V10/doc/csnet/adr-1
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Subject: adr-1 CSNET ADDRESSES
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From: CSNET INFO SERVER <rutgers!SH.CS.NET!info>
To: doug@research.att.com
Request: info
Topic: adr-1
Document Updated: 1 Jun 87
Subject: CSNET Addresses and the Domain System
==============================================================================
CSNET Coordination and Information Center (CIC) Hotline: 617/497-2777
10 Moulton Street Email: cic@sh.cs.net
Cambridge, MA 02238 Info-Server requests to: info-server@sh.cs.net
==============================================================================
CSNET ADDRESSES AND THE DOMAIN SYSTEM
In July 1986, the CSNET CIC instituted a new policy of assigning a
domain-style name to each new CSNET PhoneNet site before the site begins
to send messages on PhoneNet. This starts the site off right, with a
name that conforms to the DDN Domain Naming System , and avoids the pain
of changing names in the future.
Assigning a domain name involves registering a second-level domain name
with the Network Information Center (NIC) of the Defense Data Network
(DDN). The CSNET CIC has arranged with the DDN NIC to take care of the
registration process for organizations that are members of the CSNET
PhoneNet. In a companion document, "adr-2 Transition to Domains",
CSNET-FORUM v2 #6 (23 May 86), we explained how to choose a domain name
and register with the NIC. In this article, we talk about the
background of the domain naming system.
The DDN is composed of the ARPANET, an experimental research and
development network, and the MILNET, an operational military network.
The Internet is composed of the DDN, plus other networks that share the
same protocols, including the CSNET X25Net. CSNET PhoneNet sites are
not part of the Internet, but they can exchange electronic messages with
the Internet through a "gateway" on RELAY.CS.NET (also known as
CSNET-RELAY.ARPA). This is the same host that also connects PhoneNet
sites to each other, and to other networks, public and private, that
have gateways on CSNET PhoneNet sites.
The domain naming scheme represents an important step in making all
these networks easier to use and allowing them to expand in the future.
In the past, the addressing scheme used by the DDN was based upon
individual host computers. As spelled out in RFC733 (the ARPANET
standard for text messages adopted in 1977), and in earlier standards,
the basic pattern for an ARPANET address was:
user@host
On the ARPANET, the hostnames must be translated into network numbers
before the mail delivery programs can make connections between hosts.
The original system stored the machine addresses with the host names in
a host table. The NIC Host Table was distributed by the DDN NIC to all
hosts on the network.
The RFC733 system was firmly based on hardware. If a single
organization had multiple host computers on the ARPANET, all of the
hosts had to be registered with the Network Information Center of the
DDN (NIC) and entered in the NIC Host Table. But if there were hosts
that were not on the ARPANET themselves, but could be reached through an
ARPANET host, the NIC Host Table could not handle them. It was up to
the person sending the messages to spell out the route explicitly in the
mailing address.
RFC733 did not, however, specify the rules for addressing messages to
hosts on other networks. The address form 'user%host1@host2', was
generally used on the CSNET PhoneNet and other networks associated with
the ARPANET, but was never adopted as a network standard, although the
current network standard allows it (as one of many possible options). Some
networks that gatewayed to the ARPANET used somewhat different rules,
which were also acceptable.
The new domain system is quite different. It is based on RFC822, which
was adopted in 1983. RFC822 specifies a system of names, not physical
addresses. It sets up a domain, corresponding to an organizational
unit, not the physical hardware, as the basic unit of the name. All
users at the same organization have addresses in the same domain,
regardless of the hardware they use or the network or route that
delivers their messages.
This means that only one name, the domain name for the organization,
must be registered with the NIC. Names below the level of the
organization as a whole are left up to the organizations to manage. The
responsibility for keeping track of the names and locations of host
computers is thus delegated, and information is no longer centralized at
the DDN NIC. Eventually, when the domain system is fully implemented,
the NIC will discontinue the NIC Host Table altogether.
Distributed responsibility for names requires a distributed database.
Domain names must still be translated into network numbers and routes
between hosts, but this information is no longer kept in one place.
Instead, it is split up and stored on different Internet host computers
in dynamic database programs called "domain servers". When an Internet
host is about to send a message, it no longer checks its own copy of the
NIC Host Table for the recipient host. Instead, it queries other
Internet hosts, over real-time connections, to determine which Internet
host runs the domain server that stores the routing information for the
recipient's domain. The sending host then retrieves the routing
information, and routes the message to its destination (or at least to
the next Internet host along its route).
CSNET PhoneNet sites cannot run their own domain servers, since they do
not have direct real-time access to the Internet; instead, they must
arrange with other hosts that are on the Internet to perform this
function. The CSNET CIC runs a domain server on RELAY.CS.NET, with a
backup on SH.CS.NET, and offers to be the domain server for any PhoneNet
site that needs this service.
Domain names are organized in a hierarchical tree structure. At the top
is a set of top-level domain names chosen and administered by the DDN
NIC. The top-level domains for the United States represent classes of
organizations:
COM Commercial organizations MIL Department of Defense
EDU Educational organizations NET Administrative organizations
GOV Civilian government for networks, e.g., CSNET.
ORG Nonprofit organizations ARPA Temporary top-level domain,
Non-US countries have their own top-level domains, named for the
2-letter country codes of the International Standards Organization
(ISO standard 3166). The DDN NIC has announces country domains, as they
are registered.
Second-level domains represent individual organizations. Hypothetical
examples of domains for organizations might be:
OXBRIDGE.EDU Oxbridge University
WIDGETS.COM Widgets Mfg. Co., Inc.
THINKTANK.ORG The Thinktank Institute
Each new CSNET organization forms its domain name by selecting the
appropriate top-level domain, and choosing a second-level domain. The
CSNET CIC checks the combination to make sure that it is unique.
Once an organization has been registered as a second-level domain, it
may freely create lower-level domains, according to its needs.
The pattern for an electronic mail address is:
local-part@domain
The "local-part" is ideally a simple username, but RFC822 allows any
string in the local part (to the left of the @), so long as it can be
interpreted by the machine that takes the responsibility for
interpreting the complete "domain" (to the right of the @).
The domain is a string of words separated by periods. Lower-level
domains are arranged left to right, in ascending order, ending with a
top-level domain. Any number of lower-level domains may be used.
A domain may represent a subdivision of an organization (at any level),
or an individual host computer.
Examples of addresses might be:
laura@oxbridge.edu oxbridge.edu either has no subdomains,
or hides its subdomains from the
outside world.
diane@eng.oxbridge.edu Engineering Dept within oxbridge.edu.
dennis@ai.cs.oxbridge.edu AI project in the Computer Science
Department
craig@wk15.ccc.oxbridge.edu Workstation in the Campus Computer
Center.
The transition to domains is not yet complete. Some Internet hosts are
not yet running domain servers, and some have not yet made the switch
from the temporary ".arpa" domain to the full domain naming scheme.
The CSNET relay, originally CSNET-RELAY, then CSNET-RELAY.ARPA, and now
RELAY.CS.NET, rewrites all CSNET PhoneNet addresses to include the
relay's name when the message is destined for the Internet:
From: @RELAY.CS.NET:laura@oxbridge.edu
Later, when all Internet hosts have domain servers, and are able to
recognize all domain names, whether or not they are on the Internet,
the address will change to:
From: laura@oxbridge.edu
THE RFC822 ROUTING ADDRESS
For a long time, even after the RFC822 conventions were formally
adopted for use on the Internet, the most common method of writing the
routing address was the "%" notation:
user%oxbridge.csnet@relay.cs.net
However, this type of routing address is an informal convention,
and is not specified in any official RFC or other Internet document.
The routing address specified in RFC822 is of the form:
@relay.cs.net:user@oxbridge.csnet
This is the form that relay.cs.net uses for messages that are sent
from CSNET PhoneNet sites to Internet hosts.
You will also see routing addresses for CSNET PhoneNet sites with
domain-style names. The reason is that there are still some Internet
hosts that cannot handle domain server entries for off-Internet hosts.
@relay.cs.net:joe@cs.somewhere.edu
Other examples of RFC822 routing addresses are:
@wiscvm.wisc.edu:XXYYZZ@XYZVS.BITNET
@seismo.css.gov:bill@uuhost.uucp
It is possible for a routing address to route through multiple hosts:
@elsewhere.com,@nowhere.edu,@relay.cs.net:joe@cs.somewhere.edu
REFERENCES (Available from the CSNET Info Server):
RFC733 "Standard for the Format of ARPA Network Text Messages", David H.
Crocker (The Rand Corporation), John J. Vittal (Bolt Beranek and
Newman Inc.), Kenneth T. Pogran (Massachusetts Institute of
Technology), D. Austin Henderson, Jr.(Bolt Beranek and Newman Inc.) 21
November 1977.
RFC822 "Standard for the Format of ARPA Internet Text Messages", David H.
Crocker (University of Delaware), August 1982.
RFC882 "Domain Names - Concepts and Facilities", P. Mockapetris
(Information Sciences Institute, University of Southern California),
November 1983.
RFC883 "Domain Names - Implementation and Specification", P. Mockapetris
(Information Sciences Institute, University of Southern California),
November 1983.
RFC920 "Domain Requirements", J. Postel and J. Reynolds (Information
Sciences Institute, University of Southern California), October 1984.
RFC921 "Domain Names System Implementation Schedule - Revised", Jon Postel
(Information Sciences Institute, University of Southern California),
October 1984.
RFC973 "Domain System Changes and Observations", Paul Mockapetris
(Information Sciences Institute, University of Southern California),
January 1983. Supplements RFC 883.
RFC974 "Mail Routing and the Domain System", Craig Partridge (CSNET CIC,
BBN Laboratories, Inc.), January 1986.
-----------
This article appeared in CSNET-FORUM v2 #8 (8 Aug 86) with the title
"Background on the Domain Naming System".