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rfc:rfc3367

Network Working Group N. Popp Request for Comments: 3367 M. Mealling Category: Standards Track VeriSign, Inc.

                                                            M. Moseley
                                                         Netword, Inc.
                                                           August 2002
               Common Name Resolution Protocol (CNRP)

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

 People often refer to things in the real world by a common name or
 phrase, e.g., a trade name, company name, or a book title.  These
 names are sometimes easier for people to remember and type than URLs.
 Furthermore, because of the limited syntax of URLs, companies and
 individuals are finding that the ones that might be most reasonable
 for their resources are being used elsewhere and so are unavailable.
 For the purposes of this document, a "common name" is a word or a
 phrase, without imposed syntactic structure, that may be associated
 with a resource.
 This effort is about the creation of a protocol for client
 applications to communicate with common name resolution services, as
 exemplified in both the browser enhancement and search site
 paradigms.  Although the protocol's primary function is resolution,
 it is also intended to address issues of internationalization and
 localization.  Name resolution services are not generic search
 services and thus do not need to provide complex Boolean query,
 relevance ranking or similar capabilities.  The protocol is a simple,
 minimal interoperable core.  Mechanisms for extension are provided,
 so that additional capabilities can be added.

Popp, et. al. Standards Track [Page 1] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

Table of Contents

 1.      Introduction . . . . . . . . . . . . . . . . . . . . . . .  3
 2.      Important Notes  . . . . . . . . . . . . . . . . . . . . .  4
 2.1     Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
 2.2     DTD is Definitive  . . . . . . . . . . . . . . . . . . . .  4
 2.3     Uniform Resource Identifiers . . . . . . . . . . . . . . .  5
 3.      Interaction Model  . . . . . . . . . . . . . . . . . . . .  5
 3.1     Services, Servers, Datasets and Referrals  . . . . . . . .  5
 3.2     Requests and Responses . . . . . . . . . . . . . . . . . .  5
 3.3     Transport Independence . . . . . . . . . . . . . . . . . .  6
 3.4     Character encoding . . . . . . . . . . . . . . . . . . . .  6
 3.5     Queries  . . . . . . . . . . . . . . . . . . . . . . . . .  7
 3.6     Hints  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
 4.      Object Model . . . . . . . . . . . . . . . . . . . . . . .  8
 4.1     Properties . . . . . . . . . . . . . . . . . . . . . . . .  8
 4.1.1   Core properties  . . . . . . . . . . . . . . . . . . . . .  8
 4.1.2   Abstract and custom properties . . . . . . . . . . . . . .  9
 4.1.3   Base properties  . . . . . . . . . . . . . . . . . . . . .  9
 4.1.4   Common name string encoding and equivalence rules  . . . . 11
 4.2     Objects  . . . . . . . . . . . . . . . . . . . . . . . . . 11
 4.2.1   Query  . . . . . . . . . . . . . . . . . . . . . . . . . . 11
 4.2.1.1 Logical operations within a Query  . . . . . . . . . . . . 12
 4.2.2   Results  . . . . . . . . . . . . . . . . . . . . . . . . . 13
 4.2.2.1 ResourceDescriptor . . . . . . . . . . . . . . . . . . . . 13
 4.2.3   Service  . . . . . . . . . . . . . . . . . . . . . . . . . 14
 4.2.3.1 Datasets . . . . . . . . . . . . . . . . . . . . . . . . . 14
 4.2.3.2 Servers  . . . . . . . . . . . . . . . . . . . . . . . . . 16
 4.2.4   Status Messages  . . . . . . . . . . . . . . . . . . . . . 19
 4.2.4.1 Status of CNRP, Not the Transport  . . . . . . . . . . . . 19
 4.2.4.2 Codes and Description  . . . . . . . . . . . . . . . . . . 19
 4.2.4.3 Status Codes . . . . . . . . . . . . . . . . . . . . . . . 19
 4.2.5   Referral . . . . . . . . . . . . . . . . . . . . . . . . . 21
 4.2.5.1 Loop Detection and Dataset Handling in Servers . . . . . . 22
 4.2.6   Discoverability: ServiceQuery and Schema . . . . . . . . . 24
 5.      XML DTD for CNRP . . . . . . . . . . . . . . . . . . . . . 26
 6.      Examples . . . . . . . . . . . . . . . . . . . . . . . . . 28
 6.1     Service Description Request  . . . . . . . . . . . . . . . 28
 6.2     Sending A Query and Getting A Response . . . . . . . . . . 29
 7.      Transport  . . . . . . . . . . . . . . . . . . . . . . . . 30
 7.1     HTTP Transport . . . . . . . . . . . . . . . . . . . . . . 30
 7.2     SMTP Transport . . . . . . . . . . . . . . . . . . . . . . 31
 8.      Registration: application/cnrp+xml . . . . . . . . . . . . 31
 9.      Security Considerations  . . . . . . . . . . . . . . . . . 32
 10.     IANA Considerations  . . . . . . . . . . . . . . . . . . . 32
         References . . . . . . . . . . . . . . . . . . . . . . . . 33

Popp, et. al. Standards Track [Page 2] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 A.      Appendix A: Well Known Property and Type Registration
         Templates  . . . . . . . . . . . . . . . . . . . . . . . . 35
 A.1     Properties . . . . . . . . . . . . . . . . . . . . . . . . 35
 A.2     Types  . . . . . . . . . . . . . . . . . . . . . . . . . . 35
 B.      Status Codes . . . . . . . . . . . . . . . . . . . . . . . 37
 B.1     Level 1 (Informative) Codes  . . . . . . . . . . . . . . . 37
 B.2     Level 2 (Success) Codes  . . . . . . . . . . . . . . . . . 38
 B.3     Level 3 (Partial Success) Codes  . . . . . . . . . . . . . 38
 B.4     Level 4 (Transient Failure) Codes  . . . . . . . . . . . . 40
 B.5     Level 5 (Permanent Failures) Codes . . . . . . . . . . . . 40
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . 41
         Full Copyright Statement . . . . . . . . . . . . . . . . . 42

1. Introduction

 Services are arising that offer a mapping from common names to
 Internet resources (e.g., as identified by a URI).  These services
 often resolve common name categories such as company names, trade
 names, or common keywords.  Thus, such a resolution service may
 operate in one or a small number of categories or domains, or may
 expect the client to limit the resolution scope to a limited number
 of categories or domains.  For example, the phrase "Internet
 Engineering Task Force" is a common name in the "organization"
 category, as is "Moby Dick" in the book category.
 Two classes of clients of such services are being built, browser
 improvements and web accessible front-end services.  Browser
 enhancements modify the "open" or "address" field of a browser so
 that a common name can be entered instead of a URL.  Internet search
 sites integrate common name resolution services as a complement to
 search.  In both cases, these may be clients of back-end resolution
 services.  In the browser case, the browser must talk to a service
 that will resolve the common name.  The search sites are accessed via
 a browser.  In some cases, the search site may also be the back-end
 resolution service, but in others, the search site is a front-end to
 a collection of back-end services.
 This effort is about the creation of a protocol for client
 applications to communicate with common name resolution services, as
 exemplified in both the browser enhancement and search site
 paradigms.  Name resolution services are not generic search services
 and thus do not need to provide complex Boolean query, relevance
 ranking or similar capabilities.  The protocol is a simple, minimal
 interoperable core.  Mechanisms for extension are provided, so that
 additional capabilities can be added.

Popp, et. al. Standards Track [Page 3] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 Several other issues, while of importance to the deployment of common
 name resolution services, are outside of the resolution protocol
 itself and are not in the initial scope of the proposed effort.
 These include discovery and selection of resolution service
 providers, administration of resolution services, name registration,
 name ownership, and methods for creating, identifying or insuring
 unique common names.
 For the purposes of this document, a "common name" is a word or a
 phrase, without imposed syntactic structure, that may be associated
 with a resource.  These common names will be used primarily by
 humans, as opposed to machine agents.  A common name "resolution
 service" handles these associations between common names and data
 (resources, information about resources, pointers to locations,
 etc.).  A single common name may be associated with different data
 records, and more than one resolution service is expected to exist.
 Any common name may be used in any resolution service.
 Common names are not URIs (Uniform Resource Identifiers) in that they
 lack the syntactic structure imposed by URIs; furthermore, unlike
 URNs, there is no requirement of uniqueness or persistence of the
 association between a common name and a resource.  (Note: common
 names may be expressed in a URI, the syntax for which is described in
 RFC 3368 [9].)
 This document will define a protocol for the parameterized resolution
 necessary to make common names useful.  "Resolution" is defined as
 the retrieval of data associated (a priori) with descriptors that
 match the input request.  "Parameterized" means the ability to have a
 multi-property descriptor.  Descriptors are not required to provide
 unique identification, therefore 0 or more records may be returned to
 meet a specific input query.

2. Important Notes

2.1 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 [7].

2.2 DTD is Definitive

 The descriptive portions of this document contain pieces of XML that
 are *illustrative examples only*.  Section 5 of this document
 contains the XML DTD for CNRP, which is definitive.  If any
 discrepancies are found, the DTD wins.

Popp, et. al. Standards Track [Page 4] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

2.3 Uniform Resource Identifiers

 All URIs used within the CNRP protocol MUST adhere to the
 'absoluteURI' production found in the ABNF of [3].  CNRP does not
 define the semantics of a Base and therefore is not capable of
 expressing the 'URI-Reference' production.

3. Interaction Model

3.1 Services, Servers, Datasets and Referrals

 CNRP assumes a particular interaction model where a generalized
 "service" provides common name resolution at one or more actual
 "servers".  If the data contained in all its servers is identical
 (mirrors), the service need not identify any particular subset of
 data.  If, however, the service provides different collections of
 data through different servers (e.g., subsets, specialized
 collections, etc.), it SHOULD indicate what subsets of its data that
 each server offers.  This is done by using URIs to uniquely
 disambiguate one dataset from another.  If the service offers a copy
 of a collection of data on agreement with a foreign service, the
 foreign service SHOULD provide a dataset URI to allow the collection
 to be identified as related to its own offerings.
 CNRP supports the concept of referrals.  This is where a server can
 know that another Service exists, within the same Service or
 elsewhere, that can provide further answers to a particular query but
 decides to forward that fact onto the client instead of chaining the
 query for the client.  A referral is sent along with the rest of the
 results from a server (if any).  Referrals to a service SHOULD
 indicate the particular dataseturi that triggered the referral, if it
 is known.  See Section 4.2.5 for details on referrals and loop
 detection.

3.2 Requests and Responses

 The protocol consists of a simple request/response mechanism.  A
 client sends one of a few types of requests to a server which
 responds with the results of that request.  All requests and
 responses are encoded with XML [8] using the DTD found in Section 5.
 There are two types of requests.  One is a general query for a
 common-name.  The other is a request for an object that describes the
 service and its capabilities.  There is only one type of response
 which is a set of results.  Results can contain actual result items,
 referrals and/or status messages.

Popp, et. al. Standards Track [Page 5] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

3.3 Transport Independence

 CNRP is completely encapsulated within its XML definition, and is
 therefore transport-independent in its specification.  However,
 clients need to have a clearly defined means of bootstrapping a
 connection with a server.
 It is possible to define special-purpose applications that use CNRP
 but which never need the HTTP bootstrapping method outlined below;
 those applications MUST define how to find the appropriate
 server/port/protocol.  CNRP servers dedicated to those applications
 may provide service only on the ports/transport protocols defined by
 the application.
 All other (generic) CNRP clients and servers MUST support the HTTP
 (Section 7.1) transport on the default CNRP port of 1096.
 Note that a particular service may choose to change to a different
 transport or port via statements within a CNRP service description
 request, but with initial contacts between a client and a server
 being over HTTP on port 1096.  For a short explanation of how CNRP
 employs HTTP, see Section 7.1 of this document.  If other transports
 are used, they MUST be handled over a port other than the default
 CNRP port.

3.4 Character Encoding

 To guarantee interoperability, the following provisions apply:
 o  XML queries and responses MUST be encoded as UTF-8.
    Note: As in any XML document, numeric character references may be
    used.
 o  The encoding of characters in the CNRP URI is based on UTF-8; for
    details, please see [9].
 Any interfaces electing to present/accept protocol elements in other
 representations are responsible for accurate transcoding for use in
 CNRP protocol elements, per the above provisions.

Popp, et. al. Standards Track [Page 6] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

3.5 Queries

 Queries are sent by the client to the server.  There are two types of
 queries.
 1.  A `special' initial query that establishes the schema for a
     particular CNRP database and communicates that to the client.
     The CNRP client will send this query, and in turn receive an XML
     document defining the query properties that the database
     supports.  (In CNRP, XML [8] is used to define and express all
     objects.)  This query is called the 'servicequery' in the DTD.
     In the case where a client does not know anything about the
     Service, the client MAY assume that it can at least issue the
     request via HTTP.
 2.  A `standard' query, which is the submission of the CNRP search
     string to the database.  The query will conform to the schema
     that MAY have been previously retrieved from the service.
 There will be a set of query properties, listed below, treated as
 hints by the server.  Note: a CNRP database will accept any correctly
 encoded CNRP query property; the extent to which a query result is
 responsive to those properties is a service differentiator.  The base
 properties that are always supported are common name, language,
 geography, category, and range (start and length of the result set).
 CNRP allows database service providers to create unique data types
 and expose them to any CNRP client via the CNRP schema XML documents.

3.6 Hints

 A hint is an assertion by the user about himself, herself or itself
 and the context in which he/she/it is operating.  There is no data
 type `hint'; a hint is expressed within the structure of the query
 itself and is limited or enabled by the richness of the defined query
 namespace.  In effect, a query and any property within it is a hint.
 For example, the "language" property can be given as a hint in a
 query; this may be used to order search results.  If one wants
 results first in US English followed by European French and finally
 South American Spanish, the following can be included in the query:
    <property name="language" type="rfc1766">en-US</property>
    <property name="language" type="rfc1766">fr-FR</property>
    <property name="language" type="rfc1766">sp-MX</property>

Popp, et. al. Standards Track [Page 7] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 Note that the property statements say nothing about whether the
 language is primary, secondary, etc.  In this example, the ordering
 of the statement controls that--the first statement, being first,
 means that US English is the primary language.  The second statement
 specifies the second region/language, and so on.  *But this is only
 an example.*  The extent to which hints are supported (or not) is a
 service differentiator.
 The fact that a hint exists does not mean that a CNRP database must
 respond to it.  This best-effort approach is similar to relevance
 ranking in a search engine (high precision, low recall); hints are
 similar to a search engine's selection criteria.  CNRP services will
 attempt to return the results "closest" to the selection criteria.
 This is quite different from a SQL database approach where a SQL
 query returns the entire results set and each result in the set must
 match all the requirements expressed by the qualifier (the SQL WHERE
 clause).

4. Object Model

4.1 Properties

 In CNRP, objects are property lists.  A property is a named
 attribute.  A property also has a well-defined type.  Some properties
 can be part of the query or the results list or both.  For
 simplicity, CNRP is limiting property values to string values.

4.1.1 Core Properties

 CNRP introduces a set of core properties.  Core properties are the
 minimal set of properties that all CNRP services MUST support in
 order to reach CNRP compliance.  Hence, the core properties define
 the level of interoperability between all CNRP services.  The core
 properties are:
 1.  CommonName: the common name associated with a resource.
 2.  ID: an opaque string that serves as a unique identifier for a
     result from a Service (typically a database ID).  The ID is not
     globally unique, nor necessarily persistent (e.g., between
     queries at a given Service).
 3.  resourceURI: An 'absoluteURI' as defined in the collected ABNF
     found in RFC 2396 [3].
 4.  description: A free text description of the resource.

Popp, et. al. Standards Track [Page 8] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

4.1.2 Abstract and Custom Properties

 In addition to core properties, CNRP introduces the notion of
 abstract properties.  The abstract property element provides schema
 extensibility beyond the core properties.  The notion of abstract
 property is extremely important in CNRP since it enables a wider
 range of CNRP based services than those based on the core properties.
 To create concrete custom properties, a CNRP service must define a
 property name and a property type.  Therefore, there are really two
 ways to create a custom property.  The first way is to create a new
 property name and define at least one type for it.  Another way is to
 extend an existing property by defining a new type.  The "geography"
 property discussed in the next section is an example of a multi-type
 property.  Note that a type is only applicable to the property it is
 defined for.  If a new property is defined, a new type MUST be
 defined even though the value set for that type may be identical to
 an existing type for an existing property.  In other words, types are
 scoped to a given property.  Custom properties MUST be registered
 with IANA.  Details about the registration process for new properties
 can be found in Section 10.
 For example, let us assume that a CNRP service specialized on online
 books would like to introduce the ISBN property of type "number".
 This property would encapsulate the ISBN number of the book online
 and would have he following XML representation:
    <property name="isbn" type="number">92347231</property>

4.1.3 Base Properties

 Illustrating the use of abstract property to extend the core schema,
 CNRP also defines a set of custom properties called base properties.
 In order to keep the requirements extremely simple, these properties
 are not mandatory to implement to reach CNRP compliance.  Although,
 these properties are not required, it is expected that many services,
 especially large ones, will implement them.  An equally important
 goal for introducing additional properties is to provide a results
 filtering mechanism.  This is a requirement for large namespaces that
 contain several million names.

Popp, et. al. Standards Track [Page 9] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 The base properties and their types are defined in Appendix A but
 listed here for clarity:
 o  Language:
    The language associated with a resource.  The default type of this
    property is 'RFC1766' and the vocabulary is drawn from the list of
    languages in RFC 1766 [4].  If RFC 1766 is updated, then the
    values listed in the updated version are also valid for this type.
 o  Geography:
    The geographical region or location associated with a resource.
    Some of the possible types are listed below.  See Appendix A for a
    complete list of types specified by this document.
  • 'freeform': a free form expression for a geographical location

(e.g., "palo alto in california").

  • 'ISO3166-1': geographical region expressed using a standard

country code as defined by ISO3166-1 (e.g., "US").

  • 'ISO3166-2': value = a geographical region expressed using a

standard region and country codes as defined by ISO3166-2

       (e.g., "US-CA").
  • 'lat-long': the latitude and longitude of a geographical

location.

 o  Category:
    The category associated with a resource.  There are large numbers
    of possible types for this property.  Two possible ones are:
    1.  'freeform': a free form expression for a category (e.g.,
        "movies").
    2.  'NAICS': The North American Industry Code System.
 o  Range:
    The range is a results set control property.  The range property
    is used to specify the starting point and the length of a results
    set (e.g., I want 5 records starting at the 10th record).  It
    should only ever have one type but, in the interest of
    extensibility and consistency, others can be created if there is a
    need.  The default type is 'start-length' which takes the form of
    two integers separated by a dash.  The first integer is the
    starting number and the second is the number of values to include.

Popp, et. al. Standards Track [Page 10] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 o  Dataseturi: An absoluteURI (as defined in [3] that identifies a
    defined set of Common Names and associated data.
 Note: For many properties the default "type" is "freeform".  The free
 form type value is important because it allows very simple user
 interface where the user can enter a value in a text field.  It is up
 to the service to interpret the value correctly and take advantage of
 it to increase the relevance of results (using specialized
 dictionaries for instance).

4.1.4 Common Name String Encoding and Equivalence Rules

 CNRP specifies that common name strings should be encoded using UTF-
 8.  CNRP does not specify any string equivalence rules for matching a
 common name in the query against a common name of a Resource.  String
 equivalence rules are language and service dependent.  They are
 specific to relevance ranking algorithms, hence treated as CNRP
 services.  Consequently, string equivalence rules are not part of the
 CNRP protocol specification.  For example, the query member:
    <commonname>bmw</commonname>
 should be read as a selection criterion for a resource with a common
 name LIKE (similar to) the string "bmw" where the exact definition of
 the LIKE operator is intuitive, yet specific to the queried CNRP
 service.
 It is also important to note that XML treats whitespace as a special
 case in many situations.  In some cases, it collapses whitespace into
 a single space.  Both client and server Implementors are warned to
 reference the XML standard for the various ramifications of using
 whitespace in queries and/or results.

4.2 Objects

4.2.1 Query

 The Query object encapsulates all the query components such as
 CommonName, ID, and any properties.  A Query cannot be empty.  A
 Query must contain either one and only one common name, or one and
 only one ID.  A Query can also contain the custom properties defined
 by a specific CNRP service.

Popp, et. al. Standards Track [Page 11] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 For example, a query for the first 5 resources whose common name is
 like "bmw" would be expressed as:
 <query>
         <commonname>bmw</commonname>
         <property name="range" type="start-length">1-5</property>
 </query>

4.2.1.1 Logical Operations Within a Query

 The Query syntax is extremely simple.  CNRP does not extensively
 support Boolean logic operator such as OR, AND or NOT.  However,
 there exist two implicit logical operations that can be expressed
 through the Query object and its properties.  First, a query with
 multiple property-value pairs implicitly expresses an AND operation
 on the query terms.  For instance, the CNRP query to request all the
 resources whose common name is like "bmw", AND whose language is
 "German" can be expressed as:
 <query>
      <commonname>bmw</commonname>
      <property name="language" type="rfc1766">
         de-DE
      </property>
 </query>
 Note however, that because the server is only trying to best match
 the Query criteria, there is no guarantee that all or any of the
 resources in the results match both requirements.
 In addition, CNRP allows the client to express a logical OR by
 specifying multiple values for the same property within the Query.
 For example, the logical expression:
    property = value1 OR property = value2 OR property = valueN
 Will be expressed as:
 <property>value1</property>
 <property>value2</property>
 <property>valueN</property>
 So if there are different properties expressed, CNRP ANDs them; if
 there are multiples instances of the same property expressed, CNRP
 ORs them.

Popp, et. al. Standards Track [Page 12] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 It is important to underline that this form is only applicable to
 properties (with the exception of the CommonName itself which, even
 though it is a property, is the entire point of the query).  In
 particular, logical OR operations on the common name are not
 supported.  Note that the ordering or the property-value pairs in the
 query implies a precedence.  As a consequence, CNRP also introduces
 one special string value: "*".  Not surprisingly, "*" means all
 admissible values for the typed property.  For example, the following
 query requests all the resources whose common name is like BMW and
 whose language is preferably in German or French or any other
 language.
 <query>
      <commonname>bmw</commonname>
      <property name="language" type="rfc1766">de-DE</property>
      <property name="language" type="rfc1766">fr-FR</property>
      <property name="language" type="rfc1766">*</property>
 </query>

4.2.2 Results

 The results object is a container for CNRP results.  The type of
 objects contained in Results can be: ResourceDescriptor, Error,
 Referral and Schema.  Results from a CNRP service are ordered by
 decreasing relevance.  When the results set contains results from
 multiple CNRP services, the results can no longer be ordered (since
 relevance ranking is specific to a given service).  In that case,
 however, note that results originating from the same service remain
 ordered.

4.2.2.1 ResourceDescriptor

 The ResourceDescriptor object describes an Internet resource (e.g., a
 Web page, a person, any object identified by a URI).  Therefore, the
 ResourceDescriptor MUST always include the resourceURI property.  The
 ResourceDescriptor can also contain the commonname, URI, ID (the ID
 of this entry in the service's database), description, language,
 geography, and category of the resource.  A ResourceDescriptor can
 also be augmented using custom properties and can reference a service
 object to indicate its origin (using the serviceRef element).  As
 with referrals, a resourcedescriptor block can also contain an ID
 attribute that is used by a status message to refer to a particular
 resourcedescriptor.  Be careful not to confuse this ID with the id
 tag itself which refers to the database id of the actual database
 entry.

Popp, et. al. Standards Track [Page 13] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 <results>
      <service id="i0">
           <serviceuri>http://cnrp.bar.com/</serviceuri>
      </service>
      <resourcedescriptor id="i1">
           <commonname>bmw</commonname>
           <id>foo.com:234364</id>
           <resourceuri>http://www.bmw.de/</resourceuri>
           <serviceref ref="i0" />
           <description>BMW Motorcycles, International</description>
           <property name="language" type="rfc1766">de-DE</property>
      </resourcedescriptor>
      <referral>
           <serviceref ref="i0" />
      </referral>
 </results>

4.2.3 Service

 The Service object provides an encapsulation of an instance of a CNRP
 service.  A service is uniquely identified through the serviceuri tag
 which MUST be included in the Service object.  A Service object MAY
 include a a brief textual description of the service.  It MAY include
 datasets, servers and custom properties.
 <service>
      <serviceuri>http://cnrp.foo.com</serviceuri>
      <description>foo.com is a CNRP service specialized on cocktail
         recipes</description>
 </service>
 The service object MAY also be extended by including existing
 properties to further describe the service.  For instance, a service
 that focuses on French companies could be expressed as:
 <service>
      <serviceuri>http://cnrp.foo.com</serviceuri>
      <property name="category" type="freeform">companies</property>
      <property name="geography" type="ISO3166-1">FR</property>
 </service>

4.2.3.1 Datasets

 The dataset object represents a set of CN-to-URI mappings.  For
 example, the database of AOL keywords and their URIs constitute a
 dataset.  The dataset object allows a CNRP implementation to uniquely
 identify the database(s) of mappings that it resolves.  In that
 respect, the notion of dataset allows a separation between resolution

Popp, et. al. Standards Track [Page 14] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 and data, providing the mechanism for a CNRP service to resolve
 common-names on behalf of another CNRP service or even multiple
 services.  Conversely, the same dataset can be served by two distinct
 CNRP services.  Since a CNRP service can resolve names within one or
 more datasets, the service object can contain one or more dataset
 objects (zero if the dataset is not formally declared).
 Within the service object, a dataset is uniquely defined using the
 dataseturi property.  Other properties, such as language and
 description, can describe the dataset further.  Like the service
 object, the dataset object has an ID attribute associated with it
 that is unique within a particular XML message.  Like the service
 object's ID attribute, this ID is used by resourcedescriptors and
 referrals to specify which service and/or dataset they came from or
 are referring to.
 Any service can be said to have a 'default dataset' which is the
 dataset that considered to have been used if a server simply responds
 to a client's query that didn't contain a dataset.  The 'default
 dataset' can also be said to be the only dataset that is used by
 Services that don't support datasets at all.  This concept is useful
 for clients that intend on doing rigorous loop detection by way of
 keeping a list of visited service/dataset nodes.
 This example illustrates how the service object would look as it
 defines two datasets:
 <service id="i0">
  <serviceuri>http://acmecorp.com</serviceuri>
  <dataset id="i1">
    <property name="dataseturi">
       urn:oid:1.2.3.4.666.5.4.3.1
    </property>
    <property name="language">en-us</property>
    <property name="language">en-gb</property>
  </dataset>
  <dataset id="i2">
    <property name="dataseturi">
       urn:oid:1.2.3.4.666.10.9.8.7.6
    </property>
    <property name="language">fr</property>
  </dataset>
 </service>
 The dataseturi property can also be used within the query as a hint
 to the service for the dataset within which the commonname should be
 resolved:

Popp, et. al. Standards Track [Page 15] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 <query>
    <commonname>toys r us</commonname>
    <property name="dataseturi">urn:oid:1.2.3.4.666.5.4.3.1</property>
 </query>
 It is important to note that resolution rules (i.e., string
 equivalence, relevance ranking, etc.) are likely to be dataset
 specific.  This is true even if the resolution is provided by the
 same service.
 Another use of the dataseturi property is in a referral.  In that
 case, the datasetref tag is used to pinpoint a specific dataset
 within the service.
 <referral>
    <serviceref ref="i0" /><datasetref ref="i1" />
 </referral>
 While the concept of datasets is important for services wishing to
 make their data available via other services, it is important to
 remember that the declaration and use of datasets is completely
 optional.  Compliance with the CNRP protocol does not require a
 service object to define or reference any dataset object.  The only
 requirement for compliance is that a client and/or server know the
 format of the particular XML tags and deal with them syntactically.
 If it chooses to ignore them, then this is well within its rights.

4.2.3.2 Servers

 The service object also encapsulates a list of server objects.  The
 server object is used to describe a CNRP server or set of servers.  A
 server is identified through its serveruri.  The URI used to identify
 a server is not a CNRP URI [9], but instead, is a URI of the scheme
 used as the CNRP transport mechanism.  I.e., for a CNRP server that
 will communicate via the HTTP protocol to the host foo.com on port
 6543, the serveruri would be http://foo.com:6543.  If some other
 information is required in order for the correct transport to be
 used, then that information can be communicated via other properties.
 Note that a Service MUST have at least one Server that responds on
 the default CNRP port in order for a client to get the initial
 Service object.
 A server can serve one or more datasets declared by its service.  The
 served databases are specified using the dataseturi property.  As for
 other objects, a server can be further described using descriptive
 properties such as geography and description.  The following XML
 completes the service definition from the previous example by
 defining two CNRP servers.  One server is located in the US and the

Popp, et. al. Standards Track [Page 16] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 other is located in France.  The US server is specialized and only
 serves the French dataset.
   <servers>
      <server>
         <serveruri>cnrp://router.us.widgetco.com:4321</serveruri>
         <property name="geography" type="ISO3166-1">US</property>
      </server>
      <server>
         <serveruri>cnrp://router.fr.acmeco.com:4321</serveruri>
         <property name="geography" type="ISO3166-1">FR</property>
      </server>
   </servers>
 As we will see in a following section, the Service object can contain
 Schema objects.  These Schema objects fully describe the query and
 response interfaces implemented by a CNRP service.  In that regard,
 the Service object is essential to discoverability.  It constitutes
 the main entry point for a CNRP client to dynamically discover the
 capabilities of a resolution service.  For that purpose, the Service
 object can be returned as part of the response to any resolution
 query.  Furthermore, the Service object is the dedicated response to
 the specialized servicequery (see Section 4.2.6).
 Another use of Service is for other objects to indicate their CNRP
 service of origin.  System messages, referrals and
 resourcedescriptors can include a reference to their Service object.
 For example, imagine a CNRP service that acts as a proxy for multiple
 CNRP services.  For example, it is a requirement that CNRP allows
 aggregation of results from different sources.  Consider one such
 CNRP service that acts as a proxy for multiple CNRP services.  In
 this mode, the proxy service contacts each CNRP sub-service in
 parallel or serially.  Then, the proxy combines the individual result
 sets into a unique response returned to the CNRP client.  Since the
 aggregate result set contains resourcedescriptors from different
 services, the proxy adds a servicereference tag within each
 individual result to indicate their service of origin.  In the event
 one of the referred services resolves names within multiple datasets,
 it is possible for these objects to refer to a specific dataset
 within the service by using the datasetref tag.  This example is of a
 hybrid result set with resourcedescriptors referencing their service
 and dataset of origin:

Popp, et. al. Standards Track [Page 17] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 <?xml version="1.0"?>
 <!DOCTYPE cnrp PUBLIC "-//IETF//DTD CNRP 1.0//EN"
     "http://ietf.org/dtd/cnrp-1.0.dtd">
 <cnrp>
      <results>
           <service id="i0">
                <serviceuri>http://acmecorp.com</serviceuri>
                <dataset id="i1">
                   <property name="dataseturi">
                    urn:oid:1.2.3.4.666.5.4.3.1
                   </property>
                </dataset>
                <dataset id="i2">
                   <property name="dataseturi">
                    urn:oid:1.2.3.4.666.10.9.8.7.6
                   </property>
                </dataset>
           </service>
           <service id="i3">
              <serviceuri>http://serverfarm.acmecorp.com</serviceuri>
           </service>
           <service id="i4">
               <serviceuri>http://servers.acmecorp.co.uk</serviceuri>
               <dataset id="i5">
                   <property name="dataseturi">
                     urn:oid:1.2.3.4.666.5.4.3.1
                   </property>
               </dataset>
           </service>
           <resourcedescriptor>
                     <commonname>Fidonet</commonname>
                     <id>1333459455</id>
                     <resourceuri>http://www.fidonet.ca</resourceuri>
                     <serviceref ref="i0" /><datasetref ref="i1" />
                     <description>This is ye olde Canadian
                      Fidonet</description>
           </resourcedescriptor>
           <resourcedescriptor>
                     <commonname>Fidonet</commonname>
                     <id>1333459455</id>
                     <resourceuri>http://host:port/bla</resourceuri>
                     <serviceref ref="i3" />
                     <description>An old Fidonet node</description>
           </resourcedescriptor>
           <referral>
               <serviceref ref="i0" /><datasetref ref="i2" />
           </referral>
      </results>

Popp, et. al. Standards Track [Page 18] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 </cnrp>

4.2.4 Status Messages

4.2.4.1 Status of CNRP, Not the Transport

 The status messages defined here are only applicable to operations
 defined by CNRP itself.  If some feature or operation is defined by
 the transport (security via HTTP, mail failure via SMTP, etc.), then
 any status messages about that operation MUST be sent in accordance
 with that transport's reporting mechanism and not via CNRP.

4.2.4.2 Codes and Description

 A Status object indicates a message to the client in the results set.
 The object encapsulates two values: a status code and a description.
 The description can contain a textual description of the status being
 communicated.  In many cases, additional diagnostic information can
 also be included.  No attempt is made to standardize the description
 of a given status code since the only programmatic element that
 matters is the actual code.
 A status message can also specify which other CNRP element it refers
 to by including a reference to the ID of the element in question.
 For example, if a Service block has an ID of "i2" and a status
 message refers to that block, then it can put that ID in its ref
 attribute.
          <status code="x.y.z" ref="i2">
               The CNRP foo.com database is temporarily unreachable
          </status>

4.2.4.3 Status Codes

 The organization of status codes is taken from RFC 1893 [10] which
 structures its codes in the form of x.yyy.zzz.  Taken from RFC 1893
 is the ABNF for the codes:
           status-code = class "." subject "." detail
           class = "2"/"3"/"4"/"5"
           subject = 1*3digit
           detail = 1*3digit

Popp, et. al. Standards Track [Page 19] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 The top level codes denote levels of severity of the status:
 o  1.X.X Informational
  • The information conveyed by the code has no bearing or

indication of the success or failure of any request. It is

       strictly for informational purposes only.
 o  2.X.X Success
  • The request was processed and results were returned. In most

cases, this status class won't be sent since actual results

       themselves denote success.  In other cases, results were
       returned but some information needs to be returned to the
       client.
 o  3.X.X Partial Success
  • The request was processed and results were returned. In this

case though, some values sent with the request were either

       invalid or ignored but in a way that the server still considers
       the response to be a successful one and not indicative of any
       true error condition.
 o  4.X.X Transient Failure
  • The request was valid as sent, but some temporary event

prevents the successful completion of the request and/or

       sending of the results.  Sending in the future may be possible.
 o  5.X.X Permanent Failure
  • A permanent failure is one which is not likely to be resolved

by re-sending the request in its current form. Some change to

       the request or the destination must be made for successful
       request.
 The second level codes denote the subject of the status messages.
 This value applies to each of the five classifications.  The subject
 sub-code, if recognized, must be reported even if the additional
 detail provided by the detail sub-code is not recognized.  The
 enumerated values for the subject sub-code are:

Popp, et. al. Standards Track [Page 20] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 o  X.0.X Other or Undefined Status
  • No specific information is available about what subject class

this message belongs to.

 o  X.1.X Query Related
  • Any status related to some specific way in which the query was

encoded or its values with the exception of properties.

 o  X.2.X Service Related
  • Any status related to the service in which this server is

cooperating in providing.

 Appendix B contains a list of all predefined status codes

4.2.5 Referral

 A Referral object in the results set is a place holder for un-fetched
 results from a different service and possibly dataset.  Referrals
 typically occur when a CNRP server knows of another service capable
 of providing relevant results for the query and wants to notify the
 client about this possibility.  The client can decide whether it
 wants to follow the referral and resolve the extra results by
 contacting the referred-to service using the information contained
 within the Referral object (a Service object and possible
 properties).  The Referral is a simple mechanism to enable
 hierarchical resolution as well as to join multiple resolution
 services together.

Popp, et. al. Standards Track [Page 21] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 <results>
      <service id="i0">
           <serviceuri>http://cnrp.bar.com/</serviceuri>
           <dataset id="i1">
              <property name="dataseturi">
                urn:oid:1.3.6.1.4.1.782.1
              </property>
           </dataset>
           <dataset id="i2">
              <property name="dataseturi">
                urn:oid:1.3.6.1.4.1.782.2
              </property>
           </dataset>
      </service>
      <resourcedescriptor>
           <commonname>bmw</commonname>
           <id>foo.com:234364</id>
           <resourceuri>http://www.bmw.de/</resourceuri>
           <serviceref ref="i0" /><datasetref ref="i1" />
           <description>BMW Motorcycles, International</description>
           <property name="language" type="iso646">de-DE</property>
      </resourcedescriptor>
      <referral>
           <serviceref ref="i0" /><datasetref ref="i2" />
      </referral>
 </results>
 Like other CNRP objects, a referral can be further described using
 custom properties.  Like a resourcedescriptor, a referral can have an
 ID attribute that is used by a status message to talk about a
 particular referral block.

4.2.5.1 Loop Detection and Dataset Handling in Servers

 Referrals in CNRP can be handled in three ways:
 o  application specific,
 o  as hints only,
 o  rigorous loop detection.
 In the first two cases, the behavior of the client, when it receives
 a referral, is not defined in this memo.  The client can chase the
 referral in such a way as to treat it as a hint only.  In this case,
 datasets may or may not be handled.  Loop detection can be nothing
 more than, "Have I talked to this hostname before?" or "Stop after
 the 3rd referral".  These two cases are most likely to apply to

Popp, et. al. Standards Track [Page 22] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 simple or constrained implementations where the clients and servers
 have some a priori knowledge of their capabilities.  Without such
 knowledge there is too much ambiguity vis-a-vis services and datasets
 for clients to do reliable loop detection.
 The last case is where the client expects to talk to multiple servers
 that may know nothing about each other.  This case expresses the
 basic semantics of what a server should tell a client if it
 understands datasets or referrals.  Since a referral specifies the
 exact dataset to which it is referring, a node in the list of visited
 nodes is made up of a serviceuri and a dataseturi.  Both of these
 values need to be considered during loop detection.  In the case
 where a service does not support datasets, the visited node is made
 up of the service and the 'default dataset'.
 The major thing to remember when doing loop detection across servers
 is that some servers may not understand datasets at all, while others
 specifically rely on them.  To help determine how loop detection
 nodes should be marked, three specific status messages have been
 defined:
 The 3.1.3 (Datasets not supported) status message is used to denote
 that the server does not support datasets at all.  It is sent in
 response to a query containing datasets.  The client should consider
 that the server ignored the datasets and the client should consider
 this node to have been visited for all possible datasets (including
 the 'default' dataset).
 The 3.1.4 (First dataset only supported) status message is used by a
 server to indicate the situation where a client has included several
 dataseturis in its query and the server can only support one at a
 time.  In this case, the server is explicitly stating that it used
 the first dataseturi only.  The client should consider that only the
 first dataseturi specified was processed correctly.  The client
 should consider that the remaining datasets in the query were ignored
 completely.  They would need to be sent individually as referrals if
 the client really cares about those results.  Only the first
 serviceuri/dataseturi pair should be marked as visited.
 The 3.1.5 (This dataset not supported) status message is used to
 indicate that a specific dataseturi sent in a query by a client is
 not supported by the server.  This serviceuri/dataseturi pair should
 be considered as visited by the client.  If this message is sent in
 reply to a query specifying multiple datasets, the client should
 behave the same as if it received the 3.1.3 message from above.  It
 should be considered bad form for a server to send this status
 message back in response to a query with multiple datasets because it
 is ambiguous.

Popp, et. al. Standards Track [Page 23] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 While there is no exact algorithm for loop detection that clients are
 encouraged to support, these status messages can be used by the
 server to be clear about what Services and Datasets it considers to
 have been queried.  It is up to the client to decide what to do with
 these messages and how closely it attempts to do loop detection.

4.2.6 Discoverability: ServiceQuery and Schema

 A subclass of Query, the ServiceQuery object supports the dynamic
 discovery of a specific CNRP service's characteristics.  Note that
 CNRP compliance does not require that a service fully implements
 discoverability.  In particular, returning the Service object with
 its serviceuri constitutes a minimal yet sufficient compliant
 implementation.  Nevertheless, we expect that advanced CNRP services
 will choose to return a full description of their supported
 interfaces.
 The complete response to a servicequery returns the Service object
 described in section 5.3.2 with the following schema information:
 1.  The base and custom properties used by the CNRP service (Property
     schema),
 2.  The properties used to describe the Service object (Service
     schema),
 3.  The properties that belong to the query interface (Query schema),
 4.  The properties that belong to a resource within the results
     (Resource schema).
 These leads to the following new object definitions:
 o  propertyschema -- A property schema describes all the custom
    properties that are part of the service.
 o  propertydeclaration -- A property declaration describes a base or
    custom property used by the CNRP service.  A property declaration
    has a name and a type (the name and the type of the property that
    it refers to).  Note that as part of the property schema, one MUST
    declare both existing and newly defined properties.
 o  propertyreference -- A property reference is a reference to a
    property declaration so that a given schema (a service, query or
    resource schema) can declare the property within its interface.
    Note that a property reference specify whether the use of the
    property is required or optional only.

Popp, et. al. Standards Track [Page 24] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 o  serviceschema -- The service schema defines the properties used to
    describe the service.
 o  queryschema -- A query schema describes the structure of a query
    handled by the CNRP service.  The properties referred within the
    query schema are part of the query interface of the resolution
    service.
 o  resourcedescriptorschema -- A ResourceDescriptor schema describes
    the resource returned as a result by the CNRP service.
 For example, a CNRP query to discover a service's capabilities will
 be in the form:
 <cnrp> <servicequery/> </cnrp>
 And for a CNRP service for cocktail recipes in French, the
 corresponding response would be:
 <service>
      <serviceuri>http://cnrp.recipe.com</serviceuri>
      <propertyschema>
         <propertydeclaration id="i1">
               <propertyname>language</propertyname>
               <propertytype>rfc1766</propertytype>
         </propertydeclaration>
         <propertydeclaration id="i2">
               <propertyname>cocktailrecipe</propertyname>
               <propertytype>freeform</propertytype>
         </propertydeclaration>
      </propertyschema>
      <queryschema>
           <propertyreference required="yes" ref="i1"/>
      </queryschema>
      <resourcedescriptorschema>
           <propertyreference required="yes" ref="i1"/>
           <propertyreference required="yes" ref="i2"/>
      </resourcedescriptorschema>
 </service>
 This response stipulates that the service accepts the property
 language as part of the query interface and returns
 resourcedescriptors that contain both the language and cocktailRecipe
 properties.

Popp, et. al. Standards Track [Page 25] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

5. XML DTD for CNRP

 <!-- The document tag -->
 <!ELEMENT cnrp (query|results|servicequery)>
 <!-- Used to request a Service object -->
 <!ELEMENT servicequery EMPTY>
 <!-- A query can either request a schema, a specific record by -->
 <!-- id, or a common-name with a set of properties (or         -->
 <!-- assertions) about the entity doing the query.             -->
 <!ELEMENT query (id|(commonname,property*))>
 <!ELEMENT id (#PCDATA)>
 <!ELEMENT commonname (#PCDATA)>
 <!-- NOTE: CNRP defines several well known properties          -->
 <!-- and types. See Appendix A for details.                    -->
 <!ELEMENT property (#PCDATA)>
 <!-- The name of the property -->
 <!ATTLIST property name CDATA #REQUIRED>
 <!-- The type of the property -->
 <!ATTLIST property type CDATA "freeform">
 <!ELEMENT results (status? |
                    ( service+,
                         ( status  | resourcedescriptor | referral )*
                    )*
                   )>
 <!ELEMENT resourcedescriptor (commonname,id,resourceuri,
     serviceref, datasetref?,
     description,
     property*)>
 <!ATTLIST resourcedescriptor id ID #IMPLIED>
 <!-- The entire point of all this... -->
 <!ELEMENT resourceuri (#PCDATA)>
 <!ELEMENT description (#PCDATA)>
 <!ELEMENT referral (serviceref, datasetref?)>
 <!ATTLIST referral id ID #IMPLIED>
 <!ELEMENT status (#PCDATA)>
 <!ATTLIST status code CDATA #REQUIRED>
 <!ATTLIST status ref IDREF #IMPLIED>
 <!-- serviceRef is used to point to one of a set of provided   -->
 <!-- service objects. This is so that a resource can point to  -->

Popp, et. al. Standards Track [Page 26] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 <!-- which service it came from. We could include the entire   -->
 <!-- service object but then we would be repeating large       -->
 <!-- amounts of information.                                   -->
 <!ELEMENT serviceref EMPTY>
 <!ATTLIST serviceref ref IDREF #IMPLIED>
 <!ELEMENT service (serviceuri, dataset*,
    servers?,
    description?,
    property*,propertyschema?,queryschema?,resourcedescriptorschema?,
    serviceschema?)>
 <!-- The time to live of the schema in seconds since it was   -->
 <!-- retrieved -->
 <!ATTLIST service ttl CDATA "0">
 <!ATTLIST service id ID #IMPLIED>
 <!ELEMENT serviceuri (#PCDATA)>
 <!ELEMENT servers (server+)>
 <!ELEMENT server (serveruri, property*)>
 <!ELEMENT serveruri (#PCDATA)>
 <!ELEMENT dataset (property*)>
 <!ATTLIST dataset id ID #IMPLIED>
 <!ELEMENT datasetref EMPTY>
 <!ATTLIST datasetref ref IDREF #IMPLIED>
 <!ELEMENT propertyschema (propertydeclaration*)>
 <!ELEMENT propertydeclaration (propertyname, propertytype*)>
 <!ATTLIST propertydeclaration id ID #IMPLIED>
 <!ELEMENT propertyname (#PCDATA)>
 <!ELEMENT propertytype (#PCDATA)>
 <!-- This specifies if the type is meant to be the default -->
 <!-- type. This is usually reserved for "freeform".        -->
 <!ATTLIST propertytype default (no|yes) "no">
 <!-- The properties you can use in a query -->
 <!ELEMENT queryschema (propertyreference*)>
 <!-- The properties you can expect to see in an Resource -->
 <!ELEMENT resourcedescriptorschema (propertyreference*)>
 <!-- The properties you can expect to find in a Service  -->
 <!-- definition -->
 <!ELEMENT serviceschema (propertyreference*)>
 <!ELEMENT propertyreference EMPTY>

Popp, et. al. Standards Track [Page 27] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 <!-- This specifies if a property is required as part of -->
 <!-- the query. -->
 <!ATTLIST propertyreference ref IDREF #REQUIRED>
 <!ATTLIST propertyreference required (no|yes) "no">

6. Examples

6.1 Service Description Request

 This is what the client sends when it is requesting a servers schema.
 <?xml version="1.0"?>
 <!DOCTYPE cnrp PUBLIC "-//IETF//DTD CNRP 1.0//EN"
  "http://ietf.org/dtd/cnrp-1.0.dtd">
 <cnrp>
      <servicequery />
 </cnrp>
 This is the result.  Notice how the Service tag is used to allow the
 service to describe itself in its own terms.
 <?xml version="1.0"?>
 <!DOCTYPE cnrp PUBLIC "-//IETF//DTD CNRP 1.0//EN"
  "http://ietf.org/dtd/cnrp-1.0.dtd">
 <cnrp>
  <results>
   <service ttl="43200">
     <serviceuri>urn:foo:bar</serviceuri>
     <servers>
       <server>
           <serveruri>http://host1.acmecorp.com:4321/foo?</serveruri>
       </server>
       <server>
           <serveruri>smtp://host2.acmecorp.com:4321/foo?</serveruri>
       </server>
     </servers>
     <description>This is the Acme CNRP Service</description>
     <!-- This property means that Acme specializes in
          tradename services -->
     <property name="category" type="naics">544554</property>
     <property name="BannerAdServer" type="uri">
               http://adserver.acmecorp.com/
     </property>
     <propertyschema>
       <propertydeclaration id="i1">
         <propertyname>workgroupID</propertyname>
         <propertytype default="yes">freeform</propertytype>
         <propertytype default="no">domainname</propertytype>

Popp, et. al. Standards Track [Page 28] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

       </propertydeclaration>
       <propertydeclaration id="i2">
         <propertyname>BannerAdServer</propertyname>
         <propertytype default="yes">URI</propertytype>
       </propertydeclaration>
     </propertyschema>
     <queryschema>
         <propertyreference ref="i1" required="yes" />
     </queryschema>
     <resourcedescriptorschema>
         <propertyreference ref="i1" required="yes" />
     </resourcedescriptorschema>
     <serviceschema>
         <propertyreference ref="i2" required="yes" />
     </serviceschema>
   </service>
  </results>
 </cnrp>

6.2 Sending A Query and Getting A Response

 This is the query that is sent from the client to the server:
 <?xml version="1.0"?>
 <!DOCTYPE cnrp PUBLIC "-//IETF//DTD CNRP 1.0//EN"
  "http://ietf.org/dtd/cnrp-1.0.dtd">
 <cnrp>
  <query>
     <commonname>Fido</commonname>
     <property name="geography" type="iso3166-2">
        CA-QC</property>
     <property name="geography" type="iso3166-1">CA</property>
     <property name="language" type="rfc1766">fr-CA</property>
  </query>
 </cnrp>
 This is the result set.  It is sent back in response to the query.
 This result set includes a referral and a non-fatal error.
 <?xml version="1.0"?>
 <!DOCTYPE cnrp PUBLIC "-//IETF//DTD CNRP 1.0//EN"
  "http://ietf.org/dtd/cnrp-1.0.dtd">
 <cnrp>
   <results>
     <service id="i0">
       <serviceuri>http://acmecorp.com</serviceuri>
     </service>
     <service id="i1">

Popp, et. al. Standards Track [Page 29] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

       <serviceuri>http://serverfarm.acmecorp.com</serviceuri>
     </service>
     <service id="i2">
       <serviceuri>http://servers.acmecorp.co.uk</serviceuri>
     </service>
     <resourcedescriptor>
       <commonname>Fidonet</commonname>
       <id>1333459455</id>
       <resourceuri>http://www.fidonet.ca</resourceuri>
       <serviceref ref="i0" />
       <description>This is ye olde Canadian Fidonet</description>
     </resourcedescriptor>
     <resourcedescriptor>
       <commonname>Fidonet</commonname>
       <id>1333459455</id>
       <resourceuri>http://host:port/bla</resourceuri>
       <serviceref ref="i1" />
       <description>An old Fidonet node</description>
     </resourcedescriptor>
     <referral><serviceref ref="i2" /></referral>
     <status code="3.1.1">
         The language property 'fr-CA' was ignored
     </status>
   </results>
 </cnrp>

7. Transport

 Two CNRP transport protocols are specified.  HTTP is used due to its
 popularity and ease of integration with other web applications.  SMTP
 is also used as a way to illustrate a protocol that has a much
 different range of  latency than most protocols.
 In the cases where transports use MIME Media Types (HTTP and SMTP
 being examples of such), the CNRP payload MUST use the
 'application/cnrp+xml' media type.  See Section 8 for the
 registration template for this media type.  One important note about
 this media type is that, since CNRP always uses UTF-8, there is no
 charset attribute.

7.1 HTTP Transport

 The HTTP transport is fairly simple.  The client connects to an HTTP
 based CNRP server and issues a request using the POST method to the
 "/" path with the Content-type and Accept header set to
 "application/cnrp+xml".  The content of the POST body is the CNRP XML
 document that is being sent.  All HTTP 1.1 features are allowed
 during the request.

Popp, et. al. Standards Track [Page 30] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 The results are sent back to the client with a Content-Type of
 "application/cnrp+xml".  The body of the result is the CNRP XML
 document being  sent to the client.

7.2 SMTP Transport

 The SMTP transport is very similar to the HTTP transport.  Since
 there is no method to specify, the CNRP XML document is simply sent
 to a particular SMTP endpoint with its Content-Type set to
 "application/cnrp+xml".  The server responds by sending a response to
 the originator of the request with the results in the body and the
 Content-Type set to "application/cnrp+xml".  The Service MUST specify
 at least one SMTP target (email address) to contact.

8. Registration: application/cnrp+xml

 This is the registration template for 'application/cnrp+xml' per [6].
 MIME media type name: application
 MIME subtype name: cnrp+xml
 Required parameters: none
 Optional parameters: none
 Encoding considerations: This media type consists of 8bit text which
    may necessitate the use of an appropriate content transfer
    encoding on some transports.  Since these considerations are the
    same as XML in general, RFC3023's [6] discussion of XML and MIME
    is applicable.
 Security considerations: none specific to this media type.  See
    Section 9 for general CNRP considerations.
 Interoperability considerations: n/a
 Published specification: This media type is a proper subset of the
    the XML 1.0 specification [8] except for the limitations placed on
    tags and encodings by this document.
 Applications which use this media type: any CNRP client/server
    wishing to send or receive CNRP requests or responses
 Additional Information: none
 Contact for further information: c.f., the "Author's Address" section
    of this memo

Popp, et. al. Standards Track [Page 31] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 Intended usage: limited use
 Author/Change controller: the IESG

9. Security Considerations

 Three security threats exist for CNRP or applications that depend on
 it:  Man in the Middle attacks, malicious agents posing as a service
 by spoofing a Service object, and denial of service attacks caused by
 adding a new level of indirection for resolution of a resource.
 The proposed solution for man in the middle attacks is to utilize
 transport level authentication and encryption, where available.  In
 the case where the transport can't provide the level of required
 authentication, individual entries or the entire response can be
 signed/encrypted using XML signature methods being developed by the
 XMLDSIG Working Group.
 In the case of where a service attempts to pose as another by
 spoofing the serviceuri in the Service object, the Service object
 should be signed.  A client can then verify the Service object's
 veracity by verifying the signature.  How the client obtains that
 authoritative public key is out of scope since it depends on the
 service discovery problem.
 While this document cannot propose a solution for Denial Of Service
 (DOS) attacks, it can illustrate that, like many other cases, any
 time a new level of indirection is created, an opportunity for a DOS
 attack is created.  Service providers are encouraged to be aware of
 this and to act accordingly to mitigate the effects of a DOS attack.

10. IANA Considerations

 The major consideration for the IANA is that the IANA will be
 registering well known properties, property types and status
 messages.  It will not register values.  Since this document does not
 discuss CNRP service discovery, the IANA will not be registering the
 existence of servers or Server objects.
 There are three types of entities the IANA can register: properties,
 property types, and status messages.  If a property or type is not
 registered with the IANA, then they must start with "x-".  Status
 messages can be created for local consumption and not registered.
 There is no requirement that new status messages are mandatory to
 implement unless this document is updated.  Status message
 registrations are more for informational purposes.

Popp, et. al. Standards Track [Page 32] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 The required information for the registration of a new property is
 the property's name, its default type, and a general description.  A
 new type requires the type's name, what properties it is valid for,
 and a description.  A new status message requires the X.Y.ZZZ code
 and a brief description of the state being communicated.
 All properties, types and status messages are registered on a First
 Come First Served basis with no review by the IANA or any group of
 experts.  The consensus opinion of the CNRP Working Group is that
 review of property registrations should occur once there is
 operational experience with the protocol and an actual need for the
 review.  If, at some future date, this policy needs to change, this
 document will be updated.
 The property and type registration templates found in Appendix A
 should be registered by the IANA at publication time of this
 document.
 The IANA is also directed to register the Media Type specified in
 Section 8.

References

 [1]   United States, "North American Industry Classification System",
       January 1997, <http://www.census.gov/epcd/www/naics.html>.
 [2]   Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
       Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol --
       HTTP/1.1", RFC 2616, June 1999.
 [3]   Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
       Resource Identifiers (URI): Generic Syntax", RFC 2396, August
       1998.
 [4]   Alvestrand, H., "Tags for the Identification of Languages", RFC
       1766, March 1995.
 [5]   Moats, R., "URN Syntax", RFC 2141, May 1997.
 [6]   Murata, M., St. Laurent, S. and D. Kohn, "XML Media Types", RFC
       3023, January 2001.
 [7]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [8]   Bray, T., Paoli, J. and C. Sperberg-McQueen, "Extensible Markup
       Language (XML) 1.0", February 1998.

Popp, et. al. Standards Track [Page 33] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 [9]   Mealling, M., "The 'go' URI Scheme for the Common Name
       Resolution Protocol", RFC 3368, August 2002.
 [10]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 1893,
       January 1996.
 [11]  "Country and Region Codes", ISO 3166, January 1996.

Popp, et. al. Standards Track [Page 34] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

Appendix A. Well Known Property and Type Registration Templates

A.1 Properties

 Property Name: geography
 Default Type: iso3166-1
 Description: A geographic location
 Property Name: language
 Default Type: rfc1766
 Description: A language specification
 Property Name: category
 Default Type: freeform
 Description: A node in some system of semantic relationships that is
 considered relevant to the common-name.
 Property Name: range
 Default Type: range
 Description: A range given in the format "x,y" where x is the
 starting point and y is the length.  This property is used by the
 client to tell the server that is is requesting a subrange of the
 results.
 Property Name: dataseturi
 Default Type: uri
 Description: A URI used to disambiguate between two Datasets offered
 by the same Service.

A.2 Types

 Type: freeform
 Property: category
 Description: The value is to be interpreted by the server the best
 way it knows how.  This value has no defined structure.

Popp, et. al. Standards Track [Page 35] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 Type: freeform
 Property: geography
 Description: The value is to be interpreted by the server the best
 way it knows how.  This value has no defined structure.
 Type: freeform
 Property: language
 Description: The value is to be interpreted by the server the best
 way it knows how.  This value has no defined structure.
 Type: iso3166-2
 Property: geography
 Description: The combination of country and sub-region codes found in
 ISO 3166-2 [11].
 Type: iso3166-1
 Property: Geography
 Description: Country Codes found in ISO 3166-1 [11].
 Type: postalcode
 Property: Geography
 Description: A postal code that is valid for some region.  A good
 example is the Zip code system used in the US.
 Type: lat-long
 Property: Geography
 Description:
    Values for latitude and longitude shall be expressed as decimal
    fractions of degrees.  Whole degrees of latitude shall be
    represented by a two-digit decimal number ranging from 0 through
    90.  Whole degrees of longitude shall be represented by a decimal
    number ranging from 0 through 180.  When a decimal fraction of a
    degree is specified, it shall be separated from the whole number
    of degrees by a decimal point.  Decimal fractions of a degree may
    be expressed to the precision desired.
    Latitudes north of the equator shall be specified by a plus sign
    (+), or by the absence of a minus sign (-), preceding the
    designating degrees.  Latitudes south of the Equator shall be
    designated by a minus sign (-) preceding the two digits
    designating degrees.  A point on the Equator shall be assigned to
    the Northern Hemisphere.
    Longitudes east of the prime meridian shall be specified by a plus
    sign (+), or by the Longitudes west of the meridian shall be
    designated by minus sign (-) preceding the digits designating
    degrees.  A point on the prime meridian shall be assigned to the

Popp, et. al. Standards Track [Page 36] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

    Eastern Hemisphere.  A point on the 180th meridian shall be
    assigned to the Western Hemisphere.  One exception to this last
    convention is permitted.  For the special condition of describing
    a band of latitude around the earth, the East Bounding Coordinate
    data element shall be assigned the value +180 (180) degrees.
    Any spatial address with a latitude of +90 (90) or -90 degrees
    will specify the position at the North or South Pole,
    respectively.  The component for longitude may have any legal
    value.
    With the exception of the special condition described above, this
    form is specified in Department of Commerce, 1986, Representation
    of geographic point locations for information interchange (Federal
    Information Processing Standard 70-1):  Washington, Department of
    Commerce, National Institute of Standards and Technology.
          DEGREES   = *PLUSMINUS DIGITS '.' DIGITS
          PLUSMINUS = + | -
          DIGITS    = DIGIT *DIGIT
          DIGIT     = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9
 Type: rfc1766
 Property: Language
 Description: language codes as defined by RFC 1766 [4]
 Type: naics
 Property: Category
 Description: North American Industry Code System [1]
 Type: uri
 Property: dataseturi
 Description: A URI adhering to the 'absoluteURI' production of the
 Collected ABNF found in [3]

Appendix B. Status Codes

B.1 Level 1 (Informative) Codes

 1.0.0 -- Undefined Information
    This code is used for any non-categorizable and informative
    message.  If, for example, the server wanted to tell the client
    that the systems administrator's cat has blue hair, then this code
    would be the appropriate place for this information.

Popp, et. al. Standards Track [Page 37] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 1.1.0 -- Query related information
    This code is used for any informative information concerning the
    query that client sent.  For example, "The query you sent was
    rather interesting!".
 1.2.0 -- An informative message pertaining to the Service
    This message concerns the Service in the general sense.

B.2 Level 2 (Success) Codes

 2.0.0 -- Something undefined succeeded
    There was success but the situation that this message concerns is
    undefined.
 2.1.0 -- Query succeeded
    The query succeeded.  This message MUST be returned when there
    were no results that matched the query.  I.e., the query was
    successfully handled and the correct set of results contained no
    resources or referrals.  The lack of results is not an error but a
    successful statement about the common-name.
 Note: The apparent lack of 2.X.X level codes is caused by success
 usually being indicated not by a status message but by the server
 returning only the objects that the client requested.

B.3 Level 3 (Partial Success) Codes

 3.0.0 -- Something undefined was only partially successful
    Some request by the client was only partially successful.  The
    exact situation or cause of that partial failure is not defined.
 3.1.0 -- The query was only partially successful.
 3.1.1 -- The query contained invalid or unsupported properties
    The query contained invalid or unsupported property names, types
    or values.  The invalid properties were ignored and the query
    processed.
 3.1.2 -- The XML was well formed but invalid
    The XML sent by the client was well formed but invalid.  The
    server was smart enough to figure out what the client was talking
    about and return some results.

Popp, et. al. Standards Track [Page 38] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

 3.1.3 Server does not support datasets
    This status should be generated by servers that do not handle
    datasets.  A server can send this status message at any time, but
    it especially useful for when a server receives a query from a
    client that contains a dataseturi.  In this case and if the client
    is doing rigorous loop detection, the client should consider this
    entire service to have been visited.
 3.1.4 The first dataset in the list of datasets you gave in the
       query was the only one used.
    This status message is used by a server to indicate the situation
    where a client has included several dataseturis in its query and
    the server can only support one at a time.  In this case the
    server is explicitly stating that it used the first dataseturi
    only.  The client should consider that only the first dataseturi
    specified was processed correctly.  The client should consider
    that the remaining datasets in the query were ignored completely.
    They would need to be sent individually as referrals if the client
    really cares about those results.  Only the first
    serviceuri/dataseturi pair should be marked as visited if loop
    detection is being handled.
 3.1.5 This dataset not supported.
    This message is used to indicate that a specific dataseturi sent
    in a query by a client is not supported by the server.  This
    serviceuri/dataseturi pair should be considered as visited by the
    client.  If this message is sent in reply to a query specifying
    multiple datasets, the client should behave the same as if it
    received the 3.1.3 message from above.  It should be considered
    bad form for a server to send this status message back in response
    to a query with multiple datasets because it is ambiguous.
 3.2.0 -- The server caused a partially successful event
    Due to some internal server error, the results returned were
    incomplete.
 3.2.1 -- Some referral server was unavailable
    This status message is used to denote that one or more of the
    referral services that are normally queried was unavailable.
    Results were generated, but they may not be representative of a
    complete answer.

Popp, et. al. Standards Track [Page 39] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

B.4 Level 4 (Transient Failure) Codes

 4.0.0 -- Something undefined caused a persistent transient failure.
 4.1.0 -- There was an error in the query that made it unable to be
          interpreted.
 4.2.0 -- The query was to complex
    The query as specified was too complex for this Service to handle.
 4.2.1 -- The Service was too busy
    Due to resource constraints, the entire service is too busy to
    handle requests.  This means that any of the Servers cooperating
    in providing this Service would have also returned this same
    message.
 4.2.2 -- The Server is in maintenance
    This server is now in maintenance mode.  Try another server from
    this service or try again at a later time.
 4.2.3 -- The Server had an internal error
    There was an internal error that caused the server to fail
    completely.

B.5 Level 5 (Permanent Failures) Codes.

 5.0.0 -- Something undefined caused a permanent failure.
 5.1.0 -- The query permanently failed.
 5.2.0 -- The service had a permanent failure.
 5.2.1 -- This Service is no longer available.
    This Service has decided to no longer make itself available.
 5.2.2 -- The Server had a permanent failure.
    This server has permanently failed.  Try another server from this
    service.

Popp, et. al. Standards Track [Page 40] RFC 3367 Common Name Resolution Protocol (CNRP) August 2002

Authors' Addresses

 Nico Popp
 VeriSign, Inc.
 487 East Middlefield Road
 Mountain View, CA 94043
 Phone: (650) 426-3291
 EMail: npopp@verisign.com
 Michael Mealling
 VeriSign, Inc.
 21345 Ridgetop Circle
 Sterling, VA  20166
 US
 EMail: michael@verisignlabs.com
 Marshall Moseley
 Netword, Inc.
 702 Russell Avenue
 Gaithersburg, MD  20877-2606
 US
 Phone: (240) 631-1100
 EMail: marshall@netword.com

Popp, et. al. Standards Track [Page 41] RFC 3367 Common Name Resolution Protocol (CNRP) August 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.

Popp, et. al. Standards Track [Page 42]

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