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+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.
+
+
+
+Alvestrand                   Informational                      [Page 1]
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+RFC 3254       Definitions for talking about directories      April 2002
+
+
+   -  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.
+
+
+
+
+Alvestrand                   Informational                      [Page 2]
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+RFC 3254       Definitions for talking about directories      April 2002
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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.
+
+
+
+
+Alvestrand                   Informational                      [Page 3]
+
+RFC 3254       Definitions for talking about directories      April 2002
+
+
+   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
+
+
+
+Alvestrand                   Informational                      [Page 4]
+
+RFC 3254       Definitions for talking about directories      April 2002
+
+
+   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
+
+
+
+
+
+Alvestrand                   Informational                      [Page 5]
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+RFC 3254       Definitions for talking about directories      April 2002
+
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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.
+
+
+
+
+Alvestrand                   Informational                      [Page 6]
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+RFC 3254       Definitions for talking about directories      April 2002
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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.
+
+
+
+
+
+
+Alvestrand                   Informational                      [Page 7]
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+RFC 3254       Definitions for talking about directories      April 2002
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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.
+
+
+
+
+
+
+Alvestrand                   Informational                      [Page 8]
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+RFC 3254       Definitions for talking about directories      April 2002
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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
+
+
+
+
+Alvestrand                   Informational                      [Page 9]
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+RFC 3254       Definitions for talking about directories      April 2002
+
+
+   [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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+Alvestrand                   Informational                     [Page 10]
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+RFC 3254       Definitions for talking about directories      April 2002
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+8.  Full Copyright Statement
+
+   Copyright (C) The Internet Society (2002).  All Rights Reserved.
+
+   This document and translations of it may be copied and furnished to
+   others, and derivative works that comment on or otherwise explain it
+   or assist in its implementation may be prepared, copied, published
+   and distributed, in whole or in part, without restriction of any
+   kind, provided that the above copyright notice and this paragraph are
+   included on all such copies and derivative works.  However, this
+   document itself may not be modified in any way, such as by removing
+   the copyright notice or references to the Internet Society or other
+   Internet organizations, except as needed for the purpose of
+   developing Internet standards in which case the procedures for
+   copyrights defined in the Internet Standards process must be
+   followed, or as required to translate it into languages other than
+   English.
+
+   The limited permissions granted above are perpetual and will not be
+   revoked by the Internet Society or its successors or assigns.
+
+   This document and the information contained herein is provided on an
+   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
+
+Acknowledgement
+
+   Funding for the RFC Editor function is currently provided by the
+   Internet Society.
+
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