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Internet-Draft                                       H. Alvestrand
                                                     Cisco Systems
Target Category: Informational
                                                        April 2001
                                             Expires: October 2001

Definitions for talking about directories

Status of this Memo
The following text is food for the I-D machinery.
     The file name of this memo is draft-alvestrand-directory-defs-
     This document is an Internet-Draft and is in full conformance with
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When discussing systems for making information accessible through the
Internet in standardized ways, it may be useful if the people
discussing have a common understanding of the terms they use.
One group of such systems is known under the term "directories".
This document is not intended to be either comprehensive or definitive,
but is intended to give some aid in mutual comprehension when
discussing information access methods to be incorporated into Internet
Standards-Track documents.
Reference to this document would, for instance, give one the power to
agree that the Domain Name Service is a global lookup repository with
perimeter integrity and loose, converging consistency, while an LDAP 

Definitions for talking about directories        Harald Alvestrand
draft-alvestrand-directory-defs-02.txt        Expires October 2001

directory server is a local, centralized repository with both lookup
and search capability.

1. Introduction and basic terms
We suggest using the following terms for the remainder of this
- Information: Facts and ideas which can be represented (encoded) as
  data in various forms.
- Data: Information in a specific physical representation, usually a
  sequence of symbols that have meaning; especially a representation of
  information that can be processed or produced by a computer. (from
- Repository: An amount of data that is accessible through one or more
  access methods.
  Again, this term is deliberately not defined more strictly.
- Requester: Entity that may (try to) access data in a repository. Note
  that no assumption is made that the requester is animal, vegetable or
- Maintainer: Entity that causes changes to the data in the repository.
  Usually, all maintainers are requesters, since they need to look at
  the data too, but the roles are distinct.
- Access method: Well-defined series of operations that will cause data
  available from a repository to be obtained by the requester.
- Site: Entity that hosts all or part of a repository, and makes it
  available through one or more access methods. A site may in various
  contexts be a machine, a datacenter, a network of datacenters, or a
  single device.

2. Dimensions of classification

2.1 Uniqueness and scope
Some information systems are global, in the sense that only one can
sensibly exist in the world.
Others are inherently local, in that each locality, site or even box
will run its own information store, independent of all others.

The following terms are suggested:
- Global repository: A repository that there can be only one of in the
  world. The world itself is a prime example; the public telephone
  system's number assignments according to E.164 is another.

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Definitions for talking about directories        Harald Alvestrand
draft-alvestrand-directory-defs-02.txt        Expires October 2001

- Local repository: A class of repository of which multiple instances
  can exist, each with information relevant to that particular
  repository, with no need for coordination between them.
- Centralized repository: A repository where all access to data has to
  pass through some single site.
- Distributed repository: A repository that is not centralized; that
  is, access to data can occur through multiple sites.
- Replicated repository: A distributed repository where all sites have
  the same information
- Cooperative repository: A distributed repository where not all sites
  have all the information, but where mechanisms exist to get the info
  to the requester, even when it is not available to the site
  originally asked

Note: The term "global" is often a matter of social or legal context;
for instance, the E.164 telephone numbering system is global by
international treaty, while the debate about whether the Domain Name
System is global in fact or just a local repository with ambitions has
proved bait for too many discussions to enumerate.
Some claim that globality is in the eye of the beholder; "everything is
local to some context". When discussing technology, it may be wise to
use "very widely deployed" instead.
Note: Locating the repositories changes with the scale of
consideration. For instance, the global DNS service is considered a
distributed cooperative repository, built out of zone repositories that
themselves may be distributed, and are always replicated when

2.2 Search, Lookup, Query and Notify
A different consideration when describing repositories is the types of
method they offer to find information.
The chief classifications are:
- Lookup repositories require the user to know or guess some exact
  value before asking for information, sometimes called a "lookup key"
  and sometimes called a "name".
  The response to a successful lookup is a single group of information,
  often called "information about the named entity".
- Search repositories require the user to know some approximate value
  of some information. They usually return zero, one or more responses
  that match the information supplied according to some algorithm.
  Where the repository is structured around "entities", the information
  can be about zero, one or many entities.
An orthogonal dimension has to do with time:

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Definitions for talking about directories        Harald Alvestrand
draft-alvestrand-directory-defs-02.txt        Expires October 2001

- Query repositories will answer a request with a response, and once
  that is over with, will do nothing more.
- Notify repositories will get a request from an user to have
  information returned at some later time when it becomes available,
  current or whatever, and will respond at that time with a
  notification that information is available.
- Subscription repositories are like notify repositories, but will
  transfer the actual information when available.

2.3 Consistency models
Consistency (or the lack thereof) is a property of distributed
repositories; for this particular discussion, we ignore the subject of
semantically inconsistent data (such as an assertion that a man is
blind and has a valid driver's license), and focus on the problem of
consistency where inconsistency is defined as having the same request,
using the same credentials, be answered with different data at
different sites.
Distributed repositories may have:
- Strict consistency, where the problem above never arises. This is
  quite expensive.
- Strict internal consistency, where the replies always reflect a
  consistent picture of the total repository, but some sites may
  reflect an earlier version of the repository than others
- Loose, converging consistency, where different parts of the
  repository may be updated at different times as seen from a single
  site, but the process is designed in such a way that if one stops
  making changes to the repository, all sites will sooner or later
  present the same information
- Inconsistency, where no guarantee can be made whatsoever
One interesting variant is subset consistency, where the system is
consistent (according to one of the definitions above), but not all
questions will be answered at all sites; possibly because different
sites have different policies on what they make available (NetNews), or
because different sites only need different subsets of the "whole
picture" (BGP).

2.4 Security models
It's harder to describe security models in a few sentences than other
properties of information systems. There also exists a large
specialized literature on terminology for security, including [SEC].
Some thoughts, though:
On trust in information: Why do we trust a piece of information to be

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- Because it's in the repository (and therefore must have been
  This is perimeter (or Eggshell) integrity.
- Because it contains internal integrity checks, usually involving
  digital signatures by verifiable identities
  This is item integrity; the granularity of the integrity and the
  ability to do integrity checks on the relationships between objects
  is extremely important and extremely hard to get right, as is
  establishing the roots of the trust chains.
- Because it fits other available information, and causes the right
  things to happen when I use it.
  This is hopeful integrity.
Which integrity model to choose is a matter of evaluating the cost of
implementing the integrity, the cost of having the integrity break on
you, and the impact of cost on doing business.
On access to information, the usual categories apply:
- Open access: Anyone can get the information.
- Property-based access: Access because of what you are, or where you
  are. For example limited to "same network", "physically present" or
  "resolvable DNS name"
- Identity-based access: Access because of who you are (or successfully
  claim to be). username/password, certificatesà..
  These are then backed up by a layer specifying what the identity you
  have proven yourself to be has access to
- Token-based access: Access because of what you have. Hardware tokens,
  smartcards, certificates, capability keysà.
  In this case, access is given to all who can present that credential,
  without caring about their identity.
The most common approaches are identity-based and open access; however,
"what you have" access is commonly used informally in, for example,
password-protected FTP or Web sites where the password is shared
between all members of a group.

2.5 Update models
A few examples:
- Read-only repositories have no standard means of changing the
  information in them. This is usually accomplished through some other
  interface than the standard interface.
- Read-mostly repositories are designed based on a theory that reads
  will greatly outnumber updates; this may, for instance, be reflected
  in relatively slow consistency-updating protocols.
- Read-write repositories assume that the updates and the read
  operations are of the same order of magnitude.

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Definitions for talking about directories        Harald Alvestrand
draft-alvestrand-directory-defs-02.txt        Expires October 2001

2.6 The term "Directory"
The definitions above never used the term "Directory".
In most common usages, the properties that a repository must have in
order to be worthy of being called a directory are:
- Search
- Convergent consistency
All the other terms above may vary across the set of things that are
called "directories".

3. Classification of some real systems

3.1 The Domain Name System
The DNS [DNS] is a global cooperative lookup repository with loose,
converging consistency and query capability only.
It is either strictly read-only or read-mostly (with Dynamic DNS), has
an open access model, and mainy perimeter integrity (some would say
hopeful integrity). DNSSEC aims to give it item integrity.
The DNS is built out of zone repositories that themselves may be
distributed, and are always replicated when distributed.
Note that like many other systems, the DNS has some features that do
not fit neatly in the classification; for instance, there is a
(deprecated and not widely used) function called IQUERY, which allows a
very limited query capability.
If one opens up the box and looks at the relationship between primary
and secondary nameservers, that can be seen as a limited form of notify
capability, but this is not available to end-users of the total system.

3.2 The (imagined) X.500 Global Directory
X.500 [X500] was intended to be a global search repository with loose,
converging consistency.
It was intended to be read-mostly, perimeter secure and query-capable.

3.3 The Global BGP Routing Information Database
The Global or top-level BGP routing information database [BGP1] is
often viewed as a global read-write repository with loose, converging
subset consistency (not all routes are carried everywhere) and very
limited integrity control, mostly intended to be perimeter integrity
based on "access control based on what you are".
One can argue that BGP [BGP2] is better viewed as a global mechanism
for updating a set of local read/write repositories, since far from all

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Definitions for talking about directories        Harald Alvestrand
draft-alvestrand-directory-defs-02.txt        Expires October 2001

routing information is carried everywhere, and the decision on what
routes to accept is always considered a local policy matter. But from a
security model perspective, a lot of the controls are applied at the
periphery of the routing system, not at each local repository; this
still makes it interesting to consider properties that apply to the BGP
system as a whole.

3.4 The NetNews system
NetNews [NEWS] is a global read-write repository with loose (non-
converging) subset consistency (not all sites carry all articles, and
article retention times differ). Between sites it offers subscription
capability; to users it offers both search and lookup functionality.

An SNMP [SNMP] agent can be thought of as a local, centralized
repository offering lookup functionalty.
With SNMPv3, it offers all kinds of access models, but mostly "access
because of what you have" seems popular.

4. Security Considerations
Security is a very relevant question when considering information
access systems.
Some issues to consider are:
- Controlled access to information
- Controlled rights to update information
- Protection of the information path from provider to consumer
- With personal information, privacy issues
- Interactions between multiple ways to access the same information

It is proably a Good Thing to consider carefully the security models
from section 2.4 when designing repositories or repository access

5. Acknowledgement
The author wishes to thank all who contributed to this document,
including Patrik Faltstrom, Eric A. Hall, James Benedict, Ted Hardie,
Urs Eppenberger and many others.

6. References
[SEC]     Internet Security Glossary. R. Shirey. May 2000

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Definitions for talking about directories        Harald Alvestrand
draft-alvestrand-directory-defs-02.txt        Expires October 2001

[DNS]     RFC 1034 "Domain names - concepts and facilities". P.V.
Mockapetris. Nov-01-1987
[E164]    ITU-T Recommendation E.164/I.331 (05/97): The International
Public Telecommunication Numbering Plan. 1997.
[BGP1]    "Analyzing the Internet's BGP Routing Table", published in
"The Internet Protocol Journal", Volume 4, No 1, April 2001. At the
time of writing, available at http://www.telstra.net/gih/papers/ipj/4-
[BGP2]    RFC 1771, "A Border Gateway Protocol 4 (BGP-4)". RFC 1771. Y.
Rekhter, T. Li. March 1995.
[NEWS] RFC 977, "Network News Transfer Protocol". B. Kantor, P.
Lapsley,  February 1986
[SNMP] RFC 2570, "Introduction to Version 3 of the Internet-standard
Network Management Framework" J. Case, R. Mundy, D. Partain, B.
Stewart. April 1999.
[X500] RFC 1308, "Executive Introduction to Directory Services Using
the X.500 Protocol". C. Weider, J. Reynolds. March 1992

7. Author's Address
Harald Tveit Alvestrand
Cisco Systems
Weidemanns vei 27
N-7043 Trondheim
EMail: Harald@alvestrand.no
Phone: +41 44 29 94

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