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Network Working Group                                      H. AlvestrandRequest for Comments: 3254                                 Cisco SystemsCategory: Informational                                       April 2002Definitions for talking about directoriesStatus 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]

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 classification2.1 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]

RFC 3254       Definitions for talking about directories      April 2002   -  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 isAlvestrand                   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 doAlvestrand                   Informational                      [Page 5]

RFC 3254       Definitions for talking about directories      April 2002      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]

RFC 3254       Definitions for talking about directories      April 2002   -  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 systems3.1 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]

RFC 3254       Definitions for talking about directories      April 2002   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]

RFC 3254       Definitions for talking about directories      April 20024. 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   fromsection 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,RFC2828, 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 athttp://www.telstra.net/gih/papers/ipj/4-1-bgp.pdfAlvestrand                   Informational                      [Page 9]

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,RFC1308, 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.noAlvestrand                   Informational                     [Page 10]

RFC 3254       Definitions for talking about directories      April 20028.  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.Alvestrand                   Informational                     [Page 11]