Hitchhikers Guide to the Internet | Page 8

documentation for RIP appears to be the code to
BSD "routed".
Routed
The ROUTED program, which does RIP for 4.2BSD systems, has
many options. One of the most frequently used is: "routed -q" (quiet
mode) which means listen to RIP infor- mation but never broadcast it.
This would be used by a machine on a network with multiple RIP
speaking gate- ways. It allows the host to determine which gateway is
best (hopwise) to use to reach a distant network. (Of course you might
want to have a default gateway to prevent having to pass all the
addresses known to the Internet around with RIP).
There are two ways to insert static routes into "routed", the
"/etc/gateways" file and the "route add" command. Static routes are
useful if you know how to reach a distant network, but you are not
receiving that route using RIP. For the most part the "route add"
command is preferable to use. The reason for this is that the command
adds the route to that machine's routing table but does not export it
through RIP. The "/etc/gateways" file takes precedence over any
routing information received through a RIP update. It is also broadcast
as fact in RIP updates produced by the host without question, so if a
mistake is made in the "/etc/gateways" file, that mistake will soon
permeate the RIP space and may bring the network to its knees.
One of the problems with "routed" is that you have very little control
over what gets broadcast and what doesn't. Many times in larger
networks where various parts of the network are under different
administrative controls, you would like to pass on through RIP only
nets which you receive from RIP and you know are reasonable. This
prevents people from adding IP addresses to the network which may be
illegal and you being responsible for passing them on to the Internet.
This

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type of reasonability checks are not available with "routed" and leave it
usable, but inadequate for large networks.
Hello (RFC-891)
Hello is a routing protocol which was designed and implemented in a
experimental software router called a "Fuzzball" which runs on a
PDP-11. It does not have wide usage, but is the routing protocol
currently used on the NSFnet backbone. The data transferred between
nodes is similar to RIP (a list of networks and their metrics). The metric,
however, is milliseconds of delay. This allows Hello to be used over
nets of various link speeds and performs better in congestive situations.
One of the most interesting side effects of Hello based networks is their
great timekeeping ability. If you consider the problem of measuring
delay on a link for the metric, you find that it is not an easy thing to do.
You cannot measure round trip time since the return link may be more
congested, of a different speed, or even not there. It is not really
feasible for each node on the network to have a builtin WWV
(nationwide radio time standard) receiver. So, you must design an
algorithm to pass around time between nodes over the network links
where the delay in transmission can only be approximated. Hello
routers do this and in a nationwide network maintain synchronized time
within milliseconds.
Exterior Gateway Protocol (EGP RFC-904)
EGP is not strictly a routing protocol, it is a reacha- bility protocol. It
tells only if nets can be reached through a particular gateway, not how
good the connec- tion is. It is the standard by which gateways to local
nets inform the ARPAnet of the nets they can reach. There is a metric
passed around by EGP but its usage is not standardized formally. Its
typical value is value is 1 to 8 which are arbitrary goodness of link
values understood by the internal DDN gateways. The smaller the value
the better and a value of 8 being unreach- able. A quirk of the protocol
prevents distinguishing between 1 and 2, 3 and 4..., so the usablity of
this as a metric is as three values and unreachable. Within NSFnet the
values used are 1, 3, and unreachable. Many routers talk EGP so they
can be used for ARPAnet gateways.
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Gated

So we have regional and campus networks talking RIP among
themselves, the NSFnet backbone talking Hello, and the DDN speaking
EGP. How do they interoperate? In the beginning there was static
routing, assembled into the Fuzzball software configured for each site.
The problem with doing static routing in the middle of the network is
that it is broadcast to the Internet whether it is usable or not. Therefore,
if a net becomes unreachable and you try to get there, dynamic routing
will immediately issue a net unreachable to you. Under static routing
the routers would think the net could be reached