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Versions: 00 01

Network Working Group                                        M. Mealling
Internet-Draft                                   Network Solutions, Inc.
Expires: December 24, 1999                                     R. Daniel
                                                        DATAFUSION, Inc.
                                                           June 25, 1999

    Resolution of Uniform Resource Identifiers using the Domain Name
                                 System
                       draft-ietf-urn-dns-rds-01

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on December 24, 1999.

Abstract

   The architectural principles laid out in RFC2276[9] defines the
   concept of a "resolver discovery service". This document describes
   an immediately-deployable RDS. It is implemented by a new DNS
   Resource Record, NAPTR (Naming Authority PoinTeR)[10], that provides
   a method for encoding incrementally discovered rules within DNS. By
   using these incrementally discovered rules to re-map parts of a URI,
   we can change the host that is contacted to resolve a URI.  This
   will allow a more graceful handling of URLs over long time periods,
   and forms the foundation for a new proposal for Uniform Resource
   Names.

   In addition to locating resolvers, the NAPTR provides for other
   naming systems to be grandfathered into the URN world, provides
   independence between the name assignment system and the resolution
   protocol system, and allows multiple services (Identifier to
   Location, Identifier to Description, Identifier to Resource, ...) to

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   be offered.  In conjunction with the SRV RR, the NAPTR record allows
   those services to be replicated for the purposes of fault tolerance
   and load balancing.

Copyright Notice

   Copyright (C) The Internet Society (1999). All Rights Reserved.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.    Overview of the NAPTR Record . . . . . . . . . . . . . . . .  5
   4.    The Distinction between URNs and URLs  . . . . . . . . . . .  7
   5.    The DNS RDS Algorithm  . . . . . . . . . . . . . . . . . . .  8
   5.1   The First Known Key  . . . . . . . . . . . . . . . . . . . .  8
   5.1.1 URI Example  . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.1.2 URN Example  . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.2   Services . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.3   Protocols  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   6.    Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.1   Example 1  . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.2   Example 2  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.3   Example 3  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.    Notes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 16
         References . . . . . . . . . . . . . . . . . . . . . . . . . 17
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17
   A.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 19
   B.    Security Considerations  . . . . . . . . . . . . . . . . . . 20
   C.    Appendix A -- Psuedo Code  . . . . . . . . . . . . . . . . . 21

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1. Introduction

   Uniform Resource Locators have been a significant advance in
   retrieving Internet-accessible resources. However, their  brittle
   nature over time has been recognized for several years. The Uniform
   Resource Identifier working group proposed the development of
   Uniform Resource Names to serve as persistent, location-independent
   identifiers for Internet resources in order to overcome most of the
   problems with URLs. RFC-1737[1] sets forth requirements on URNs.

   During the lifetime of the URI-WG, a number of URN proposals were
   generated. The developers of several of those proposals met in a
   series of meetings, resulting in a compromise known as the Knoxville
   framework.  The major principle behind the Knoxville framework is
   that the resolution system must be separate from the way names are
   assigned. This is in marked contrast to most URLs, which identify
   the host to contact and the protocol to use. Readers are referred to
   [2] for background on the Knoxville framework and for additional
   information on the context and purpose of this proposal.

   Separating the way names are resolved from the way they are
   constructed provides several benefits. It allows multiple naming
   approaches and resolution approaches to compete, as it allows
   different protocols and resolvers to be used. There is just one
   problem with such a separation - how do we resolve a name when it
   can't give us directions to its resolver?

   For the short term, DNS is the obvious candidate for the resolution
   framework, since it is widely deployed and understood. However, it
   is not appropriate to use DNS to maintain information on a
   per-resource basis. First of all, DNS was never intended to handle
   that many records. Second, the limited record size is inappropriate
   for catalog information. Third, domain names are not appropriate as
   URNs.

   Therefore our approach is to use DNS to locate "resolvers" that can
   provide information on individual resources, potentially including
   the resource itself. To accomplish this, we "rewrite" the URI into a
   domain name following the rules found in NAPTR records. Rewrite
   rules provide considerable power, which is important when trying to
   meet the goals listed above. However, collections of rules can
   become difficult to understand. To lessen this problem, the NAPTR
   rules are *always* applied to the original URI, *never* to the
   output of previous rules.

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2. Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC 2119.

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3. Overview of the NAPTR Record

   The NAPTR record is defined in RFCXXXX[10]. To summarize, the key
   fields in the NAPTR RR are Order, Preference, Service, Flags,
   Regexp, and Replacement:

   o  The order field specifies the order in which records MUST be
      processed when multiple NAPTR records are returned in response to
      a single query.  A naming authority may have delegated a portion
      of its namespace to another agency. Evaluating the NAPTR records
      in the correct order is necessary for delegation to work
      properly.

   o  The preference field specifies the order in which records SHOULD
      be processed when multiple NAPTR records have the same value of
      "order".  This field lets a service provider specify the order in
      which resolvers are contacted, so that more capable machines are
      contacted in preference to less capable ones.

   o  The service field specifies the resolution protocol and
      resolution service(s) that will be available if the rewrite
      specified by the regexp or replacement fields is applied.
      Resolution protocols are the protocols used to talk with a
      resolver. They will be specified in other documents, such as [5].
      Resolution services are operations such as I2R (URI to Resource),
      I2L (URI to URL), I2C (URI to URC), etc.  These are specified in
      the URI Resolution Services document[6], and their behavior in a
      particular resolution protocol will be given in the specification
      for that protocol (see [5] for a concrete example).

   o  The flags field contains modifiers that affect what happens in
      the next DNS lookup, typically for optimizing the process. Flags
      may also affect the interpretation of the other fields in the
      record, therefore, clients MUST skip NAPTR records which contain
      an unknown flag value.

   o  The regexp field is one of two fields used for the rewrite rules,
      and is the core concept of the NAPTR record. The regexp field is
      a String containing a sed-like substitution expression. (The
      actual grammar for the substitution expressions is given later in
      this draft). The substitution expression is applied to the
      original URN to determine the next domain name to be queried. The
      regexp field should be used when the domain name to be generated
      is conditional on information in the URI. If the next domain name
      is always known, which is anticipated to be a common occurrence,
      the replacement field should be used instead.

   o  The replacement field is the other field that may be used for the
      rewrite rule. It is an optimization of the rewrite process for

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      the case where the next domain name is fixed instead of being
      conditional on the content of the URI. The replacement field is a
      domain name (subject to compression if a DNS sender knows that a
      given recipient is able to decompress names in this RR type's
      RDATA field). If the rewrite is more complex than a simple
      substitution of a domain name, the replacement field should be
      set to . and the regexp field used.

   Note that the client applies all the substitutions and performs all
   lookups, they are not performed in the DNS servers. Note also that
   it is the belief of the developers of this document that regexps
   should rarely be used. The replacement field seems adequate for the
   vast majority of situations. Regexps are only necessary when
   portions of a namespace are to be delegated to different resolvers.
   Finally, note that the regexp and replacement fields are, at
   present, mutually exclusive. However, developers of client software
   should be aware that a new flag might be defined which requires
   values in both fields.

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4. The Distinction between URNs and URLs

   From the point of view of this system, there is no theoretical
   difference between resolving URIs in the general case and URNs in
   the specific case. Operationally however, there is a difference that
   stems from the unknown case of URI resolution not becoming
   widespread. If URN resolution is collapsed into generic URI
   resolution, URNs may suffer by the lack of adoption of URI
   resolution. The technically correct solution however should
   discourage such a case.

   The solution is to allow for shortcutting for URN resolution. In the
   following specification generic URI resolution starts by inserting
   rules for known URI shemes into the 'uri.net' registry. For URN
   resolution one of the rules would be for the 'urn' URI scheme. This
   rule would simply delegate to the 'urn.net' zone for additional
   NAPTRS based on the URN namespace.

   Since this rule is the basis for the entire URN RDS, it can be
   shortcutted by simply starting URN resolution at the 'urn.net'
   registry. This the distinction between the 'uri.net' and 'urn.net'
   well known keys seen below.

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5. The DNS RDS Algorithm

   Since the general RDS framework was the basis for the original NAPTR
   algorithm, the two match very well. The only pieces missing from the
   general NAPTR specification are the original key, protocols and
   services.

5.1 The First Known Key

   In the generic URI case, the first known key is created by taking
   the URI scheme and appending 'uri.net' to the end. In the specific,
   shortcutted URN case, the first known key is created by taking the
   Namespace Identifier and appending 'urn.net' to the end.

5.1.1 URI Example

      http://www.foo.com/ would have a first known key of
      'http.uri.net'.

5.1.2 URN Example

      urn:foo:12345 would have a first known key of 'foo.urn.net'.

5.2 Services

   The services that make sense for URI resolution are generic for both
   URI and URN resolution since the input value types itself based on
   the URI scheme. Some valid services are defined in RFCXXXX
   (draft-ietf-urn-resolution-services-07.txt).

   Examples of some of these services are:

   I2L: given a URI return one URL that identifies a location where the
      original URI can be found

   I2Ls: given a URI return one or more URLs that identify multiple
      locations where the original URI can be found

   I2R: given a URI return one instance of the resource identified by
      that URI.

   I2Rs: given a URI return one or more instances of the resources
      identified by that URI.

   I2C: given a URI return one instance of a description of that
      resource.

   I2N: given a URI return one URN that names the resource (Caution:
      equality with respect to URNs is non-trivial. See [9] for

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      examples of why.)

5.3 Protocols

   The protocols used in the Services field are currently limited to
   THTTP[5]. Simply specifying any protocol in the services field is
   insufficient since there are additional semantics surrounding URI
   resolution that are not specified within the protocols.

   For example, if Z39.50 were to be specified as a valid protocol it
   would have to define how it would encode requests for specific
   services, how the URI is encoded, and what information is returned.

   Thus, for this document the only valid value used in the examples is
   'thttp'.

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6. Examples

6.1 Example 1

   Consider a URN that uses the hypothetical DUNS namespace. DUNS
   numbers are identifiers for approximately 30 million registered
   businesses around the world, assigned and maintained by Dunn and
   Bradstreet. The URN might look like:

                      urn:duns:002372413:annual-report-1997

   The first step in the resolution process is to find out about the
   DUNS namespace. The namespace identifier[3], "duns", is extracted
   from the URN, prepended to urn.net, and the NAPTRs for duns.urn.net
   looked up. It might return records of the form:

   duns.urn.net.
   ;;      order pref flags service          regexp        replacement
    IN NAPTR 100  10  "s" "dunslink+I2L+I2C" ""  dunslink.udp.isi.dandb.com.
    IN NAPTR 100  20  "s" "rcds+I2C"         ""  rcds.udp.isi.dandb.com.
    IN NAPTR 100  30  "s" "thttp+I2L+I2C+I2R" ""  thttp.tcp.isi.dandb.com.

   The order field contains equal values, indicating that no name
   delegation order has to be followed. The preference field indicates
   that the provider would like clients to use the special dunslink
   protocol, followed by the RCDS protocol, and that HTTP is offered as
   a last resort. All the records specify the "s" flag, which will be
   explained momentarily.  The service fields say that if we speak
   dunslink, we will be able to issue either the I2L or I2C requests to
   obtain a URL or a URC (description) of the resource. The Resource
   Cataloging and Distribution Service (RCDS)[7] could be used to get a
   URC for the resource, while HTTP could be used to get a URL, URC, or
   the resource itself.  All the records supply the next domain name to
   query, none of them need to be rewritten with the aid of regular
   expressions.

   The general case might require multiple NAPTR rewrites to locate a
   resolver, but eventually we will come to the "terminal NAPTR". Once
   we have the terminal NAPTR, our next probe into the DNS will be for
   a SRV or A record instead of another NAPTR. Rather than probing for
   a non-existent NAPTR record to terminate the loop, the flags field
   is used to indicate a terminal lookup. If it has a value of "s", the
   next lookup should be for SRV RRs, "a" denotes that A records should
   sought.  A "p" flag is also provided to indicate that the next
   action is Protocol-specific, but that looking up another NAPTR will
   not be part of it.

   Since our example RR specified the "s" flag, it was terminal.
   Assuming our client does not know the dunslink protocol, our next

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   action is to lookup SRV RRs for rcds.udp.isi.dandb.com, which will
   tell us hosts that can provide the necessary resolution service.
   That lookup might return:

      ;;                          Pref Weight Port Target
      _rcds._udp.isi.dandb.com IN SRV 0    0    1000 defduns.isi.dandb.com.
                               IN SRV 0    0    1000 dbmirror.com.au.
                               IN SRV 0    0    1000 ukmirror.com.uk.

    telling us three hosts that could actually do the resolution, and
   giving us the port we should use to talk to their RCDS server.  (The
   reader is referred to the SRV specification[4] for the
   interpretation of the fields above).

   There is opportunity for significant optimization here. We can
   return the SRV records as additional information for terminal NAPTRs
   (and the A records as additional information for those SRVs). While
   this recursive provision of additional information is not explicitly
   blessed in the DNS specifications, it is not forbidden, and BIND
   does take advantage of it [8]. This is a significant optimization.
   In conjunction with a long TTL for *.urn.net records, the average
   number of probes to DNS for resolving DUNS URNs would approach one.
   Therefore, DNS server implementors SHOULD provide additional
   information with NAPTR responses. The additional information will be
   either SRV or A records.  If SRV records are available, their A
   records should be provided as recursive additional information.

   Note that the example NAPTR records above are intended to represent
   the reply the client will see. They are not quite identical to what
   the domain administrator would put into the zone files. For one
   thing, the administrator should supply the trailing '.' character on
   any FQDNs.

   Also note that there could have been an additional first step where
   the URN was resolved as a generic URI by looking up urn.uri.net. The
   resulting rule would have specified that the NID be extracted from
   the URN and 'urn.net' appended to it resulting in the new key
   'duns.urn.net' which is the first step from above.

6.2 Example 2

   Consider a URN namespace based on MIME Content-Ids. The URN might
   look like this:

                    urn:cid:199606121851.1@mordred.gatech.edu

    (Note that this example is chosen for pedagogical purposes, and
   does not conform to the CID URL scheme.)

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   The first step in the resolution process is to find out about the
   CID namespace. The namespace identifier, cid, is extracted from the
   URN, prepended to urn.net, and the NAPTR for cid.urn.net looked up.
   It might return records of the form:

    cid.urn.net.
     ;;       order pref flags service        regexp           replacement
      IN NAPTR 100   10   ""  ""  "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i"    .

   We have only one NAPTR response, so ordering the responses is not a
   problem.  The replacement field is empty, so we check the regexp
   field and use the pattern provided there. We apply that regexp to
   the entire URN to see if it matches, which it does.  The \2 part of
   the substitution expression returns the string "gatech.edu". Since
   the flags field does not contain "s" or "a", the lookup is not
   terminal and our next probe to DNS is for more NAPTR records:
   lookup(query=NAPTR, "gatech.edu").

   Note that the rule does not extract the full domain name from the
   CID, instead it assumes the CID comes from a host and extracts its
   domain.  While all hosts, such as mordred, could have their very own
   NAPTR, maintaining those records for all the machines at a site as
   large as Georgia Tech would be an intolerable burden. Wildcards are
   not appropriate here since they only return results when there is no
   exactly matching names already in the system.

   The record returned from the query on "gatech.edu" might look like:

     gatech.edu.
     ;;       order pref flags service           regexp  replacement
     IN NAPTR 100  50  "s"  "z3950+I2L+I2C"     ""
_z3950._tcp.gatech.edu.
     IN NAPTR 100  50  "s"  "rcds+I2C"          ""    _rcds._udp.gatech.edu.
     IN NAPTR 100  50  "s"  "thttp+I2L+I2C+I2R" ""
_thttp._tcp.gatech.edu.

   Continuing with our example, we note that the values of the order
   and preference fields are equal in all records, so the client is
   free to pick any record. The flags field tells us that these are the
   last NAPTR patterns we should see, and after the rewrite (a simple
   replacement in this case) we should look up SRV records to get
   information on the hosts that can provide the necessary service.

   Assuming we prefer the Z39.50 protocol, our lookup might return:

      ;;                        Pref Weight   Port Target
      _z3950._tcp.gatech.edu IN SRV 0    0      1000 z3950.gatech.edu.
                             IN SRV 0    0      1000 z3950.cc.gatech.edu.
                             IN SRV 0    0      1000 z3950.uga.edu.

    telling us three hosts that could actually do the resolution, and

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   giving us the port we should use to talk to their Z39.50 server.

   Recall that the regular expression used \2 to extract a domain name
   from the CID, and \. for matching the literal '.' characters
   seperating the domain name components. Since '\' is the escape
   character, literal occurances of a backslash must be escaped by
   another backslash. For the case of the cid.urn.net record above, the
   regular expression entered into the zone file should be
   "/urn:cid:.+@([^\\.]+\\.)(.*)$/\\2/i".  When the client code
   actually receives the record, the pattern will have been converted
   to "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i".

6.3 Example 3

   Even if URN systems were in place now, there would still be a
   tremendous number of URLs.  It should be possible to develop a URN
   resolution system that can also provide location independence for
   those URLs.  This is related to the requirement in [1] to be able to
   grandfather in names from other naming systems, such as ISO Formal
   Public Identifiers, Library of Congress Call Numbers, ISBNs, ISSNs,
   etc.

   The NAPTR RR could also be used for URLs that have already been
   assigned.  Assume we have the URL for a very popular piece of
   software that the publisher wishes to mirror at multiple sites
   around the world:

           http://www.foo.com/software/latest-beta.exe

   We extract the prefix, "http", and lookup NAPTR records for
   http.uri.net. This might return a record of the form:

      http.uri.net. IN NAPTR
      ;;  order   pref flags service      regexp             replacement
           100     90   ""      ""   "!http://([^/:]+)!\1!i"       .

   This expression returns everything after the first double slash and
   before the next slash or colon. (We use the '!' character to delimit
   the parts of the substitution expression. Otherwise we would have to
   use backslashes to escape the forward slashes, and would have a
   regexp in the zone file that looked like
   "/http:\\/\\/([^\\/:]+)/\\1/i".).

   Applying this pattern to the URL extracts "www.foo.com". Looking up
   NAPTR records for that might return:

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      www.foo.com.
      ;;       order pref flags   service  regexp     replacement
       IN NAPTR 100  100  "s"   "thttp+L2R"   ""    _thttp._tcp.foo.com.
       IN NAPTR 100  100  "s"   "ftp+L2R"    ""    _ftp._tcp.foo.com.

   Looking up SRV records for thttp.tcp.foo.com would return
   information on the hosts that foo.com has designated to be its
   mirror sites. The client can then pick one for the user.

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7. Notes

   o  Registration procedures for the urn.net and uri.net DNS zones is
      specified in "Assignment Procedures for the URI Resolution using
      DNS (RFC2168)"[11]

   o  A client MUST process multiple NAPTR records in the order
      specified by the "order" field, it MUST NOT simply use the first
      record that provides a known protocol and service combination.

   o  If a record at a particular order matches the URI, but the client
      doesn't know the specified protocol and service, the client
      SHOULD continue to examine records that have the same order. The
      client MUST NOT consider records with a higher value of order.
      This is necessary to make delegation of portions of the namespace
      work.  The order field is what lets site administrators say "all
      requests for URIs matching pattern x go to server 1, all others
      go to server 2".  A match is defined as:

      1.  The NAPTR provides a replacement domain name

      2.  or the regular expression matches the URN

   o  When multiple RRs have the same "order", the client should use
      the value of the preference field to select the next NAPTR to
      consider. However, because of preferred protocols or services,
      estimates of network distance and bandwidth, etc. clients may use
      different criteria to sort the records.

   o  If the lookup after a rewrite fails, clients are strongly
      encouraged to report a failure, rather than backing up to pursue
      other rewrite paths.

   o  When a namespace is to be delegated among a set of resolvers,
      regexps must be used. Each regexp appears in a separate NAPTR RR.
      Administrators should do as little delegation as possible,
      because of limitations on the size of DNS responses.

   o  Note that SRV RRs impose additional requirements on clients.

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8. Acknowledgments

   The editors would like to thank Keith Moore for all his
   consultations during the development of this draft. We would also
   like to thank Paul Vixie for his assistance in debugging our
   implementation, and his answers on our questions. Finally, we would
   like to acknowledge our enormous intellectual debt to the
   participants in the Knoxville series of meetings, as well as to the
   participants in the URI and URN working groups.

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References

   [1]  Sollins, S., Masinter, L., "Functional Requirements for Uniform
        Resource Names", RFC 1737, December 1994.

   [2]  Arms, B., "The URN Implementors, Uniform Resource Names: A
        Progress Report", D-Lib Magazine, February 1996.

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

   [4]  Eastlake, D., Gulbrandsen, A., "A DNS RR for specifying the
        location of services (DNS SRV)", January 1999.

   [5]  Daniel, R., "A Trivial Convention for using HTTP in URN
        Resolution", RFC 2169, June 1997.

   [6]  Mealling, M., Daniel, R., "URI Resolution Services Necessary
        for URN Resolution", RFC 2483, January 1999.

   [7]  Moore, K., Browne, S., Cox, J., Gettler, J., "Resource
        Cataloging and Distribution System", Technical Report
        CS-97-346, December 1996.

   [8]  Vixie, P., "Personal communication", January 1996.

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

   [10]  Mealling, M., Daniel, R., "The Naming Authority Pointer
         (NAPTR) DNS Resource Record", draft-ietf-urn-naptr-rr-03.txt,
         July 1999.

   [11]  Mealling, M., "Assignment Procedures for the URI Resolution
         using DNS (RFC2168)",
         draft-ietf-urn-urn.net-procedures-01.txt, November 1998.

Authors' Addresses

   Michael Mealling
   Network Solutions, Inc.
   505 Huntmar Park Drive
   Herndon, VA  22070
   US

   Phone: +1 770 935 5492
   EMail: michaelm@netsol.com
   URI:   http://www.netsol.com

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   Ron Daniel
   DATAFUSION, Inc.
   139 Townsend Street, Ste. 100
   San Francisco, CA  94107
   US

   Phone: +1 415 222 0100
   EMail: rdaniel@datafusion.net
   URI:   http://www.datafusion.net

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Appendix A. IANA Considerations

   The use of the "urn.net" and "uri.net" zones requires registration
   policies and procedures to be followed and for the operation of
   those DNS zones to be maintained. These policies and procedures are
   spelled out in a "Assignment Procedures for the URI Resolution
   using DNS (RFC2168)"[11]. The operation of those zones imposes
   operational and adminstrative responsibilities on the IANA.

   The registration methods used for specifying values for the Services
   (both protocols and services) and Flags fields that are specific to
   URI resolution is for a specification to be published as an RFC and
   approved by the IESG.

   The registration policies for URLs and URNs are also specified
   elsewhere and thus those impacts on the IANA are spelled out there.

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Appendix B. Security Considerations

   The use of "urn.net" and "uri.net" as the registry for namespaces is
   subject to denial of service attacks, as well as other DNS spoofing
   attacks. The interactions with DNSSEC are currently being studied.
   It is expected that NAPTR records will be signed with SIG records
   once the DNSSEC work is deployed.

   The rewrite rules make identifiers from other namespaces subject to
   the same attacks as normal domain names. Since they have not been
   easily resolvable before, this may or may not be considered a
   problem.

   Regular expressions should be checked for sanity, not blindly passed
   to something like PERL.

   This document has discussed a way of locating a resolver, but has
   not discussed any detail of how the communication with the resolver
   takes place. There are significant security considerations attached
   to the communication with a resolver. Those considerations are
   outside the scope of this document, and must be addressed by the
   specifications for particular resolver communication protocols.

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Appendix C. Appendix A -- Psuedo Code

   For the edification of implementers, pseudocode for a client routine
   using NAPTRs is given below. This code is provided merely as a
   convience, it does not have any weight as a standard way to process
   NAPTR records. Also, as is the case with pseudocode, it has never
   been executed and may contain logical errors. You have been warned.

       //
       // findResolver(URN)
       // Given a URN, find a host that can resolve it.
       //
       findResolver(string URN) {
         // prepend prefix to urn.net
         sprintf(key, "%s.urn.net", extractNS(URN));
         do {
           rewrite_flag = false;
           terminal = false;
           if (key has been seen) {
             quit with a loop detected error
           }
           add key to list of "seens"
           records = lookup(type=NAPTR, key); // get all NAPTR RRs for 'key'

           discard any records with an unknown value in the "flags" field.
           sort NAPTR records by "order" field and "preference" field
               (with "order" being more significant than "preference").
           n_naptrs = number of NAPTR records in response.
           curr_order = records[0].order;
           max_order = records[n_naptrs-1].order;

           // Process current batch of NAPTRs according to "order" field.
           for (j=0; j < n_naptrs && records[j].order <= max_order; j++) {
             if (unknown_flag) // skip this record and go to next one
                continue;
             newkey = rewrite(URN, naptr[j].replacement, naptr[j].regexp);
             if (!newkey) // Skip to next record if the rewrite didn't
                match continue;
             // We did do a rewrite, shrink max_order to current value
             // so that delegation works properly
             max_order = naptr[j].order;
             // Will we know what to do with the protocol and services
             // specified in the NAPTR? If not, try next record.
             if(!isKnownProto(naptr[j].services)) {
               continue;
             }
             if(!isKnownService(naptr[j].services)) {
               continue;
             }

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             // At this point we have a successful rewrite and we will
             // know how to speak the protocol and request a known
             // resolution service. Before we do the next lookup, check
             // some optimization possibilities.
             if (strcasecmp(flags, "S")
              || strcasecmp(flags, "P"))
              || strcasecmp(flags, "A")) {
                terminal = true;
                services = naptr[j].services;
                addnl = any SRV and/or A records returned as additional
                        info for naptr[j].
             }
             key = newkey;
             rewriteflag = true;
             break;
           }
         } while (rewriteflag && !terminal);

         // Did we not find our way to a resolver?
         if (!rewrite_flag) {
            report an error
            return NULL;
         }

         // Leave rest to another protocol?
         if (strcasecmp(flags, "P")) {
            return key as host to talk to;
         }

         // If not, keep plugging
         if (!addnl) { // No SRVs came in as additional info, look them up
           srvs = lookup(type=SRV, key);
         }

         sort SRV records by preference, weight, ...
         foreach (SRV record) { // in order of preference
           try contacting srv[j].target using the protocol and one of the
               resolution service requests from the "services" field of the
               last NAPTR record.
           if (successful)
             return (target, protocol, service);
             // Actually we would probably return a result, but this
             // code was supposed to just tell us a good host to talk to.
         }
         die with an "unable to find a host" error;
       }

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Full Copyright Statement

   Copyright (C) The Internet Society (1999).  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 implmentation 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
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