How to Calculate Network Maximum Transfer Unit in omnet++
To calculate the Maximum Transfer Unit (MTU) in a OMNeT++ which is the largest size of a packet that can be transmitted through a network link deprived of needing to be fragmented. In OMNeT++, MTU is naturally build as a parameter in the configuration of the network, and it can be stated by checking the configured MTU value on a network interface or by mimicing the transmission of packets of multiple sizes.
Steps to Calculate or Determine MTU in OMNeT++:
- Set or Inspect the MTU in the Network Configuration:
- The MTU is normally described in the network configuration like in the NED file or as a parameter in the INI file. We can verify this value directly.
- Simulate Packet Transmission:
- We can simulate the sending packets of increasing size and monitor when fragmentation occurs to empirically define the MTU. The largest packet that can be sent except fragmentation is the MTU.
Example Implementation: Simulating Packet Transmission to Determine MTU
Follow the sample to simulate the transmission packets to define MTU in OMNeT++:
#include <omnetpp.h>
#include “inet/common/packet/Packet.h”
#include “inet/networklayer/contract/INetfilter.h”
using namespace omnetpp;
using namespace inet;
class MTUModule : public cSimpleModule, public INetfilter::IHook {
private:
int currentPacketSize; // Size of the current packet being tested
bool isFragmented; // Flag to check if the packet was fragmented
protected:
virtual void initialize() override {
// Start with a small packet size and increase
currentPacketSize = 500; // Initial packet size in bytes
// Register this module as a network layer hook
INetfilter *networkLayer = getModuleFromPar<INetfilter>(par(“networkLayerModule”), this);
networkLayer->registerHook(0, this);
// Schedule the first packet transmission
scheduleAt(simTime() + par(“sendInterval”).doubleValue(), new cMessage(“sendPacket”));
}
virtual void handleMessage(cMessage *msg) override {
if (strcmp(msg->getName(), “sendPacket”) == 0) {
// Create and send a packet of the current size
Packet *packet = new Packet(“dataPacket”);
const auto& payload = makeShared<ByteCountChunk>(B(currentPacketSize));
packet->insertAtBack(payload);
send(packet, “out”);
// Schedule the next packet transmission with an increased size
currentPacketSize += 500; // Increase packet size by 500 bytes
scheduleAt(simTime() + par(“sendInterval”).doubleValue(), new cMessage(“sendPacket”));
// Check if the last packet was fragmented
if (isFragmented) {
EV << “MTU is ” << currentPacketSize – 500 << ” bytes\n”; // Last successful size
cancelAndDelete(msg); // Stop the simulation if fragmentation occurs
} else {
isFragmented = false; // Reset flag for the next packet
}
}
}
// This method is called to inspect and possibly modify a packet before it is sent
virtual Result datagramForwardHook(INetworkDatagram *datagram, const InterfaceEntry *inIE, const InterfaceEntry *&outIE, L3Address& nextHop) override {
if (datagram->getByteLength() > outIE->getMTU()) {
isFragmented = true; // Mark as fragmented if it exceeds the MTU
}
return ACCEPT;
}
virtual void finish() override {
EV << “Final MTU determined: ” << currentPacketSize – 500 << ” bytes\n”;
}
// Necessary to implement the IHook interface
virtual void refreshDisplay() const override {}
virtual Result datagramLocalInHook(INetworkDatagram *datagram, const InterfaceEntry *inIE) override { return ACCEPT; }
virtual Result datagramLocalOutHook(INetworkDatagram *datagram, const InterfaceEntry *&outIE, L3Address& nextHop) override { return ACCEPT; }
virtual Result datagramPostRoutingHook(INetworkDatagram *datagram, const InterfaceEntry *inIE, const InterfaceEntry *&outIE, L3Address& nextHop) override { return ACCEPT; }
virtual Result datagramPreRoutingHook(INetworkDatagram *datagram, const InterfaceEntry *&inIE, const InterfaceEntry *&outIE, L3Address& nextHop) override { return ACCEPT; }
};
Define_Module(MTUModule);
Explanation:
- MTUModule:
- currentPacketSize: Begins with an initial packet size (e.g., 500 bytes) and raises with every transmission to examine the MTU.
- isFragmented: A flag to state whether the current packet was fragmented.
- initialize() Function:
- Check outgoing packets by registering the module as a hook.
- Lists the first packet transmission.
- handleMessage() Function:
- Sends packets of increasing size until fragmentation is sensed.
- When a packet is fragmented, the module logs the final successful size as the MTU.
- datagramForwardHook() Function:
- Verify if a packet surpasses the MTU and would be fragmented. If fragmentation occurs, it sets the isFragmented flag.
- finish() Function:
- Once the simulation is completed, logs the final determined MTU value.
- Run the Simulation:
- Compile and run OMNeT++ project. The simulation will send packets of increasing size and stop when fragmentation occurs, specifying the MTU.
- Analyze and Interpret Results:
- The logged MTU value signifies the largest packet size that can be transferred without fragmentation in the network. This value is crucial for enhancing packet sizes and ignoring the overhead associated with fragmentation.
Additional Considerations:
- Network Interfaces: Various network interfaces may have multiple MTU values. While the MTU is defined, we have to make certain that the correct interface is being used.
- Traffic Types: While determining MTU, consider the distinctive traffic kinds and patterns in the network to make sure that the chosen MTU is suitable.
In this demonstration, we offered the details on how to simulate the transmission of packets of different sizes to determine the MTU and how to calculate the network maximum transfer unit in OMNeT++.
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