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

Chapter 22
Introduction to Routing Protocols (RIP)

There are a few protocols that handle for a single autonomous system. RIP is the easier of the two (RIP or OSPF) and came from the Xerox Network System (XNS) protocol. The origins of RIP are based in the origins of the Internet, but historically it came from Xerox and its XNS protocol. RIP was freely distributed in the Unix operating system and, because of its simplicity, gained widespread acceptance. Unfortunately, there are many deficiencies associated with this protocol, and there have been many “patches” applied to it to make it work more reliably in large networks. For smaller networks, the protocol works just fine.

Since, IP is a routable protocol, it needs a routing protocol to enable it to route between networks. RIP is known as a distance vector protocol. Its database (the routing table) contains two fields needed for routing: a vector (a known IP address) and the distance (how many routers away) to the destination. Actually, the table contains more fields than that, but we will discuss that later.

RIP simply builds a table in memory that contains all the routes that it knows about and the distance to that network. When the protocol initializes, it simply places the IP addresses of its local interfaces into the table. It associates a cost with those interfaces and that cost is usually set to 1 (explained in a moment). The router will then solicit (or it may wait for information to be supplied to it) information from other routers on its locally attached subnets. Eventually, as other routers report (send their tables) to other routers, each router will have the information needed about all routes on its subnets or internetwork.

Any IP datagrams that must traverse a router in the path to its destination is said to have traversed one hop for each router traversed. Therefore, when a router receives a packet and examines the destination address in the datagram, it will then perform a table lookup based on that destination address. The router will also find the port associated with this destination address in the database and will forward the datagram out of that port and onward to the final destination. In RIP, all routers compute their tables and then give each other their tables (just the IP network address and the cost). Routers that receive this table will add the cost assigned to the incoming interface (received port) to each of the entries in the table. The router then decides whether to keep any of the information in the received table. This information is then passed to other routers.