INTRODUCTION

The DARPA Internet Architecture.

The Internet system consists of a number of interconnected packet networks supporting communication among host computers using the Internet protocols. These protocols include the Internet Protocol (IP), the Internet Control Message Protocol (ICMP), the Transmission Control Protocol (TCP), and application protocols depending upon them .

All Internet protocols use IP as the basic data transport mechanism. IP is a datagram, or connectionless, internetwork service and includes provision for addressing, type-of-service specification, fragmentation and reassembly, and security information. ICMP is considered an integral part of IP, although it is architecturally layered upon IP. ICMP provides error reporting, flow control and first-hop gateway redirection.

Reliable data delivery is provided in the Internet protocol suite by transport-level protocols such as the Transmission Control Protocol (TCP), which provides end-end retransmission, resequencing and connection control. Transport-level connectionless service is provided by the User datagram Protocol (UDP).

Networks and gateways

Constituent networks may generally be divided into two classes.

1. Local-Area Networks (LANs)
2. Wide-Area Networks (WANs)

A gateway is connected to two or more networks, appearing to each of these networks as a connected host. Thus, it has a physical interface and an IP address on each of the connected networks. Forwarding an IP datagram generally requires the gateway to choose the address of the next-hop gateway or (for the final hop) the destination host. This choice, called "routing", depends upon a routing data-base within the gateway. This routing data-base should be maintained dynamically to reflect the current topology of the Internet system; a gateway normally accomplishes this by participating in distributed routing and reachability algorithms with other gateways. gateways provide datagram transport only, and they seek to minimize the state information necessary to sustain this service in the interest of routing flexibility and robustness.

Autonomous Systems

For technical, managerial, and sometimes political reasons, the gateways of the Internet system are grouped into collections called "autonomous systems" . The gateways included in a single autonomous system (AS) are expected to

• Be under the control of a single operations and maintenance (O&M) organization;
• Employ common routing protocols among themselves, to maintain their routing data-bases dynamically.

An IP datagram may have to traverse the gateways of two or more autonomous systems to reach its destination, and the autonomous systems must provide each other with topology information to allow such forwarding. The Border Gateway Protocol (BGP) is used for this purpose, between gateways of different autonomous systems.

Routing Information Protocol (RIP)

RIP is one protocol in a series of routing protocols based on the Bellman-Ford (or distance vector) algorithm. This algorithm has been used for routing computations in computer networks since the early days of the ARPANET. The particular packet formats and protocol described here are based on the program "routed", which is included with the Berkeley distribution of Unix. It has become a de facto standard for exchange of routing information among gateways and hosts. It is implemented for this purpose by most commercial vendors of IP gateways. Note, however, that many of these vendors have their own protocols which are used among their own gateways.

This protocol is most useful as an "interior gateway protocol". In a nationwide network such as the current Internet, it is very unlikely that a single routing protocol will used for the whole network. Rather, the network will be organized as a collection of "autonomous systems". An autonomous system will in general be administered by a single entity, or at least will have some reasonable degree of technical and administrative control. Each autonomous system will have its own routing technology. This may well be different for different autonomous systems. The routing protocol used within an autonomous system is referred to as an interior gateway protocol, or "IGP". A separate protocol is used to interface among the autonomous systems. The earliest such protocol, still used in the Internet, is "EGP" (exterior gateway protocol). Such protocols are now usually referred to as inter-AS routing protocols. RIP was designed to work with moderate-size networks using reasonably homogeneous technology. Thus it is suitable as an IGP for many campuses and for regional networks using serial lines whose speeds do not vary widely.

RIP is intended for use within the IP-based Internet. The Internet is organized into a number of networks connected by gateways. The networks may be either point-to-point links or more complex networks such as Ethernet or the ARPANET. hosts and gateways are presented with IP datagrams addressed to some host. Routing is the method by which the host or gateway decides where to send the datagram. It may be able to send the datagram directly to the destination, if that destination is on one of the networks that are directly connected to the host or gateway. However, the interesting case is when the destination is not directly reachable. In this case, the host or gateway attempts to send the datagram to a gateway that is nearer the destination. The goal of a routing protocol is very simple. It is to supply the information that is needed to do routing.

This protocol does not solve every possible routing problem. As mentioned above, it is primary intended for use as an IGP, in reasonably homogeneous networks of moderate size. In addition, the following specific limitations should be mentioned

• The protocol is limited to networks whose longest path involves 15 hops. Note that this statement of the limit assumes that a cost of 1 is used for each network.
• The protocol depends upon "counting to infinity" to resolve certain unusual situations.
• This protocol uses fixed "metrics" to compare alternative routes.