Network Working Group                                        M. Mealling
Internet-Draft                                                  Verisign
Expires: April 28, 2002                                 October 28, 2001

   Dynamic Delegation Discovery System (DDDS) Part Two: The Algorithm

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   This document describes the Dynamic Delegation Discovery System
   (DDDS) algorithm for applying dynamically retrieved string
   transformation rules to an application-unique string.  Well-formed
   transformation rules will reflect the delegation of management of
   information associated with the string.  This document is also part
   of a series that is completely specified in "Dynamic Delegation
   Discovery System (DDDS) Part One: The Comprehensive DDDS Standard"
   (RFC WWWW).  It is very important to note that it is impossible to
   read and understand any document in this series without reading the

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The Algorithm  . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.1 Components of a Rule . . . . . . . . . . . . . . . . . . . . .  7
   3.2 Substitution Expression Syntax . . . . . . . . . . . . . . . .  8
   3.3 The Complete Algorithm . . . . . . . . . . . . . . . . . . . . 10
   4.  Specifying An Application  . . . . . . . . . . . . . . . . . . 11
   5.  Specifying A Database  . . . . . . . . . . . . . . . . . . . . 13
   6.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.1 An Automobile Parts Identification System  . . . . . . . . . . 15
   6.2 A Document Identification Service  . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 21
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 22

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1. Introduction

   The Dynamic Delegation Discovery System is used to implement lazy
   binding of strings to data, in order to support dynamically
   configured delegation systems.  The DDDS functions by mapping some
   unique string to data stored within a DDDS Database by iteratively
   applying string transformation rules until a terminal condition is

   This document describes the general DDDS algorithm, not any
   particular application or usage scenario.  The entire series of
   documents is specified in "Dynamic Delegation Discovery System (DDDS)
   Part One:  The Comprehensive DDDS Standard" (RFC WWWW) [1].  It is
   very important to note that it is impossible to read and understand a
   single document in that series without reading the related documents.

   The DDDS's history is an evolution from work done by the Uniform
   Resource Name Working Group.  When Uniform  Resource Names [6] were
   originally formulated there was the desire to locate an authoritative
   server for a URN that (by design) contained no information about
   network locations.  A system was formulated that could use a database
   of rules that could be applied to a URN to find out information about
   specific chunks of syntax.  This system was originally called the
   Resolver Discovery System [7] and only applied to URNs.

   Over time other systems began to apply this same algorithm and
   infrastructure to other, non-URN related, systems (see Section 6 for
   examples of other ways of using the DDDS).  This caused some of the
   underlying assumptions to change and need clarification.  These
   document are an update of those original URN specifications in order
   to allow new applications and rule databases to be developed in a
   standardized manner.

   This document obsoletes RFC 2168 [11] and RFC 2915 [9] as well as
   updates RFC 2276 [7].

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2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

   Application Unique String
      A string that is the initial input to a DDDS application.  The
      lexical structure of this string must imply a unique delegation
      path, which is analyzed and traced by the repeated selection and
      application of Rewrite Rules.

   Rewrite Rule
      A rule that is applied to an Application Unique String to produce
      either a new key to select a new rewrite rule from the rule
      database, or a final result string that is returned to the calling
      application.  Also simply known as a Rule.

   First Well Known Rule
      This is a rewrite rule that is defined by the application and not
      actually in the Rule Database.  It is used to produce the first
      valid key.

   Terminal Rule
      A Rewrite Rule that, when used, yields a string that is the final
      result of the DDDS process, rather than another database key.

      A set of protocols and specifications that specify actual values
      for the various generalized parts of the DDDS algorithm.  An
      Application must define the syntax and semantics of the
      Application Unique String, the First Well Known Rule, and one or
      more Databases that are valid for the Application.

   Rule Database
      Any store of Rules such that a unique key can identify a set of
      Rules that specify the delegation step used when that particular
      Key is used.

      A common rule database may be used to associate different services
      with a given Application Unique String;  e.g.  different protocol
      functions, different operational characteristics, geographic
      segregation, backwards compatibility, etc.  Possible service
      differences might be message receiving services for
      email/fax/voicemail, load balancing over web servers, selection of
      a nearby mirror server, cost vs performance trade-offs, etc.
      These Services are included as part of a Rule to allow the

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      Application to make branching decisions based on the applicability
      of one branch or the other from a Service standpoint.

      Most Applications will require a way for a Rule to signal to the
      Application that some Rules provide particular outcomes that
      others do not; e.g.  different output formats, extensibility
      mechanisms, terminal rule signaling, etc.  Most Datatabases will
      define a Flags field that an Application can use to encode various
      values that express these signals.

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3. The Algorithm

   The DDDS algorithm is based on the concept of Rewrite Rules.  These
   rules are collected into a DDDS Rule Database, and accessed by given
   unique keys.  A given Rule, when applied to an Application Unique
   String, transforms that String into new Key that can be used to
   retrieve a new Rule from the Rule Database.  This new rule is then
   re-applied to the original Application Unique String and the cycle
   repeats itself until a terminating condition is reached.  An
   Application MUST NOT apply a Rule to the output of a previous Rule.
   All Rewrite Rules for all Applications must ALWAYS apply to the exact
   same Application Unique String that the algorithm started with.

   It is a fundamental assumption that the Application Unique String has
   some kind of regular, lexical structure that the rules can be applied
   to.  It is an assumption of the DDDS that the lexical element used to
   make a delegation decision is simple enough to be contained within
   the Application Unique String itself.  The DDDS does not solve the
   case where a delegation decision is made using knowledge contained
   outside the AUS and the Rule (time of day, financial transactions,
   rights management, etc).

   Diagramatically the algorithm looks like this:

          +--------- Application Unique String
          |                 +-----+
          |                 |input|
          |         +-------+     +---------+
          |         | First Well Known Rule |
          |         +-------+      +--------+
          |                 |output|
          |                 +------+
          |                First Key
          |                    |
          |                    |
          |                    +----<--------------<--------------+
          |                    |                                  |
          |                   key     (a DDDS database always     |
          |                 +-----+    takes a key and returns    |
          |                 |input|    a rule)                    ^
          |       +---------+     +------------+                  |
          |       | Lookup key in DDDS Database|                  |
          |       +---------+      +-----------+                  |
          |                 |output|                              |
          |                 +------+                              |
          |                 rule set                              |
          |                    |                                  |

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          |                    |      (the input to a rule        |
          |                 rule set  is the rule and the AUS.    ^
          |                 +-----+   The output is always        |
          +---------------->|input|   either a key or the result) |
            +---------------+     +------------------+            |
            | Apply Rules to Application Unique String|           |
            | until non-empty result are obtained     |           |
            | that meet the applications requirements |           |
            +---------------+      +-----------------+            |
                            |output|                              |
                            +------+                              ^
                              key                                 |
                               |                                  |
                               |                                  |
                               |                                  |
                               |                                  |
                               v                                  |
               +--------------------------------------+           |
               | Was the last matching rule terminal? | No >------+
                              Yes             (if the rule isn't terminal
                               |              then its output is the new
                               |              key which is used to find a
                               |              new rule set)
              | The output of the last rule is the |
              | result desired by the application  |

3.1 Components of a Rule

   A Rule is made up of 4 pieces of information:

   A Priority
      Simply a number used to show which of two otherwise equal rules
      may have precedence.  This allows the database to express rules
      that are equivalent but weighted for load balancing reasons.

   A set of Flags
      Flags are used to specify attributes of the rule that determine if
      this rule is the last one to be applied.  The last rule is called
      the terminal rule and its output should be the intended result for
      the application.  Flags are unique across Applications.  An
      Application may specify that it is using a flag defined by yet
      another Application but it must use that other Application's

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      definition.  One Application cannot re-define a Flag used by
      another Application.  This may mean that a registry of Flags will
      be needed in the future but at this time it is not a requirement.

   A description of Services
      Services are used to specify semantic attributes of a particular
      delegation branch.  There are many cases where two delegation
      branches are identical except that one delegates down to a result
      that provides one set of features while another provides some
      other set.  Features may include operational issues such as load
      balancing, geographically based traffic segregation, degraded but
      backwardly compatibile functions for older clients, etc.  For
      example, two rules may equally apply to a specific delegation
      decision for a string.  One rule can lead to a terminal rule that
      produces information for use in high availability environments
      while another may lead to an archival service that may be slower
      but is more stable over long periods of time.

   A Substitution Expression
      This is the actual string modification part of the rule.  It is a
      combination of a POSIX Extended Regular Expression [8] and a
      replacement string similar to Unix sed-style substitution

3.2 Substitution Expression Syntax

   The character set(s) that the substitution expression is in and can
   act on are dependent both on the Application and on the Database
   being used.  An Application must define what the allowed character
   sets are for the Application Unique String.  A DDDS Database
   specification must define what character sets are required for
   producing its keys and for how the substitution expression itself is
   encoded.  The grammar-required characters below only have meaning
   once a specific character set is defined for the Database and/or

   The syntax of the Substitution Expression part of the rule is a sed-
   style substitution expression.  True sed-style substitution
   expressions are not appropriate for use in this application for a
   variety of reasons, therefore the contents of the regexp field MUST
   follow this grammar:

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      subst-expr   = delim-char  ere  delim-char  repl  delim-char  *flags
      delim-char   = "/" / "!" / <Any octet not in 'POS-DIGIT' or 'flags'>
                         ; All occurances of a delim_char in a subst_expr must
                         ; be the same character.>
      ere          = <POSIX Extended Regular Expression>
      repl         = *(string / backref)
      string       = *(anychar / escapeddelim)
      anychar      = <any character other than delim-char>
      escapeddelim = "\" delim-char
      backref      = "\" POS-DIGIT
      flags        = "i"
      POS-DIGIT    = "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9"

   The result of applying the substitution expression to the String MUST
   result in a key which obeys the rules of the Database (unless of
   course it is a Terminal Rule in which case the output follows the
   rules of the application).  Since it is possible for the regular
   expression to be improperly specified, such that a non-conforming key
   can be constructed, client software SHOULD verify that the result is
   a legal database key before using it.

   Backref expressions in the repl portion of the substitution
   expression are replaced by the (possibly empty) string of characters
   enclosed by '(' and ')' in the ERE portion of the substitution
   expression.  N is a single digit from 1 through 9, inclusive.  It
   specifies the N'th backref expression, the one that begins with the
   N'th '(' and continues to the matching ')'.  For example, the ERE


    has backref expressions:

                         \1  = ABCDEFG
                         \2  = BCDE
                         \3  = C
                         \4  = F
                         \5..\9  = error - no matching subexpression

   The "i" flag indicates that the ERE matching SHALL be performed in a
   case-insensitive fashion.  Furthermore, any backref replacements MAY
   be normalized to lower case when the "i" flag is given.  This flag
   has meaning only when both the Application and Database define a
   character set where case insensitivity is valid.

   The first character in the substitution expression shall be used as
   the character that delimits the components of the substitution
   expression.  There must be exactly three non-escaped occurrences of
   the delimiter character in a substitution expression.  Since escaped

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   occurrences of the delimiter character will be interpreted as
   occurrences of that character, digits MUST NOT be used as delimiters.
   Backrefs would be confused with literal digits were this allowed.
   Similarly, if flags are specified in the substitution expression, the
   delimiter character must not also be a flag character.

3.3 The Complete Algorithm

   The following is the exact DDDS algorithm:

   1.  The First Well Known Rule is applied to the Application Unique
       String which produces a Key

   2.  The Application asks the Database for the ordered set of Rules
       that are bound to that Key (see NOTE below on order details)

   3.  The Substitution Expression for each Rule in the list is applied
       to the Application Unique String until a non-empty string is
       yielded.  The rule that produced the non-empty string is used for
       the next step.  If the list is exhausted without a valid match
       then the application is notified that no valid output was

   4.  If the Service description of the rule does not match the
       Application requirements, go back to step 3 and continue through
       the already retrieved list of rules.

   5.  If the Flags part of the Rule designate that this Rule is NOT
       Terminal, go back to step 2 with the substitution result as the
       new Key.

   6.  Notify the Application that the process has finished and provide
       the Application with the Flags and Services part of the Rule
       along with the output of the last Substitution Expression.

   NOTE: In some applications and/or databases the result set can
   express the case where two or more Rules are considered equal.  These
   Rules are treated as the same Rule, each one possibly having a
   Priority which is used to weight a random selection among the
   equivalent Rules (this allows for Rules to 'load balance'

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4. Specifying An Application

   In order for this algorithm to have any usefulness, a specification
   must be written describing an application and one or more databases.
   In order to specify an application the following pieces of
   information are required:

   Application Unique String:
      This is the only string that the rewrite rules will apply to.  The
      string must have some regular structure and be unique within the
      application such that anyone applying Rules taken from the same
      Database will end up with the same Keys.  For example, the URI
      Resolution application defines the Application Unique String to be
      a URI.

      No application is allowed to define an Application Unique String
      such that the Key obtained by a rewrite rule is treated as the
      Application Unique String for input to a new rule.  This leads to
      sendmail style rewrite rules which are fragile and error prone.
      The one single exception to this is when an Application defines
      some flag or state where the rules for that application are
      suspended and a new DDDS Application or some other arbitrary set
      of rules take over.  If this is the case then, by definition, none
      of these rules apply.  One such case can be found in the URI
      Resolution application which defines the 'p' flag which states
      that the next step is 'protocol specific' and thus outside of the
      scope of DDDS.

   First Well Known Rule:
      This is the first rule that, when applied to the Application
      Unique String, produces the first valid Key.  It can be expressed
      in the same form as a Rule or it can be something more complex.
      For example, the URI Resolution application might specify that the
      rule is that the sequence of characters in the URI up to but not
      including the first colon (the URI scheme) is the first Key.

   Valid Databases:
      The application can define which Databases are valid.  For each
      Database the Application must define how the First Well Known
      Rule's output (the first Key) is turned into something that is
      valid for that Database.  For example, the URI Resolution
      application could use the Domain Name System (DNS) as a Database.
      The operation for turning this first Key into something that was
      valid for the database would be to to turn it into some DNS-valid
      domain-name.  Additionally, for each Database an Application
      defines, it must also specify what the valid character sets are
      that will produce the correct Keys.  In the URI Resolution example
      shown here, the character set of a URI is 7 bit ASCII which

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      matches fairly well with DNS's 8 bit limitation on characters in
      its zone files.

   Expected Output:
      The Application must define what the expected output of the
      Terminal Rule should be.  For example, the URI Resolution
      application is concerned with finding servers that contain
      authoritative data about a given URI.  Thus the output of the
      terminal rule would be information (hosts, ports, protocols, etc)
      that would be used to contact that authoritative server.

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5. Specifying A Database

   Additionally, any Application must have at least one corresponding
   Database from which to retrieve the Rules.  It is important to note
   that a given Database may be used by more than one Application.  If
   this is the case, each rule must be use some combination of its
   Services and/or substitution expression to match only those
   Application Unique Strings for which it is valid.

   A Database specification must include the following pieces of

   General Specification:
      The Database must have a general specification.  This can
      reference other standards (SQL, DNS, etc) or it can fully specify
      a novel database system.  This specification MUST be clear as to
      what allowed character sets exist in order to know in which
      character set the Keys and Rules are encoded.

   Lookup Procedure:
      This specifies how a query is formulated and submitted to the
      database.  In the case of databases that are used for other
      purposes (such as DNS), the specification must be clear as to how
      a query is formulated specifically for the database to be a DDDS
      database.  For example, a DNS based Database must specify which
      Resource Records or Query Types are used.

   Key Format:
      If any operations are needed in order to turn a Key into something
      that is valid for the database then these must be clearly defined.
      For example, in the case of a DNS database, the Keys must be
      constructed as valid domain-names.

   Rule Format:
      The specification for the output format of a rule.

   Rule Insertion Procedure:
      A specification for how a Rule is inserted into the database.
      This can include policy statements about whether or not a Rule is
      allowed to be added.

   Rule Collision Avoidance:
      Since a Database may be used by multiple Applications (ENUM and
      URI Resolution for example), the specification must be clear about
      how rule collisions will be avoided.  There are usually two
      methods for handling this: 1) disallow one key from being valid in
      two different Applications; 2) if 1 isn't possible then write the
      substitution expression such that the regular expression part

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      contains enough of the Application Unique String as part of its
      match to differentiate between the two Applications.

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6. Examples

   The examples given here are for pedagogical purposes only.  They are
   specifically taken from fictious applications that have not been
   specified in any published document.

6.1 An Automobile Parts Identification System

   In this example imagine a system setup where by all automobile
   manufacturers come together and create a standardized part numbering
   system for the various parts (nuts, bolts, frames, instruments, etc)
   that make up the automobile manufacturing and repair process.  The
   problem with such a system is that the auto industry is a very
   distributed system where parts are built by various third parties
   distributed around the world.  In order to find information about a
   given part a system must be able to find out who makes that part and
   contact them about it.

   To facilitate this distributed system the identification number
   assigned to a part is assigned hierarchically such that the first 5
   digits make up a parts manufacturer ID number.  The next 3 digits are
   an auto line identifier (Ford, Toyota, etc).  The rest of the digits
   are assigned by the parts manufacturer according to rules that the
   manufacturer decides.

   The auto industry decides to use the DDDS to create a distributed
   information retrieval system that routes queries to the actual owner
   of the data.  The industry specifies a database and a query syntax
   for retrieving rewrite rules (the APIDA Network) and then specifies
   the Auto Parts Identification DDDS Application (APIDA).

   The APIDA specification would define the following:

   o  Application Unique String: the part number

   o  First Well Known Rule: take the first 5 digits (the manufacturers
      ID number) and use that as the Key

   o  Valid Databases: The APIDA Network

   o  Expected Output: EDIFAC information about the part

   The APIDA Network Database specification would define the following:

   o  General Specification: a network of EDI enabled databases and
      services that, when given a subcomponent of a part number will
      return an XML encoded rewrite rule

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   o  Lookup Procedure: following normal APIDA Network protocols, ask
      the network for a rewrite rule for the Key.

   o  Key Format: no conversion is required

   o  Rule Format: see APIDA Network documentation for the XML DTD

   o  Rule Insertion Procedure: determined by the authority that has
      control over each section of the part number.  I.e.  in order to
      get a manufacturer ID you must be a member of the Auto Parts
      Manufacturers Association

   In order to illustrate how the system would work, imagine the part
   number "4747301AB7D".  The system would take the first 5 digits,
   '47473' and ask the network for that Rewrite Rule.  This Rule would
   be provided by the parts manufacturers database and would allow the
   manufacturer to either further sub-delegate the space or point the
   querier directly at the EDIFAC information in the system.

   In this example let's suppose that the manufacturer returns a Rule
   that states that the next 3 digits should be used as part of a query
   to their service in order to find a new Rule.  This new Rule would
   allow the parts manufacturer to further delegate the query to their
   parts factories for each auto line.  In our example part number the
   number '01A' denotes the Toyota line of cars.  The Rule that the
   manufacturer returns further delegates the query to a supply house in
   Japan.  This rule also denotes that this Rule is terminal and thus
   the result of this last query will be the actual information about
   the part.

6.2 A Document Identification Service

   This example is very similar to the last since the documents in this
   system can simply be thought of as the auto part in the last example.
   The difference here is that the information about the document is
   kept very close to the author (usually on their desktop).  Thus there
   is the probability that the number of delegations can be very deep.
   Also, in order to keep from having a large flat space of authors, the
   authors are organized by organizations and departments.

   Let's suppose that the Application Unique String in this example
   looks like the following:


   The Application specification would look like this:

   o  Application Unique String: the Document ID string given above

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   o  First Well Known Rule: the characters up to but not including the
      first '-' is treated as the first Key.

   o  Valid Databases: the DIS LDAP Directory

   o  Expected Output: a record from an LDAP server containing
      bibliographic information about the document in XML

   The Database specification for the DIS LDAP Directory would look like

   o  General Specification: the Database uses the LDAP directory
      service.  Each LDAP server has a record that contains the Rewrite
      Rule.  Rules refer to other LDAP servers using the LDAP URL

   o  Lookup Procedure: using standard LDAP queries, the client asks the
      LDAP server for information about the Key.

   o  Key Format: no conversion is necessary.

   o  Rule Format: See the LDAP Rewrite Rule specification

   o  Rule Insertion Procedure: See the procedures published by the
      entity that has authority over that section of the DIS tree.  The
      first section, the organization, is owned by the DIS Agency.

   In this example, the first lookup is for the organization's Rule.  At
   that point the organization may point the client directly at some
   large, organization wide database that contains the expected output.
   Other organizations may decentralize this process so that Rules end
   up delegating the query all the way down to the authors document
   management environment of choice.

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7. Security Considerations

   This document simply defines the DDDS algorithm and thus, by itself,
   does not imply any security issues.  It is when this algorithm is
   coupled with a Database and an Application that security
   considerations can be known well enough to enumerate them beyond
   simply saying that dynamic delegation points are a possible point of

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8. IANA Considerations

   This document does not create any requirements on the IANA.  Database
   and Application specifications may have considerable requirements but
   they cannot be enumerated here.

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   [1]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         One: The Comprehensive DDDS Standard", RFC WWWW, draft-ietf-
         urn-ddds-toc-00.txt (work in progress), October 2001.

   [2]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Two: The Algorithm", RFC XXXX, draft-ietf-urn-ddds-05.txt (work
         in progress), May 2000.

   [3]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Three: The DNS Database", RFC ZZZZ, draft-ietf-urn-dns-ddds-
         database-07.txt (work in progress), May 2000.

   [4]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Four: The URI Resolution Application", RFC YYYY, draft-ietf-
         urn-uri-res-ddds-05.txt (work in progress), October 2000.

   [5]   Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
         Five: URI.ARPA Assignment Procedures", RFC VVVV, draft-ietf-
         urn-net-procedures-09.txt (work in progress), October 2001.

   [6]   Moats, R., "URN Syntax", RFC 2141, May 1997.

   [7]   Sollins, K., "Architectural Principles of Uniform Resource Name
         Resolution", RFC 2276, January 1998.

   [8]   The Institute of Electrical and Electronics Engineers, "IEEE
         Standard for Information Technology - Portable Operating System
         Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1)", IEEE
         Std 1003.2-1992, ISBN 1-55937-255-9, January 1993.

   [9]   Mealling, M. and R. Daniel, "The Naming Authority Pointer
         (NAPTR) DNS Resource Record", RFC 2915, August 2000.

   [10]  Faltstrom, P., "E.164 number and DNS", RFC 2916, September

   [11]  Daniel, R. and M. Mealling, "Resolution of Uniform Resource
         Identifiers using the Domain Name System", RFC 2168, June 1997.

Mealling                 Expires April 28, 2002                [Page 20]

Internet-Draft                    DDDS                      October 2001

Author's Address

   Michael Mealling
   505 Huntmar Park Drive
   Herndon, VA  22070

   Phone: +1 770 921-2251

Mealling                 Expires April 28, 2002                [Page 21]

Internet-Draft                    DDDS                      October 2001

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Mealling                 Expires April 28, 2002                [Page 22]