draft-young-md-query.txt

Network Working Group                                    I.A. Young, Ed.
Internet-Draft                                               Independent
Intended status: Informational                            7 January 2025
Expires: 11 July 2025


                        Metadata Query Protocol
                        draft-young-md-query-22

Abstract

   This document defines a simple protocol for retrieving metadata about
   named entities, or named collections of entities.  The goal of the
   protocol is to profile various aspects of HTTP to allow requesters to
   rely on certain, rigorously defined, behaviour.

   This document is a product of the Research and Education Federations
   (REFEDS) Working Group process.

Editorial Note (To be removed by RFC Editor before publication)

   Discussion of this draft takes place on the MDX mailing list
   (mdx@lists.iay.org.uk), which is accessed from [MDX.list].

   XML versions, latest edits and the issues list for this document are
   available from [md-query].

   The changes in this draft are summarized in Appendix A.23.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 July 2025.






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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Notation and Conventions  . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Protocol Transport  . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Transport Protocol  . . . . . . . . . . . . . . . . . . .   4
     2.2.  HTTP Version  . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  HTTP Method . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Request Headers . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Response Headers  . . . . . . . . . . . . . . . . . . . .   5
     2.6.  Status Codes  . . . . . . . . . . . . . . . . . . . . . .   6
     2.7.  Base URL  . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.8.  Content Negotiation . . . . . . . . . . . . . . . . . . .   7
   3.  Metadata Query Protocol . . . . . . . . . . . . . . . . . . .   7
     3.1.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Request by Identifier . . . . . . . . . . . . . . . .   7
       3.2.2.  Request All Entities  . . . . . . . . . . . . . . . .   8
       3.2.3.  Response  . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.4.  Example Request and Response  . . . . . . . . . . . .   9
   4.  Efficient Retrieval and Caching . . . . . . . . . . . . . . .   9
     4.1.  Conditional Retrieval . . . . . . . . . . . . . . . . . .   9
     4.2.  Content Caching . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Content Compression . . . . . . . . . . . . . . . . . . .  10
   5.  Protocol Extension Points . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     6.1.  Integrity . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.2.  Confidentiality . . . . . . . . . . . . . . . . . . . . .  10
     6.3.  Authentication  . . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Change Log (to be removed by RFC Editor before
           publication)  . . . . . . . . . . . . . . . . . . . . . .  13



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     A.1.  Since draft-lajoie-md-query-01  . . . . . . . . . . . . .  13
     A.2.  Since draft-young-md-query-00 . . . . . . . . . . . . . .  13
     A.3.  Since draft-young-md-query-01 . . . . . . . . . . . . . .  14
     A.4.  Since draft-young-md-query-02 . . . . . . . . . . . . . .  14
     A.5.  Since draft-young-md-query-03 . . . . . . . . . . . . . .  14
     A.6.  Since draft-young-md-query-04 . . . . . . . . . . . . . .  14
     A.7.  Since draft-young-md-query-05 . . . . . . . . . . . . . .  15
     A.8.  Since draft-young-md-query-06 . . . . . . . . . . . . . .  15
     A.9.  Since draft-young-md-query-07 . . . . . . . . . . . . . .  15
     A.10. Since draft-young-md-query-08 . . . . . . . . . . . . . .  15
     A.11. Since draft-young-md-query-09 . . . . . . . . . . . . . .  15
     A.12. Since draft-young-md-query-10 . . . . . . . . . . . . . .  15
     A.13. Since draft-young-md-query-11 . . . . . . . . . . . . . .  15
     A.14. Since draft-young-md-query-12 . . . . . . . . . . . . . .  16
     A.15. Since draft-young-md-query-13 . . . . . . . . . . . . . .  16
     A.16. Since draft-young-md-query-14 . . . . . . . . . . . . . .  16
     A.17. Since draft-young-md-query-15 . . . . . . . . . . . . . .  16
     A.18. Since draft-young-md-query-16 . . . . . . . . . . . . . .  16
     A.19. Since draft-young-md-query-17 . . . . . . . . . . . . . .  16
     A.20. Since draft-young-md-query-18 . . . . . . . . . . . . . .  16
     A.21. Since draft-young-md-query-19 . . . . . . . . . . . . . .  16
     A.22. Since draft-young-md-query-20 . . . . . . . . . . . . . .  16
     A.23. Since draft-young-md-query-21 . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   Many clients of web-based services are capable of consuming
   descriptive metadata about a service in order to customize or obtain
   information about the client's connection parameters.  While the form
   of the metadata (e.g., JSON, XML) and content varies between services
   this document specifies a set of semantics for HTTP ([RFC7230] et
   seq.) that allow clients to rely on certain behavior.  The defined
   behavior is meant to make it easy for clients to perform queries, to
   be efficient for both requesters and responders, and to allow the
   responder to scale in various ways.

   The Research and Education Federations group ([REFEDS]) is the voice
   that articulates the mutual needs of research and education identity
   federations worldwide.  It aims to represent the requirements of
   research and education in the ever-growing space of access and
   identity management.









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   From time to time REFEDS will wish to publish a document in the
   Internet RFC series.  Such documents will be published as part of the
   RFC Independent Submission Stream [RFC4844]; however the REFEDS
   working group sign-off process will have been followed for these
   documents, as described in the REFEDS Participant's Agreement
   [REFEDS.agreement].

   This document is a product of the REFEDS Working Group process.

1.1.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document makes use of the Augmented BNF metalanguage defined in
   [STD68].

1.2.  Terminology

   entity:  A single logical construct for which metadata may be
      asserted.  Generally this is a network accessible service.

   metadata:  A machine readable description of certain entity
      characteristics.  Generally metadata provides information such as
      end point references, service contact information, etc.

2.  Protocol Transport

   The metadata query protocol seeks to fully employ the features of the
   HTTP protocol.  Additionally this specification makes mandatory some
   optional HTTP features.

2.1.  Transport Protocol

   The metadata query protocol makes use of the HTTP protocol
   ([RFC7230]) to transmit requests and responses.  The underlying HTTP
   connection MAY make use of any appropriate transport protocol.  In
   particular, the HTTP connection MAY make use of either TCP or TLS at
   the transport layer.  See the Security Considerations section for
   guidance in choosing an appropriate transport protocol.








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2.2.  HTTP Version

   Requests from clients MUST NOT use an HTTP version prior to version
   1.1.  Responders MUST reply to such requests using status code 505,
   "HTTP Version Not Supported".

   Protocol responders MUST support requests using HTTP version 1.1, and
   MAY support later versions.

2.3.  HTTP Method

   All metadata query requests MUST use the GET method.

2.4.  Request Headers

   All metadata query requests MUST include the following HTTP headers:

      Accept - this header MUST contain the content-type identifying the
      type, or form, of metadata to be retrieved.  See section 5.3.2 of
      [RFC7231].

   All metadata query requests SHOULD include the following HTTP
   headers:

      Accept-Charset, see section 5.3.3 of [RFC7231]

      Accept-Encoding, see section 5.3.4 of [RFC7231]

   A metadata request to the same URL, after an initial request, MUST
   include the following header:

      If-None-Match, see section 3.2 of [RFC7232].

2.5.  Response Headers

   All successful metadata query responses (even those that return no
   results) MUST include the following headers:

      Content-Encoding - required if, and only if, content is
      compressed.  See section 3.1.2.2 of [RFC7231].

      Content-Type, see section 3.1.1.5 of [RFC7231].

      ETag, see section 2.3 of [RFC7232].

   All metadata retrieval responses SHOULD include the following
   headers:




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      Cache-Control, see section 5.2 of [RFC7234].

      Content-Length, see section 3.3.2 of [RFC7230]

      Last-Modified, see section 2.2 of [RFC7232].

2.6.  Status Codes

   This protocol uses the following HTTP status codes:

      200 "OK" - standard response code when returning requested
      metadata

      304 "Not Modified" - response code indicating requested metadata
      has not been updated since the last request

      400 "Bad Request" - response code indicating that the requester's
      request was malformed in some fashion

      401 "Unauthorized" - response code indicating the request must be
      authenticated before requesting metadata

      404 "Not Found" - indicates that the requested metadata could not
      be found; this MUST NOT be used in order to indicate a general
      service error.

      405 "Method Not Allowed" - response code indicating that a non-GET
      method was used

      406 "Not Acceptable" - response code indicating that metadata is
      not available in the request content-type

      505 "HTTP Version Not Supported" - response code indicating that
      HTTP/1.1 was not used

2.7.  Base URL

   Requests defined in this document are performed by issuing an HTTP
   GET request to a particular URL ([STD66]).  The final component of
   the path to which requests are issued is defined by the requests
   specified within this document.  A base URL precedes such paths.
   Such a base URL:

   *  MUST contain the scheme and authority components.

   *  MUST contain a path component ending with a slash ('/') character.

   *  MUST NOT include a query component.



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   *  MUST NOT include a fragment identifier component.

2.8.  Content Negotiation

   As there may be many representations for a given piece of metadata,
   agent-driven content negotiation is used to ensure the proper
   representation is delivered to the requester.  In addition to the
   required usage of the Accept header a responder SHOULD also support
   the use of the Accept-Charset header.

3.  Metadata Query Protocol

   The metadata query protocol retrieves metadata either for all
   entities known to the responder or for a named collection based on a
   single "tag" or "keyword" identifier.  A request returns information
   for none, one, or a collection of entities.

3.1.  Identifiers

   The query protocol uses identifiers to "tag" metadata for single- and
   multi-entity metadata collections.  The assignment of such
   identifiers to a particular metadata document is the responsibility
   of the query responder.  If a metadata collection already contains a
   well known identifier it is RECOMMENDED that such a natural
   identifier is used when possible.  Any given metadata collection MAY
   have more than one identifier associated with it.

   An identifier used in the query protocol is a non-empty sequence of
   arbitrary 8-bit characters:

   id       = 1*idchar
   idchar   = %x00-ff ; any encodable character

3.2.  Protocol

3.2.1.  Request by Identifier

   A metadata query request for all entities tagged with a particular
   identifier is performed by issuing an HTTP GET request to a URL
   constructed as the concatenation of the following components:

   *  The responder's base URL.

   *  The string "entities/".

   *  A single identifier, percent-encoded appropriately for use as a
      URL path segment (see sections 2.1 and 3.3 of [STD66]).




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   For example, with a base URL of http://example.org/mdq/, a query for
   the identifier foo would be performed by an HTTP GET request to the
   following URL:

   http://example.org/mdq/entities/foo

   Correct encoding of the identifier as a URL path segment is critical
   for interoperability.  In particular:

      The character '/' MUST be percent-encoded.

      The space character MUST be encoded as '%20' and MUST NOT be
      encoded as '+' as would be required in a query parameter.

   For example, with a base URL of http://example.org/mdq/, a query for
   the identifier "blue/green+light blue" would be performed by an HTTP
   GET request to the following URL:

   http://example.org/mdq/entities/blue%2Fgreen+light%20blue

3.2.2.  Request All Entities

   A metadata query request for all entities known to the responder is
   performed by issuing an HTTP GET request to a URL constructed as the
   concatenation of the following components:

   *  The responder's base URL.

   *  The string "entities".

   For example, with a base URL of http://example.org/mdq/, a query for
   all entities would be performed by an HTTP GET request to the
   following URL:

   http://example.org/mdq/entities

3.2.3.  Response

   The response to a metadata query request MUST be a document that
   provides metadata for the given request in the format described by
   the request's Accept header.

   The responder is responsible for ensuring that the metadata returned
   is valid.  If the responder can not create a valid document it MUST
   respond with a 406 status code.  An example of such an error would be
   the case where the result of the query is metadata for multiple
   entities but the request content type does not support returning
   multiple results in a single document.



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3.2.4.  Example Request and Response

   The following example demonstrates a metadata query request using a
   base URL of http://metadata.example.org/service/ and the identifier
   http://example.org/idp.

   GET /service/entities/http:%2F%2Fexample.org%2Fidp HTTP/1.1
   Host: metadata.example.org
   Accept: application/samlmetadata+xml

                  Figure 1: Example Metadata Query Request

   HTTP/1.x 200 OK
   Content-Type: application/samlmetadata+xml
   ETag: "abcdefg"
   Last-Modified: Thu, 15 Apr 2010 12:45:26 GMT
   Content-Length: 1234

   <?xml version="1.0" encoding="UTF-8"?>
   <EntityDescriptor entityID="http://example.org/idp"
       xmlns="urn:oasis:names:tc:SAML:2.0:metadata">
   ....

                 Figure 2: Example Metadata Query Response

4.  Efficient Retrieval and Caching

4.1.  Conditional Retrieval

   Upon a successful response the responder MUST return an ETag header
   and MAY return a Last-Modified header as well.  Requesters SHOULD use
   either or both, with the ETag being preferred, in any subsequent
   requests for the same resource.

   In the event that a resource has not changed since the previous
   request, the responder SHOULD send a 304 (Not Modified) status code
   as a response.

4.2.  Content Caching

   Responders SHOULD include cache control information with successful
   (200 status code) responses, assuming the responder knows when
   retrieved metadata is meant to expire.  The responder SHOULD also
   include cache control information with 404 Not Found responses.  This
   allows the requester to create and maintain a negative-response
   cache.  When cache controls are used only the 'max-age' directive
   SHOULD be used.




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4.3.  Content Compression

   As should be apparent from the required request and response headers
   this protocol encourages the use of content compression.  This is in
   recognition that some metadata documents can be quite large or
   fetched with relatively high frequency.

   Requesters SHOULD support, and advertise support for, gzip
   compression unless such usage would put exceptional demands on
   constrained environments.  Responders MUST support gzip compression.
   Requesters and responders MAY support other compression algorithms.

5.  Protocol Extension Points

   The Metadata Query Protocol is extensible using the following
   protocol extension points:

   *  Profiles of this specification may assign semantics to specific
      identifiers, or to identifiers structured in particular ways.

   *  Profiles of this specification may define additional paths (other
      than entities and entities/) below the base URL.

6.  Security Considerations

6.1.  Integrity

   As metadata often contains information necessary for the secure
   operation of interacting services it is RECOMMENDED that some form of
   content integrity checking be performed.  This may include the use of
   TLS at the transport layer, digital signatures present within the
   metadata document, or any other such mechanism.

6.2.  Confidentiality

   In many cases service metadata is public information and therefore
   confidentiality is not required.  In the cases where such
   functionality is required, it is RECOMMENDED that both the requester
   and responder support TLS.  Other mechanisms, such as XML encryption,
   MAY also be supported.

6.3.  Authentication

   All responders which require client authentication to view retrieved
   information MUST support the use of HTTP basic authentication
   ([RFC7235], [RFC7617]) over TLS.  Responders SHOULD also support the
   use of X.509 client certificate authentication.




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7.  IANA Considerations

   This document has no actions for IANA.

8.  Acknowledgements

   The editor would like to acknowledge the following individuals for
   their contributions to this document:

      Scott Cantor (The Ohio State University)

      Leif Johansson (SUNET)

      Thomas Lenggenhager (SWITCH)

      Joe St Sauver (University of Oregon)

      Tom Scavo (Internet2)

   Special acknowledgement is due to Chad LaJoie (Covisint) for his work
   in editing previous versions of this specification.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/info/rfc7232>.






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   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,
              <https://www.rfc-editor.org/info/rfc7617>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [STD66]    Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [STD68]    Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

9.2.  Informative References

   [md-query] Young, I.A., Ed., "md-query Project",
              <https://github.com/iay/md-query>.

   [MDX.list] Young, I.A., Ed., "MDX Mailing List",
              <http://lists.iay.org.uk/listinfo.cgi/mdx-iay.org.uk>.

   [REFEDS]   Research and Education Federations, "REFEDS Home Page",
              <http://www.refeds.org/>.

   [REFEDS.agreement]
              Research and Education Federations, "REFEDS Participant's
              Agreement",
              <https://refeds.org/about/about_agreement.html>.

   [RFC4844]  Daigle, L., Ed. and IAB, "The RFC Series and RFC Editor",
              RFC 4844, DOI 10.17487/RFC4844, July 2007,
              <https://www.rfc-editor.org/info/rfc4844>.




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Appendix A.  Change Log (to be removed by RFC Editor before publication)

A.1.  Since draft-lajoie-md-query-01

   Adopted as base for draft-young-md-query-00.

   Updated author and list of contributors.

   Changed ipr from "pre5378Trust200902" to "trust200902", submission
   type from IETF to independent and category from experimental to
   informational.

   Added empty IANA considerations section.

   Minor typographical nits but (intentionally) no substantive content
   changes.

A.2.  Since draft-young-md-query-00

   Split into two documents: this one is as agnostic as possible around
   questions such as metadata format and higher level protocol use
   cases, a new layered document describes the detailed requirements for
   SAML support.

   Rewrite Section 3.2.1 to clarify construction of the request URL and
   its relationship to the base URL.

   Added Section 2.1 to clarify that the transport protocol underlying
   HTTP may be either TCP or SSL/TLS.

   Clarify position on HTTP versions (Section 2.2) which may be used to
   underly this protocol.

   Added Change Log modelled on draft-ietf-httpbis-http2.

   Added a reference to [STD68].  Use ABNF to describe request syntax.
   Replace transformed identifier concept with extended identifiers
   (this also resulted in the removal of any discussion of specific
   transformed identifier formats).  Add grammar to distinguish basic
   from extended identifiers.

   Changed the required response when the result can not be validly
   expressed in the requested format from 500 to 406.

   Removed the '+' operator and all references to multiple identifiers
   in queries.  If more complex queries are required, these will be
   reintroduced at a different path under the base URL.




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   Added a section describing Protocol Extension Points.

A.3.  Since draft-young-md-query-01

   Added REFEDS RFC stream boilerplate.

   Tidied up some normative language.

A.4.  Since draft-young-md-query-02

   Introduced a normative reference to [STD66].

   Reworked the definition of the base URL so that a non-empty path
   ending with '/' is required.  This allows the definition of request
   URLs to be simplified.

   Clarified the definition of the base URL to exclude a query
   component; corrected the terminology for the fragment identifier
   component.

   Added the definition for the query for all entities in Section 3.2.2.

   Corrected an example in Section 3.2.4 to include the required double
   quotes in the value of an ETag header.  Added text to clarify the
   base URL and identifier being used in the example.

   Simplified the definition of identifiers, so that any non-empty
   identifier is accepted and no semantics are defined for particular
   structures.  Extended syntaxes such as the {sha1} notation for
   transformed identifiers are now left to profiles.

   Remove incidental references to SSL.

   Remove status code 501 ("not implemented") as it is no longer
   referenced.

A.5.  Since draft-young-md-query-03

   Correct a typo in the identifier grammar.

A.6.  Since draft-young-md-query-04

   Updated to rely on the new definition of HTTP/1.1 in [RFC7230] et
   seq.  instead of RFC 2616.

   Corrected Section 3.2.3 to indicate that the request contains an
   Accept header, not a Content-Type header.




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   Added an Editorial Note to help direct readers back to the
   discussion.

A.7.  Since draft-young-md-query-05

   Remove unnecessary percent-encoding of a ':' character in the example
   in Section 3.2.4.

   Removed use of the ambiguous term "URL-encoded" in Section 3.2.1.
   Instead, indicate that the encoding must correspond to the rules for
   encoding a URL path segment specifically, and call out some of the
   more important implications arising from that.  Added a new example
   illustrating these implications.

   Updated the description of conditional retrieval in Section 4.1 to
   make the use of a 304 (Not Modified) status code a normative but non-
   mandatory obligation on the responder, not simply a description of
   what the requester will receive.

A.8.  Since draft-young-md-query-06

   No substantive changes.

A.9.  Since draft-young-md-query-07

   No substantive changes.

A.10.  Since draft-young-md-query-08

   Modernise normative language to include [RFC8174].

   Reference [RFC7617] instead of the Internet-Draft.

   Improved references to RFCs.

A.11.  Since draft-young-md-query-09

   No substantive changes.

A.12.  Since draft-young-md-query-10

   No substantive changes.

A.13.  Since draft-young-md-query-11

   No substantive changes.





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A.14.  Since draft-young-md-query-12

   No substantive changes.

A.15.  Since draft-young-md-query-13

   No substantive changes.

A.16.  Since draft-young-md-query-14

   No substantive changes.

A.17.  Since draft-young-md-query-15

   No substantive changes.

A.18.  Since draft-young-md-query-16

   No substantive changes.

A.19.  Since draft-young-md-query-17

   No substantive changes.

A.20.  Since draft-young-md-query-18

   Formatting changes to allow rendering with xml2rfc version 3.

A.21.  Since draft-young-md-query-19

   No substantive changes.

A.22.  Since draft-young-md-query-20

   No substantive changes.

A.23.  Since draft-young-md-query-21

   No substantive changes.

Author's Address

   Ian A. Young (editor)
   Independent
   Email: ian@iay.org.uk






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