GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3404

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.

Abstract

 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
 others.

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

Mealling Standards Track [Page 1] RFC 3404 DDDS Based URI Resolution October 2002

 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
 reached.
 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.

Mealling Standards Track [Page 2] RFC 3404 DDDS Based URI Resolution October 2002

 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 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.

Mealling Standards Track [Page 3] RFC 3404 DDDS Based URI Resolution October 2002

 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"

Mealling Standards Track [Page 4] RFC 3404 DDDS Based URI Resolution October 2002

 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
 all.
 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.

Mealling Standards Track [Page 5] RFC 3404 DDDS Based URI Resolution October 2002

 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
 [11].
 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
       resource.
 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.

Mealling Standards Track [Page 6] RFC 3404 DDDS Based URI Resolution October 2002

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
 'better'.
 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.

Mealling Standards Track [Page 7] RFC 3404 DDDS Based URI Resolution October 2002

 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:
    urn:foo:002372413:annual-report-1997
 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:

foo.urn.arpa. ;; 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.

Mealling Standards Track [Page 8] RFC 3404 DDDS Based URI Resolution October 2002

 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:
    cid:199606121851.1@bar.example.com

Mealling Standards Track [Page 9] RFC 3404 DDDS Based URI Resolution October 2002

 (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:

cid.uri.arpa. ;; 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:

example.com. ;; 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.

Mealling Standards Track [Page 10] RFC 3404 DDDS Based URI Resolution October 2002

 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:.+@([^\\.]+\\.)(.*)$!\\2!i".  When the client code actually
 receives the record, the pattern will have been converted to
 "!^cid:.+@([^\.]+\.)(.*)$!\2!i".

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:
 http://www.example.com/software/latest-beta.exe
 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:
 "/^http:\\/\\/([^\\/:]+)/\\1/i").
 Applying this pattern to the URI extracts "www.example.com".  Looking
 up NAPTR records for that might return:
 www.example.com.
 ;;       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.

Mealling Standards Track [Page 11] RFC 3404 DDDS Based URI Resolution October 2002

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
 problem.

Mealling Standards Track [Page 12] RFC 3404 DDDS Based URI Resolution October 2002

 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.

References

 [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
      2002.
 [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.

Mealling Standards Track [Page 13] 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.

Mealling Standards Track [Page 14] 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
          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;

Mealling Standards Track [Page 15] 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;
       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, ...
   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

Mealling Standards Track [Page 16] 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
 VeriSign
 21345 Ridgetop Circle
 Sterling, VA  20166
 US
 EMail: michael@neonym.net
 URI:   http://www.verisignlabs.com

Mealling Standards Track [Page 17] RFC 3404 DDDS Based URI Resolution October 2002

Full Copyright Statement

 Copyright (C) The Internet Society (2002).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Mealling Standards Track [Page 18]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3404.txt · Last modified: 2002/10/16 19:22 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki