Build Your Own Router

A simple IPv4 router with static routing, ARP, and ICMP support, implemented in C++11 and running on Mininet.

What It Does

This router receives raw Ethernet frames and processes them like a real router:

• ARP: Responds to ARP requests, sends ARP requests to discover next-hop MACs, maintains a cache with 30-second TTL
• IPv4 Forwarding: Validates checksums, decrements TTL, looks up routing table with longest-prefix-match, forwards packets to the correct interface
• ICMP: Generates Echo Reply (for ping), Time Exceeded (for traceroute), and Port/Host Unreachable messages

Network Topology

          server1 (192.168.2.2/24)
              │
         sw0-eth1 (192.168.2.1/24)
              │
client ── sw0-eth3 (10.0.1.1/8) ── [Router sw0]
(10.0.1.100/8)                      │
                              sw0-eth2 (172.64.3.1/16)
                                     │
                              server2 (172.64.3.10/16)

Routing Table

Destination	Gateway	Mask	Interface
0.0.0.0	10.0.1.100	0.0.0.0	sw0-eth3
192.168.2.2	0.0.0.0	255.255.255.0	sw0-eth1
172.64.3.10	0.0.0.0	255.255.0.0	sw0-eth2

What You Need to Implement

The starter code provides the framework. Three methods need to be filled in:

Method	File	What It Does
handlePacket()	simple-router.cpp	Entry point: dispatches Ethernet frames by type, validates MAC, calls ARP/IP handlers
periodicCheckArpRequestsAndCacheEntries()	arp-cache.cpp	Called every second: resends ARP requests, times out after 5 attempts, cleans expired cache
lookup()	routing-table.cpp	Longest-prefix-match lookup across the static routing table

Additional helper methods in simple-router.cpp handle ARP requests/replies, IP validation/forwarding, and ICMP message generation.

Build & Run

# Prerequisites (Ubuntu 16.04)
sudo bash setup.sh

# Compile
make

# Run (3 terminals required)
# Terminal 1: POX controller
/opt/pox/pox.py --verbose ucla_cs118

# Terminal 2: Mininet topology
chmod +x run.py
sudo ./run.py

# Terminal 3: Router
./router

Once all three are running, test in the Mininet CLI:

mininet> client ping server1
mininet> client traceroute server2
mininet> client wget http://192.168.2.2/index.html

Implementation Overview

Layer	File	Responsibility
Ethernet	simple-router.cpp	Frame dispatch by type (ARP/IPv4) and MAC filtering
ARP	arp-cache.cpp	IP↔MAC resolution, request/reply, cached entries with 30s TTL
IPv4	simple-router.cpp	Checksum validation, TTL decrement, longest-prefix-match forwarding
ICMP	simple-router.cpp	Echo Reply (type 0), Time Exceeded (type 11), Port/Host Unreachable (type 3)
Routing	routing-table.cpp	Longest-prefix-match lookup over static routing table

Key Design Decisions

• ARP timeout handling: Requests retry every 1 second, up to 5 times. On timeout, an ICMP Host Unreachable is sent back to the source via a reverse route lookup.
• Interface selection for local replies: Echo Reply and ICMP errors are sent via the incoming interface (not the destination IP's interface), with the destination IP as the source address.
• MAC preservation: Forwarded packets queued for ARP resolution retain their original source MAC, ensuring ICMP errors can reach the original sender.

Test Results

Test	Score	Status
Public autograde	45/45	✅
Ping TTL correctness	10 pts	✅ ttl=63 after forwarding
Traceroute 1-hop	10 pts	✅
ARP timeout (no crash)	5 pts	✅
Host Unreachable	5 pts	✅ Verified with modified RTABLE
Large file download (10 MB)	10 pts	✅ md5 matched
Total	85/85	All passed

Documentation

• Implementation Guide — Detailed walkthrough with code references
• Testing Guide — Step-by-step testing procedures (including autograde notes)

Acknowledgement

Based on the CS118 project by Alexander Afanasyev, UCLA.