Routing Information Protocol (RIP) Cont...

RIP Message Format

RIP updates are placed as UDP payload inside an IP datagram. The format is as follows:
- Command : It indicates whether the packet is a request or a response. The request asks that a router send all or part of its routing table. The response can be an unsolicited regular routing update or a reply to a request. Responses contain routing table entries. Multiple RIP packets are used to convey information from large routing tables.
- Version number : It specifies the RIP version used. This field can signal different potentially incompatible versions.
- Zero : This field is not actually used by RFC 1058 RIP; it was added solely to provide backward compatibility with prestandard varieties of RIP. Its name comes from its defaulted value: zero.
- Address family identifier (AFI) : It specifies the address family used. RIP is designed to carry routing information for several different protocols. Each entry has an address-family identifier to indicate the type of address being specified. The AFI for IP is 2.
- Address : It specifies the IP address for the entry.
- Metric : It indicates how many internetwork hops (routers) have been traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.

RIPv2 header explanation

- Command : Indicates whether the packet is a request or a response.
- Version : Version of RIP.
- Unused : It has a value set to zero.
- Address family identifier (AFI) : It specifies the address family used.
- Route tag : It provides a method for distinguishing between internal routes (learned by RIP) and external routes (learned from other protocols).
- IP address : It specifies the IP address for the entry.
- Subnet mask : It contains the subnet mask for the entry. If this field is zero, no subnet mask has been specified for the entry.
- Next hop : It indicates the IP address of the next hop to which packets for the entry should be forwarded.
- Metric : It indicates how many internetwork hops (routers) have been traversed in the trip to the destination. This value is between 1 and 15 for a valid route, or 16 for an unreachable route.

Routing Information Protocol (RIP)

Characteristics of RIP

- RIP (Routing Information Protocol) is a standard for exchange of routing information among gateways and hosts.
- RIP is most useful as an "interior gateway protocol".
- All RIP routing protocols are based on a distance vector algorithm called the Bellman-Ford algorithm, after Bellman's development of the equation used as the basis of dynamic programming, and Ford's early work in the area.
- RIP is considered an effective solution for small homogeneous networks.
- For larger, more complicated networks, RIP's transmission of the entire routing table every 30 seconds may put a heavy amount of extra traffic in the network.
- RIP sends routing-update messages at regular intervals and when the network topology changes. When a router receives a routing update that includes changes to an entry, it updates its routing table to reflect the new route.
- Each router sends a list of distance-vectors each of its neighbours periodically.
- The metric must be a positive integer. This metric measures the cost to get to the destination. In RIP, this cost describes number of hops.
- Each hop is assigned a hop count value, which is typically 1. The router adds 1 to the metric value when a router receives a routing update and enters the network in the routing table.
- RIP prevents routing loops from continuing indefinitely by implementing a limit on the number of hops allowed in a path from the source to a destination. The maximum number of hops in a path is 15.
- RIP implements the split horizon and holddown mechanisms to prevent incorrect routing information from being propagated.
- RIP uses numerous timers to regulate its performance. These include a routing-update timer, a route-timeout timer, and a route-flush timer.

Limitations of the protocol

- The protocol is limited to networks whose longest path involves 15 hops.
- The protocol depends upon "counting to infinity" to resolve certain unusual situations.
- This protocol uses fixed "metrics" to compare alternative routes.
- It is not appropriate for situations where routes need to be chosen based on real-time parameters such a measured delay, reliability, or load.