Mini Router Simulator

A C++ project that simulates a Layer 3 Router using Object-Oriented Programming (OOP). It emulates packet forwarding with a complete TCP/IP lifecycle, including TTL aging, routing table lookup, ARP resolution, and Layer 2 encapsulation.

This project demonstrates system-level networking concepts and modular C++ design.

Features

• Packet Structure — simulates real network packets with Layer 3 (IP, TTL) and Layer 2 (MAC) headers.
• TCP/IP Lifecycle Management — implements TTL (Time-To-Live) aging to prevent routing loops.
• Encapsulation — simulates Layer 2 header rewriting (Source/Destination MAC updates) at each hop.
• Routing Table — stores Destination IP → Next Hop IP mappings (Layer 3).
• ARP Table — stores IP → MAC address mappings (Layer 2).
• Modular OOP Design — separate classes for Router, Tables, and Packet structures.

Packet Flow & Encapsulation Logic

The simulator processes a Packet object through the following lifecycle stages:

graph TD
    A[Ingress: Packet In] -->|Check TTL| B{TTL > 1?}
    B -- No --> C[Drop Packet: TTL Expired]
    B -- Yes --> D[Decrement TTL]
    D --> E{L3 Lookup: Route Exists?}
    E -- No --> F[Drop Packet: No Route]
    E -- Yes --> G{L2 Lookup: ARP Exists?}
    G -- No --> H[Drop Packet: ARP Failed]
    G -- Yes --> I[Encapsulation]
    
    subgraph Encapsulation Process
    I1[Set SrcMAC = Router MAC]
    I2[Set DstMAC = Next Hop MAC]
    end
    
    I --> I1 --> I2 --> J[Egress: Forward Packet]

Loading

System-Level Operations Simulated:

1. Validation & Aging: Checks if the packet is valid; decrements TTL (Time-To-Live).
2. Routing (L3): Determines the next hop IP based on the destination.
3. Encapsulation (L2): Rewrites the Ethernet header:
   • SrcMAC becomes the Router's MAC.
   • DstMAC becomes the Next Hop's MAC.

Example Output

=== Test Case 1: Normal Forwarding ===

[Router] Ingress: Packet destined for 10.0.0.2
         Current TTL: 64
-> L3 Routing Decision: Next hop is 192.168.1.1
[Forwarding] Packet forwarded successfully!
             L2 Header Updated -> SrcMAC: AA:BB:CC:DD:EE:01 | DstMAC: AA:BB:CC:DD:EE:01
             L3 Header Updated -> TTL: 63

=== Test Case 2: TTL Expired (Lifecycle Simulation) ===

[Router] Ingress: Packet destined for 10.0.0.9
         Current TTL: 1
x Dropped: TTL expired (Lifecycle ended).

=== Test Case 3: No Route ===

[Router] Ingress: Packet destined for 10.0.1.1
         Current TTL: 64
x Dropped: No Route found for 10.0.1.1

Build & Run

Ensure you have a C++17 compatible compiler (e.g., g++ or clang++).

# Compile the project
make

# Run the simulator
make run

# Clean build files
make clean

Learning Goals

• Emulate Packet Forwarding: Implemented a simulator that handles packet objects rather than simple strings.
• TCP/IP Lifecycle: Applied TTL logic to simulate packet aging and network loop prevention.
• Encapsulation: Practiced system-level operations by rewriting Layer 2 headers (MAC addresses) during forwarding.
• Network Layering: Clear separation between Layer 3 (IP Routing) and Layer 2 (ARP/Mac) logic using OOP.
• C++ Proficiency: Utilized structs, classes, and standard libraries to build maintainable network code.