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DNS Privacy, Authorization, Special Uses, Encoding, Characters, Matching, and Root Structure: Time for Another Look?
draft-klensin-dns-function-considerations-02

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Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8324.
Author Dr. John C. Klensin
Last updated 2017-06-19
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draft-klensin-dns-function-considerations-02
Network Working Group                                         J. Klensin
Internet-Draft                                             June 19, 2017
Intended status: Informational
Expires: December 21, 2017

    DNS Privacy, Authorization, Special Uses, Encoding, Characters,
          Matching, and Root Structure: Time for Another Look?
              draft-klensin-dns-function-considerations-02

Abstract

   The basic design of the Domain Name System was completed almost 30
   years ago.  The last half of that period has been characterized by
   significant changes in requirements and expectations, some of which
   either require changes to how the DNS is used or that can be
   accommodated only poorly or not at all.  This document asks the
   question of whether it is time to either redesign and replace the DNS
   to match contemporary requirements and expectations (rather than
   continuing to try to design and implement incremental patches that
   are not fully satisfactory) or to draw some clear lines about
   functionality that is not really needed or that should be performed
   somewhere else.

Author's Note

   This draft is intended to draw a number of issues and references
   together in one place and to start a discussion.  It is obviously
   incomplete, particularly with regard to the list of perceived issues
   and deficiencies with that DNS.  To avoid misunderstanding, I don't
   completely believe some of the deficiencies listed below but am
   merely providing information about claims of deficiencies.  Input is
   welcome, especially about what is missing (or plain wrong) and would
   be greatly appreciated.

   This document should be discussed on the IETF list or by private
   conversation with the author.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 21, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Background and Hypothesis . . . . . . . . . . . . . . . . . .   4
   3.  Warts and Tensions With The Current DNS . . . . . . . . . . .   5
     3.1.  Multiple address types  . . . . . . . . . . . . . . . . .   5
     3.2.  Matching Part I: Case Sensitivity in Labels and Other
           Anomalies . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Matching Part II: Non-ASCII ("internationalized") Domain
           Name Labels . . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Matching Part III: Label Synonyms, Equivalent Names, and
           Variants  . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Query Privacy . . . . . . . . . . . . . . . . . . . . . .   8
     3.6.  Alternate Name Spaces for Public Use in the DNS
           Framework: The CLASS Problem  . . . . . . . . . . . . . .   9
     3.7.  Loose Synchronization . . . . . . . . . . . . . . . . . .   9
     3.8.  Private Name Spaces and Special Names . . . . . . . . . .  10
     3.9.  Alternate Query or Response Encodings . . . . . . . . . .  11
     3.10. Distribution and Managment of Root Servers  . . . . . . .  11
     3.11. Identifiers Versus Brands and Other Convenience Names . .  11
     3.12. A Single Hierarchy with a Centrally-controlled Root . . .  12
     3.13. Newer Application Protocols and New Requirements  . . . .  13
       3.13.1.  The Extensions . . . . . . . . . . . . . . . . . . .  13
       3.13.2.  Extensions and Deployment Pressures -- The TXT
                RRTYPE . . . . . . . . . . . . . . . . . . . . . . .  14
       3.13.3.  Periods and Zone Cut Issues  . . . . . . . . . . . .  15

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     3.14. Scaling of Reputation and Other Ancillary Information . .  15
   4.  Searching and the DNS - An Historical Note  . . . . . . . . .  16
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  22
     A.1.  Changes from version -00 (2017-06-02) to -01  . . . . . .  22
     A.2.  Changes from version -01 (2017-06-06) to -02  . . . . . .  22
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   This document explores contemporary expectations of the Internet's
   domain system (DNS) and compares them to the assumptions and
   properties of the DNS design.  It is primarily intended to ask the
   question of whether the differences are causing enough stresses on
   the system, stresses that cannot be resolved satisfactorily by
   further patching, that the Internet community should be considering
   designing a new system, one that is better adapted to current needs
   and expectations, and developing a deployment and transition strategy
   for it.  For those (perhaps the majority of us) for whom actually
   replacing the DNS is too radical to be realistic, the document may be
   useful in two other ways.  It may provide a foundation for discussing
   what functions the DNS should not be expected to support and how
   those functions can be supported in other ways, perhaps via an
   intermediate system that then calls on the DNS or by using some other
   type of database technology for some set of functions while leaving
   the basic DNS functions intact.  Or it may provide a basis for
   "better just get used to that and the way it works" discussions to
   replace fantasies about what the DNS might do in some alternate
   reality.

   There is a key design or philosophical question associated with the
   analysis in this document that the document does not address.  It is
   whether changes to perceived requirements to DNS functionality as
   described here are, in most respects, evolutionary or whether many of
   them are instances of trying to utilize the DNS for new requirements
   because it exists and is already deployed independent of whether the
   DNS is really appropriate or not.  The latter might be an instance of
   a problem often described in the IETF as "when all you have is a
   hammer, everything looks like a nail".

   While this document does not assume deep technical or operational
   knowledge of the DNS, it does assume some knowledge and at least
   general familiarity with the concepts of RFC 1034 [RFC1034] and RFC

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   1035 [RFC1035] and the terminology discussed in RFC 7719 [RFC7719]
   and elsewhere.  Although some of the comments it contains might be
   taken as hints or examples of different ways to think about the
   design issues, it makes no attempt to explore, much less offer a
   tutorial on, alternate naming systems or database technologies.

   It is perhaps worth noting that, while the perspective is different
   and more than a dozen years have passed, many of the issues discussed
   in this document were analyzed and described (most of them with more
   extensive explanations) in a 2005 US National Research Council report
   [NRC-Signposts].

   Readers should note that several references are to obsolete
   documents.  That was done because they are intended to show the
   documents and dates that introduced particular features or concepts.
   When current versions are intended, they are referenced.

2.  Background and Hypothesis

   The domain name system (DNS) [RFC1034] was designed starting in the
   early 1980s [RFC0799] [RFC0881] [RFC0882] with the main goal of
   replacing the flat, centrally-administered, host table system
   [RFC0810] [RFC0952] [RFC0953] with a hierarchical, administratively-
   distributed, system.  The DNS design included some features that were
   judged to be unworkable and either replaced (e.g., the mail
   destination (MD) and mail forwarder (MF) approach [RFC0882] that were
   replaced by the MX approach [RFC0974]), abandoned (e.g., the
   mechanism for using email local parts as labels described in RFC 1034
   Section 3.3), or deprecated (e.g., the WKS RR TYPE [RFC1123].  Newer
   ideas and requirements have identified a number of other features,
   some of which were less developed than others.  Of course the
   original designers could not anticipate everything that has come to
   be expected of the DNS in the last 30 years.

   In recent years, demand for new and extended services and uses of the
   DNS have, in turn, led to proposals for DNS extensions or changes of
   various sorts.  Some have been adopted, including a model for
   negotiating extended functionality [RFC2671], others were found to be
   impracticable, and still others continue to be under consideration.
   A few features of the original DNS specification, such as the CLASS
   property and label types, have also been suggested to be so badly
   specified that they should be deprecated [Sullivan-Class].

   Unlike earlier changes such as the IDNA mechanisms for better
   incorporating non-ASCII labels without modifying the DNS structure
   itself [RFC3490] [RFC5890], some recent proposals require or strongly
   suggest changes to APIs, formats, or interfaces by programs that need
   to retrieve information from the DNS or interpret that information.

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   Requirements for such changes suggest that it may be time to stop
   patching the DNS or trying to extend it in small increments, but to
   consider development of a system that better meets today's needs and
   a transition strategy to it.

   The next section of this document discusses a number of issues with
   the current DNS design that could appropriately be addressed by a
   different and newer design model.  In at least some cases, changing
   the model and protocols could bring significant benefits to the
   Internet and/or its administration.

   This document is not a proposal for a new protocol.  It is intended
   to stimulate thought about how far we want to try to push the
   existing DNS, to examine whether expectations of it are already
   exceeding its plausible capabilities, and to start discussion of a
   redesign or alternatives to one if the time for that discussion has
   come.

3.  Warts and Tensions With The Current DNS

   As suggested above, there are many signs that the DNS is incapable of
   meeting contemporary expectations of how it should work and
   functionality it should support.  Some of those expectations are
   unrealistic under any imaginable circumstances; others are impossible
   (or merely problematic) in the current DNS structure but could be
   accommodated in a redesign.  These are examples, rather than a
   comprehensive list, and do not appear in any particular order.

3.1.  Multiple address types

   While returning both TYPE A (IPv4 address) and AAAA (IPv6 address)
   records as additional information in response to any of several query
   types (see RFC 3596 [RFC3596]) was a useful patch, it is easy to
   imagine better choices.  For example, except that it would have
   required DNS modifications, we could have established a single
   "address" query type (QTYPE) that could return whatever IPv4 and/or
   IPv6 addresses were available, perhaps with preference information if
   that were stored in the database, and without requiring the "ANY" be
   used.  Other solutions would have been plausible; that one is offered
   only to combine an existence proof of at least one possibility and an
   example of how the existing DNS design and implementations are
   preventing us from thinking more broadly about possible solutions.

3.2.  Matching Part I: Case Sensitivity in Labels and Other Anomalies

   The DNS specifications assume that labels are octet strings and
   octets with the high bit zero have seven-bit ASCII codes in the
   remaining bits.  They require that, when a domain name used in a

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   query is matched to one stored in the database, those ASCII
   characters be interpreted in a case-independent way, i.e., upper and
   lower case letters are treated as equivalent (digits and symbols are
   not affected).  For non-ASCII octets, i.e., octets in labels with the
   first bit turned on, there are no assumptions about the character
   coding used, much less any rules about character case equivalence --
   strings must be compared by matching bits in sequence.  Even though
   the current model for handling non-ASCII (i.e., "internationalized")
   domain name labels (IDNs) [RFC5890] (and see Section 3.3 below)
   encodes information so the DNS is not directly affected, the notion
   that some characters in labels are handled in a case-insensitive way
   and that others are case-sensitive (or that upper case must be
   prohibited entirely as IDNA does) has caused a good deal of confusion
   and resentment.  Those concerns about inconsistent behavior and
   mishandling (or suboptimal handling) of case relationships for some
   languages have not been mitigated by repeated explanations that the
   relationships between "decorated" lower-case characters and their
   upper-case equivalent are often sensitive to language and locality
   and therefore not deterministic with information available to DNS
   servers.

3.3.  Matching Part II: Non-ASCII ("internationalized") Domain Name
      Labels

   Quite independent of the case-sensitivity problem, one of the
   fundamental properties of Unicode [Unicode] is that some abstract
   characters can be represented in multiple ways, such as by a single,
   precomposed, code point or by a base code point followed by one or
   more code points that specify combining characters.  While Unicode
   Normalization can be used to eliminate many (but not all) of those
   distinctions for comparison (matching) purposes, it is best applied
   during matching rather than by changing one string into another.  The
   first version of IDNA ("IDNA2003") made the choice to change strings
   during processing for either storage or retrieval [RFC3490]
   [RFC3491]; the second ("IDNA2008") required that all strings be
   normalized [RFC5891].  Neither is optimal, if only because
   transforming the strings themselves implies that the input string in
   an application may not be the same as the string used in processing
   and perhaps later display.

   It would almost certainly be preferable, and more consistent with
   Unicode recommendations, to use normalization (and perhaps other
   techniques) at matching time rather than altering the strings at all,
   even if there were still only a single matching algorithm, i.e.,
   normalization were added to the existing ASCII-only case folding.
   However, even Unicode's discussion of normalization [Unicode-UAX15]
   indicates that there are special, language-dependent, cases (the most
   commonly-cited example is the dotless "i" (U+0131)).  Not only does

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   the DNS lack any information about languages that could be used in a
   mapping algorithm, but, as long as there is a requirement that there
   be only one mapping algorithm for the entire system, that information
   could not be used even if it were available.  One could imagine a
   successor system that would use information stored at nodes in the
   hierarchy to specify different matching rules for subsidiary nodes
   (or equivalent arrangements for non-hierarchical systems).  It is not
   clear whether that would be a good idea, but it certainly is not
   possible with the DNS as we know it.

3.4.  Matching Part III: Label Synonyms, Equivalent Names, and Variants

   As the initial phases of work on IDNs started to conclude, it became
   obvious that the nature and evolution of human language and writing
   systems required treating some names as "the same as" others.  The
   first important example of this involved the relatively recent effort
   to simplify the Chinese writing system, thereby creating a
   distinction between "Simplified" and "Traditional" Chinese even
   though the meaning of the characters remained the same in almost all
   cases (in so-called ideographic character sets, characters have
   meaning rather than representing sounds).  A joint effort among the
   relevant country code top level domain (TLD) registries and some
   other interested parties produced a set of recommendations for
   dealing with the issues with that script [RFC3743] and introduced the
   concept of "variant" characters and domain names.

   However, when names are seen as having meanings, rather than merely
   being mnemonics, and especially when they represent brands or the
   equivalent, or when spelling for a particular written language is not
   completely standardized, there is an immediate demand to treat
   different strings as exact equivalents.  As a trivial English-
   language example, it is widely understood that "colour" and "color"
   represent the same word, so does that imply that, if they are used as
   DNS labels in domain names all of whose other labels are identical,
   should the two domain names be treated as identical?  Examples for
   other languages or writing systems, especially ones in which some or
   all markings that distinguish characters by sound or that change the
   pronunciation of words are optional, are often more numerous and more
   problematic than national spelling differences in English, but they
   are harder to explain to those unfamiliar with those other languages
   or writing systems (and hard to illustrate in ASCII-only Internet-
   Drafts and RFCs).  Although approximations are possible, the DNS
   cannot handle that requirement: not only do its aliasing mechanisms
   (CNAME, DNAME, and various proposals for newer and different types of
   aliasing [DNS-Aliases] [DNS-BNAME], not provide a strong enough
   binding, but the ability to use those aliases from a subtree
   controlled by one administrative entity to that of another one,
   implies that there is little or no possibility of the owner (in

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   either the DNS sense or the registrar-registrant one) of a particular
   name to control the synonyms for it.  Some of that issue can be dealt
   with at the application level, e.g., by redirects in web protocols,
   but taking that approach, which is the essential characteristic of
   "if both names belong to the same owner, everything is ok"
   approaches, results in names being handled in inconsistent ways in
   different protocols.

   A different way of looking at part of this issue (and, to some
   degree, of the one discussed above in Section 3.3) is that these
   perceived equivalences and desired transformations are context-
   dependent, but the DNS resolution process is not [RFC6912].

   Similar problems arise as people notice that some characters are
   easily mistaken for others and that might be an opportunity for user
   confusion and attacks.  Commonly-cited examples include the Latin and
   Cyrillic script "a" characters, which are identical [CACM-Homograph],
   the characters in many scripts that look like open circles or
   vertical or horizontal lines, and even the Latin script letter "l"
   and the European digit "1", but examples abound in other scripts and
   combinations of scripts as well.  The most common proposed solution
   within the DNS context has been to treat these cases, as well as
   those involving orthographic variations, as "variants" and either ban
   all but one (or a few) of the possible labels from the DNS (possibly
   on a first come first served basis) or by ensuring that any
   collection of such strings that are delegated as assigned to the same
   ownership (see above).  Neither solution is completely satisfactory:
   if all but one string is excluded, users who guess at a different
   form, perhaps in trying to transcribe characters from written or
   printed form, don't find what they are looking for and, as pointed
   out above, "same ownership" is sufficient only with carefully-
   designed and administered applications protocol support and sometimes
   not then.

   Some of these issues are discussed at more length in an ICANN report
   [ICANN-VIP].

3.5.  Query Privacy

   There has been growing concern in recent years that DNS queries occur
   in clear text on the public Internet and that, if those queries can
   be intercepted, they can expose a good deal of information about
   interests and contacts that could compromise individual privacy.
   While a number of proposals, including query name minimization
   [RFC7816] and running DNS over an encrypted tunnel [RFC7858] have
   been made to mitigate that problem, they all appear to share the
   common properties of security patches rather than designed-in
   security or privacy mechanisms.  While experience may prove otherwise

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   once (and if) they are widely deployed, it does not appear that any
   of them are as satisfactory as a system with query privacy designed
   in might be.  More general tutorials on this issue have appeared
   recently [Huston-DNSPrivacy].

3.6.  Alternate Name Spaces for Public Use in the DNS Framework: The
      CLASS Problem

   The DNS standards include specification of a CLASS value to "identify
   a protocol family or instance of a protocol" RFC 1034, Section 3.6
   and elsewhere [RFC1034].  While it was used effectively in the early
   days of the DNS to manage different protocol families within the same
   administrative environment, recent attempts to use it to either
   partition the DNS namespace in other ways such as for non-ASCII names
   (partially to address the issues in Section 3.2 Section 3.3) or to
   use DNS mechanisms for entirely different namespaces have exposed
   fundamental problems with the mechanism [Sullivan-Class], leading to
   recommendations that it be dropped entirely.

   Whether either the function CLASS was originally intended to provide
   or the ones for which there have been attempts to use it more
   recently are actually needed is a separate question; it is clear that
   the current DNS technical and administrative model is unsuitable for
   either function.

3.7.  Loose Synchronization

   The DNS model of master and slave servers, with the latter initiating
   updates based on TTL values, together with more local caches, depends
   heavily on an approach that has come to be called "loose
   synchronization", i.e., that there can be no expectation that all of
   the servers that might reasonably answer a query will have exactly
   the same data unless those data have been unchanged for a rather long
   period.  Put differently, if some or all of the records associated
   with a particular node in the DNS (informally, a fully-qualified
   domain name (FQDN)) change, one cannot expect those changes to be
   propagated immediately.

   That model has worked rather well since the DNS was first deployed,
   protecting the system from requirements, that are typical where
   simultaneous update of multiple systems is needed, such as elaborate
   locking, complex update mechanisms, or journaling.  As has often been
   pointed out with the Internet, implementation and operational
   complexity are often the enemy of stability, security, and
   robustness.  Loose synchronization has helped keep the DNS as simple
   and robust as possible.

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   A number of recent ideas about using the DNS to store data that
   change very rapidly and where the changes are important are, however,
   largely incompatible with loose synchronization.  Efforts to use very
   short (or zero) refresh times (in SOA records for slave updates from
   masters) and TTLs (for caches) to simulate nearly-simultaneous
   updating may work up to a point but appear to impose very heavy loads
   on servers and distribution mechanisms that were not designed to
   accommodate that style of working.  Similar observations can be made
   about attempts to use the NOTIFY extension [RFC1996] or dynamic,
   "server-push", updating rather than the traditional DNS mechanisms.
   While the NOTIFY and push mechanisms normally provide refresh times
   and update mechanisms faster than those specified in RFC 1034 and
   1035, they imply that a "master" server must know the identities of
   (and have good connectivity to all of) its slaves, defeating at least
   some of the advantages associated with stealth slaves, particularly
   those associated with reduction of query traffic across the Internet.
   Those mechanisms do nothing for cache updates: unless servers keep
   track of every query for names associated with a specific zone and
   somehow notify the query source systems, the only alternative to
   having information that might be obsolete stored in caches is to use
   very short or zero TTLs so the cached data time out almost
   immediately after being stored (or are not stored at all), requiring
   a new query to an authoritative server each time a resolver makes a
   query.

3.8.  Private Name Spaces and Special Names

   Almost since the DNS was first deployed, there have been situations
   in which it is desirable to use DNS-like names, and often DNS
   resolution mechanisms or modifications of them, with name spaces for
   which globally-available and consistent resolution using the public
   DNS is either unfeasible or undesirable (and for which the use of
   CLASS is not an appropriate mechanism).  The need to isolate names
   and addresses on LANs from the public Internet, typically via "split
   horizon" approaches, is one example of this requirement although
   often not recognized as such.  Another example that has generated a
   good deal of controversy involves "special names" -- labels or
   pseudo-labels, often in TLD positions, that signal that the full name
   should not be subject to normal DNS resolution or other processing
   [RFC6761] [DNSOP-Sutld].

   Independent of troublesome policy questions about who should allocate
   such names and the procedures to be used, they almost inherently
   require either a syntax convention to identify them (there actually
   was such a convention, but it was abandoned many years ago and there
   is no plausible way to re-institute it) or tables of such names that
   are known to, and kept updated on, every resolver on the Internet, at
   least if spurious queries to the root servers are to be avoided.

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   If the DNS were to be redesigned and replaced, we could recognize
   this requirement as part of the design and handle it much better than
   it is possible to handle it today.

3.9.  Alternate Query or Response Encodings

   The DNS specifies formats for queries and data responses, based on
   the state of the art and best practices at the time it was designed.
   Recent work has suggested that there would be significant advantages
   to supporting at least a description of the DNS messages in one or
   more alternate formats, such as JSON [Hoffman-DNS-JSON]
   [Hoffman-SimpleDNS-JSON].  While that work has been carefully done to
   avoid requiring changes to the DNS, much of the argument for having
   such a JSON-based description format could easily be turned into an
   argument that, if the DNS were being revised, that format might be
   preferable as a more direct alternative to having DNS queries and
   responses in the original form.

3.10.  Distribution and Managment of Root Servers

   The DNS model requires a collection of root servers that hold, at
   minimum, information about top-level domains.  Over the years, that
   requirement has evolved from a technically fairly minor function,
   normally carried out as a service to the broader Internet community
   and its users and systems, to a subject that is intensely
   controversial with regard to who should control those servers, how
   they should be distributed and where they should be located.  While a
   number of mechanisms have been proposed and one (anycast [RFC7094])
   is in very active use to mitigate some of the real and perceived
   problems, it seems obvious that a DNS successor, designed for today's
   perceived requirements, could handle these problems in a technically
   more appropriate and less controversial way.

3.11.  Identifiers Versus Brands and Other Convenience Names

   A key design element of the original network object naming systems
   for the ARPANET, largely inherited by the DNS, was that the names,
   while expected to be mnemonic, were identifiers and their being
   highly distinguishable and not prone to ambiguity was important.
   That led to very restrictive rules about what could appear in a name.
   In the case of the host table, the restrictions that came to the DNS
   (largely via SMTP) as the "preferred syntax" [RFC 1034 Section 3.5]
   or what we now often call the letter-digit-hyphen (LDH) rule.
   Similar rules to make identifiers easier to use, less prone to
   ambiguity, or less likely to interfere with syntax in more formal
   languages occur frequently.  For example, almost every programming
   language has restrictions on what can appear in an identifier and
   Unicode provides general recommendations about identifier composition

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   [Unicode-USA31].  Both are quite restrictive as compared to the
   number of characters and total number of strings that can be written
   using that character coding system.

   In the last decade or two, another perspective has emerged, largely
   without being explicitly understood or acknowledged.  In it, the DNS
   is really (and primarily) a system for expressing thoughts and
   concepts.  Those include free expression of ideas in as close to
   natural language as possible as well as representation of product
   names and brands.  That view requires letter-like characters that
   might not be reasonable in identifiers along with a variety of
   symbols and punctuation and might require indicators of preferred
   type styles to provide information in a form that exactly matches
   personal or legal preferences.  That perspective would argue for
   standardizing word and sentence separators, removing the 63 octet per
   label limit and probably the limit of 255 octets on the total length
   of a domain name, and maybe even eliminating the hierarchy or
   allowing separators for labels in presentation form (now fixed at "."
   for the DNS) to be different according to context.  At least it
   suggests that the original design was defective in not prioritizing
   those uses over support for unique and unambiguous identifiers.

   So we have two, or, depending on how one counts, three very different
   use cases.  The historical one is support for unique identifiers.
   The other is expression of ideas and, if one considers it separate,
   presentation of brand and product names.  Because they inherently
   involve different constraints, priorities, and success criteria,
   these perspectives are, at best, only loosely compatible.

   We cannot simultaneously optimize both the identifier perspective and
   either or both of the others in the same system.  At best, there are
   some complex trade-offs involved.  Even then, it is not clear that
   the same DNS (or other system) can accommodate all of them.  Until we
   come to terms with that, the differences manifest themselves with
   friction among communities, most often with tension between "we want
   to do (or use or sell) these types of labels" and "not good for the
   operational Internet or the DNS".

3.12.  A Single Hierarchy with a Centrally-controlled Root

   A good many Internet policy discussions in the last two decades have
   revolved around such questions of how many top level domains there
   should be and what they should be, who should control them and how,
   how (or if) their individual operations and policy decisions should
   be accountable to others, and what processes should be used (and by
   what entities or organizational structures) to make those decisions.
   Several people have pointed out that, if we were designing a next-
   generation DNS using today's technology, it should be possible to

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   remove the technical requirement for a central authority over the
   root (some people have suggested that blockchain approaches would be
   helpful for this purpose; others believe they just would not scale
   adequately, at least at acceptable cost).  Whether that would be
   desirable or not is fairly obviously a question of perspective and
   priorities.

3.13.  Newer Application Protocols and New Requirements

   New work done in other areas has led to demands for new DNS features,
   many of them involving data values that require recursively
   referencing the DNS.  Early record types that did that were
   accompanied by restrictions that reduced the risk of looping
   references or other difficulties.  For example, while the MX RRTYPE
   has a fully-qualified domain name as its data, SMTP imposes "primary
   name" restrictions that prevent the name used from being, e.g., a
   CNAME.  While loops with CNAMEs are possible, RFC 1034, Section 3.6,
   includes a discussion about ways to avoid problems and how they
   should be handled.  Some newer protocols and conventions can cause
   more stress.  There are separate issues with additions and with how
   the DNS has been extended to try to deal with them.

3.13.1.  The Extensions

   Some examples of DNS extensions for new protocol demands that
   illustrate, or have led to, increased stress include:

   NAPTR  Requires far more complex data in the DNS for ENUM (e.g., VoIP
      and SIP) support, including URI information and hence recursive or
      repeated lookups, than any of the RRTYPEs originally supported.
      The RRSET associate with these records can become quite large
      becaues the separator between the various records is part of the
      RDATA, and not the {owner, class, type} triple (a problem slightly
      related to the problem with overloading of TXT RTYPE discussed in
      Section 3.13.2.  This problem, and similar ones for some of the
      cases below. may suggest that any future design is in need of a
      different TYPE model such as systematic arrangements for subtypes
      or some explicit hierarchy in the TYPEs.

   URI  Has a URI as its data, typically also requiring recursive or
      repeated lookups.

   Service location (SRV) and credential information (including SPF)
      Require structured data and, especially for the later,
      significantly more data, than most original RRTYPEs.

   URI/URL  The early design decision for the World Wide Web that its
      mechanism for identifying digital web content (now known as

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      Uniform Resource Identifiers [RFC3986]) did so by using domain
      names and hence the network location of the information or other
      material.  That, in turn, has required systems intended to improve
      web performance by locating and retrieving a "nearest copy" rather
      than the single copy designated by the URL to intercept DNS
      queries and respond with values that are not precisely those
      stored for the designated domain name in the DNS or to otherwise
      access information in a way not supported by the DNS itself.

   In addition to the stresses these new functions cause, incremental
   deployment of systems that utilize them means that some functions
   will work in some environments and not others.  This has been
   especially problematic with complex, multi-record, functions like
   DNSSEC that provide or require special validation mechanisms such as
   DNNSEC.

3.13.2.  Extensions and Deployment Pressures -- The TXT RRTYPE

   Unfortunately (but unsurprisingly) and despite IETF efforts to make
   things easier [RFC6895], DNS support libraries have often been slow
   to add full support for new RRTYPEs, impeding deployment of
   applications that depend on them.  Both to get faster deployment and,
   at least until recently, to avoid burdensome IETF approval
   procedures, many application designers have chosen to push protocol-
   critical information into records with TXT RRTYPE, a record type that
   was originally intended to include only information equivalent to
   comments.

   This causes two problems.  First, TXT records used this way tend to
   get long and complex, which is a problem in itself if one is trying
   to minimize TCP connections.  Second, applications that are
   attempting to obtain data cannot merely ask for the relevant QTYPE,
   they must obtain all of the records with QTYPE TXT and parse them to
   determine which ones are of interest.  That would be easier if there
   was some standard for how to do that parsing but, at least in part
   because the clear preference in the DNS design is for distinct
   RRTYPEs for different kinds of information, there is no such
   standard.

   On the other hand, this issue is somewhat different from most of the
   others described in this document because (as the IETF has
   recommended several times) the problem is easily solved within the
   current DNS design by allocating and supporting new RRTYPEs when
   needed rather than using TXT as a workaround (that does not mean that
   other solutions are impossible, either with the current DNS or some
   other design).  The problem then lies in the implementations and/or
   mechanisms that deter or impede rapid deployment of support for new
   RRTYPEs.

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3.13.3.  Periods and Zone Cut Issues

   One of the DNS characteristics that is poorly understood by non-
   experts is that the period (".", U+002E) character can be used in
   three different ways:

   o  As a label separator in the presentation form that also designates
      a "zone break" (delegation boundary).  For example,
      foo.bar.example.com indicates the owner, "foo", of records in the
      "bar.example.com" zone.

   o  As a label separator in the presentation form that does not
      designate a zone break.  For example, foo.bar.example.com
      indicates the owner, "foo.bar", of records in the "example.com"
      zone.

   o  As a character within a label, including as a substitute for an
      at-sign ("@") when an email address appears in an SOA record or in
      a label that denotes such an address (see Section 2).

   In general, these cases cannot be distinguished by looking at them.
   The third is problematic for non-DNS reasons, e.g.,
   "john.doe.example.net" is ambiguous as to whether it should be
   interpreted as a simple FQDN, as a notation for john.doe@example.net,
   for john@doe.example.net, and so on.

   The distinction between the first two cases was probably not
   important as the DNS was originally intended to be used.  However, as
   soon as RRTYPEs (other than NS records that define the zone cut) are
   used that are sensitive to the boundaries between zones, the
   distinctions become important to people other than the relevant zone
   administrators.  DNSSEC involves one such set of relationships.  It
   increases the importance of questions about what should go in a
   parent zone and what should go in child zones and how much difference
   it makes if NS records in a parent zone for a child zone are
   consistent with the records and data in the child zone.  This also
   causes application issues, may raise questions about relationships
   between registrars and one or more registries or, if they are
   separate, DNS operators.

3.14.  Scaling of Reputation and Other Ancillary Information

   The original design for DNS administration, reflected in RFC 1591
   [RFC1591] and elsewhere, assumed that all domains would exhibit a
   very high level of responsibility toward and for the community and
   that level of responsibility would be enforced if necessary.  More
   recent decisions have taken things in the direction of "registrant
   beware" and even "user and applications beware" even some recent

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   protocols at least partially reflect the original model (see, e.g.,
   IDNA [RFC5890] and the discussion in a recent Internet-Draft
   [Klensin-5891bis].  One possible approach to the problems, especially
   security problems, that are enabled by the new environment is to
   establish reputation systems associated with clearly-defined
   administrative boundaries and with warnings to users.

   The IETF DBOUND WG [IETF-DBOUND] addressed ways to establish and
   document boundaries more precise than simple dependencies on TLDs but
   it was not successful in producing a standard.

   A TLD reputation-based approach was adopted by some web browsers
   after IDNs and a growing number of gTLDs were introduced; that
   approach was based on a simple list and does not scale to the current
   size of the DNS or even the DNS root.

4.  Searching and the DNS - An Historical Note

   Some of the issues identified above might reasonably be addressed,
   not by changing the DNS itself but by changing our model of what it
   is about and how it is used.  Specifically, one key assumption when
   the DNS (and the host table system before it) was designed was that
   it was a naming system for network resources, not, e.g., digital
   content.  As such, exact matching was important, it was reasonable to
   have labels treated as mnemonics that did not necessarily have
   linguistic or semantic meaning except to those using them, and so on.
   A return to that model, presumably by having user-facing applications
   call on an intermediate layer to disambiguate user-friendly names and
   map them to DNS names (network object locators) would significantly
   reduce stress on the DNS and would also allow dealing with types of
   matching and similar or synonymous strings that cannot be handled
   algorithmically no matter how much DNS matching rules were altered.

   In the early part of the last decade, the IETF explored that approach
   a little bit in the context of IDNs and what were then called
   "Internet keywords" [DNS-search].  It may be time to look at that
   approach again and more deeply in the context of more recent
   developments.

   It is worth noting that, while that "search" approach, or some other
   approach that abstracted and separated several of the issues
   identified in Section 3 from the DNS protocol and database
   themselves, it does not address all of them.  At least some elements
   of several of those issues, such as the synchronization ones
   described in Section 3.7, are inherent in the DNS design and, if we
   are not going to replace the DNS, we had best get used to them.

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5.  Acknowledgements

   Many of the concerns and ideas described in this document reflect
   conversations over a period of many years, some rooted in DNS
   "keyword" and "search" discussions that paralleled the development of
   Internationalized Domain Names (IDNs).  Conversations with, or
   writings of, Rob Austein, Christine Borgman, Carolina Carvalho, Vint
   Cerf, Lyman Chapin, Patrik Faltstrom, Geoff Huston, Xiaodong Lee,
   Karen Liu, Yaqub Mueller, Andrew Sullivan, Paul Twomey, Suzanne
   Woolf, Jiankang Yao, other participants in the circa 2003 "DNS
   Search" effort and in the ICANN SSAC Working Party on IDNs, and some
   others whose names were sadly forgotten were particularly important
   to either the content of this document or the motivation for writing
   it even though they may not agree with the conclusions I have reached
   and bear no responsibility for them.

   Many of the subsections of Section 3 were extracted from comments
   first made in conjunctions with recent email discussions.  Comments
   from Suzanne Woolf about an early draft were particularly important
   as was material developed with suggestions from Patrik Faltstrom,
   especially Section 3.13.  Feedback and suggestions from several of
   the above and from Stephane Bortzmeyer, Tony Finch, and George
   Sadowsky were extremely helpful for improving the clarity and
   accuracy of parts of the document, especially so for a broader
   audience.

6.  IANA Considerations

   [[CREF1: RFC Editor: Please remove this section before publication.]]

   This memo includes no requests to or actions for IANA.

7.  Security Considerations

   From both security and privacy perspectives, a replacement for the
   DNS would not have to go very far to be a significant improvement.

8.  References

8.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

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8.2.  Informative References

   [CACM-Homograph]
              Gabrilovich, E. and A. Gontmakher, "The Homograph Attack",
              Communications of the ACM 45(2):128, February 2002,
              <http://www.cs.technion.ac.il/~gabr/papers/
              homograph_full.pdf>.

   [DNS-Aliases]
              Woolf, S., Lee, X., and J. Yao, "Problem Statement: DNS
              Resolution of Aliased Names", March 2011,
              <https://datatracker.ietf.org/doc/draft-ietf-dnsext-
              aliasing-requirements/>.

   [DNS-BNAME]
              Yao, J., Lee, X., and P. Vixie, "Bundled DNS Name
              Redirection", May 2016, <https://datatracker.ietf.org/doc/
              draft-yao-dnsext-bname/>.

   [DNS-search]
              IETF, "Internet Resource Name Search Service", 2003,
              <https://datatracker.ietf.org/wg/irnss/about/>.

              While it met several times informally and as one or more
              BOFs, this effort never really got off the ground.  That
              was due in part to the IETF decision to go forward with
              the IDNA approach and in part by signs that the "keyword"
              efforts were beginning to fall apart.

   [DNSOP-Sutld]
              Lemon, T., Droms, R., and W. Kumari, "Special-Use Domain
              Names Problem Statement", June 2017,
              <https://datatracker.ietf.org/doc/draft-ietf-dnsop-sutld-
              ps>.

   [Hoffman-DNS-JSON]
              Hoffman, P., "Representing DNS Messages in JSON", May
              2017, <https://datatracker.ietf.org/doc/draft-hoffman-dns-
              in-json/>.

   [Hoffman-SimpleDNS-JSON]
              Hoffman, P., "Simple DNS Queries and Responses in JSON",
              June 2017, <https://datatracker.ietf.org/doc/draft-
              hoffman-simplednsjsonn/>.

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   [Huston-DNSPrivacy]
              Huston, G. and J. Silva Dama, "DNS Privacy", Internet
              Protocol Journal Vol 20, No 1, March 2017,
              <http://ipj.dreamhosters.com/wp-
              content/uploads/issues/2017/ipj20-1.pdf>.

   [ICANN-VIP]
              ICANN, "IDN Variant Issues Project: Final Integrated
              Issues Report Published and Proposed Project Plan for Next
              Steps is Now Open for Public Comment", February 2012,
              <https://www.icann.org/news/announcement-2012-02-20-en>.

   [IETF-DBOUND]
              IETF, "Domain Boundaries (dbound)", 2017,
              <https://datatracker.ietf.org/wg/dbound/about/>.

   [Klensin-5891bis]
              Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Registry Restrictions and
              Recommendations", March 2017,
              <https://datatracker.ietf.org/doc/draft-klensin-idna-
              rfc5891bis/>.

   [NRC-Signposts]
              National Research Council, "Signposts in Cyberspace: The
              Domain Name System and Internet Navigation"", 2005,
              <https://www.nap.edu/catalog/11258/signposts-in-
              cyberspace-the-domain-name-system-and-internet-
              navigation>.

   [RFC0799]  Mills, D., "Internet name domains", RFC 799,
              DOI 10.17487/RFC0799, September 1981,
              <http://www.rfc-editor.org/info/rfc799>.

   [RFC0810]  Feinler, E., Harrenstien, K., Su, Z., and V. White, "DoD
              Internet host table specification", RFC 810,
              DOI 10.17487/RFC0810, March 1982,
              <http://www.rfc-editor.org/info/rfc810>.

   [RFC0881]  Postel, J., "Domain names plan and schedule", RFC 881,
              DOI 10.17487/RFC0881, November 1983,
              <http://www.rfc-editor.org/info/rfc881>.

   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
              RFC 882, DOI 10.17487/RFC0882, November 1983,
              <http://www.rfc-editor.org/info/rfc882>.

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   [RFC0952]  Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
              host table specification", RFC 952, DOI 10.17487/RFC0952,
              October 1985, <http://www.rfc-editor.org/info/rfc952>.

   [RFC0953]  Harrenstien, K., Stahl, M., and E. Feinler, "Hostname
              Server", RFC 953, DOI 10.17487/RFC0953, October 1985,
              <http://www.rfc-editor.org/info/rfc953>.

   [RFC0974]  Partridge, C., "Mail routing and the domain system",
              STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986,
              <http://www.rfc-editor.org/info/rfc974>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
              Application and Support", STD 3, RFC 1123,
              DOI 10.17487/RFC1123, October 1989,
              <http://www.rfc-editor.org/info/rfc1123>.

   [RFC1591]  Postel, J., "Domain Name System Structure and Delegation",
              RFC 1591, DOI 10.17487/RFC1591, March 1994,
              <http://www.rfc-editor.org/info/rfc1591>.

   [RFC1996]  Vixie, P., "A Mechanism for Prompt Notification of Zone
              Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
              August 1996, <http://www.rfc-editor.org/info/rfc1996>.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, DOI 10.17487/RFC2671, August 1999,
              <http://www.rfc-editor.org/info/rfc2671>.

   [RFC3490]  Faltstrom, P., Hoffman, P., and A. Costello,
              "Internationalizing Domain Names in Applications (IDNA)",
              RFC 3490, DOI 10.17487/RFC3490, March 2003,
              <http://www.rfc-editor.org/info/rfc3490>.

   [RFC3491]  Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
              Profile for Internationalized Domain Names (IDN)",
              RFC 3491, DOI 10.17487/RFC3491, March 2003,
              <http://www.rfc-editor.org/info/rfc3491>.

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              DOI 10.17487/RFC3596, October 2003,
              <http://www.rfc-editor.org/info/rfc3596>.

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   [RFC3743]  Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
              Engineering Team (JET) Guidelines for Internationalized
              Domain Names (IDN) Registration and Administration for
              Chinese, Japanese, and Korean", RFC 3743,
              DOI 10.17487/RFC3743, April 2004,
              <http://www.rfc-editor.org/info/rfc3743>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <http://www.rfc-editor.org/info/rfc5891>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <http://www.rfc-editor.org/info/rfc6761>.

   [RFC6895]  Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
              April 2013, <http://www.rfc-editor.org/info/rfc6895>.

   [RFC6912]  Sullivan, A., Thaler, D., Klensin, J., and O. Kolkman,
              "Principles for Unicode Code Point Inclusion in Labels in
              the DNS", RFC 6912, DOI 10.17487/RFC6912, April 2013,
              <http://www.rfc-editor.org/info/rfc6912>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,
              <http://www.rfc-editor.org/info/rfc7094>.

   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <http://www.rfc-editor.org/info/rfc7719>.

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
              <http://www.rfc-editor.org/info/rfc7816>.

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   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <http://www.rfc-editor.org/info/rfc7858>.

   [Sullivan-Class]
              Sullivan, A., "The DNS Is Not Classy: DNS Classes
              Considered Useless", July 2016,
              <https://datatracker.ietf.org/doc/draft-sullivan-dns-
              class-useless/>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard, Version
              9.0.0,", ISBN 978-1-936213-13-9, 2016,
              <http://www.unicode.org/versions/Unicode9.0.0/>.

   [Unicode-UAX15]
              Davis, M. and K. Whistler, "Unicode Normalization Forms",
              February 2016, <http://unicode.org/reports/tr15/>.

   [Unicode-USA31]
              Davis, M., "Unicode Identifier and Pattern Syntax", May
              2016, <http://unicode.org/reports/tr31/>.

Appendix A.  Change Log

   RFC Editor: Please remove this appendix before publication.

A.1.  Changes from version -00 (2017-06-02) to -01

   o  Many editorial corrections

   o  Addition of new (some replacing prior placeholder) sections,
      especially to the list of issues with the current DNS design and
      notably including Section 3.13.

A.2.  Changes from version -01 (2017-06-06) to -02

   o  Improved the discussion ins several sections, including a somewhat
      muddled description in Section 3.7

   o  Revised the Introduction to make the context for this document
      somewhat more clear.

   o  Added several more references even though still not nearly enough
      to make this document a comprehensive bibliography (which is not
      intended).

   o  Many editorial corrections and a few added references.

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Author's Address

   John C Klensin
   1770 Massachusetts Ave, Ste 322
   Cambridge, MA  02140
   USA

   Phone: +1 617 245 1457
   Email: john-ietf@jck.com

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