Hitchhikers Guide to the Internet | Page 7

The distribution of the database (what
each node knows), the form of the updates, and metrics used to
measure the value of a connection, are the parameters which determine
the characteristics of a routing protocol.
Under some algorithms each node in the network has complete
knowledge of the state of the network (the adult algorithm). This
implies the nodes must have larger amounts of local storage and
enough CPU to search the large tables in a short enough time
(remember this must be done for each packet). Also, routing updates
usually contain only changes to the existing information (or you spend
a large amount of the network capacity passing around megabyte
routing updates). This type of algorithm has several problems. Since
the only way the routing information can be passed around is across the
network and the propagation time is non-trivial, the view of the
network at each node is a correct historical view of the network at
varying times in the past. (The adult algorithm, but rather than looking
directly at the dining area, looking at a photograph of the dining room.
One is likely to pick the optimal route and find a bus-cart has moved in
to block the path after the photo was taken). These inconsistencies can
cause circular routes (called routing loops) where once a packet enters
it is routed in a closed path until its time to live (TTL) field expires and
it is discarded.
Other algorithms may know about only a subset of the network. To
prevent loops in these protocols, they are usually used in a hierarchical
network. They know completely about their own area, but to leave that
area they go to one particular place (the default gateway). Typically
these are used in smaller networks (campus, regional...).
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Routing protocols in current use:
Static (no protocol-table/default routing)

Don't laugh. It is probably the most reliable, easiest to implement, and
least likely to get one into trouble for a small network or a leaf on the
Internet. This is, also, the only method available on some
CPU-operating system combinations. If a host is connected to an
Ethernet which has only one gateway off of it, one should make that the
default gateway for the host and do no other routing. (Of course that
gateway may pass the reachablity information somehow on the other
side of itself).
One word of warning, it is only with extreme caution that one should
use static routes in the middle of a network which is also using
dynamic routing. The routers passing dynamic information are
sometimes confused by conflicting dynamic and static routes. If your
host is on an ethernet with multiple routers to other networks on it and
the routers are doing dynamic routing among themselves, it is usually
better to take part in the dynamic routing than to use static routes.
RIP
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
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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