The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from source to destination. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support. RIP implements the split horizon, route poisoning, and holddown mechanisms to prevent incorrect routing information from being propagated. In RIPv1 routers broadcast updates with their routing table every 30 seconds. In the early deployments, routing tables were small enough that the traffic was not significant. As networks grew in size, however, it became evident there could be a massive traffic burst every 30 seconds, even if the routers had been initialized at random times. In most networking environments, RIP is not the preferred choice of routing protocol, as its time to converge and scalability are poor compared to EIGRP, OSPF, or IS-IS. However, it is easy to configure, because RIP does not require any parameters, unlike other protocols. RIP uses the User Datagram Protocol (UDP) as its transport protocol, and is assigned the reserved port number 520.
Development of distance-vector routing Based on the Bellman–Ford algorithm and the Ford–Fulkerson algorithm, distance-vector routing protocols started to be implemented from 1969 onwards in data networks such as the ARPANET and CYCLADES. The predecessor of RIP was the Gateway Information Protocol (GWINFO) which was developed by Xerox in the mid-1970s to route its experimental network. As part of the Xerox Network Systems (XNS) protocol suite GWINFO transformed into the XNS Routing Information Protocol. This XNS RIP in turn became the basis for early routing protocols, such as Novell's IPX RIP, AppleTalk's Routing Table Maintenance Protocol (RTMP), and the IP RIP. The 1982 Berkeley Software Distribution of the UNIX operating system implemented RIP in the routed daemon. The 4.2BSD release proved popular and became the basis for subsequent UNIX versions, which implemented RIP in the routed or gated daemon. Ultimately, RIP had been extensively deployed before the standard, written by Charles Hedrick, was passed as RIPv1 in 1988.
The RIP hop count The routing metric used by RIP counts the number of routers that need to be passed to reach a destination IP network. The hop count 0 denotes a network that is directly connected to the router. 16 hops denote a network that is unreachable, according to the RIP hop limit.
Versions There are three standardized versions of the Routing Information Protocol: RIPv1 and RIPv2 for IPv4, and RIPng for IPv6.
RIP version 1 The original specification of RIP was published in 1988. When starting up, and every 30 seconds thereafter, a router with RIPv1 implementation broadcasts to 255.255.255.255 a request message through every RIPv1 enabled interface. Neighbouring routers receiving the request message respond with a RIPv1 segment, containing their routing table. The requesting router updates its own routing table, with the reachable IP network address, hop count and next hop, that is the router interface IP address from which the RIPv1 response was sent. As the requesting router receives updates from different neighbouring routers it will only update the reachable networks in its routing table, if it receives information about a reachable network it has not yet in its routing table or information that a network it has in its routing table is reachable with a lower hop count. Therefore, a RIPv1 router will in most cases only have one entry for a reachable network, the one with the lowest hop count. If a router receives information from two different neighbouring router that the same network is reachable with the same hop count but via two different routes, the network will be entered into the routing table two times with different next hop routers. The RIPv1 enabled router will then perform what is known as equal-cost load balancing for IP packets. RIPv1 enabled routers not only request the routing tables of other routers every 30 seconds, they also listen to incoming requests from neighbouring routers and send their own routing table in turn. RIPv1 routing tables are therefore updated every 25 to 35 seconds. The RIPv1 protocol adds a small random time variable to the update time, to avoid routing tables synchronizing across a LAN. It was thought, as a result of random initialization, the routing updates would spread out in time, but this was not true in practice. Sally Floyd and Van Jacobson showed in 1994 that, without slight randomization of the update timer, the timers synchronized over time. RIPv1 can be configured into silent mode, so that a router requests and processes neighbouring routing tables, and keeps its routing table and hop count for reachable networks up to date, but does not needlessly send its own routing table into the network. Silent mode is commonly implemented to hosts. RIPv1 uses classful routing. The periodic routing updates do not carry subnet information, lacking support for variable length subnet masks (VLSM). This limitation makes it impossible to have different-sized subnets inside of the same network class. In other words, all subnets in a network class must have the same size. There is also no support for router authentication, making RIP vulnerable to various attacks.
… excerpt ends here. Continue reading the full article.
