Hitchhikers Guide to the Internet | Page 9

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 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.
-15-
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