Dataplane-router

Dataplane – the part that implements the actual routing process, based on entries in the routing table.
This project implements a simple Layer 3 router capable of forwarding IPv4 packets, performing Longest Prefix Match (LPM), handling ARP (requests, replies, and caching), and generating ICMP messages.
It is based on the skeleton provided in Laboratory 4 of the Communication Protocols course.

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1. Packet Forwarding Process

When the router receives an IPv4 packet, it follows these steps:

• Destination check
  If the destination IP matches one of the router’s interfaces, the router does not forward the packet but replies with an ICMP Echo Reply.

• Checksum verification
  The IP header checksum is recalculated. If it differs from the original, the packet is dropped (corrupted data).

• TTL (Time To Live)

  • If TTL <= 1: the packet is dropped and an ICMP Time Exceeded message is sent.
  • Otherwise, TTL is decremented and the checksum is recalculated.

• Route lookup (LPM)
  The function get_best_route(IP_node *root, uint32_t IP) searches the Trie to find the most specific route (Longest Prefix Match).
  If no valid route is found, an ICMP Destination Unreachable message is sent.

• Next-hop MAC resolution
  After identifying the route, the router looks for the next hop’s MAC address in the ARP table before forwarding the packet.

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2. Longest Prefix Match (LPM)

Implemented using a binary Trie structure:

• Each node represents one bit of the IP address (0 or 1).
• Each node can store a pointer to a valid routing table entry.
• The insert_ip_route() function inserts a route bit by bit according to its mask.
• The get_best_route() function traverses the Trie and returns the longest matching prefix for a given IP.

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3. ARP Protocol

After determining the best route, the router must find the MAC address of the next hop:

• If the MAC address is cached:
  The packet is immediately sent.

• If the MAC address is missing:

  • The packet is stored in a waiting queue.
  • An ARP Request is sent for the next hop’s IP.

• Upon receiving an ARP Reply:

  • The router updates the ARP table.
  • All queued packets waiting for that IP are transmitted.
  • Packets without a resolved address remain in the queue.

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4. ICMP Protocol

Implemented in the send_icmp() function, which builds and sends ICMP messages in the following cases:

• TTL expired → ICMP Time Exceeded
• No valid route → ICMP Destination Unreachable
• Packet addressed to the router → ICMP Echo Reply

Steps to build and send an ICMP packet:

1. Construct a new IP + ICMP packet based on the original.
2. Set the source MAC to the router’s interface MAC.
3. Set the destination MAC to the original sender’s MAC.
4. Swap source/destination IPs.
5. Set ICMP header fields (type, code, checksum).
6. Recalculate the IP checksum.
7. Send the packet on the same interface where it was received.

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Assignment Source

This project was developed for the Communications Protocols course, based on the Dataplane router assignment:

https://pcom.pages.upb.ro/tema1/