. Do not use TCP sockets. If you use
TCP, you will not receive any marks for your assignment.
You will find it useful to review Sections 3.5.4 - 3.5.6 of the text (or the relevant parts from the
Week 5 lecture notes). It may also be useful to review the basic concepts of reliable data transfer
from Section 3.4 (or relevant parts from the Week 4 lecture notes). Section 3.5 of the textbook
which covers the bulk of the discussion on TCP is available to download on the assignment page.
Figure 1: This depicts the assignment setup. A file is to be transferred from the Sender to the Receiver, both running on the same
machine. Data segments will flow from the sender to receiver, while ACK segments will flow from the receiver to sender.
Data
Ack Sender Receiver
UDP Socket1
sender_port specified as argument
UDP Socket 2
receiver_port specified as argument
3
3.1 File Names
The main code for the sender should be contained in the following files: sender.c, or
Sender.java, or sender.py. You may create additional files such as header files or other
class files and name them as you wish.
The sender should accept the following seven arguments:
1. sender_port: the UDP port number to be used by the sender to send STP segments to the
receiver. The sender will receive ACK segments from the receiver through this same port. We
recommend using a random port number between 49152 to 65535 (dynamic port number
range) for the sender and receiver ports.
2. receiver_port: the UDP port number on which receiver is expecting to receive STP
segments from the sender. The receiver should send ACK segments to the sender through this
same port. We recommend using a random port number in the same range noted above.
3. txt_file_to_send: the name of the text file that must be transferred from sender to
receiver using your reliable transport protocol. You may assume that the file included in the
argument will be available in the current working directory of the sender with the “read”
access permissions set (execute “chmod +r txt_file_to_send” at the terminal in the
directory containing the file, where txt_file_to_send is set to the actual name of the
file).
4. max_win: the maximum window size in bytes for the sender window. This should be an
unsigned integer. Effectively, this is the maximum number of data bytes that the sender can
transmit in a pipelined manner and for which ACKs are outstanding. max_win must be
greater than or equal to 1000 bytes (MSS) and does not include STP headers. When
max_win is set to 1000 bytes, STP will effectively behave as a stop-and-wait protocol,
wherein the sender transmits one data segment at any given time and waits for the
corresponding ACK segment. While testing, we will ensure that max_win is a multiple of
1000 bytes (e.g., 5000 bytes).
5. rto: the value of the retransmission timer in milliseconds. This should be an unsigned
integer.
6. flp: forward loss probability, which is the probability that any segment in the forward
direction (DATA, FIN, SYN) is lost. This should be a float value between 0 and 1 (inclusive).
If flp is 0.1, then the sender will drop approximately 10% of the segments that it intends to
send to the receiver.
7. rlp: reverse loss probability, which is the probability of a segment in the reverse direction
(i.e., ACKs) being lost. This should be a float value between 0 and 1 (inclusive). If rlp is
0.05, then the sender will drop approximately 5% of the ACK segments received.
The sender should be initiated as follows:
If you use Java:
java Sender sender_port receiver_port txt_file_to_send max_win rto flp rlp
If you use C:
./sender sender_port receiver_port txt_file_to_send max_win rto flp rlp
If you use Python:
python3 sender.py sender_port receiver_port txt_file_to_send max_win rto flp rlp
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During testing, we will ensure that the 7 arguments provided are in the correct format. We will not
test for erroneous arguments, missing arguments, etc. That said, it is good programming practice to
check for such input errors.
The main code for the receiver should be contained in the following files: receiver.c, or
Receiver.java, or receiver.py. You may create additional files such as header files or
other class files and name them as you wish.
The receiver should accept the following four arguments:
1. receiver_port: the UDP port number to be used by the receiver to receive STP segments
from the sender. This argument should match the second argument for the sender.
2. sender_port: the UDP port number to be used by the sender to send STP segments to the
receiver. This argument should match the first argument for the sender.
3. txt_file_received: the name of the text file into which the data sent by the sender
should be stored (this is the file that is being transferred from sender to receiver). You may
assume that the receiver program will have permission to create files in its working directory
(execute “chmod +w .” at the terminal to allow the creation of files in the working
directory), and that a file with this name does not exist in the working directory or may be
overwritten.
4. max_win: the receive window size. This argument should match the fourth argument for the
sender. It is provided to the receiver to ensure that the receiver can initialise a buffer
sufficient to hold all the data that the sender can transmit in a pipelined manner.
The receiver should be initiated as follows:
If you use Java:
java Receiver receiver_port sender_port txt_file_received max_win
If you use C:
./receiver receiver_port sender_port txt_file_received max_win
If you use Python:
python3 receiver.py receiver_port sender_port txt_file_received max_win
During testing, we will ensure that the 4 arguments provided are in the correct format. We will not
test for erroneous arguments, missing arguments, etc. That said, it is good programming practice to
check for such input errors.
The receiver must be initiated before initiating the sender. The two programs will be executed on
the same machine. Pay attention to the order of the port numbers to be specified in the arguments
for the two programs as they are in reverse order (sender port is first for the sender while receiver
port is first for the receiver). If you receive an error that one or both port numbers are in use, then
choose different values from the dynamic port number range (49152 to 65535) and try again.
The sender and receiver should exit after the file transfer is complete and the required information,
as stated in the subsequent sections of this document, is written to the sender and receiver log files.
3.2 Segment Format
STP segments must have 2 *two*-byte fields: "type" and "seqno" headers. Each of these store
unsigned integer values in network byte order, i.e., big-endian.
type (2 bytes) seqno (2 bytes) Data (0 to MSS bytes)
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The "type" field takes on 4 possible values. DATA = 0, ACK = 1, SYN = 2, FIN = 3.
Unlike TCP, in which multiple types can be set simultaneously, STP segments must be of exactly
one of the types specified above.
The "seqno" field indicates the sequence number of the segment. This field is used in all segments.
For DATA segments, the sequence number increases by the size (in bytes) of each segment
(excluding headers). For ACK segments, the sequence number acts as a cumulative
acknowledgment, and indicates the number of the next byte expected by the receiver. For SYN
segments, the sequence number is the initial sequence number (ISN), which should be a randomly
chosen integer from 0 to 2^16 - 1, which is the maximum sequence number. The sequence number
of the first DATA segment of the connection should thus be ISN+1. For FIN segments, the
sequence number is one larger than the sequence number of the last byte of the last data segment of
the connection. The Maximum Segment Size (MSS) (excluding headers) for STP segment is 1000
bytes. A DATA segment can thus be up to 1004 bytes long. The last DATA segment for the file
being transferred may contain less than 1000 bytes as the file size may not be a multiple of 1000
bytes. All segments excluding DATA segments should only contain the headers and must thus be
exactly 4 bytes long.
The logic for determining the sequence number and ack number in STP is like TCP. However, STP
does not use a separate ack number header field. Rather, the “seqno” field contains the ack number
for the ACK segments.
3.3 State Diagram
The asymmetry between sender and receiver leads to somewhat different state diagrams for the two
endpoints. The state diagram for STP is shown below, which depicts the normal behaviour for both
end points.
Figure 2: State diagram depicting the normal behaviour of STP.
The receiver can be in four possible states: CLOSED, LISTEN, ESTABLISHED and TIME_WAIT.
Initially, it is in the CLOSED state. Upon issuing a passive open, it enters the LISTEN state. Note
that the receiver is the passive host in our protocol and is initiated first, while the sender is initiated
next and actively opens the connection. While in the LISTEN state, the receiver waits for a SYN
segment to arrive on the correct port number. When it does, it responds with an ACK, and moves to
the ESTABLISHED state. Should a duplicate SYN arrive while the receiver is in the
ESTABLISHED state, it reissues the ACK and remains in the ESTABLISHED state. The ACKs
sent by the receiver are cumulative (like TCP). After the sender has reliably transmitted all data
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(and received acknowledgments), it will send a FIN segment to the receiver. Upon receipt of the
FIN, the receiver moves to the TIME_WAIT state. As in TCP, it remains in TIME_WAIT for two
maximum segment lifetimes (MSLs) before re-entering the CLOSED state. This is to ensure that
the receiver can respond to potentially retransmitted FIN segments from the sender. You may
assume that the MSL is 1 seconds. In other words, the receiver should remain in TIME_WAIT for 2
seconds and then transition to CLOSED.
The sender can be in five possible states: CLOSED, SYN_SENT, ESTABLISHED, CLOSING and
FIN_WAIT. Like the receiver, the sender starts in the CLOSED state. It then issues an active open
by sending a SYN segment (to the receiver's port), thus entering the SYN_SENT state. This SYN
transmission also includes the initial sequence number (ISN) of the conversation. The ISN should
be chosen at random from the valid range of possible sequence numbers (0 to 2^16 – 1). If a
corresponding ACK is not received within rto msec, the sender should retransmit the SYN
segment. In the common case in which the SYN is acknowledged correctly (the ACK must have the
correct sequence number = ISN + 1), the sender enters the ESTABLISHED state and starts
transmitting DATA segments. The sender maintains a single timer (for rto msec) for the oldest
unacknowledged packet and only retransmits this packet if the timer expires. Once all DATA
segments have been transmitted the sender enters the CLOSING state. At this point, the sender must
still ensure that any buffered data arrives at the receiver reliably. Upon verification of successful
transmission, the sender sends a FIN segment with the appropriate sequence number (1 greater than
the sequence number of the last data byte) and enters the FIN_WAIT state. Once the FIN segment
is acknowledged, the sender re-enters the CLOSED state. If an ACK is not received before the
timer (rto msec) expires, the sender should retransmit the FIN segment.
Strictly speaking, you don’t have to implement the CLOSED state at the start for the sender. Your
sender program when executed can immediately send the SYN segment and enter the SYN_SENT
state. Also, when the sender is in the FIN_WAIT state and receives the ACK for the FIN segment,
the program can simply exit. This is because the sender only transmits a single file in one execution
and quits following the reliable file transfer.
Unlike TCP which follows a three-way handshake (SYN, SYN/ACK, ACK) for connection setup
and independent connection closures (FIN, ACK) in each direction, STP follows a two-way
connection setup (SYN, ACK) and one directional connection closure (FIN, ACK) process. The
setup and closure are always initiated by the sender.
If one end point detects behaviour that is unexpected, it should simply print a message to the
terminal indicating that the connection is being reset, and then terminate. For example, if the
receiver receives a DATA segment while it is in the LISTEN state (where it is expecting a SYN
segment). The state transition diagram on the previous page does not capture such erroneous
scenarios. You are free to specify the format of the message that is printed. Note that, we will NOT
be rigorously testing your code for such unexpected behaviour.
3.4 List of features to be implemented by the sender.
You are required to implement the following features in the sender (and equivalent functionality in
the receiver).
1. The sender should first open a UDP socket on sender_port and initiate a two-way
handshake (SYN, ACK) for the connection establishment. The sender sends a SYN segment, and
the receiver responds with an ACK. This is different to the three-way handshake implemented by
TCP. If the ACK is not received before a timeout (rto msec), the sender should retransmit the
SYN.
2. The sender must choose a random initial sequence number (ISN) between 0 and 216 – 1.
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Remember to perform sequence number arithmetic modulo 216. The sequence numbers should cycle
back to zero after reaching 216 – 1.
3. A one-directional (forward) connection termination (FIN, ACK). The sender will initiate the
connection close once the entire file has been reliably transmitted by sending the FIN segment and
the receiver will respond with an ACK. This is different to the bi-directional close implemented by
TCP. If the ACK is not received before a timeout (rto msec), the sender should retransmit the FIN.
The sender should terminate after connection closure.
4. STP implements a sliding window protocol like TCP, whereby multiple segments can be
transmitted by the sender in a pipelined manner. The sender should maintain a buffer to store all
unacknowledged segments. The total amount of data that the sender can transmit in a pipelined
manner and for which acknowledgments are pending is limited by max_win. Similar to TCP, as
the sender receives ACK segments, the left edge of the window can slide forward, and the sender
can transmit the next DATA segments (if there is pending data to be sent).
5. Each STP segment transmitted by the sender (including DATA, SYN, FIN) must be encapsulated
in a UDP segment and transmitted through the UDP socket.
6. The sender must maintain a single timer for retransmission of data segments (Section 3.5.4 of the
text). The value of the timeout will be supplied as a command-line argument to the sender program
(rto msec). This timer is for the oldest unacknowledged data segment. In the event of a timeout,
only the oldest unacknowledged data segment should be retransmitted (like TCP). The sender
should not retransmit all unacknowledged segments. Remember that you are NOT implementing
Go-Back-N.
7. The sender should implement all the features mentioned in Section 3.5.4 of the text, except for
doubling the timeout interval. You are expected to implement the functionality of the simplified
TCP sender (Figure 3.33 of the text) and fast retransmit (i.e., the sender should retransmit the oldest
unacknowledged data segment on three duplicate ACKs) (pages 247-248).
8. The use of the “seqno” field was outlined in Section 3.2. For DATA segments, the sequence
number increases by the size (in bytes) of each segment (excluding headers). For ACK segments,
the sequence number acts as a cumulative acknowledgment, and indicates the number of the next
byte expected by the receiver. The logic is thus like TCP, except that STP does not use a separate
ACK header field. The ACK segments use the seqno header field to indicate the ACK numbers.
9. The sender will receive each ACK segment from the receiver through the same socket which the
sender uses to transmit data. The ACK segment will be encapsulated in a UDP segment. The sender
must first extract the ACK segment from the UDP segment and then process it as per the operation
of the STP protocol. ACK segments have the same format as DATA segments but do not contain
any data.
10. The sender program should emulate the behaviour of an unreliable communication channel
between the sender and receiver. UDP segments can occasionally experience loss in a network, but
the likelihood is very low when the sender and receiver are executed on the same machine.
Moreover, to properly test the implementation of your sender program, we would like to control the
unreliable behaviour of the underlying channel. The sender emulates loss of STP segments in both
directions which can be controlled through two command line argument: (i) flp: determines the
probability of a DATA, SYN, or FIN segment created by the sender being dropped. In other words,
each such segment created by the sender will be dropped with a probability flp. If the segment is
not dropped, then it will be transmitted through the socket to the receiver. (ii) rlp: determines the
probability of an ACK segment in the reverse direction from the sender being dropped. In other
words, each ACK segment arriving at the sender socket will be dropped with a probability rlp. If
the segment is not dropped, then it will be processed as per the STP protocol.
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Note about Random Number Generation
You will need to generate random numbers to establish an initial sequence number (ISN) and
implement segment loss. If you have not learnt about the principles behind random number
generators, you need to know that random numbers are in fact generated by a deterministic formula
by a computer program. Therefore, strictly speaking, random number generators are called pseudorandom number generators because the numbers are not truly random. The deterministic formula
for random number generation in Python, Java and C use an input parameter called a seed. If a fixed
seed is used, then the same sequence of random numbers will be produced, each time the program is
executed. This will thus likely generate the same ISN and the same sequence of segment loss in
each execution of the sender. While this may be useful for debugging purposes, it is not a realistic
representation of an unreliable channel. Thus, you must ensure that you do not use a fixed seed in
your submitted program. A simple way to use a different seed for each execution is to base the seed
on the system time.
The following code fragments in Python, Java and C generate random numbers between 0 and 1
with a different seed in each execution.
• In Python, you initialise a random number generator by using random.seed(). By default,
the random number generator uses the current system time. After that you can generate a
random floating point number between (0,1) by using random.random().
• In Java, you initialise a random number generator by using Random random = new
Random(). This constructor sets the seed of the random number generator to a value very
likely to be distinct from any other invocation of this constructor. After that, you can generate a
random floating point number between (0,1) by using float x =
random.nextFloat().
• In C, you initialise a random number generator by using srand(time(NULL)). After that,
you can generate a random floating point number between (0,1) by using float x =
rand()/((float)(RAND_MAX)+1). Note that, RAND_MAX is the maximum value
returned by the rand() function.
11. The sender should maintain a log file named sender_log.txt where it records the
information about each segment that it sends and receives. Information about dropped segments
should also be included. You may assume that the sender program will have permission to create
files in its current working directory. If the file already exists, then it should be overwritten. Start
each entry on a new line. The format should be as follows: