# Protocol Overview

Routing protocols determine the "best" route to each destination and distribute routing information among the systems on a network. Routing protocols are divided into two general groups: interior and exterior protocols. GateD software combines management of the interior and exterior routing protocols in one software daemon.

## Interior Routing Protocols

Interior protocols are used to exchange reachability information within an autonomous system (AS). They are refered to as a class by the acronym igp. There are several interior protocols:

RIP
  The Routing Information Protocol, Version 1 and Version 2, is the most commonly used interior protocol. RIP selects the route with the lowest metric as the best route. The metric is a hop count representing the number of gateways through which data must pass to reach its destination. The longest path that RIP accepts is 15 hops. If the metric is greater than 15, a destination is considered unreachable and GateD discards the route. RIP assumes the best route is the one that uses the fewest gateways i.e., the shortest path, not taking into account congestion or delay on route.

  The RIP version 1 protocol is described in RFC 1058 and the RIP version 2 protocol is described in RFC 1388.

HELLO
  HELLO , another interior protocol, uses delay as the deciding factor in choosing the best route. Round-trip time is the length of time it takes a datagram to travel from the source and destination. HELLO is historically significant for the Internet as it was the protocol used among the original prototype NSFNET backbone fuzzball gateways. Today, like fuzzballs, HELLO is a little-used protocol.

  An earlier version of the HELLO protocol is described in RFC 891.

OSPF
  Open Shortest Path First is a link-state protocol. OSPF is better suited than RIP for complex networks with many routers. OSPF provides equal cost multipath routing.

  OSPF is described in RFC 1583, the MIB is defined in RFC 1253. Other related documents are RFC 1245, RFC 1246 and RFC 1370.

IS-IS
  Intermediate System to Intermediate System (ISIS) is a link state interior gateway protocol (IGP) originally developed for routing ISO/CLNP (International Organization for Standardization/Connectionless Network Protocol) packets. The version distributed with GateD can route IP packets as well.