Hitchhiker's Guide to the Internet - Ed Krol (read along books .txt) 📗

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 (fedor@devvax.tn.cornell.edu) 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 thing since sliced bread, but there is a catch called metric conversion. You have RIP measuring in hops, Hello measuring in milliseconds, and EGP using arbitrary small numbers. The big questions is how many hops to a millisecond, how many milliseconds in the EGP number 3…. Also, remember that infinity (unreachability) is 16 to RIP, 30000 or so to Hello, and 8 to the DDN with EGP. Getting all these metrics to work well together is no small feat. If done incorrectly and you translate an RIP of 16 into an EGP of 6, everyone in the ARPAnet will still think your gateway can reach the unreachable and will send every packet in the world your way. For these reasons, Mark requests that you consult closely with him when configuring and using "gated".

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"Names"

All routing across the network is done by means of the IP address associated with a packet. Since humans find it difficult to remember addresses like 128.174.5.50, a symbolic name register was set up at the NIC where people would say "I would like my host to be named 'uiucuxc'". Machines connected to the Internet across the nation would connect to the NIC in the middle of the night, check modification dates on the hosts file, and if modified move it to their local machine. With the advent of workstations and micros, changes to the host file would have to be made nightly. It would also be very labor intensive and consume a lot of network bandwidth. RFC-882 and a number of others describe domain name service, a distributed data base system for mapping names into addresses.

We must look a little more closely into what's in a name. First, note that an address specifies a particular connec- tion on a specific network. If the machine moves, the address changes. Second, a machine can have one or more names and one or more network addresses (connections) to different networks. Names point to a something which does useful work (i.e. the machine) and IP addresses point to an interface on that provider. A name is a purely symbolic representation of a list of addresses on the network. If a machine moves to a different network, the addresses will change but the name could remain the same.

Domain names are tree structured names with the root of the tree at the right. For example:

uxc.cso.uiuc.edu

is a machine called 'uxc' (purely arbitrary), within the subdomains method of allocation of the U of I) and 'uiuc' (the University of Illinois at Urbana), registered with 'edu' (the set of educational institutions).

A simplified model of how a name is resolved is that on the user's machine there is a resolver. The resolver knows how to contact across the network a root name server. Root servers are the base of the tree structured data retrieval system. They know who is responsible for handling first level domains (e.g. 'edu'). What root servers to use is an installation parameter. From the root server the resolver finds out who provides 'edu' service. It contacts the 'edu' name server which supplies it with a list of addresses of servers for the subdomains (like 'uiuc'). This action is repeated with the subdomain servers until the final sub- domain returns a list of addresses of interfaces on the host in question. The user's machine then has its choice of which of these addresses to use for communication.

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A group may apply for its own domain name (like 'uiuc' above). This is done in a manner similar to the IP address allocation. The only requirements are that the requestor have two machines reachable from the Internet, which will act as name servers for that domain. Those servers could also act as servers for subdomains or other servers could be designated as such. Note that the servers need not be located in any particular place, as long as they are reach- able for name resolution. (U of I could ask Michigan State to act on its behalf and that would be fine). The biggest problem is that someone must do maintenance on the database. If the machine is not convenient, that might not be done in a timely fashion. The other thing to note is that once the domain