How to Implement Transport Layer in NS2

 

To implement the Transport Layer in Network Simulator 2 (NS2), we will be employed with protocols like TCP and UDP that are already executed in NS2. These protocols control how data is routed among nodes over a network, handles the congestion, flow control, and reliability. Since NS2 deliver the built-in support for TCP and UDP, we can also generate or customize transport layer protocols for particular purposes. This will demonstrate configuring and customizing the transport layer in NS2.

Step-by-Step Implementation:

  1. Understanding the Transport Layer in NS2

In NS2 is the transport layer that is mainly liable for handling the connections and data transmission among applications. NS2 delivers numerous flavours of TCP like TCP Tahoe, TCP Reno, TCP New Reno and UDP for connectionless communication. The transport layer protocols handles packet flows, acknowledgment, and congestion control mechanisms.

  1. Using Existing Transport Layer Protocols

We need to mimic the behaviour of TCP or UDP by configuring the agents in TCL scripts that denotes the transport layer entities. The following is a simple sample of how to use TCP and UDP in NS2.

Example TCL Script for Using TCP and UDP

# Create a new simulator instance

set ns [new Simulator]

# Open trace and nam files

set tracefile [open “transport_layer_trace.tr” w]

$ns trace-all $tracefile

set namfile [open “transport_layer.nam” w]

$ns namtrace-all-wireless $namfile

# Define the network topology

set n0 [$ns node]

set n1 [$ns node]

set n2 [$ns node]

# Create TCP agents

set tcp0 [new Agent/TCP]

set sink0 [new Agent/TCPSink]

$ns attach-agent $n0 $tcp0

$ns attach-agent $n1 $sink0

$ns connect $tcp0 $sink0

# Set up a TCP application

set ftp0 [new Application/FTP]

$ftp0 attach-agent $tcp0

# Create UDP agents

set udp0 [new Agent/UDP]

set null0 [new Agent/Null]

$ns attach-agent $n1 $udp0

$ns attach-agent $n2 $null0

$ns connect $udp0 $null0

# Set up a UDP application (CBR traffic)

set cbr0 [new Application/Traffic/CBR]

$cbr0 set packetSize_ 512

$cbr0 set rate_ 1Mb

$cbr0 attach-agent $udp0

# Schedule FTP traffic over TCP

$ns at 1.0 “$ftp0 start”

$ns at 2.0 “$ftp0 stop”

# Schedule CBR traffic over UDP

$ns at 1.5 “$cbr0 start”

$ns at 3.0 “$cbr0 stop”

# End the simulation after 5 seconds

proc finish {} {

global ns tracefile namfile

$ns flush-trace

close $tracefile

close $namfile

exec nam transport_layer.nam &

exit 0

}

$ns at 5.0 “finish”

# Run the simulation

$ns run

Key Elements of the Script:

  1. TCP and TCPSink Agents: Agent/TCP and Agent/TCPSink are used to mimic TCP communication among nodes. The TCP agent is attached to one node, and the TCPSink (the receiver) is attached to another.
  2. FTP Application: Replicate a file transfer using the FTP application over TCP.
  3. UDP and Null Agents: Agent/UDP and Agent/Null are used to mimic UDP communication.
  4. CBR Application: A Constant Bit Rate (CBR) traffic generator is used to send packets via UDP.
  1. Customizing the Transport Layer

If we need to execute the own transport layer protocol or customize an existing one such as modify the behaviour of TCP, we will need to adjust the C++ source code for the transport layer protocols.

Steps for Customizing or Creating a New Transport Layer Protocol:

  1. Locate the Transport Layer Files: Transport layer protocols such as TCP and UDP are executed in the C++ files located in the ~/ns-allinone-2.35/ns-2.35/ directory.
    • TCP Implementation Files: tcp.cc, tcp.h
    • UDP Implementation Files: udp.cc, udp.h
  2. Modify the Protocol Logic: We can modify the transport layer behaviour by editing the proper files. For instance, to change the congestion control algorithm in TCP, we would adapt the tcp.cc file.

Example: Adding a simple congestion control modification to TCP:

void TcpAgent::slowdown() {

// Custom congestion control logic

cwnd_ = max(cwnd_ / 2, 1);  // Halve the congestion window

}

  1. Recompile NS2: After adjusting the C++ files, we need to recompile NS2 to implement the changes:

cd ns-allinone-2.35/ns-2.35/

./configure

make clean

make

  1. Test the Custom Protocol: Generate a TCL script to validate the modifications. We can require custom TCP agents in script to see the new behaviour.

Example:

set tcp0 [new Agent/TCP/MyCustomTCP]

  1. Implementing a New Transport Layer Protocol

If we want to execute a completely new transport layer protocol, follow these steps:

  1. Create New Protocol Files:
    • Generate new C++ files like my_tcp.cc and my_tcp.h according to an existing protocol such as TCP.
    • Execute the custom logic for congestion control, flow control, etc., in these files.
  2. Modify the Makefile:
    • Add new protocol files to the Makefile in NS2 to make sure they are accumulated with the NS2 code.

Example:

my_tcp.o: my_tcp.cc

$(CC) $(CFLAGS) -c my_tcp.cc

  1. Register the New Protocol in NS2:
    • Add an entry for new protocol in the tcl/lib/ns-default.tcl file so that it can be used in TCL scripts.

Example:

Agent/MyTCP set packetSize_ 512

  1. Test the New Protocol: After registering new protocol, we need to generate TCL scripts to validate it.

Example:

set tcp0 [new Agent/MyTCP]

  1. Analysing the Transport Layer Behavior

We need to evaluate the transport layer behaviour by investigating the trace file (transport_layer_trace.tr) and NAM visualization (transport_layer.nam). These files will help you to track:

  • Packet Loss: Validate for lost packets because of congestion or errors.
  • Congestion Control: Monitor on how the congestion window changes over time (for TCP).
  • Packet Delivery: Make sure the packets are distributed reliably for protocols such as TCP, and compare it with the behaviour of UDP.

We can also make scripts to parse the trace file and extract parameters such as:

  • Throughput
  • Delay
  • Packet loss rate
  1. Advanced Customizations (Optional)
  • Reliability in UDP: We need to modify UDP to contain the reliability characteristics such as packet acknowledgment and retransmission, generating a custom reliable transport protocol.
  • New Congestion Control Algorithms: Apply custom congestion control techniques in TCP by adjusting on how the congestion window is modified.
  • Multipath Transport: Execute a multipath transport protocol in which the traffic can be divided through multiple paths (similar to MPTCP).

In the conclusion, we completely learn and implicit about how the Transport Layer will analyse the outcomes in the network simulation using ns2 tool. More information regarding the Transport Layer will also be provided.

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