The University of New South Wales
COMP3331/9331 Computer Networks and Applications
Assignment for Session 2, 2018
Version 1.0
1. Change Log
Version 1.0 released on 17th August 2018.
See the changes marked in Red color.
2. Due date:
Due: 11:59pm Friday, 19th October 2018 (Week 12).
Early bird incentive: 10% bonus marks if the assignment is submitted before 11:59 pm Friday, 12th
October 2018 (Week 11). Note that the final marks of assignment component are capped at maximum 20.
E.g., if you get 19 marks and have submitted as an early bird, your marks are increased (and capped to)
20 marks.
2. Goal and learning objectives
For this assignment, you will be asked to implement a reliable transport protocol over the UDP protocol.
We will refer to the reliable transport protocol that you will be programming in this assignment as Simple
Transport Protocol (STP). STP will include most (but not all) of the features that are described in Sections
3.5.3, 3.5.4 and 3.5.6 of the text Computer Networking (7th edition). Examples of these features include
timeout, ACK, sequence number 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 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 existing
multimedia delivery services in the Internet, where they have implemented their own proprietary transport
protocol over UDP.
Note that it is mandatory that you implement STP over UDP. Do not use TCP sockets. You will not
Updates to the assignment, including any corrections and clarifications, will be posted on the
WebCMS. Please make sure that you check the subject website regularly for updates.receive any mark for this assignment if you use TCP socket.
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.
3. Overview
As part of this assignment, you will have to implement Simple Transport Protocol (STP), a piece of
software that consists of a sender and receiver component that allows reliable unidirectional data transfer.
STP includes some of the features of the TCP protocols that are described in sections 3.5.3, 3.5.4 and 3.5.6
of the textbook (7th edition). You will use your STP protocol to transfer pdf files from the sender to the
receiver. You should implement STP as two separate programs: Sender and Receiver. You only have to
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. 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 3.5.3, 3.5.4 and 3.5.6 of the text. It may also be useful to review
the basic concepts of reliable data transfer from section 3.4.
Figure 1: The basic setup of your assignment. A file is to be transferred from the Sender to the Receiver.
Sender will run on the sender side while Receiver will run on the receiver side. Note that data segments
will flow from the sender to receiver, while ACK segments will flow from the receiver to sender.
4. Assignment Specifications
This section gives detailed specifications of the assignment. Total marks for this assignment are 20. You are free to choose C, JAVA or Python as the programming language (please see Section 5). The programs
will be tested on CSE Linux machines. So please make sure that your entire application runs
correctly on these machines (i.e. your lab computers). We are unable to mark your assignment if it
does not compile or run correctly on CSE lab computers, resulting in loss of significant marks.
4.1 File Names
The main code for the sender and receiver should be contained in the following files: sender.c or
Sender.java or sender.py, and receiver.c or Receiver.java or receiver.py. You are free to create additional
files such as header files or other class files and name them as you wish.
4.2 List of features provided by the Sender and Receiver
You are required to implement the following features in the Sender and Receiver:
1. A three-way handshake (SYN, SYN+ACK, ACK) for the connection establishment. The ACK sent by
the sender to conclude the three-way handshake should not contain any payload (i.e. data). See Section
3.5.6 of text for further details.
2. A four-segment (FIN, ACK, FIN, ACK) connection termination. The Sender will initiate the connection
close once the entire file has been successfully transmitted. See Section 3.5.6 of text for further details.
3. Sender must maintain a single-timer for timeout operation. You are required to implement round-triptime
estimation and RTO estimation discussed in Section 3.5.3 of the text. The timeout is not a constant
value but is given by the formula on page 243 of the text (TimeoutInterval = EstimatedRTT + 4 *
DevRTT). Use the initial value of EstimatedRTT = 500 milliseconds and DevRTT = 250 milliseconds.
4. Sender should implement all the features mentioned in Section 3.5.4 of the text, with the exception of
doubling the timeout. The STP protocol must include the simplified TCP sender (Figure 3.33 of the text)
and fast retransmit (pages 249-251). You will need to use a number of concepts that we have discussed in
class, e.g., sequence numbers, cumulative acknowledgements, timers, buffers, etc. for implementing your
protocol.
5. Receiver should implement the features mentioned in Section 3.5.4 of the text. However, you do not
need to follow Table 3.2 for ACK generation. All packets should be immediately acknowledged, i.e. you
do not have to implement delayed ACKs.
6. STP is a byte-stream oriented protocol. You will need to include sequence number and
acknowledgement number fields in the STP header for each segment. The meaning of sequence number
and acknowledgment number are the same as in TCP. 7. MSS (Maximum segment size) is the maximum number of bytes of data that your STP segment can
contain. In other words, MSS counts data ONLY and does NOT include header. Sender must be able to
deal with different values of MSS. The value of MSS will be supplied to Sender as an input argument.
8. Another input argument for Sender is Maximum Window Size (MWS). MWS is the maximum number
of un-acknowledged bytes that the Sender can have at any time. MWS counts ONLY data. Header length
should NOT be counted as part of MWS.
Remarks: Note that TCP does not explicitly define a maximum window size. In TCP, the maximum number
of un-acknowledged bytes is limited by the smaller of receive window and the congestion control window.
Since you will not be implementing flow or congestion control, you will be limiting the number of unacknowledged
bytes by using the MWS parameter. In other words, you will need to ensure that during the
lifetime of the connection, the following condition is satisfied:
LastByteSent – LastByteAcked ≤ MWS
9. Even though you will use UDP, since the sender and receiver will mostly be running on machines that
are within close proximity of each other (e.g.: on the same Ethernet LAN or even on the same physical
machine), there will be no real possibility of datagrams being dropped/delayed/corrupted. In order to test
the reliability of your protocol, it is imperative to introduce artificially induced packet loss, delays and
corruption etc. For this purpose, you must also implement a Packet Loss and Delay (PLD) Module as part
of the Sender program. The details for this module are explained later in the specification.
4.3 Features excluded
You do not need to implement any flow nor congestion control features for this assignment.
4.4 Packet header and MSS
In designing the segment header, you only need to include the fields that you think are necessary for STP.
You can draw inspiration from TCP but the exact format of the STP packet header is for you to decide.
The header portion can include as many fields as you think are necessary. Two important fields that will
be needed are the sequence number and acknowledgement number. You will also need a number of flags
for connection establishment and teardown.
The data portion must not contain more than MSS bytes of data. You must use the same STP segment
format for data transfer as well as for the acknowledgements flowing back from the receiver to the sender.
The only difference will be that the acknowledgement segments will not contain any data. All information
that is necessary for the proper functioning of your protocol must be provided in the STP headers. You
should not use any information from the header of the UDP datagram that will encapsulate the STP
segments. 4.5 Sender
This section provides details on the Sender.
The Sender should accept the following fourteen (14) arguments (note that the last eight arguments are
used exclusively by the PLD module):
1. receiver_host_ip: The IP address of the host machine on which the Receiver is running.
2. receiver_port: The port number on which Receiver is expecting to receive packets from the
sender.
3. file.pdf: The name of the pdf file that has to be transferred from sender to receiver using your STP.
4. MWS: The maximum window size used by your STP protocol in bytes.
5. MSS: Maximum Segment Size which is the maximum amount of data (in bytes) carried in each STP
segment.
6. gamma: This value is used for calculation of timeout value. See Section 7 of the specification for
details.
The following 8 arguments are used exclusively by the PLD module:
7. pDrop: The probability that a STP data segment which is ready to be transmitted will be dropped. This
value must be between 0 and 1. For example if pDrop = 0.5, it means that 50% of the transmitted
segments are dropped by the PLD.
8. pDuplicate: The probability that a data segment which is not dropped will be duplicated. This value
must also be between 0 and 1.
9. pCorrupt: The probability that a data segment which is not dropped/duplicated will be corrupted.
This value must also be between 0 and 1.
10. pOrder: The probability that a data segment which is not dropped, duplicated and corrupted will be
re-ordered. This value must also be between 0 and 1.
11. maxOrder: The maximum number of packets a particular packet is held back for re-ordering
purpose. This value must be between 1 and 6.
12. pDelay: The probability that a data segment which is not dropped, duplicated, corrupted or re-ordered
will be delayed. This value must also be between 0 and 1.13. maxDelay: The maximum delay (in milliseconds) experienced by those data segments that are
delayed.
14. seed: The seed for your random number generator. The use of seed will be explained in Section 4.5.2
of the specification.
The Sender should be initiated as follows:
If you use Java:
java Sender receiver_host_ip receiver_port file.pdf MWS MSS gamma pDrop
pDuplicate pCorrupt pOrder maxOrder pDelay maxDelay seed
If you use C:
sender receiver_host_ip receiver_port file.pdf MWS MSS gamma pDrop
pDuplicate pCorrupt pOrder maxOrder pDelay maxDelay seed
If you use Python:
python sender.py receiver_host_ip receiver_port file.pdf MWS MSS gamma
pDrop pDuplicate pCorrupt pOrder maxOrder pDelay maxDelay seed
Note that, you should first start the Receiver before initiating the Sender.
4.5.1 The PLD Module
The PLD module should be implemented as part of your Sender program. The function of the PLD is to
emulate some of the events that can occur in the Internet such as loss of packets, packet corruption, packet
re-ordering and delays. Even though theoretically UDP packets will get lost and delayed on their own, in
our test environment these events will occur very rarely. Further to test the reliability of your STP protocol
we would like to be able to control the percentage of packets being lost, corrupted, re-ordered and delayed.
Your PLD module should take care of the following events;
Drop packets
Duplicate packets
Create bit errors within packets (a single bit error)
Transmits out of order packets
Delays packets
The following describes the sequence of steps that the PLD should perform on receiving a STP segment:
1. If the STP segment is for connection establishment or teardown, then pass the segment to UDP without
going through PLD.Remark: In order to reduce the complexity of connection setup, the connection establishment and
teardown segments from the Sender can bypass the PLD module.
2. If the STP segment is not for connection establishment or teardown, the PLD must do one of the
following:
(a) With probability pDrop, drop the segment. To implement this simply generate a random number
between 0 and 1. If the chosen number is less than pDrop, drop the STP segment.
(b) If the segment is not dropped, with probability pDuplicate, forward the STP segment twice
back-to-back to UDP.
(c) If the packet is not dropped or duplicated, with probability pCorrupt, introduce one bit error
(you can simply flip any one bit of data) and forward the STP segment to UDP.
(d) If the packet is not dropped, duplicated or corrupted, with probability pOrder save the current
STP segment and wait for forwarding of maxOrder segments to UDP before forwarding the saved
STP segment to UDP. If there is a segment already waiting for re-ordering, forward the new STP
segment without any delay i.e., there should be only one segment waiting for re-ordering at a time.
(e) If the STP segment is not dropped, duplicated, corrupted or re-ordered, with probability pDelay
the segment is to be delayed by anywhere between 0 to MaxDelay milliseconds before forwarding
to UDP. In other words, the amount of the delay that is experienced by the segment is in the interval
[0, MaxDelay] with a uniform distribution.
(f) If the STP segment is not dropped, duplicated, corrupted, re-ordered or delayed, forward the STP
segment to UDP.
Once the PLD is ready to transmit a STP segment, the Sender should encapsulate the STP segment in a
UDP datagram (i.e. create a UDP datagram with the STP segment as the payload). It should then transmit
this datagram to the Receiver through the UDP socket created earlier. (Use the RECEIVER_HOST_IP
and RECEIVER_PORT as the destination IP address and port number respectively). Once the entire file
has been transmitted reliably (i.e. the sender window is empty and the final ACK of STP closure is
received) the Sender can close the UDP socket and terminate the program.
Note that the ACK segments from the receiver must completely bypass the PLD modules. In other words,
there is no PLD module on the receiver. ACK segments are thus never dropped, duplicated, corrupted, reordered
or delayed.
4.5.2 Seed for random number generators
You will be asked to run your Sender and Receiver pair to show us that they are running correctly, see
Section 8 of the specification for the experiments that you need to conduct. In order for us to check your
results, we will be asking you to initialise your random number generator with a specific seed in Section
8 so that we can repeat your experiments. 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 pseudo-random number generators because the
numbers are not truly random. The deterministic formula for random number generation in Python, Java
and C uses an input parameter called a seed. If the same seed is used, then the same sequence of random
numbers will be produced.
The following code fragment in Python, Java and C will generate random numbers between 0 and 1 using
a supplied seed.
In Python, you initialise a random number generator (assuming the seed is 50) by using
random.seed(50);. After that you can generate a random floating point number between (0,1)
by using random.random();
In Java, you initalise a random number generator (assuming the seed is 50) by using Random
random = new Random(50);. After that, you can generate a random floating point number
between (0,1) by using float x = random.nextFloat();
In C, you initalise a random number generator (assuming the seed is 50) by using srand(50);.
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.
You will find that if you specify different seeds, a different sequence of pseudo-random numbers will be
produced.
4.5.3 Additional requirements for Sender
Your Sender will receive acknowledgements from the Receiver through the same socket, which the sender
uses to transmit data (it is using a UDP socket). The Sender must first extract the STP acknowledgement
from the UDP datagram that it receives and then process it as per the operation of your STP protocol. The
format of the acknowledgement segments should be exactly the same as the data segments except that
they should not contain any data. Note that these ACKs should bypass the PLD module.
Recall that the sender has a single-timer for timeout operation which measures timeout for the oldest
segment in the window. The sender is allowed to have other timers in PLD e.g, to cater for the pDelay
parameter. Additionally, a segment being delayed due to pDelay would not have any effect on the
following packets. For example, if segment 10 is to be delayed by 100 msec, segments following it (11
onwards) continues getting processed, segment 10 is processed again by PLD (transmitted to UDP without
getting any further error) when the 100 msec has passed. In other words, pDelay can also cause reordering.Sender will not measure the sampleRTT (for maintaining its timer for timeout) for any segment that it retransmits.
If a segment is marked for reordering due to pOrder, the PLD checks if there is already a segment waiting
in queue to be re-ordered, if so, it sends the new segment immediately to UDP.
If a segment is dropped by PLD, the sender consider that it has transmitted that segment (and loss has
occurred out in the network) and update the total bytes sent and total segments dropped.
The sender should maintain a log file titled Sender_log.txt where it records the information about each
segment that it sends and receives. Information about dropped, delayed, corrupted segments should also
be included. Start each entry on a new line. The format should be as follows: