RIP is a routing protocol based on XNS (Xerox Network
System) adapted for IP networks. It is used by many
routers (Proteon, cisco, UB...) and many BSD Unix systems
BSD systems typically run a program called "routed" to
exchange information with other systems running
RIP. RIP works best for nets of small diameter
where the links are of equal speed. The reason for
this is that the metric used to determine which path is
best is the hop-count. A hop is a traversal across a
gateway. So, all machines on the same Ethernet are
zero hops away. If a router connects connects two net-
works directly, a machine on the other side of the
router is one hop away.... As the routing information
is passed through a gateway, the gateway adds one to
the hop counts to keep them consistent across the net-
work. The diameter of a network is defined as the
largest hop-count possible within a network. Unfor-
tunately, a hop count of 16 is defined as infinity in
RIP meaning the link is down. Therefore, RIP will not
allow hosts separated by more than 15 gateways in the
RIP space to communicate.
The other problem with hop-count metrics is that if
links have different speeds, that difference is not
-11-
reflected in the hop-count. So a one hop satellite link
(with a .5 sec delay) at 56kb would be used instead of
a two hop T1 connection. Congestion can be viewed as a
decrease in the efficacy of a link. So, as a link gets
more congested, RIP will still know it is the best
hop-count route and congest it even more by throwing
more packets on the queue for that link.
The protocol is not well documented. A group of people
are working on producing an RFC to both define the
current RIP and to do some extensions to it to allow it
to better cope with larger networks. Currently, the
best documentation for RIP appears to be the code to
BSD "routed".