Lec30-Routing Information Protocol |Computer Networks

Key Concepts

• Intra-domain routing protocol
• RIP (Routing Information Protocol)
• Distance vector routing
• Cost (number of hops)
• Routing table (destination network, next router, cost)
• Application layer protocol
• UDP encapsulation
• RIP message format (request, response - solicited, unsolicited)
• Autonomous system

Routing Information Protocol (RIP) Explained

Introduction

The video focuses on RIP, a widely used intra-domain routing protocol based on distance vector routing. Unlike previous examples using weighted graphs, RIP is explained in the context of an actual network.

Cost Definition

The cost in RIP is defined as the number of hops, representing the number of networks a packet must traverse from the source router to the destination host.

• Example: In a network with routers R1, R2, R3 and networks N1, N2, N3, N4, the number of hops from R1 to a host in N4 is 3 (N2 -> N3 -> N4).

Routing Table Construction

RIP uses the distance vector routing principle to build routing tables. The routing table contains three key columns:

1. Destination Network: The network the router wants to reach.
2. Next Router: The next router in the path to the destination network.
3. Cost: The number of hops to reach the destination network.

• Example: Routing table for R1:

  | Destination Network | Next Router | Cost | | :------------------ | :---------- | :--- | | N1 | - | 1 | | N2 | - | 1 | | N3 | R2 | 2 | | N4 | R2 | 3 |

Implementation Details

RIP operates at the application layer and relies on UDP (User Datagram Protocol) at the transport layer to encapsulate its messages. UDP messages are then encapsulated within IP datagrams at the network layer.

RIP Message Format

Routers exchange routing information using RIP messages. The message format includes:

• Command (8 bits): Specifies the message type (request or response).
  • Request Message: A router asks for routing information from a neighbor.
  • Response Message: A router sends its routing information.
    • Solicited Response: Sent in response to a request.
    • Unsolicited Response: Sent periodically (every 30 seconds) without a prior request.
• Version (8 bits): RIP version number (e.g., RIP version 2).
• Reserved (16 bits): Reserved for future use.
• Address Family Identifier (16 bits): Specifies the protocol suite (e.g., TCP/IP, value is 2).
• Route Tag (16 bits): Contains information about the autonomous system.
• Network Address (variable): The IP address of the destination network.
• Next Hop Address (variable): The IP address of the next router in the path.
• Distance (variable): The cost (number of hops) to reach the destination network.
• Subnet Mask (variable): The subnet mask of the destination network.

The fields for network address, next hop address, distance, and subnet mask are repeated for each route in the routing table.

Conclusion

RIP is a distance vector routing protocol that constructs routing tables based on the number of hops to reach destination networks. It operates at the application layer, using UDP for message encapsulation. Routers exchange routing information through request and response messages, with unsolicited responses sent periodically to maintain routing table accuracy. The message format includes fields for network addresses, next hop routers, costs, and subnet masks.