Update behavioral design of network implemented in Layered Architecture Assignment OMNet

Computer Networks Lab · OMNeT++ · The LNMIIT

Layered Architecture — Packet Loss, ACK & Timeout

10% packet loss · ACK-based retransmission · Timeout handling

Network Layer Data Link Layer PDU Encapsulation Retransmission OMNeT++ Simulation

Overview

This post presents the corrected solution for the OMNeT++ Layered Architecture Lab — implementing a two-layer (Network Layer + Data Link Layer) simulation with 10% packet loss, ACK-based retransmission, and timeout handling.

Task Summary

a

Network Layer: Define number of packets as a state variable using par()

b

Data Link Layer: Simulate 10% packet loss at the destination

c

Data Link Layer: Retransmit on timeout; reset timeout after each transmission; cancel on ACK

Bugs Fixed — Summary

✓ 10% loss fix ✓ decapsulate() memory leak ✓ ACK leak on drop ✓ copy_message nullptr ✓ old copy_message deleted ✓ self-event leak fixed ✓ unused cGate* in removed

#	Bug	Wrong Code	Fixed Code
1	Packet loss % wrong	uniform(0,1) <= 0.8 → 80% drop	uniform(0,1) < 0.1 → 10% drop
2	decapsulate() memory leak	Return value ignored; outer wrapper sent	inner = decapsulate(); send inner; delete outer
3	ACK leak in drop case	ACK created but never deleted on drop	ACK created only when packet is accepted
4	copy_message garbage init	copy_message = new Dll_pkt() — empty packet	copy_message = nullptr with null-check guard
5	Old copy_message not deleted	Memory leak on every new packet	delete copy_message before storing new one
6	Self-event not deleted (NL)	cMessage event leaked after use	delete msg after every self-message
7	Unused variable cGate* in	Declared in nl_node.h but never used	Removed from header and .cc file

How to Run in OMNeT++

1

Create a new OMNeT++ project and add all files above.

2

Compile packets.msg first — it auto-generates nl_pkt_m.h and dll_pkt_m.h.

3

Build the project and run with omnetpp.ini.

4

Open the Event Log to see packet flow, ACKs, timeouts, and retransmissions.

Source Code

NED nldll.ned

simple Nl_node
{
    parameters:
        int nl_id;
        int remaining_packets;
        int source;

    gates:
        input  nl_in;
        output nl_out;
}

simple Dll_node
{
    parameters:
        int dll_id;

    gates:
        input  dlln_in;
        output dlln_out;
        input  dllc_in;
        output dllc_out;
}

module Comp
{
    parameters:
        int c_id;

    gates:
        input  c_in;
        output c_out;

    submodules:
        Nl:  Nl_node  { nl_id  = c_id; }
        Dll: Dll_node { dll_id = c_id; }

    connections:
        Nl.nl_in   <-- Dll.dlln_out;
        Nl.nl_out  --> Dll.dlln_in;
        Dll.dllc_in  <-- c_in;
        Dll.dllc_out --> c_out;
}

network Comp_network
{
    parameters:
        int source;
        int dest;

    submodules:
        C1: Comp { c_id = 1; }
        C2: Comp { c_id = 2; }

    connections:
        C1.c_out --> { delay = 100ms; } --> C2.c_in;
        C2.c_out --> { delay = 100ms; } --> C1.c_in;
}

MSG packets.msg

packet Nl_pkt
{
    int Nl_pkt_id;
    int Nl_pkt_type;   // 1 = data
}

packet Dll_pkt
{
    int Dll_pkt_id;
    int Dll_pkt_type;  // 1 = data frame,  0 = ACK frame
}

C++ nl_node.h

#ifndef __NLDLL_NL_NODE_H_
#define __NLDLL_NL_NODE_H_

#include <omnetpp.h>
#include "nl_pkt_m.h"

using namespace omnetpp;

class Nl_node : public cSimpleModule
{
  protected:
    int    id;
    int    remaining_packets;
    int    source;
    cGate* out;           // unused 'in' gate removed

    virtual void initialize();
    virtual void handleMessage(cMessage *msg);
};

#endif

C++ nl_node.cc

#include "nl_node.h"

Define_Module(Nl_node);

void Nl_node::initialize()
{
    id                = par("nl_id");
    remaining_packets = par("remaining_packets");
    source            = par("source");
    out               = gate("nl_out");

    if (id == source && remaining_packets > 0) {
        cMessage* startEvent = new cMessage("startEvent");
        scheduleAt(0, startEvent);
    }
}

void Nl_node::handleMessage(cMessage *msg)
{
    if (msg->isSelfMessage())
    {
        if (remaining_packets <= 0) {
            delete msg;
            return;
        }

        Nl_pkt* data = new Nl_pkt("Nl_pkt");
        data->setNl_pkt_id(remaining_packets);
        data->setNl_pkt_type(1);
        --remaining_packets;

        EV << "[NL id=" << id << "] Sending packet id="
           << data->getNl_pkt_id()
           << "  remaining=" << remaining_packets << endl;

        send(data, out);

        if (remaining_packets > 0) {
            cMessage* nextEvent = new cMessage("sendEvent");
            scheduleAt(simTime() + 200, nextEvent);
        }

        delete msg;   // always delete consumed self-event
    }
    else
    {
        EV << "[NL id=" << id << "] Received packet from remote node." << endl;
        delete msg;
    }
}

C++ dll_node.h

#ifndef __NLDLL_DLL_NODE_H_
#define __NLDLL_DLL_NODE_H_

#include <omnetpp.h>
#include "dll_pkt_m.h"
#include "nl_pkt_m.h"

using namespace omnetpp;

class Dll_node : public cSimpleModule
{
  protected:
    int    id;
    cGate* in_n;         // gate from NL (upper layer)
    cGate* out_n;        // gate to   NL (upper layer)
    cGate* in_c;         // gate from channel
    cGate* out_c;        // gate to   channel

    Dll_pkt*  copy_message;   // saved copy for retransmission
    cMessage* timeoutEvent;

    virtual void initialize();
    virtual void handleMessage(cMessage *msg);
};

#endif

C++ dll_node.cc

#include "dll_node.h"

Define_Module(Dll_node);

void Dll_node::initialize()
{
    id    = par("dll_id");
    in_n  = gate("dlln_in");
    out_n = gate("dlln_out");
    in_c  = gate("dllc_in");
    out_c = gate("dllc_out");

    copy_message = nullptr;          // FIX: nullptr, not empty packet
    timeoutEvent = new cMessage("timeoutEvent");
}

void Dll_node::handleMessage(cMessage *msg)
{
    // ── CASE 1: Timeout — retransmit saved copy ──
    if (msg == timeoutEvent)
    {
        if (copy_message != nullptr) {
            EV << "[DLL id=" << id << "] Timeout! Retransmitting frame id="
               << copy_message->getDll_pkt_id() << endl;

            send(copy_message->dup(), out_c);
            scheduleAt(simTime() + 200, timeoutEvent);
        }
        return;
    }

    // ── CASE 2: New data from NL (upper layer) ──
    if (msg->getArrivalGate() == in_n)
    {
        Nl_pkt* nlPkt = check_and_cast<Nl_pkt*>(msg);

        Dll_pkt* frame = new Dll_pkt("Dll_pkt");
        frame->setDll_pkt_id(nlPkt->getNl_pkt_id());
        frame->setDll_pkt_type(1);
        frame->encapsulate(nlPkt);   // nlPkt now owned by frame

        EV << "[DLL id=" << id << "] Sending DATA frame id="
           << frame->getDll_pkt_id() << endl;

        send(frame->dup(), out_c);   // send copy, keep original

        if (copy_message != nullptr)
            delete copy_message;    // FIX: delete old before saving new
        copy_message = frame;

        cancelEvent(timeoutEvent);
        scheduleAt(simTime() + 200, timeoutEvent);
        return;
    }

    // ── CASE 3: Message from channel ──
    if (msg->getArrivalGate() == in_c)
    {
        Dll_pkt* incoming = check_and_cast<Dll_pkt*>(msg);

        // 3a: DATA frame — destination side
        if (incoming->getDll_pkt_type() == 1)
        {
            // FIX: 10% packet loss (was 80% in buggy code)
            if (uniform(0, 1) < 0.1)
            {
                EV << "[DLL id=" << id << "] Packet DROPPED." << endl;
                delete incoming;
                return;
            }

            // Send ACK
            Dll_pkt* ack = new Dll_pkt("ACK");
            ack->setDll_pkt_id(incoming->getDll_pkt_id());
            ack->setDll_pkt_type(0);
            send(ack, out_c);

            EV << "[DLL id=" << id << "] Received DATA id="
               << incoming->getDll_pkt_id() << ". Sent ACK." << endl;

            // FIX: correct decapsulation
            cPacket* inner = incoming->decapsulate();
            delete incoming;          // delete outer DLL wrapper
            send(inner, out_n);
            return;
        }

        // 3b: ACK — source side
        if (incoming->getDll_pkt_type() == 0)
        {
            EV << "[DLL id=" << id << "] ACK received for frame id="
               << incoming->getDll_pkt_id() << ". Timeout cancelled." << endl;

            cancelEvent(timeoutEvent);
            delete incoming;
            return;
        }

        delete incoming;   // unknown type
    }
}

INI omnetpp.ini

[General]
network           = Comp_network
sim-time-limit    = 5000s
record-eventlog   = true

**.remaining_packets = 10
**.source            = 1
**.dest              = 2