Hitchhikers Guide to the Internet | Page 8

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 and would continue trying until the application gave up (in 2 or more minutes). Mark Fedor of Cornell ([email protected]) attempted to solve these problems with a replacement for "routed" called "gated".
"Gated" talks RIP to RIP speaking hosts, EGP to EGP speakers, and Hello to Hello'ers. These speakers frequently all live on one Ethernet, but luckily (or unluckily) cannot understand each others ruminations. In addition, under configuration file control it can filter the conversion. For example, one can produce a configuration saying announce RIP nets via Hello only if they are specified in a list and are reachable by way of a RIP broadcast as well. This means that if a rogue network appears in your local site's RIP space, it won't be passed through to the Hello side of the world. There are also configuration options to do static routing and name trusted gateways.
This may sound like the greatest