How to Start Simple Mail Transfer Protocol Using OMNeT++

To start a Simple Mail Transfer Protocol (SMTP) project in OMNeT++ that needs to replicate the SMTP system behavior within a network environment. SMTP is designed for transmitting emails among the servers and clients. Since OMNeT++ environment offers simple networking functionalities then it doesn’t have direct support for SMTP. Then, we will want to execute the SMTP from scratch or prolong existing modules replicating the behavior of SMTP.

Below is a structured outline for SMTP project in OMNeT++:

Steps to Start SMTP Project in OMNeT++

  1. Set Up OMNeT++ Environment

Download and Install OMNeT++

  1. We should download and install OMNeT++ on the system.
  2. Adhere to the installation instruction based on the OS.
  3. After installation, we confirm that OMNeT++ is properly functioning by executing a sample simulation like an example from the INET or Veins framework.

Install INET Framework

  1. INET is support to replicate network protocols with basic protocols for networking such as TCP, UDP, IP, and so on.
  2. We have to download INET using GitHub repository.
  3. Construct and set INET in OMNeT++.
  1. Understand SMTP Basics

SMTP is a protocol which is designed for transmitting emails among the servers. Below is a basic outline of how SMTP operates:

  • SMTP Client (Mail User Agent, MUA): The email application of user such as Outlook or Thunderbird, which transmits email demands to the SMTP server.
  • SMTP Server (Mail Transfer Agent, MTA): A server that executes and sends email messages to the email server of recipient.
  • Message Transfer: SMTP is frequently utilised for sending email messages among these systems. It employs port 25 for interaction.

SMTP functions mainly contain:

  • Sending Emails: To transfer email messages from the client to the server.
  • Relaying Messages: The server might transmit messages to the recipient’s server.
  • Error Handling: SMTP servers transmit error messages such as “User not found” or “Mailbox full”.
  1. Create a New OMNeT++ Project
  1. Create a Project:
    • Go to the OMNeT++ IDE and select File > New > OMNeT++ Project.
    • Name it to the project like SMTPProject.
  2. Link to INET Framework:
    • Right-click on the project, choose Properties > Project References, and connect the project to the INET framework.
  1. Design the Network Topology

We create a network topology for an SMTP simulation:

  • SMTP Client: A mail user agent (MUA), which transmits email demands to the SMTP server.
  • SMTP Server: A mail transfer agent (MTA) that obtains emails and sends them as required.
  • Network Links: It associates to the client and server across TCP/IP.

Below is an instance of a simple network using a .ned file for OMNeT++:

Example SMTPNetwork.ned:

network SMTPNetwork {

submodules:

smtpClient: StandardHost {

@display(“p=100,100”);

}

smtpServer: StandardHost {

@display(“p=300,100”);

}

router: Router {

@display(“p=200,100”);

}

connections allowunconnected:

smtpClient.pppg++ <–> router.pppg++;

router.pppg++ <–> smtpServer.pppg++;

}

This network consists of:

  • smtpClient: A host denoting the email client (MUA).
  • smtpServer: A host to signify the SMTP server (MTA).
  • router: A router replicating network interaction among the client and server.
  1. Implement SMTP Client and Server Modules

While OMNeT++ doesn’t support for predefined SMTP protocol then we want executing own custom components for the SMTP client and SMTP server.

SMTP Client (Mail User Agent – MUA):

The client transmits an email message to the SMTP server. It requires to:

  • Launch a TCP connection including the SMTP server.
  • It transmits the proper SMTP commands such as HELO, MAIL FROM, RCPT TO, DATA, and QUIT.
  • Receive server responses.

Below is a simple structure of how the SMTP Client module can be executed:

class SMTPClient : public cSimpleModule {

protected:

cTcpSocket *socket;

cMessage *sendEmailMessage;

virtual void initialize() {

// Initialize TCP socket and the email message

socket = new cTcpSocket();

sendEmailMessage = new cMessage(“SendEmail”);

scheduleAt(simTime() + 1.0, sendEmailMessage);  // Send email after 1 second

}

virtual void handleMessage(cMessage *msg) {

if (msg == sendEmailMessage) {

// Send SMTP commands like HELO, MAIL FROM, RCPT TO, etc.

socket->connect(“smtpServer”, 25);

socket->send(“HELO client.example.com\r\n”);

socket->send(“MAIL FROM:<[email protected]>\r\n”);

socket->send(“RCPT TO:<[email protected]>\r\n”);

socket->send(“DATA\r\n”);

socket->send(“Subject: Test Email\r\n”);

socket->send(“This is a test email.\r\n.\r\n”);

socket->send(“QUIT\r\n”);

} else if (msg->isSelfMessage()) {

// Handle server response, checking for success or failure

}

}

virtual void finish() {

delete socket;

delete sendEmailMessage;

}

};

SMTP Server (Mail Transfer Agent – MTA):

The server requires to:

  • For incoming SMTP connections, pay attention on port 25.
  • Describe SMTP commands and reply with proper status messages.
  • It sends the email to another server or save it.

Following is a simple skeleton of how the SMTP Server module can execute:

class SMTPServer : public cSimpleModule {

protected:

cTcpServerSocket *serverSocket;

virtual void initialize() {

// Initialize the server socket to listen on port 25

serverSocket = new cTcpServerSocket(25, this);

}

virtual void handleMessage(cMessage *msg) {

if (msg->isSelfMessage()) {

// Process SMTP commands and respond

cTcpMessage *tcpMessage = check_and_cast<cTcpMessage*>(msg);

if (tcpMessage->getName() == “HELO”) {

sendResponse(“250 OK”);

} else if (tcpMessage->getName() == “MAIL FROM”) {

sendResponse(“250 OK”);

} else if (tcpMessage->getName() == “RCPT TO”) {

sendResponse(“250 OK”);

} else if (tcpMessage->getName() == “DATA”) {

sendResponse(“354 Start mail input”);

} else if (tcpMessage->getName() == “QUIT”) {

sendResponse(“221 Bye”);

}

}

}

void sendResponse(const char* response) {

// Send a response back to the client (SMTP client)

cTcpMessage *responseMsg = new cTcpMessage(response);

send(responseMsg, “out”);

}

virtual void finish() {

delete serverSocket;

}

};

  1. Configure Simulation Parameters in omnetpp.ini

We set the simulation metrics like:

  • SMTP Client Settings: Delineate the timing to transmit email requests.
  • SMTP Server Settings: Describe response time and other SMTP server-related sets up.

Example omnetpp.ini:

network = SMTPNetwork

sim-time-limit = 100s

# SMTP Client Settings

*.smtpClient.sendEmailMessage = true

*.smtpClient.serverAddress = “smtpServer”

*.smtpClient.port = 25

# SMTP Server Settings

*.smtpServer.listenPort = 25

# Enable debugging for SMTP communication

*.smtpClient.debug = true

*.smtpServer.debug = true

  1. Run the Simulation
  1. Make use of OMNeT++’s Qtenv or Tkenv graphical environment to execute the simulation.
  2. Observe the communication among the SMTP client and server:
    • Make sure that the client transfers SMTP commands such as HELO, MAIL FROM, RCPT TO, DATA, and so on.
    • We need to monitor the server responses such as 250 OK, 354 Start mail input, and 221 Bye.
  3. Check for Errors: Confirm that emails are effectively transmitted, and debug messages support to monitor the process.
  1. Analyze and Optimize

When the simulation executes then we can examine:

  • SMTP Message Flow: Make sure that appropriate message swap among the client and server.
  • Performance: We have to estimate the latency within sending emails and processing SMTP commands.
  • Error Handling: Verify how the system manages the failed connections or command errors.
  1. Extend the Project

We can enhance the project by integrating:

  • Multiple SMTP Clients and Servers: Mimic interaction among numerous clients and servers.
  • SMTP Authentication: Integrate the support for SMTP confirmation like LOGIN, PLAIN.
  • SMTP Extensions: Execute the extended SMTP aspects such as secure interaction across TLS (STARTTLS) or larger message handling.
  1. Documentation and Reporting

It provides detailed insights like:

  • Network Topology: Describe the SMTP clients and servers set up.
  • Protocol Simulation: It offers a step-by-step procedure of how the SMTP protocol is replicated within the model.
  • Results and Insights: Distribute performance outcomes, issues encountered, and conclusions from the simulation.

Following these steps, you’ll initiate and replicate the Simple Mail Transfer Protocol projects using OMNeT++ environment and have the opportunity to expand this project as required.

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