Question

Propose a lookup algorithm for a CIDR fowarding table that does not require a linear search of the entire table to find the longest match.

Short answer

Use a trie data structure to efficiently insert and lookup CIDR prefixes, which allows you to find the longest prefix match without performing a linear search.

Step-by-step solution

Title - Understand CIDR and Forwarding Tables

CIDR stands for Classless Inter-Domain Routing and is used for IP address allocation and route aggregation. A forwarding table contains routes that guide the forwarding of packets to their destinations. CIDR allows for variable-length subnet masking, which means subnet masks can be of any length.

Title - Conceptualize Trie-based Data Structure

Consider using a trie, a tree-like data structure, where each node represents a bit in the IP address. A trie supports fast lookups and is efficient for longest prefix matching used in CIDR forwarding.

Title - Initialize the Trie

Initialize the trie root node. Each trie node can have at most two children: one representing bit ‘0’ and the other representing bit ‘1’. The root node represents the start of any IP prefix.

Title - Insert Prefixes into the Trie

For each CIDR prefix in the forwarding table, insert it into the trie. Traverse the trie based on each bit of the prefix. When you reach the end of the prefix, mark that node as a valid endpoint.

Title - Implement the Lookup Function

For a given destination IP, traverse the trie according to its bits. Keep track of the longest prefix found during traversal. If a valid endpoint is found, update the longest prefix match before continuing.

Title - Return Longest Match

Once traversal is complete, return the longest prefix match found. This endpoint will be the longest matching prefix in the forwarding table for the given IP address.

Key concepts

CIDR (Classless Inter-Domain Routing)

CIDR stands for Classless Inter-Domain Routing, and it revolutionized IP address allocation and routing.
Instead of using the traditional classful network design (classes A, B, and C), CIDR allows for flexible and more efficient allocation, using variable-length subnet masking.
This means that an IP address and its subnet mask are noted together in the format: prefix/length (e.g., 192.168.1.0/24).
This provides flexibility and helps in preventing the wastage of IP addresses, which is crucial as the internet grows.
CIDR is essential for efficient route aggregation and routing using the CIDR forwarding algorithm.

Trie Data Structure

A trie, also known as a prefix tree, is an efficient data structure for storing a dynamic set of strings.
Each node in a trie represents a single character of a string. When used for IP routing, each node represents a bit of the IP address.
Here’s why a trie is useful:

• Fast Lookup: Tries give rapid search results and are proficient in prefix matching.
• Space-efficient: Tries can be space-efficient if they share common prefixes between strings (or IP addresses).

In the context of CIDR and IP routing, each path from the root to a node represents a potential IP address prefix, which assists in quick lookup and insertion operations.

Longest Prefix Matching

Longest prefix matching is a core principle in IP routing.
When a packet is routed, the router needs to determine which rule to apply from its forwarding table.
It does this by finding the most specific rule that matches the destination IP address - essentially the longest matching prefix.
This ensures packets are routed accurately to their destinations.
For instance, if the forwarding table has rules for 192.168.1.0/24 and 192.168.1.128/25, and a packet needs to be forwarded to 192.168.1.130, the router will use the rule 192.168.1.128/25 because it’s a longer (more specific) prefix match.

IP Routing

IP routing is the process that enables data packets to travel across different networks to reach their destination.
The router determines the best path for forwarding packets based on the destination IP address in the forwarding table.
Routers use various algorithms and protocols, like RIP, OSPF, and BGP, to keep their forwarding tables updated.
In the context of a CIDR forwarding table, the router doesn’t perform a linear search.
Instead, it uses data structures like tries to efficiently locate the longest prefix match, which significantly speeds up the lookup process.

Forwarding Table

A forwarding table, also known as a routing table, is an essential component in routers.
This table contains a set of rules that determine the next hop for incoming packets.
Each entry in the table usually includes:

• The destination network (CIDR prefix)
• The subnet mask
• The next-hop IP address or interface

The primary function of the forwarding table is to facilitate the quick lookup process.
Combining CIDR and tries, routers can efficiently match incoming packets to the longest matching prefix in the forwarding table and send them along the optimal path.