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Network Working Group                                        M. Mealling
Request for Comments: 3404                                      VeriSign
Obsoletes: 2915, 2168                                       October 2002
Category: Standards Track

              Dynamic Delegation Discovery System (DDDS)
           Part Four: The Uniform Resource Identifiers (URI)
                         Resolution Application

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This document describes a specification for taking Uniform Resource
   Identifiers (URI) and locating an authoritative server for
   information about that URI.  The method used to locate that
   authoritative server is the Dynamic Delegation Discovery System.

   This document is part of a series that is specified in "Dynamic
   Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS"
   (RFC 3401).  It is very important to note that it is impossible to
   read and understand any document in this series without reading the

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.    The Distinction between URNs and URIs  . . . . . . . . . . .  3
   4.    The URI and URN Resolution Application Specifications  . . .  4
   4.1   Application Unique String  . . . . . . . . . . . . . . . . .  4
   4.2   First Well Known Rule  . . . . . . . . . . . . . . . . . . .  4
   4.3   Flags  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.4   Services Parameters  . . . . . . . . . . . . . . . . . . . .  5
   4.4.1 Services . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.4.2 protocols  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.4.3 Applicability of Services  . . . . . . . . . . . . . . . . .  7

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   4.5   Valid Databases  . . . . . . . . . . . . . . . . . . . . . .  7
   5.    Examples . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   5.1   An example using a URN . . . . . . . . . . . . . . . . . . .  8
   5.2   CID URI Scheme Example . . . . . . . . . . . . . . . . . . .  9
   5.3   Resolving an HTTP URI Scheme . . . . . . . . . . . . . . . . 11
   6.    Notes  . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 12
   8.    Security Considerations  . . . . . . . . . . . . . . . . . . 12
   9.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 13
         References . . . . . . . . . . . . . . . . . . . . . . . . . 13
         Appendix A: Pseudo Code  . . . . . . . . . . . . . . . . . . 15
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 17
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 18

1. Introduction

   The Dynamic Delegation Discovery System (DDDS) 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 a DDDS Application for resolving Uniform
   Resource Identifiers (URI).  It does not define the DDDS Algorithm or
   a Database.  The entire series of documents that do so are specified
   in "Dynamic Delegation Discovery System (DDDS) Part One: The
   Comprehensive DDDS" (RFC 3401) [1].  It is very important to note
   that it is impossible to read and understand any document in that
   series without reading the related documents.

   Uniform Resource Identifiers (URI) 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 (URN) [8] to serve as persistent, location-independent
   identifiers for Internet resources in order to overcome most of the
   problems with URIs.  RFC 1737 [6] 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 URIs, which identify
   the host to contact and the protocol to use.  Readers are referred to
   [7] for background on the Knoxville framework and for additional
   information on the context and purpose of this proposal.

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   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, the Domain Name System (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 the DDDS to locate "resolvers" that
   can provide information on individual resources, potentially
   including the resource itself.  To accomplish this, we "rewrite" the
   URI into a Key following the rules found in the DDDS.  This document
   describes URI Resolution as an application of the DDDS and specifies
   the use of at least one Database based on DNS.

2. Terminology

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

   All capitalized terms are taken from the vocabulary found in the DDDS
   algorithm specification found in RFC 3403 [3].

3. The Distinction Between URNs and URIs

   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 URI resolution possibly not becoming of widespread use.
   If URN resolution is collapsed into generic URI resolution, URNs may
   suffer by the lack of adoption of URI resolution.

   The solution is to allow for shortcutting for URN resolution.  In the
   following specification generic URI resolution starts by inserting
   rules for known URI schemes into the 'uri.arpa.' registry.  For the
   'URN:' URI scheme, one of the rules found in 'uri.arpa.' would be for
   the 'urn' URI scheme.  This rule would simply delegate to the
   'urn.arpa.' zone for additional NAPTRs based on the URN namespace.
   Essentially, the URI Resolution Rewrite Rule for 'URN:' is the URN
   Resolution Application's First Well Known Rule.

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   Therefore, this document specifies two DDDS Applications.  One is for
   URI Resolution and the other is for URN Resolution.  Both are
   technically identical but by separating the two URN Resolution can
   still proceed without the dependency.

4. The URI and URN Resolution Application Specifications

   This template defines the URI and URN Resolution DDDS Application
   according to the rules and requirements found in [3].  The DDDS
   database used by this Application is found in [4] which is the
   document that defines the Naming Authority Pointer (NAPTR) DNS
   Resource Record (RR) type.

4.1 Application Unique String

   The Application Unique String is the URI or URN for which an
   authoritative server is being located.  This URI or URN MUST be
   canonicalized and hex encoded according to the "absolute-uri"
   production found in the Collected ABNF from RFC 2396 [15].

4.2 First Well Known Rule

   In the URI case, the first known key is created by taking the URI
   scheme.  In the URN case, the first known key is the Namespace
   Identifier.  For example, the URI 'http://www.example.com/' would
   have a 'http' as its Key.  The URN 'urn:foo:foospace' would have
   'foo' as its first Key.

4.3 Flags

   At this time only four flags, "S", "A", "U", and "P", are defined.
   The "S", "A" and "U" flags are for a terminal lookup.  This means
   that the Rule is the last one and that the flag determines what the
   next stage should be.  The "S" flag means that the output of this
   Rule is a domain-name for which one or more SRV [9] records exist.
   See Section 5 for additional information on how URI and URN
   Resolution use the SRV record type.  "A" means that the output of the
   Rule is a domain-name and should be used to lookup either A, AAAA, or
   A6 records for that domain.  The "U" flag means that the output of
   the Rule is a URI [15].

   The "P" flag says that the remainder of the DDDS Algorithm is ignored
   and that the rest of the process is application specific and outside
   the scope of this document.  An application can use the Protocol part
   found in the Services field to identify which Application specific
   set of rules that should be followed next.  The record that contains
   the 'P' flag is the last record that is interpreted by the rules in
   this document.  One might think that this would also make the "P"

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   flag an indicator of a terminal lookup but this would be incorrect
   since a "terminal" Rule is a DDDS concept and this flag indicates
   that anything after this rule does not adhere to DDDS concepts at

   The remaining alphabetic flags are reserved for future versions of
   this specification.  The numeric flags may be used for local
   experimentation.  The S, A, U and P flags are all mutually exclusive,
   and resolution libraries MAY signal an error if more than one is
   given.  (Experimental code and code for assisting in the creation of
   Rewrite Rules would be more likely to signal such an error than a
   client such as a browser.)  It is anticipated that multiple flags
   will be allowed in the future, so implementers MUST NOT assume that
   the flags field can only contain 0 or 1 characters.  Finally, if a
   client encounters a record with an unknown flag, it MUST ignore it
   and move to the next Rule.  This test takes precedence over any
   ordering since flags can control the interpretation placed on fields.
   A novel flag might change the interpretation of the regexp and/or
   replacement fields such that it is impossible to determine if a
   record matched a given target.

   The "S", "A", and "U" flags are called 'terminal' flags since they
   halt the looping DDDS algorithm.  If those flags are not present,
   clients may assume that another Rule exists at the Key produced by
   the current Rewrite Rule.

4.4 Services Parameters

   Service Parameters for this Application take the form of a string of
   characters that follow this ABNF:

      service_field = [ [protocol] *("+" rs)]
      protocol      = ALPHA *31ALPHANUM
      rs            = ALPHA *31ALPHANUM
      ; The protocol and rs fields are limited to 32
      ; characters and must start with an alphabetic.

   In other words, an optional protocol specification followed by 0 or
   more resolution services.  Each resolution service is indicated by an
   initial '+' character.

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   The empty string is also valid.  This will typically be seen at the
   beginning of a series of Rules, when it is impossible to know what
   services and protocols will be offered at the end of a particular
   delegation path.

4.4.1 Services

   The service identifiers that make up the 'rs' production are generic
   for both URI and URN resolution since the input value types itself
   based on the URI scheme.  The list of valid services are defined in

   Examples of some of these services are:

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

   I2Ls: given a URI return one or more URIs 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

   I2N:  given a URI return one URN that names the resource (Caution:
         equality with respect to URNs is non-trivial.  See [6] for
         examples of why.)

4.4.2 Protocols

   The protocol identifiers that are valid for the 'protocol' production
   MUST be defined by documents that are specific to URI resolution.  At
   present the THTTP [10] protocol is the only such specification.

   It is extremely important to realize that simply specifying any
   protocol in the services field is insufficient since there are
   additional semantics surrounding URI resolution that are not defined
   within the protocols.  For example, if Z39.50 were to be specified as
   a valid protocol it would have to additionally define how it would
   encode requests for specific services, how the URI is encoded, and
   what information is returned.

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4.4.3 Applicability of Services

   Since it is possible for there to be a complex set of possible
   protocols and services a client application may often need to apply a
   more complex decision making process to a set of records than simply
   matching on an ordered list of protocols.  For example, if there are
   4 rules that are applicable the last one may have a more desirable
   Service field than the first.  But since the client may be satisfied
   by the first it will never know about the 4th one which may be

   To mitigate this the client may want to slightly modify the DDDS
   algorithm (for this application only!) in order to determine if more
   applicable protocols/services exist.  This can safely be done for
   this application by using a more complex interaction between steps 3
   and 4 of the DDDS algorithm in order to find the optimal path to
   follow.  For example, once a client has found a rule who's
   Substitution Expression produces a result and who's Service
   description is acceptable, it may make note of this but continue to
   look at further rules that apply (all the while adhering to the
   Order!) in order to find a better one.  If none are found it can use
   the one it made note of.

   Keep in mind that in order for this to remain safe, the input to step
   3 and the output of step 4 MUST be identical to the basic algorithm.
   The client software MUST NOT attempt to do this optimization outside
   a specific set of Rewrite Rules (i.e., across delegation paths).

4.5 Valid Databases

   At present only one DDDS Database is specified for this Application.
   "Dynamic Delegation Discovery System (DDDS) Part Three: The Domain
   Name System (DNS) Database" (RFC 3403) [4] specifies a DDDS Database
   that uses the NAPTR DNS resource record to contain the rewrite rules.
   The Keys for this database are encoded as domain-names.

   The output of the First Well Known Rule for the URI Resolution
   Application is the URI's scheme.  In order to convert this to a
   unique key in this Database the string '.uri.arpa.' is appended to
   the end.  This domain-name is used to request NAPTR records which
   produces new keys in the form of domain-names.

   The output of the First Well Known Rule of the URN Resolution
   Application is the URN's namespace id.  In order to convert this to a
   unique key in this Database the string '.urn.arpa.' is appended to
   the end.  This domain-name is used to request NAPTR records which
   produces new keys in the form of domain-names.

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   DNS servers MAY interpret Flag values and use that information to
   include appropriate SRV and A records in the Additional Information
   portion of the DNS packet.  Clients are encouraged to check for
   additional information but are not required to do so.  See the
   Additional Information Processing section of RFC 3404 for more
   information on NAPTR records and the Additional Information section
   of a DNS response packet.

   The character set used to encode the substitution expression is
   UTF-8.  The allowed input characters are all those characters that
   are allowed anywhere in a URI.  The characters allowed to be in a Key
   are those that are currently defined for DNS domain-names.  The "i"
   flag to the substitution expression is used to denote that, where
   appropriate for the code points in question, any matches should be
   done in a case-insensitive way.

5. Examples

5.1 An Example Using a URN

   Consider a URN that uses the hypothetical FOO namespace.  FOO numbers
   are identifiers for approximately 30 million registered businesses
   around the world, assigned and maintained by Fred, Otto and Orvil,
   Inc.  The URN might look like:


   The first step in the resolution process is to find out about the FOO
   namespace.  The namespace identifier [8], "foo", is extracted from
   the URN and prepended to '.urn.arpa.', producing 'foo.urn.arpa.'.
   The DNS is queried for NAPTR records for this domain which produces
   the following results:

;;      order pref flags service          regexp        replacement
IN NAPTR 100  10  "s" "foolink+I2L+I2C"  ""   foolink.udp.example.com.
IN NAPTR 100  20  "s" "rcds+I2C"          ""  rcds.udp.example.com.
IN NAPTR 100  30  "s" "thttp+I2L+I2C+I2R" ""  thttp.tcp.example.com.

   The order field contains equal values, indicating that no order has
   to be followed.  The preference field indicates that the provider
   would like clients to use the special 'foolink' protocol, followed by
   the RCDS protocol, and that THTTP is offered as a last resort.  All
   the records specify the "s" flag which means that the record is
   terminal and that the next step is to retrieve an SRV record from DNS
   for the given domain-name.

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   The service fields say that if we speak of foolink, we will be able
   to issue either the I2L, I2C or I2R requests to obtain a URI or ask
   some complicated questions about the resource.  The Resource
   Cataloging and Distribution Service  (RCDS) [12] could be used to get
   some metadata for the resource, while THTTP could be used to get a
   URI for the current location of the resource.

   Assuming our client does not know the foolink protocol but does know
   the RCDS protocol, our next action is to lookup SRV RRs for
   rcds.udp.example.com, which will tell us hosts that can provide the
   necessary resolution service.  That lookup might return:

      ;;                          Pref Weight Port Target
      rcds.udp.example.com  IN SRV 0    0    1000 deffoo.example.com.
                            IN SRV 0    0    1000 dbexample.com.au.
                            IN SRV 0    0    1000 ukexample.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 [9] for the
   interpretation of the fields above.)

   There is opportunity for significant optimization here.  RFC 3404
   defines that Additional Information section may be available.  In
   this case the the SRV records may be returned as additional
   information for terminal NAPTRs lookups (as well as the A records for
   those SRVs).  This is a significant optimization.  In conjunction
   with a long TTL for *.urn.arpa. records, the average number of probes
   to DNS for resolving most URIs would approach one.

   Note that the example NAPTR records above are intended to represent
   the result of a NAPTR lookup using some client software like
   nslookup; zone administrators should consult the documentation
   accompanying their domain name servers to verify the precise syntax
   they should use for zone files.

   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.arpa.
   The resulting rule would have specified that the NID be extracted
   from the URN and '.urn.arpa.' appended to it resulting in the new key
   'foo.urn.arpa.' which is the first step from above.

5.2 CID URI Scheme Example

   Consider a URI scheme based on MIME Content-Ids.  The URI might look
   like this:


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

   The first step in the resolution process is to find out about the CID
   scheme.  The scheme is extracted from the URI, prepended to
   '.uri.arpa.', and the NAPTR for 'cid.uri.arpa.' looked up in the DNS.
   It might return records of the form:

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

   Since there is only one record, ordering the responses is not a
   problem.  The replacement field is empty, so the pattern provided in
   the regexp field is used.  We apply that regexp to the entire URI to
   see if it matches, which it does.  The \2 part of the substitution
   expression returns the string "example.com".  Since the flags field
   is empty, the lookup is not terminal and our next probe to DNS is for
   more NAPTR records where the new domain is 'example.com'.

   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 'bar', could have their very own
   NAPTR, maintaining those records for all the machines at a site could
   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 "example.com" might look like:

;;      order pref flags service           regexp  replacement
IN NAPTR 100 50 "s" "z3950+I2L+I2C"     ""    z3950.tcp.example.com.
IN NAPTR 100 50 "s" "rescap+I2C"        ""    rescap.udp.example.com.
IN NAPTR 100 50 "s" "thttp+I2L+I2C+I2R" ""    thttp.tcp.example.com.

   Continuing with the example, note that the values of the order fields
   are equal for all records, so the client is free to pick any record.
   The Application defines the flag 's' to mean a terminal lookup and
   that the output of the rewrite will be a domain-name for which an SRV
   record should be queried.  Once the client has done that, it has the
   following information: the host, port, the protocol, and the services
   available via that protocol.  Given these bits of information the
   client has enough to be able to contact that server and ask it
   questions about the cid URI.

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

5.3 Resolving an HTTP URI Scheme

   Even if URN systems were in place now, there would still be a
   tremendous number of host based URIs.  It should be possible to
   develop a URI resolution system that can also provide location
   independence for those URIs.

   Assume we have the URI for a very popular piece of software that the
   publisher wishes to mirror at multiple sites around the world:


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

   http.uri.arpa. 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 this:

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

   ;;       order pref flags   service  regexp     replacement
    IN NAPTR 100  100  "s"   "thttp+L2R"   ""    thttp.example.com.
    IN NAPTR 100  100  "s"   "ftp+L2R"    ""     ftp.example.com.

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

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6. Notes

   o  Registration procedures for the 'urn.arpa.' and 'uri.arpa.' DNS
      zones are specified in "Dynamic Delegation Discovery System (DDDS)
      Part Five: URI.ARPA Assignment Procedures" (RFC 3405 [5].

   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".

   o  Note that SRV RRs impose additional requirements on clients.

7. IANA Considerations

   The use of the "urn.arpa." and "uri.arpa." 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 "Dynamic Delegation Discovery System
   (DDDS) Part Five: URI.ARPA Assignment Procedures (RFC 3405)" [5].
   The operation of those zones imposes operational and administrative
   responsibilities on the IANA.

   The registration method used for values in the Services and Flags
   fields is for a specification to be approved by the IESG and
   published as either an Informational or standards track RFC.

   The registration policies for URIs is found in RFC 2717 [17].  URN
   NID registration policies are found in RFC 2611 [16].

8. Security Considerations

   The use of "urn.arpa." and "uri.arpa." 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

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   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.

9. Acknowledgments

   The editors would like to thank Keith Moore for all his consultations
   during the development of this document.  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.

   Specific recognition is given to Ron Daniel who was co-author on the
   original versions of these documents.  His early implementations and
   clarity of thinking was invaluable in clearing up many of the
   potential boundary cases.


   [1]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        One: The Comprehensive DDDS", RFC 3401, October 2002.

   [2]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Two: The Algorithm", RFC 3402, October 2002.

   [3]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Three: The Domain Name System (DNS) Database", RFC 3403, October

   [4]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Four: The Uniform Resource Identifiers (URI) Resolution
        Application", RFC 3404, October 2002.

   [5]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Five: URI.ARPA Assignment Procedures", RFC 3405y, October 2002.

   [6]  Sollins, K. and L. Masinter, "Functional Requirements for
        Uniform Resource Names", RFC 1737, December 1994.

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RFC 3404               DDDS Based URI Resolution            October 2002

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

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

   [9]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
        specifying the location of services (DNS SRV)", RFC 2782,
        February 2000.

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

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

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

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

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

   [15] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

   [16] Daigle, L., van Gulik, D., Iannella, R. and P. Falstrom, "URN
        Namespace Definition Mechanisms", RFC 2611, BCP 33, June 1999.

   [17] Petke, R. and I. King, "Registration Procedures for URL Scheme
        Names", RFC 2717, BCP 35, November 1999.

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

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RFC 3404               DDDS Based URI Resolution            October 2002

Appendix A. Pseudo Code

   For the edification of implementers, pseudocode for a client routine
   using NAPTRs is given below.  This code is provided merely as a
   convenience, 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.arpa."
     sprintf(key, "%s.urn.arpa.", 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
         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)) {
         if(!isKnownService(naptr[j].services)) {

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RFC 3404               DDDS Based URI Resolution            October 2002


         // 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
         // the flags to see if we're done.
         // Note: it is possible to rewrite this so that this valid
         // record could be noted as such but continue on in order
                // to find a 'better' record. But that code would be to
         // voluminous and application specific to be illustrative.
         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;
     } 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, ...
     for each (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

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RFC 3404               DDDS Based URI Resolution            October 2002

         // code was supposed to just tell us a good host to talk to.
     die with an "unable to find a host" error;

Author's Address

   Michael Mealling
   21345 Ridgetop Circle
   Sterling, VA  20166

   EMail: michael@neonym.net
   URI:   http://www.verisignlabs.com

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RFC 3404               DDDS Based URI Resolution            October 2002

Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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