Introduction

Routing is the method by which the host or gateway decides where to send the IP datagram to. RIP is intended to be used in the context of the internet. Its scope isn’t as large, as it’s best fit for autonomous systems (the small parts of the internet, i.e the networks that make the network of network “internet”).

Using RIP, a host or a gateway may be able to send the packet directly to the destination, if that destination is directly connected to the latter. However, the interesting case is when the destination is NOT directly connected to the host/gateway. In this case, the host/gateway tries to send the packet to a nearer host/gateway to the destination.

The goal of the routing protocol is very simple: Supply the information that is needed to do routing.

Limitation of the protocol

Organization of the RFC

Protocol Specification

As per most IP documentations, we’ll use the term ‘host’ to refer to both the host or gateway.

Sepc

  • Any host that uses RIP is assumed to have interfaces to one or more networks.
  • The protocol relies on accessing information about each of these networks
  • The most important of these information is its metric or ‘cost’ [1-15]
  • Modern implementation should allow the network admin to set this cost value.
  • In addition to the cost, each network will have its network number followed by the subnet mask.

Each host is assumed to have a routing table containing the following information at least:

  • The IP address of the destination.
  • A metric, which represents the total cost of getting a datagram from host to destination.
  • The IP address of the next gateway along the path to the destination.
  • A “route change flag” that indicates if the route to the destination has changed.
  • Various timers associated with the route.

The cost of a directly connected network is always set to 1, but as mentioned earlier, the choice can be given to the network admin to change that cost based on the bandwidth, or the reliability of that route.

In order for the protocol to provide complete infomation on routing, every gateway has to participate in it, hosts that are not gateways need not participate, but many implementations of the protocol make provisions for them to listen to the routing information to keep their routing table up-to-date.

Message formats

RIP is UDP-based containing the following datagram format:

       0                   1                   2                   3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | command (1)   | version (1)   |      must be zero (2)         |
      +---------------+---------------+-------------------------------+
      | address family identifier (2) |      must be zero (2)         |
      +-------------------------------+-------------------------------+
      |                         IP address (4)                        |
      +---------------------------------------------------------------+
      |                        must be zero (4)                       |
      +---------------------------------------------------------------+
      |                        must be zero (4)                       |
      +---------------------------------------------------------------+
      |                          metric (4)                           |
      +---------------------------------------------------------------+
                                      .
                                      .
                                      .

Few notes about the format:

  1. The metric should be a value between 1 and 15 inclusive, the metric indicate the ‘cost’ by which the destination is reachable or 16 if it’s not.

  2. The RFC describes version 1 of the protocol which contains commands mainly related to sharing all parts of the routing table of the gateway.

  3. the IP Address field is represents a normal internet address that is written in 4 octets. Note that multiple ip addresses (with thier respective costs) can be added as entries. The protocol relies on a fixed size to split them (20 bytes)

Simulation (using cisco-packettracer)

  1. drop configuration of static routing while maintaining assigned ip addresses to router interfaces.

  2. make router rip enabled. Use V2 for ALL gateways and add directly connected networks as well as stop the aggregation of routes.

  3. to configure the default static route.

Implementation

to be continued…