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 is allocated to an administrative entity, that entity can freely allocate subdomains using what ever manner it sees fit.

The Berkeley Internet Name Domain (BIND) Server implements the Internet name server for UNIX systems. The name server is a distributed data base system that allows clients to name resources and to share that information with other net- work hosts. BIND is integrated with 4.3BSD and is used to lookup and store host names, addresses, mail agents, host information, and more. It replaces the "/etc/hosts" file for host name lookup. BIND is still an evolving program. To keep up with reports on operational problems, future design decisions, etc, join the BIND mailing list by sending a request to "bind-request@ucbarp.Berkeley.EDU". BIND can also be obtained via anonymous FTP from ucbarpa.berkley.edu.

There are several advantages in using BIND. One of the most important is that it frees a host from relying on "/etc/hosts" being up to date and complete. Within the .uiuc.edu domain, only a few hosts are included in the host table distributed by SRI. The remainder are listed locally within the BIND tables on uxc.cso.uiuc.edu (the server machine for most of the .uiuc.edu domain). All are equally reachable from any other Internet host running BIND.

BIND can also provide mail forwarding information for inte- rior hosts not directly reachable from the Internet. These hosts can either be on non-advertised networks, or not con- nected to a network at all, as in the case of UUCP-reachable hosts. More information on BIND is available in the "Name Server Operations Guide for BIND" in "UNIX System Manager's Manual", 4.3BSD release.

 There are a few special domains on the network, like SRI-
 NIC.ARPA. The 'arpa' domain is historical, referring to
 hosts registered in the old hosts database at the NIC.
 There are others of the form NNSC.NSF.NET. These special
 domains are used sparingly and require ample justification.
 They refer to servers under the administrative control of

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the network rather than any single organization. This allows for the actual server to be moved around the net while the user interface to that machine remains constant. That is, should BBN relinquish control of the NNSC, the new provider would be pointed to by that name.

In actuality, the domain system is a much more general and complex system than has been described. Resolvers and some servers cache information to allow steps in the resolution to be skipped. Information provided by the servers can be arbitrary, not merely IP addresses. This allows the system to be used both by non-IP networks and for mail, where it may be necessary to give information on intermediate mail bridges.

What's wrong with Berkeley Unix

University of California at Berkeley has been funded by DARPA to modify the Unix system in a number of ways. Included in these modifications is support for the Internet protocols. In earlier versions (e.g. BSD 4.2) there was good support for the basic Internet protocols (TCP, IP, SMTP, ARP) which allowed it to perform nicely on IP ether- nets and smaller Internets. There were deficiencies, how- ever, when it was connected to complicated networks. Most of these problems have been resolved under the newest release (BSD 4.3). Since it is the springboard from which many vendors have launched Unix implementations (either by porting the existing code or by using it as a model), many implementations (e.g. Ultrix) are still based on BSD 4.2. Therefore, many implementations still exist with the BSD 4.2 problems. As time goes on, when BSD 4.3 trickles through vendors as new release, many of the problems will be resolved. Following is a list of some problem scenarios and their handling under each of these releases.

ICMP redirects

Under the Internet model, all a system needs to know to get anywhere in the Internet is its own address, the address of where it wants to go, and how to reach a gateway which knows about the Internet. It doesn't have to be the best gateway. If the system is on a network with multiple gateways, and a host sends a packet for delivery to a gateway which feels another directly connected gateway is more appropriate, the gateway sends the sender a message. This message is an ICMP redirect, which politely says "I'll deliver this message for you, but you really ought to use that gate- way over there to reach this host". BSD 4.2 ignores these messages. This creates more stress on the gate- ways and the local network, since for every packet

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sent, the gateway sends a packet to the originator. BSD 4.3 uses the redirect to update its routing tables, will use the route until it times out, then revert to the use of the route it thinks is should use. The whole process then repeats, but it is far better than one per packet.

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