RIP is intended to allow hosts and gateways to exchange information
for computing routes through an IP-based network. RIP is a distance
vector protocol. Thus, it has the general features described in
section 2. RIP may be implemented by both hosts and gateways. As in
most IP documentation, the term "host" will be used here to cover
either. RIP is used to convey information about routes to
"destinations", which may be individual hosts, networks, or a special
destination used to convey a default route.
Any host that uses RIP is assumed to have interfaces to one or more
networks. These are referred to as its "directly-connected
networks". The protocol relies on access to certain information
about each of these networks. The most important is its metric or
"cost". The metric of a network is an integer between 1 and 15
inclusive. It is set in some manner not specified in this protocol.
Most existing implementations always use a metric of 1. New
implementations should allow the system administrator to set the cost
of each network. In addition to the cost, each network will have an
IP network number and a subnet mask associated with it. These are to
be set by the system administrator in a manner not specified in this
protocol.
Note that the rules specified in section 3.2 assume that there is a
single subnet mask applying to each IP network, and that only the
subnet masks for directly-connected networks are known. There may be
systems that use different subnet masks for different subnets within
a single network. There may also be instances where it is desirable
for a system to know the subnets masks of distant networks. However,
such situations will require modifications of the rules which govern
the spread of subnet information. Such modifications raise issues of
interoperability, and thus must be viewed as modifying the protocol.
Each host that implements RIP is assumed to have a routing table.
This table has one entry for every destination that is reachable
through the system described by RIP. Each entry contains at least
the following information:
The IP address of the destination.
A metric, which represents the total cost of getting a
datagram from the host to that destination. This metric is
the sum of the costs associated with the networks that
would be traversed in getting to the destination.
The IP address of the next gateway along the path to the
destination. If the destination is on one of the
directly-connected networks, this item is not needed.
A flag to indicate that information about the route has
changed recently. This will be referred to as the "route
change flag."
Various timers associated with the route. See section 3.3
for more details on them.
The entries for the directly-connected networks are set up by the
host, using information gathered by means not specified in this
protocol. The metric for a directly-connected network is set to the
cost of that network. In existing RIP implementations, 1 is always
used for the cost. In that case, the RIP metric reduces to a simple
hop-count. More complex metrics may be used when it is desirable to
show preference for some networks over others, for example because of
differences in bandwidth or reliability.
Implementors may also choose to allow the system administrator to
enter additional routes. These would most likely be routes to hosts
or networks outside the scope of the routing system.
Entries for destinations other these initial ones are added and
updated by the algorithms described in the following sections.
In order for the protocol to provide complete information on routing,
every gateway in the system must participate in it. Hosts that are
not gateways need not participate, but many implementations make
provisions for them to listen to routing information in order to
allow them to maintain their routing tables.
