Download Figure A.1: Interface to the Network Simulator

ECP2056: Data Communications and Computer Networking
Laboratory Experiment
FACULTY OF ENGINEERING
LAB SHEET
DATA COMMUNICATIONS AND COMPUTER
NETWORKING
ECP 2056
TRIMESTER II (2010-2011)
DCC1: COMMUNICATION PROTOCOL SIMULATION
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Laboratory Experiment
Communication Protocol Simulation
1. Objectives
 To understand the process flow of simulation of communication networks.
 To study and understand the reliable transmission and congestion control mechanisms in
Transmission Control Protocol (TCP).
2. Equipment and Software Required
 Desktop computer
 NS-2 Network Simulator
3. Introduction
a) Simulation process flow
Simulation is the technique of imitating the behavior of some situation or system by means of an
analogous model, situation, or apparatus, either to gain information more conveniently or to train
personnel. From the simulation, we hope to understand the system better.
The basic phases or steps in simulation of communication networks are:
i)
Model the network by specifying important parameters
ii)
Identify data / statistics to be collected
iii)
Run the simulations and collect statistics
iv)
Post-simulation: view and analyze result
b) Transmission Control Protocol (TCP)
TCP is one of the transport protocols in the TCP/IP protocol suite. It is a Layer 4 protocol in the OSI
reference model. It provides a connection oriented, reliable, and byte stream service. The term
connection-oriented means any two applications using TCP must establish a TCP connection with
each other before they can exchange data. It is a full duplex protocol, meaning that each TCP
connection supports a pair of byte streams, one flowing in each direction. TCP includes a flowcontrol mechanism for each of these byte streams that allows the receiver to limit how much data the
sender can transmit. TCP also implements a congestion-control mechanism.
TCP provides the following facilities:
i)
Stream Data Transfer - From the application's viewpoint, TCP transfers a contiguous stream
of bytes (octets). TCP does this by grouping the bytes in TCP segments, which are passed to
Layer 3 protocol for transmission to the destination. TCP itself decides how to segment the
data and it may forward the data at its own convenience.
ii)
Reliability - TCP assigns a sequence number to each byte transmitted, and expects a positive
acknowledgment (ACK) from the receiving TCP. If the ACK is not received within a timeout
interval, the data is retransmitted. The receiving TCP uses the sequence numbers to rearrange
the segments when they arrive out of order, and to eliminate duplicate segments.
iii)
Flow Control - The receiving TCP sends an ACK packet, which includes the information of
the next expected packet’s sequence number. This indicates the packet with earlier sequence
number has been successfully received. It also gives an indication to the sender the number
of bytes the receiver can receive beyond the last received TCP segment, without causing
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overrun and overflow in its internal buffers. This is sent in the ACK in the form of the
highest sequence number it can receive without problems.
iv)
Multiplexing - To allow for many processes within a single host to use TCP communication
facilities simultaneously, the TCP provides a set of addresses or ports within each host.
Concatenated with the network and host addresses from the Internet communication layer,
this forms a socket (see Figure 1). A pair of sockets uniquely identifies each connection.
v)
Logical Connections - The reliability and flow control mechanisms described above require
that TCP initializes and maintains certain status information for each data stream. The
combination of this status, including sockets, sequence numbers and window sizes, is called a
logical connection. Each connection is uniquely identified by the pair of sockets used by the
sending and receiving processes.
vi)
Full Duplex - TCP provides for concurrent data streams in both directions.
Figure 1: Illustration of TCP Socket
c) Reliable Transmission and Congestion Control in TCP
i)
Reliable Transmission and Flow Control - TCP uses the positive acknowledgement with
retransmission as the fundamental technique to provide reliable transfer under unreliable
packet delivery. A simple way to do this is based on the stop-and-wait algorithm.
Figure 2 depicts the operation of the stop-and-wait algorithm. Refers to Figure 2.a, after the
sender transmits one packet, it waits for an acknowledgement (ACK) from the receiver.
Every time a packet with a sequence number x arrives correctly at the receiver, the receiver
acknowledges the packet #x by sending an ACK packet (containing the sequence number of
another packet which it is waiting for; this number may or may not be “x + 1”) back to the
sender. Therefore, this technique guarantees reliable transfer between end hosts. To support
this feature, the sender keeps a record of each packet it sends. In order to avoid confusion
caused by delayed or duplicated ACKs, the stop-and-wait algorithm sends each packet with
unique sequence numbers and receives that numbers in each ACKs.
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If the sender does not receive ACK for previous sent packet after a certain period of time, the
sender times out and retransmits that packet again. There are two cases when the sender does
not receive ACK; one is when the ACK is lost and the other is the frame itself is not
transmitted (see Figure 2b). To support this feature, the sender keeps timer per each packet.
The timer is usually set to the retransmission timeout period (RTO), which is determined by
some other algorithms.
(a) Event Timing Diagram Showing Normal Operation
(b) Event Timing Diagram Showing Error in Transmission
Figure 2: Operation of Stop-and-Wait Algorithm
The main shortcoming of the stop-and-wait algorithm described above is that it allows the
sender to have only one outstanding packet on the link at a time. The sender should wait until
it receives an ACK of previous packet before it sends next packet. As a result, it wastes a
substantial amount of network bandwidth. To improve efficiency, TCP uses a variant of the
sliding window strategy, called slow start. The difference between sliding window and slow
start is that slow start allows various window sizes while sliding window has fixed window
size.
Figure 3 illustrates the operation of slow start. The basic idea behind slow start is to send
packets as much as the network can accept. It starts to transmit 1 packet, and if the packet is
transmitted successfully and receives an ACK, it increases its window size to 2. After
receiving 2 ACKs, it increases its window size to 4, and then 8, and so on. Therefore, slow
start actually increases its window size exponentially (see Figure 3a).
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If there is packet loss, slow start only sends that lost packet as depicted in Figure 3b. This
mechanism is known as “Selective Repeat”. To do this, the receiver keeps a record of
correctly received packet numbers and acknowledges the last successful packet number to the
sender.
Slow start increases total throughput by keeping networks busy as in sliding window, and
also it solves the end-to-end flow control problem by allowing the receiver to restrict
transmission until it has sufficient buffer space to accommodate more data. Whenever the
receiver sends ACK, the available buffer space is attached to the ACK (which is known as
window advertisement), so that the sender can decide its sending window size.
In TCP, the window size is defined as the minimum value between cwnd (congestion
window size) and window advertisement. At any time, the window size cannot be greater
than maximum cwnd, which is fixed. TCP transmits its data in slow start when the sender
starts to transmit for first time or when packets are lost in transmission.
(a) Normal Operation
(b) Error in Transmission
Figure 3: Operation of Slow Start Algorithm
ii)
Congestion Control - When a network is getting congested and packets are lost or when a
network delay increases and packets timed out, TCP uses multiplicative-decrease additiveincrease technique to adjust its window size to fit the congested environment. The operation
of the congestion avoidance algorithm is as follows.
To support the congestion avoidance algorithm, the sender keeps two state variables:
congestion window, cwnd and a threshold ssthresh. The ssthresh is used to indicate the
adjustment of the congestion window, cwnd. The sender’s output routine always sends the
minimum of cwnd and the window advertised by the receiver. On a timeout, half the current
window size (cwnd) is recorded in ssthresh (multiplicative decrease), then cwnd is set to 1
packet (this indicates slow start). When a new data is acknowledged, the sender does:
if (cwnd < ssthresh)
cwnd = cwnd + 1
else
cwnd = cwnd + 1/cwnd
/* slow start – open window exponentially */
/* additive increase – open window slowly */
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Thus the algorithm adjusts the window to a safe operating point (half of the window that
cause the packet lost), then additive increase takes over and slowly increases the window size
to probe for more bandwidth becoming available on the path. This enables TCP to deliver its
data reliably and efficiently even in a congested environment. This feature is necessary in the
Internet.
4. Glossary
 IP – Internet Protocol
 TCP – Transmission Control Protocol
5. Material & Equipment Required
i)
Software: Network Simulator version 2 (NS-2), Network Animator (NAM)
6. Precautions
i)
All works must be save into new filenames
7. References
i)
William Stallings, "Data & Computer Communications," 8th Edition, Prentice Hall, 2007.
ii)
Fred Halsall, "Multimedia Communications," Addison-Wesley, 2001
iii)
NS-2, http://nsnam.isi.edu/nsnam/index.php/User_Information
iv)
Wikipedia, http://en.wikipedia.org/wiki
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8. Experiments
The simulation experiments are to be done on an interface to the Network Simulator (NS), named NSInterface. Refer to Appendix I for details.
Experiment 1: Stop-and-Wait & Slow Start Algorithms
The objectives of this experiment are to understand the basic reliable transmission algorithm (stop and
wait) in section (a) and the effect of window size in the slow start algorithm in section (b). A simple
simulation topology (Figure 4) is used: a sender is connected to a receiver through a bidirectional
communication link.
Figure 4: Simulation Topology Used in the Experiment 1 (a) & (b)
Section (a)
Normal Operation
i)
Invoke the NS-Interface as described in Appendix I.
ii)
From the menu [Experiment 1], select the sub-menu [Experiment 1a].
iii)
Select the “View simulation animation after run” checkbox to view the simulation animation
after the simulation has finished.
iv)
Click the “Run” button to start the simulation.
v)
When the simulation finishes and NAM window appears, play the animation by clicking on the
play button.
vi)
The time sent and received of each packet (with a unique sequence number) can be viewed at the
“annotation window”. Record the details of the first 5 packets (i.e. time of sending, time of
receiving, type (data or ACK) and sequence number) successfully sent and acknowledged.
vii)
Record the result shown in the “Result Window” of the NS-Interface.
Error in Transmission
i)
To simulate an error in transmission, select menu [Experiment 1] and sub-menu [Experiment
1a] as above.
ii)
Select the “View simulation animation after run” checkbox to view the simulation animation
after the simulation has finished.
iii)
Click on the “Configure” button. Select the “Simulate errors in transmission” checkbox from
the popup dialog. Click OK to return to the main window.
iv)
Click on the “Run” button to start the simulation.
v)
Run NAM to play the animation and record the details of the first 10 packets successfully sent
and acknowledged by the sender as displayed in the annotation window of NAM.
vi)
When complete close the NAM application window(s).
vii)
Record the results shown in the “Result Window” of the NS-Interface.
Discussion
i)
For the experiment under normal operation, draw the timing diagram (e.g. Figure 2) for the first
5 packets successfully sent and acknowledged. The diagram must include the time sent and
received, sequence number and type of each packet.
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ii)
Laboratory Experiment
For experiment with error in transmission, draw the timing diagram for 10 packets that were
successfully sent and acknowledged. The diagram must include the lost packet, time out value,
and details of each packet (time sent and received, sequence number and packet type).
Section (b)
Impact of the Sliding Window Size
i)
From the menu [Experiment 1], select the sub-menu [Experiment 1b].
ii)
Click on the “Configure” button. On the popup dialog, enter a value of 4 for the “Maximum
Window Size”. Click OK to return to the main window.
iii)
Click on the “Run” button to start the simulation. (Optional: You may wish to view the
animated result of the simulation by checking the checkbox “View simulation animation after
run” but this is not required to complete this experiment)
iv)
Record the result: total bytes successfully transmitted, as shown in the “Result Window”.
v)
Perform step ii) to step iv) for at least 8 different values of Maximum Window Size in a fixed
increment.
Discussion
i)
Plot the graph of “total bytes transmitted” versus “maximum window size”.
ii)
Explain the trend observed in the graph.
Experiment 2: Congestion Control Algorithm
The objectives of this experiment are to understand the congestion control algorithm in TCP and the
effect of window size in the slow start algorithm under a congested scenario. The simulation topology
used is shown in Figure 5 where a sender (node 0) and the receiver (node 3) communicate through a
congested link (i.e. link connecting node 1 and node 2). The congestion scenario is modeled by using a
small bandwidth value for link between node 1 and node 2 (2Mbps) and a longer propagation delay as
compared to other links. Besides, the buffer size of the outgoing interface from node 1 to node 2 is
configured to some small values, i.e. 5 or 20. In section (a), the above said buffer size is configured as 5
packets. In section (b), the buffer size may be 5 or 20 packets so as to study the effect of buffer size on
the network performance under the same range of Maximum Window Size.
Figure 5: Simulation Topology Used in the Experiment 2 (a) & (b)
Section (a)
i)
From the menu <Experiment 2>, select the sub-menu <section (a)>.
ii)
Click on the “Run” button to start the simulation. (Optional: Select the “View simulation
animation after run” checkbox to view the simulation animation after the simulation has
finished.)
iii)
Record the results for total bytes successfully transmitted, as shown in the “Result Window”.
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iv)
v)
Laboratory Experiment
Three files will be created after the simulation - “ssthresh.tr”, “cwnd.tr” and “seqno.tr”. The
location of these files is indicated at the “Result Window”. These files record the variations of
the value of ssthresh, cwnd, and received sequence number against time respectively. The files
are made up of two column values; the first column is the time, and the second column is the
corresponding value (ssthresh, cwnd, or sequence number).
Save a copy of these files (i.e. to diskette, thumbdrive, email) for the following discussion part of
this experiment.
Discussion
i)
Plot the graph of the variations of ssthresh AND cwnd against time on the same graph.
ii)
Plot the graph of the variations of the sequence number against time.
iii)
Explain the variations of the value of ssthresh, cwnd, and sequence number based on the TCP
congestion control algorithm and simulation animation.
Section (b)
i)
From the menu [Experiment 2], select the sub-menu [Experiment 2b].
ii)
Click on the “Configure” button. In the popup dialog, set the “Maximum Window Size” to 1.
iii)
Do NOT select the “Buffer Size = 20 packets (Default = 5)” checkbox. Click OK to return to
the main window.
iv)
Click on the “Run” button to start the simulation.
v)
Record the results for total bytes successfully transmitted, as shown in the “Result Window”.
vi)
Perform step ii) to step v) for at least 8 different values of Maximum Window Size (from 1 to 40
in fixed increments).
vii)
Perform step ii) to step v) for at least 8 different values of Maximum Window Size (1 to 40) but
this time, select the “Buffer Size = 20 packets (Default = 5)” checkbox at step iii). This
changes the buffer size for the bottleneck link (with 2Mbps bandwidth) to 20.
Discussion
i)
Plot the graph of the variations of total bytes transmitted against maximum window size for the
case of buffer size = 5 packets.
ii)
Plot the graph of the variations of total bytes transmitted against maximum window size for the
case of buffer size = 20 packets.
iii)
Explain the trends observed in the graphs.
Experiment 3: Performance of Multiple TCP Connections over a Congested Link
The objective of this experiment is to study the network performance using TCP as a transport protocol
under a more realistic scenario. The topology used is depicted in Figure 6. In the experiment, the senders
and the receivers are connected through a bottleneck link (link with bandwidth of 1Mbps and
propagation delay of 10ms). The connection between the sender and receiver is one-to-one, i.e., one
sender sends data only to one receiver at the other end of the network, and vice versa. The number of the
user pairs (sender-receiver pairs) is a configurable parameter. By varying this value, we can study the
performance of the network when multiple parties are involved in communicating information at the
same time.
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Figure 6: Simulation Topology Used in the Experiment 3
i)
ii)
iii)
iv)
v)
From the menu, select [Experiment 3]. By default, the number of pairs is set to 1 pair to begin
the simulation with.
Click on the “Configure” button. At the popup dialog, set the “Number of User Pairs” to 2.
Click OK to return to the main window.
Click on the “Run” button to start the simulation.
Record the results for average bytes successfully transmitted and average end-to-end delay, as
shown in the “Result Window”.
Perform step ii) to step iv) for at least 8 different values for the number of user pairs (from 2 to
20 in fixed increments).
Discussion
i)
Calculate the average throughput for each “Number of User Pairs”, where the throughput is
defined as:
Total Bytes Successfully Transmitted
Throughput 
Duration of the Transmissi on
ii)
Plot the graph for average throughput versus number of user pairs.
iii)
Plot the graph for average end-to-end delay versus number of user pairs.
iv)
Explain the trends observed in the graphs.
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Appendix I
Figure below shows a screen shot of the interface. The interface serves as the front end for NS to
perform simulations. The procedures to use the interface are as follows:
i)
Invoke the interface by double clicking on the shortcut icon named “DCC2” on the desktop. If
there is no shortcut on the desktop then proceed to Start  DCC2 and select the option “DCC2”
there instead. A window similar to Figure A.1 below will appear.
Figure A.1: Interface to the Network Simulator
ii)
iii)
iv)
v)
vi)
To start a simulation, select an experiment from the top menu. A drop-down menu allows
selection of any sub experiments where available.
Each experiment may have some configuration parameters to set. This can be done by clicking
on the “Configure” button. A pop-up dialog will then appear and prompt the user for the
appropriate input(s).
To start the simulation, click on the “Run” button. Upon completing execution of the simulation,
the simulation results will be displayed in the “Result Window” area at the bottom of the
interface.
The Network Animator (NAM) may be used to visualize the details of the simulation by playing
an animation of the events that occur during the simulation. In order to invoke NAM, select the
checkbox to “View simulation animation after run”.
After the simulation finishes, the NAM program window as in Figure A.2 will appear alongside
2 other windows (the command window and the console window).
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Figure A.2: Screen Shot of Network Animator (NAM)
vii)
viii)
ix)
x)
On the NAM window, click on the “Play” button to begin the animation in the display window.
The time slider at the bottom indicates the current point in the simulation that is being displayed.
To move to a particular point in time, drag the slider to the desired location (WARNING! This
process takes a long time to complete and may make the system appear to ‘hang’ even though it
has not)
The user can change the speed of the animation by adjusting the slider of animation time step.
The default step value is 2.0ms between each animation frame/tick. Changing the speed of the
animation DOES NOT change the outcome of the simulation.
Any additional information recorded during the simulation run will be displayed in the
annotation window of NAM (for experiments 1a and 1b)
Annotation
window
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Rewind
Play backward
Laboratory Experiment
Stop
xii)
Forward
Animation
step control
Zoom in/out
xi)
Play
Clicking on the zoom in and out buttons allow the size scale of the animation in the window to
be changed accordingly.
In Experiment 3, the number of pairs of nodes can be controlled with the “Configure” button but
when displayed on the NAM animation window might result in an awkward arrangement. In this
case, click the “re-layout” button to auto position all nodes and link to not overlap.
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xiii)
Laboratory Experiment
To see the details in an animated packet, click on the packet while it is visible (i.e. it can be seen
in the display window) to monitor the packet. The details will be displayed on the monitor
window (see Figure A.3).
Figure A.3: Monitor a Packet in NAM
xiv)
When running NAM, note that the interface of the main program (i.e. DCC2) is disabled and
cannot be interacted with. To return to the program, close all the windows associated with NAM
(the NAM window, NAM console and command window). Final results of any simulation is
shown in the result window but is not updated until the nam windows are closed and control
returns to the main interface.
Result window: Results displayed
here are updated only after control
is returned to this interface
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