Network Working Group H. Alvestrand
Request for Comments: 3254 Cisco Systems
Category: Informational April 2002
Definitions for talking about directories
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
When discussing systems for making information accessible through the
Internet in standardized ways, it may be useful if the people who are
discussing it have a common understanding of the terms they use.
For example, a reference to this document would give one the power to
agree that the DNS (Domain Name System) is a global lookup repository
with perimeter integrity and loose, converging consistency. On the
other hand, a LDAP (Lightweight Directory Access Protocol) directory
server is a local, centralized repository with both lookup and search
capability.
This document discusses one group of such systems which is known
under the term, "directories".
1. Introduction and basic terms
We suggest using the following terms for the remainder of this
document:
- 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 [SEC].)
- Repository: An amount of data that is accessible through one or
more access methods.
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- Requester: Entity that may (try to) access data in a repository.
Note that no assumption is made that the requester is animal,
vegetable, or mineral.
- 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, however, 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.
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.
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.
- 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.
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- 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 system is considered a
distributed cooperative repository, built out of zone repositories
that themselves may be distributed, and are always replicated when
distributed.
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 methods require the user to know or guess some exact value
before asking for information, sometimes called a "lookup key" or
"identifier" and sometimes called a "name". The word "name" is
NOT recommended, since it conflicts with other uses of that word
The response to a successful lookup is a single group of
information, often called "information about the identified
entity". A lookup method is binary (yes/no) in recall: It either
returns one result or no result; if it returns a result, that
result is the right result for that lookup key, so it is also of
binary precision (no info or completely relevant info).
- Search methods 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.
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In database terms, a lookup method corresponds to a query exactly
matching a unique key on a table; all other database queries would be
classified as "search" methods.
In general, repositories that offer more flexible search methods may
also give room for ad-hoc queries, refinements from a previous query,
approximate matching and other aids; this may lead to many different
combinations of precision and recall.
One may define terms to enumerate what one gets out of these
repositories:
. Precision is the degree to which what you asked for is what you
wanted (no extraneous information)
. Recall is the ability to assure oneself that all relevant data
from the repository is returned
. Type I errors occurs when relevant data exists in the
repository, but is not returned
. Type II errors occur when irrelevant data is returned with a
query result
Note that these concepts can only be applied when the property
"relevance" is well defined; that is, it depends on what the
repository is used for. A further discussion of these topics is
found in [KORFHAGE].
An orthogonal dimension has to do with time:
- 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 a 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 occurrences of pregnant
men), and focus on the problem of consistency where inconsistency is
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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 difficult; repositories that exhibit this property are
usually quite constrained and/or 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
Its 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 data: Why do we trust a piece of data to be correct?
- Because it's in the repository (and therefore must have been
authorized).
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
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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 (cost), the value to you of integrity of
the resource being protected (value), and the impact of cost on doing
business (risk).
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). (I.e., username/password, personal
certificates or other verifiable information.)
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, or capability keys.
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.
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- 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.
- Write-mostly repositories are designed to store an incoming stream
of data, and when needed reproduce a relevant piece of data from
the stream. Typical examples are insurance company databases and
audit logs.
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 mainly perimeter integrity (some would
say hopeful integrity). DNSSEC [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.
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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 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.
3.5 SNMP MIBs
An SNMP [SNMP] agent can be thought of as a local, centralized
repository offering lookup functionality.
With SNMPv3, it offers all kinds of access models, but mostly,
"access because of what you have", seems popular.
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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 probably a Good Thing to consider carefully the security models
from section 2.4 when designing repositories or repository access
protocols.
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, John Klensin, and many others.
6. References
[SEC] Shirey, R., "Internet Security Glossary", FYI 36, RFC
2828, May 2000.
[DNS] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[DNSSEC] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[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/4-1-bgp.pdf
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[BGP2] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.
[NEWS] Kantor, B. and P. Lapsley, "Network News Transfer
Protocol", RFC 977, February 1986.
[SNMP] Case, J., Mundy, R., Partain, D. and B. Stewart,
"Introduction to Version 3 of the Internet-standard
Network Management Framework", RFC 2570, April 1999.
[X500] Weider, C. and J. Reynolds, "Executive Introduction to
Directory Services Using the X.500 Protocol", FYI 13, RFC
1308, March 1992.
[KORFHAGE] "Information Storage and Retrieval", Robert R. Korfhage,
Wiley 1997. See page 194 for "precision" and "recall"
definitions.
7. Author's Address
Harald Tveit Alvestrand
Cisco Systems
Weidemanns vei 27
N-7043 Trondheim
NORWAY
Phone: +47 41 44 29 94
EMail: Harald@alvestrand.no
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8. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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