CSE 489/589 Spring 2021
Programming Assignment 2
Reliable Transport Protocols
1. Objectives
In a given simulator, implement three reliable data transport
protocols: Alternating-Bit (ABT), Go-Back-N (GBN), and SelectiveRepeat
(SR).
2. Getting Started
2.0 Your machine for testing
Please identify the testing machine assigned to you following
this spreadsheet.
Testing on a different machine may disrupt the grading for
your analysis reports.
2.1 Alternating-Bit Protocol (rdt3.0)
Text book: Page 214 – Page 217
2.2 Go-Back-N Protocol
Text book: Page 221 – Page 226
2.3 Selective-Repeat Protocol
Text book: Page 226 – Page 232
2.4 Install the PA2 template
Read the PA2 template in full and install the template.
It is mandatory to use this template.
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3. Implementation
3.1 Programming environment
You will write C (or C++) code that compiles under the GCC
(GNU Compiler Collection) environment. Furthermore, you
should ensure that your code compiles and operates correctly
on the ONE host assigned to you by the course instructor (see
section 3.2).
You should NOT use any machine other than the host assigned
to you.
Your code should successfully compile using the version of gcc
(for C code) or g++ (for C++ code) found on the host assigned
to you and should function correctly when executed. Further,
your implementation should NOT involve any disk I/O unless
explicitly mentioned in the PA description.
3.2 Dedicated hosts
As detailed in 2.0, Please identify the testing machine assigned
to you, by following this spreadsheet.
Testing on a different machine may disrupt the grading for
your analysis reports.
For the purpose of this assignment, you should only use (for
development and/or testing) the directory (/local/Spring_2021/)
created for you on the assigned host. Change the access
permission to this directory so that only you are allowed to
access the contents of the directory. This is to prevent others
from getting access to your code.
3.3 Overview
In this programming assignment, you will be writing the sending
and receiving transport-layer code for implementing a simple
reliable data transfer protocol. There are 3 versions of this
assignment, the Alternating-Bit Protocol version, the Go-Back-N
version, and the Selective-Repeat version.
Since we don't have standalone machines (with an OS that you
can modify), your code will have to execute in a simulated
hardware/software environment. However, the programming
interface provided to your routines, i.e., the code that would call
your entities from above and from below is very close to what is
done in an actual UNIX environment. Stopping/starting of timers
is also simulated, and timer interrupts will cause your timer
handling routine to be activated.
3.4 The routines you will write
The procedures you will write are for the sending entity (A) and
the receivPublished b ing entity
y (Google Driv B). Only uen–idRepor irecti
t Abuse onal transfer of data
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(from A to B) is required. Of course, the B side will have to
send packets to A to acknowledge receipt of data. Your routines
are to be implemented in the form of the procedures described
below. These procedures will be called by (and will call)
procedures which simulate a network environment. The overall
structure of the environment is shown below:
The unit of data passed between the upper layers and your
protocols is a message, which is declared as:
struct msg {
char data[20];
};
This declaration, and all other data structures and simulator
routines, as well as stub routines (i.e., those you are to
complete) are inside the template files, described later. Your
sending entity will thus receive data in 20-byte chunks from
Layer 5; your receiving entity should deliver 20-byte chunks of
correctly received data to Layer 5 at the receiving side.
The unit of data passed between your routines and the network
layer is the packet, which is declared as:
struct pkt {
int seqnum;
int acknum;
int checksum;
char payload[20];
};
Your routines will fill in the payload field from the message data
passed doPublished b wn from Ly aGoogle Driv yer 5. Thee o–thRepor er pat Abuse cket fields will be used
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by your protocols to insure reliable delivery, as we've seen in
class.
The routines you will write are detailed below. As noted above,
such procedures in real-life would be part of the operating
system, and would be called by other procedures in the
operating system.
● A_output (message)
where message is a structure of type msg, containing data
to be sent to the B-side. This routine will be called
whenever the upper layer at the sending side (A) has a
message to send. It is the job of your protocol to insure that
the data in such a message is delivered in-order, and
correctly, to the receiving side upper layer.
● A_input(packet)
where packet is a structure of type pkt. This routine will be
called whenever a packet sent from the B-side (as a result
of a tolayer3() (see section 3.5) being called by a B-side
procedure) arrives at the A-side. packet is the (possibly
corrupted) packet sent from the B-side.
● A_timerinterrupt()
This routine will be called when A's timer expires (thus
generating a timer interrupt). You'll probably want to use
this routine to control the retransmission of packets. See
starttimer() and stoptimer() below for how the timer is
started and stopped.
● A_init()
This routine will be called once, before any of your other Aside
routines are called. It can be used to do any required
initialization.
● B_input(packet)
where packet is a structure of type pkt. This routine will be
called whenever a packet sent from the A-side (as a result
of a tolayer3() (see section 3.5) being called by a A-side
procedure) arrives at the B-side. packet is the (possibly
corrupted) packet sent from the A-side.
● B_init()
This routine will be called once, before any of your other Bside
routines are called. It can be used to do any required
initialization.
These six routines are where you can implement your
protocols.
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3.5 Software Interfaces
The procedures described above are the ones that you will
write. We have written the following routines which can be
called by your routines:
● starttimer (calling_entity, increment)
where calling_entity is either 0 (for starting the A-side
timer) or 1 (for starting the B side timer), and increment is
a float value indicating the amount of time that will pass
before the timer interrupts. A's timer should only be started
(or stopped) by A-side routines, and similarly for the B-side
timer. To give you an idea of the appropriate increment
value to use: a packet sent into the network takes an
average of 5 time units to arrive at the other side when
there are no other messages in the medium.
● stoptimer (calling_entity)
where calling_entity is either 0 (for stopping the A-side
timer) or 1 (for stopping the B side timer).
● tolayer3 (calling_entity, packet)
where calling_entity is either 0 (for the A-side send) or 1
(for the B side send), and packet is a structure of type pkt.
Calling this routine will cause the packet to be sent into the
network, destined for the other entity.
● tolayer5 (calling_entity, data)
where calling_entity is either 0 (for A-side delivery to layer
5) or 1 (for B-side delivery to layer 5), and data is a char
array of size 20. With unidirectional data transfer, you
would only be calling this with calling_entity equal to 1
(delivery to the B-side). Calling this routine will cause data
to be passed up to layer 5.
● getwinsize()
returns the window size value passed as parameter to -w
(see section 3.6).
● get_sim_time()
returns the current simulation time.
3.6 The simulated network environment
A call to procedure tolayer3() sends packets into the medium
(i.e., into the network layer). Your procedures A_input() and
B_input() are called when a packet is to be delivered from the
medium to your transport protocol layer.
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The medium is capable of corrupting and losing packets.
However, it will not reorder packets. When you compile your
procedures and our procedures together and run the resulting
program, you will be asked to specify certain values regarding
the simulated network environment as command-line
arguments. We describe them below:
● Seed (-s)
The simulator uses some random numbers to reproduce
random behavior that a real network usually exhibits. The
seed value (a non-zero positive integer) initializes the
random number generator. Different seed values will make
the simulator behave slightly differently and result in
different output values.
● Window size (-w)
This only applies to Go-back-N and Selective-Repeat
binaries. Both these protocols use a finite-sized window to
function. You need to tell the simulator before hand, what
window size you want to use. Infact, your code will
internally use this value for implementing the protocols.
● Number of messages to simulate (-m)
The simulator (and your routines) will stop as soon as this
number of messages has been passed down from Layer 5,
regardless of whether or not all of the messages have been
correctly delivered. Thus, you need not worry about
undelivered or unACK'ed messages still in your sender
when the simulator stops. This value should always be
greater than 1. If you set this value to 1, your program will
terminate immediately, before the message is delivered to
the other side.
● Loss (-l)
Specify a packet loss probability [0.0,1.0]. A value of 0.1,
for example, would mean that one in ten packets (on
average) are lost.
● Corruption (-c)
You are asked to specify a packet corruption probability
[0.0,1.0]. A value of 0.2, for example, would mean that one
in five packets (on average) are corrupted. Note that the
contents of payload, sequence, ack, or checksum fields can
be corrupted. Your checksum should thus include the data,
sequence, and ack fields.
● Average time between messages from sender's layer5
(-t)
You can set this value to any non-zero, positive value. Note
that the smaller the value you choose, the faster packets
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will be arriving to your sender.
● Tracing (-v)
Setting a tracing value of 1 or 2 will print out useful
information about what is going on inside the simulation
(e.g., what's happening to packets and timers). A tracing
value of 0 will turn this off. A tracing value greater than 2
will display all sorts of odd messages that are for our own
simulator-debugging purposes. A tracing value of 2 may be
helpful to you in debugging your code. You should keep in
mind that, in reality, you would not have underlying
networks that provide such nice information about what is
going to happen to your packets!
4. Protocols
4.1 Alternating-Bit-Protocol (ABT)
You are to write the procedures which together will implement a
stop-and-wait (i.e., the alternating bit protocol, which is referred
to as rdt3.0) unidirectional transfer of data from the A-side to the
B-side. Your protocol should use only ACK messages.
You should perform a check in your sender to make sure that
when A_output() is called, there is no message currently in
transit. If there is, you should buffer the data being passed to
the A_output() routine.
4.2 Go-Back-N (GBN)
You are to write the procedures which together will implement a
Go-Back-N unidirectional transfer of data from the A-side to the
B-side, with a certain window size.
It is recommended that you first implement the easier protocol
(the Alternating-Bit version) and then extend your code to
implement the more difficult protocol (the Go-Back-N version).
Some new considerations for your GBN code (which do not
apply to ABT) are:
● A_output()
will now sometimes be called when there are outstanding,
unacknowledged messages in the medium, implying that
you will have again to buffer multiple messages in your
sender. Also, you'll need buffering in your sender because
of the nature of Go-Back-N: sometimes your sender will be
called but it won't be able to send the new message
because the new message falls outside of the window.
Rather than have you worry about buffering an arbitrary
number of messages, it will be OK for you to have some
finite, maximum number of buffers available at your sender
(say for 1000 messages) and have your sender simply
abort (give up and exit) should all 1000 buffers be in use at
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one point (Note: If the buffer size is not enough in your
experiments, set it to a larger value). In the “real-world”, of
course, one would have to come up with a more elegant
solution to the finite buffer problem!
4.3 Selective-Repeat (SR)
You are to write the procedures which together will implement a
Selective-Repeat unidirectional transfer of data from the A-side
to the B-side, with a certain window size.
It is recommended that you implement the GBN protocol before
you extend your code to implement SR. Some new
considerations for your SR code are:
● B_input(packet)
will have to buffer multiple messages in your receiver
because of the nature of Selective-Repeat. The receiver
should reply with ACKs to all packets falling inside the
receiving window.
● A_timerinterrupt()
will be called when A's timer expires (thus generating a
timer interrupt).
Even though the protocol uses multiple logical timers,
remember that you've only got one hardware timer, and
may have many outstanding, unacknowledged packets in
the medium. You will have to think about how to use
this single timer to implement multiple logical timers.
Note that an implementation that simply sets a timer
every T time units and retransmits all the packets that
should have expired within those time units is NOT
acceptable. Your implementation has to ensure that
each packet is retransmitted at the exact time at which
it would be retransmitted if you had multiple timers.
5. Testing
We will test your implementation of the three protocols with the
settings/parameters described below. We will begin with the
most basic tests and then move on to more involved tests (in
the order mentioned below).
5.1 SANITY Tests
Here we perform two types of checks.
(i) Check for duplicate and/or out-of-order packets at B.
For each of the three protocols:
Environment Settings
? Number of messages to simulate (-m): 1000
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? Average time between messages from sender's layer5 (-t):
50
? Window size (-w): 10
Test Cases
? Loss (-l): 0.1, 0.2, 0.4, 0.6, 0.8, Corruption (-c): 0.0
? Loss (-l): 0.0, Corruption (-c): 0.1, 0.2, 0.4, 0.6, 0.8
(ii) Confirm that the behavior of your protocols is the expected
one under some very simple tests (e.g. no packets is delivered
under 100% loss).
For each of the three protocols:
Environment Settings
? Number of messages to simulate (-m): 20
? Average time between messages from sender's layer5 (-t):
1000 (ABT); 50 (GBN,SR)
? Window size (-w): 50
Test Cases
? Loss (-l): 0.0, Corruption (-c): 0.0
? Loss (-l): 1.0, Corruption (-c): 0.0
? Loss (-l): 0.0, Corruption (-c): 1.0
5.2 BASIC Tests
For each of the three protocols:
Environment Settings
? Number of messages to simulate (-m): 20
? Average time between messages from sender's layer5 (-t):
1000 (ABT); 50 (GBN,SR)
? Window size (-w): 50
Test Cases
? Loss (-l): {0.1, 0.4, 0.8}, Corruption (-c): 0.0
? Loss (-l): 0.0, Corruption (-c): {0.1, 0.4, 0.8}
5.3 ADVANCED Tests
For each of the three protocols:
Environment Settings
? Number of messages to simulate (-m): 1000
? Average time between messages from sender's layer5 (-t):
50
? Window size (-w): 10
Test Cases
? Loss (-l): {0.1, 0.2, 0.4, 0.6, 0.8}, Corruption (-c): 0.0
? Loss (-l): 0.0, Corruption (-c): {0.1, 0.2, 0.4, 0.6, 0.8}
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6. Analysis and Report
For the analysis part, we provide you with a set of experiments
(see section 6.1) to compare your implementation of the three
different protocols’ performance, consisting of various loss
probabilities, corruption probabilities, and window sizes.
You need to present your results in the form of a report in a file
named as Analysis_Assignment2.pdf. The report should have
the following statement right at the top:
I have read and understood the course academic integrity
policy.
Your submission will NOT be graded without this statement.
Even if you decide not to attempt the analysis part of the
assignment, you need to have a report file (with the name given
above) with the following three items:
● Academic integrity declaration mentioned above.
● Brief description of the timeout scheme you used in your
protocol implementation and why you chose that scheme.
● Brief description (including references to the corresponding
variables and data structures in your code) of how you
implemented multiple software timers in SR using a single
hardware timer.
We expect you to use graphs to show your results for each of the
experiments in 6.1 and then write down your observations. Further,
your report, at the very least, should answer questions like: What
variations did you expect for throughput by changing those
parameters and why? Do you agree with your measurements; if
not then why?
6.1 Performance Comparison
In each of the following 2 experiments, run each of your
protocols with a total number of 1000 messages to be sent by
entity A, a mean time of 50 between message arrivals (from A’s
layer5) and a corruption probability of 0.2.
You will need to use the scripts bundled in the template to run
the experiments. Refer to the template instructions for more
details.
We will not accept results that you obtain by running your own
tests.
Read the template instructions for more details.
● Experiment 1
With loss probabilities: {0.1, 0.2, 0.4, 0.6, 0.8}, compare the
3 protocols’ throughputs at the application layer of receiver
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B. Use 2 window sizes: {10, 50} for the Go-Back-N version
and the Selective-Repeat Version.
Expected Graphs
? Window size: 10; X-axis: Loss probability; Y-axis:
Throughput (ABT, GBN and SR) in one graph/plot.
? Window size: 50; X-axis: Loss probability; Y-axis:
Throughput (ABT, GBN and SR) in one graph/plot.
● Experiment 2
With window sizes: {10, 50, 100, 200, 500} for GBN and
SR, compare the 3 protocols’ throughputs at the application
layer of receiver B. Use 3 loss probabilities: {0.2, 0.5, 0.8}
for all 3 protocols.
Expected Graphs
? Loss probability: 0.2; X-axis: Window size; Y-axis:
Throughput (ABT, GBN and SR) in one graph/plot.
? Loss probability: 0.5; X-axis: Window size; Y-axis:
Throughput (ABT, GBN and SR) in one graph/plot.
? Loss probability: 0.8; X-axis: Window size; Y-axis:
Throughput (ABT, GBN and SR) in one graph/plot.
Note that the “Expected Graphs” for each experiment constitute
the minimum expectation. You are encouraged to add any
additional graphs that help you explain your observations or
your design choices. Also, note that when grading your report,
we will consider both content and presentation. Make sure your
graphs are well-presented (good choice of curves and colors,
legible fonts, proper description of axes, etc.) and your
observations and explanations are clearly described.
7. Helpful Hints & FAQ (from Kurose-Ross)
● Checksumming
You can use whatever approach for checksumming you
want. Remember that the sequence number and ack field
can also be corrupted. We would suggest a TCP-like
checksum, which consists of the sum of the (integer)
sequence and ack field values, added to a character-bycharacter
sum of the payload field of the packet (i.e., treat
each character as if it were an 8 bit integer and just add
them together).
● Note that any shared "state" among your routines needs to
be in the form of global variables. Note also that any
information that your procedures need to save from one
invocation to the next must also be a global (or static)
variable. For example, your routines will need to keep a
copy of a packet for possible retransmission. It would
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global variable in your code. Note, however, that if one of
your global variables is used by your sender side, that
variable should NOT be accessed by the receiving side
entity, since, in real life, communicating entities connected
only by a communication channel cannot share global
variables.
● There is a float global variable called time that you can
access from within your code to help you out with your
diagnostics msgs.
● Start Simple
Set the probabilities of loss and corruption to zero and test
out your routines. Better yet, design and implement your
procedures for the case of no loss and no corruption, and
get them working first. Then handle the case of one of
these probabilities being non-zero, and then finally both
being non-zero.
● Debugging
We'd recommend that you set the tracing level to 2 and put
LOTS of printf() statements in your code while you are
debugging your procedures.
● Random Numbers
The simulator generates packet loss and errors using a
random number generator. Our past experience is that
random number generators can vary widely from one
machine to another. You may need to modify the random
number generation code in the simulator we have supplied
you. Our simulation routines have a test to see if the
random number generator on your machine will work with
our code. If you get an error message:
It is likely that random number generation on your
machine is different from what this simulator expects.
Please take a look at the routine jimsrand() in the
simulator code. Sorry.
then you'll know you'll need to look at how random numbers
are generated in the routine jimsrand(); see the comments
in that routine.
● Q&A from Kurose-Ross
1. My timer doesn't work. Sometimes it times out
immediately after I set it (without waiting), other times, it
does not time out at the right time. What's up?
The timer code is OK (hundreds of students have used it).
The most common timer problem I've seen is that students
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call the timer routine and pass it an integer time value
(wrong), instead of a float (as specified).
2. You say that we can access you time variable for
diagnostics, but it seems that accessing it in managing our
timer interrupt list would also be useful. Can we use time
for this purpose?
Yes.
3. How concerned does our code need to be with
synchronizing the sequence numbers between A and B
sides? Does our B side code assume that Connection
Establishment (three-way handshake) has already taken
place, establishing the first packet sequence number? In
other words can we just assume that the first packet should
always have a certain sequence number? Can we pick that
number arbitrarily?
You can assume that the three way handshake has already
taken place. You can hard-code an initial sequence number
into your sender and receiver.
4. When I submitted my assignment I could not get a
proper output because the program core dumped…. I could
not figure out why I was getting a segmentation fault so ….
Offhand I'm not sure whether this applies to your code, but
it seems most of the problems with seg. faults in this
assignment stemmed from programs that printed out char
*'s without ensuring those pointed to null-terminated strings.
(For example, the messages -- packet payloads -- supplied
by the network simulator were not null-terminated). This is
a classic difficulty that trips up many programmers who've
recently moved to C from a safer language.
8. Grading and Submission
The grading will be done using a combination of automated tests
(described in section 5) and manual evaluation of your analysis
report. For a detailed breakup of points associated with each
command/functions, see PA2 Grading Guidelines.
For packaging and submission, see the section Packaging and
Submission at PA2 template. .
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