Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 100
The Routing Information Protocol (Version 1)

Routing tables allow routers to determine the next hop path for a received datagram. But what builds those tables? Dynamic updating is the process by which routers update each other with reachability information. Before the advent of dynamic updates of routing tables, most commercial vendors supported manual updates for their router tables. This meant manually entering network numbers, their associated distances, and the port numbers into the router table. The Internet was then known as the ARPAnet and it employed a routing update scheme known as the Gateway Information Protocol and later the Gateway to Gateway Protocol (GGP). This is beyond the scope of this book and is not used anymore. Independent router vendors did not have that many routers and subnets to update, so placing a manual entry in the routers was not all that bad. As networks grew larger, this became a cumbersome way of building tables. Commercially, RIP was the protocol that enabled automatic updates of router tables.

The RIP algorithm is based on the distance-vector algorithms just described. RIP placed the fundamentals of distance-vector in a simple routing algorithm. Implementations of these protocols were first found on the ARPAnet in 1969 using the Gateway Information Protocol. However, it was first devised by Xerox Corporation as the routing algorithm used by Internet Datagram Protocol of XNS.

RFC 1058 first defined RIP for TCP/IP, and it was formally adopted by the IAB in 1988. Although it was not primarily intended as the routing algorithm for TCP, it gained widespread acceptance when it became embedded into the Berkeley 4BSD Unix operating system through a service known as routed (pronounced “route d”—d is for the daemon process that runs the protocol in Unix). Placing the functions of RIP into an RFC allowed for interoperability and detailed certain functions.

The Routing Information Protocol (Version 1)

With RIP information, any router knows the length of the shortest path (not necessarily the best) from each of its neighbor routers (routers located on the same network) to any other destination. There are many deficiencies in this protocol and they are discussed at the end of this section.

The RIP packet is quite simple. The slide shows the RIP header and data encapsulated in an Ethernet packet. The RIP data is the table information one router shares with another.