An Authorization Information Format (AIF) for Authentication and Authorization for Constrained Environments (ACE)
draft-ietf-ace-aif-07

ACE Working Group                                             C. Bormann
Internet-Draft                                    Universität Bremen TZI
Intended status: Standards Track                           15 March 2022
Expires: 16 September 2022

           An Authorization Information Format (AIF) for ACE
                         draft-ietf-ace-aif-07

Abstract

   Information about which entities are authorized to perform what
   operations on which constituents of other entities is a crucial
   component of producing an overall system that is secure.  Conveying
   precise authorization information is especially critical in highly
   automated systems with large numbers of entities, such as the
   "Internet of Things".

   This specification provides a generic information model and format
   for representing such authorization information, as well as two
   variants of a specific instantiation of that format for use with REST
   resources identified by URI path.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-ace-aif/.

   Discussion of this document takes place on the Authentication and
   Authorization for Constrained Environments (ace) Working Group
   mailing list (mailto:ace@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/ace/.

   Source for this draft and an issue tracker can be found at
   https://github.com/cabo/ace-aif.

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/.

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   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 16 September 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Information Model . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  REST-specific Model . . . . . . . . . . . . . . . . . . .   4
     2.2.  Limitations . . . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  REST-specific Model With Dynamic Resource Creation  . . .   6
   3.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Media Types . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Media Types . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Registries  . . . . . . . . . . . . . . . . . . . . . . .  11
     5.3.  Content-Format  . . . . . . . . . . . . . . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   Constrained Devices as they are used in the "Internet of Things" need
   security in order to operate correctly and prevent misuse.  One
   important element of this security is that devices in the Internet of
   Things need to be able to decide which operations requested of them
   should be considered authorized, need to ascertain that the
   authorization to request the operation does apply to the actual
   requester as authenticated, and need to ascertain that other devices
   they make requests of are the ones they intended.

   To transfer detailed authorization information from an authorization
   manager (such as an ACE-OAuth Authorization Server
   [I-D.ietf-ace-oauth-authz]) to a device, a compact representation
   format is needed.  This document defines such a format, the
   Authorization Information Format (AIF).  AIF is defined both as a
   general structure that can be used for many different applications
   and as a specific instantiation tailored to REST resources and the
   permissions on them, including some provision for dynamically created
   resources.

1.1.  Terminology

   This memo uses terms from CoAP [RFC7252] and the Internet Security
   Glossary [RFC4949]; CoAP is used for the explanatory examples as it
   is a good fit for Constrained Devices.

   The shape of data is specified in CDDL [RFC8610] [RFC9165].
   Terminology for Constrained Devices is defined in [RFC7228].

   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.

   The term "byte", abbreviated by "B", is used in its now customary
   sense as a synonym for "octet".

2.  Information Model

   Authorizations are generally expressed through some data structures
   that are cryptographically secured (or transmitted in a secure way).
   This section discusses the information model underlying the payload
   of that data (as opposed to the cryptographic armor around it).

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   The semantics of the authorization information defined in this
   document are that of an _allow-list_: everything is denied until it
   is explicitly allowed.

   For the purposes of this specification, the underlying access control
   model will be that of an access matrix, which gives a set of
   permissions for each possible combination of a subject and an object.
   We are focusing the AIF data item on a single row in the access
   matrix (such a row has often been called a capability list), without
   concern to the subject for which the data item is issued.  As a
   consequence, AIF MUST be used in a way that the subject of the
   authorizations is unambiguously identified (e.g., as part of the
   armor around it).

   The generic model of such a capability list is a list of pairs of
   object identifiers (of type Toid) and the permissions (of type Tperm)
   the subject has on the object(s) identified.

   AIF-Generic<Toid, Tperm> = [* [Toid, Tperm]]

                    Figure 1: Definition of Generic AIF

   In a specific data model (such as the one also specified in this
   document), the object identifier (Toid) will often be a text string,
   and the set of permissions (Tperm) will be represented by a bitset in
   turn represented as a number (see Section 3).

   AIF-Specific = AIF-Generic<tstr, uint>

              Figure 2: Commonly used shape of a specific AIF

2.1.  REST-specific Model

   In the specific instantiation of the REST resources and the
   permissions on them, for the object identifiers (Toid), we use the
   URI of a resource on a CoAP server.  More specifically, since the
   parts of the URI that identify the server ("authority" in [RFC3986])
   are what are authenticated during REST resource access (Section 4.2.2
   of [I-D.ietf-httpbis-semantics] and Section 6.2 of [RFC7252]), they
   naturally fall into the realm handled by the cryptographic armor; we
   therefore focus on the "path" ("path-abempty") and "query" parts of
   the URI (_URI-local-part_ in this specification, as expressed by the
   Uri-Path and Uri-Query options in CoAP).  As a consequence, AIF MUST
   be used in a way that it is clear who is the target (enforcement
   point) of these authorizations (note that there may be more than one
   target that the same authorization applies to, e.g., in a situation
   with homogeneous devices).

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   For the permissions (Tperm), we use a simple permissions model that
   lists the subset of the REST (CoAP or HTTP) methods permitted.  This
   model is summarized in Table 1.

                    +================+================+
                    | URI-local-part | Permission Set |
                    +================+================+
                    | /s/temp        | GET            |
                    +----------------+----------------+
                    | /a/led         | PUT, GET       |
                    +----------------+----------------+
                    | /dtls          | POST           |
                    +----------------+----------------+

                         Table 1: An authorization
                      instance in the AIF Information
                                   Model

   In this example, a device offers a temperature sensor /s/temp for
   read-only access, a LED actuator /a/led for read/write, and a /dtls
   resource for POST access.

   As will be seen in the data model (Section 3), the representations of
   REST methods provided are limited to those that have a CoAP method
   number assigned; an extension to the model may be necessary to
   represent permissions for exotic HTTP methods.

2.2.  Limitations

   This simple information model only allows granting permissions for
   statically identifiable objects, e.g., URIs for the REST-specific
   instantiation.  One might be tempted to extend the model towards URI
   templates [RFC6570] (for instance, to open up an authorization for
   many parameter values as in /s/temp{?any*}).  However, that requires
   some considerations of the ease and unambiguity of matching a given
   URI against a set of templates in an AIF data item.

   This simple information model also does not allow expressing
   conditionalized access based on state outside the identification of
   objects (e.g., "opening a door is allowed if that is not locked").

   Finally, the model does not provide any special access for a set of
   resources that are specific to a subject, e.g., that the subject
   created itself by previous operations (PUT, POST, or PATCH/iPATCH
   [RFC8132]) or that were specifically created for the subject by
   others.

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2.3.  REST-specific Model With Dynamic Resource Creation

   The REST-specific Model With Dynamic Resource Creation addresses the
   need to provide defined access to dynamic resources that were created
   by the subject itself, specifically, a resource that is made known to
   the subject by providing Location-* options in a CoAP response or
   using the Location header field in HTTP [I-D.ietf-httpbis-semantics]
   (the Location-indicating mechanisms).  (The concept is somewhat
   comparable to "ACL inheritance" in NFSv4 [RFC8881], except that it
   does not use a containment relationship but the fact that the dynamic
   resource was created from a resource to which the subject had
   access.)  In other words, it addresses an important subset of the
   third limitation mentioned in Section 2.2.

          +================+===================================+
          | URI-local-part | Permission Set                    |
          +================+===================================+
          | /a/make-coffee | POST, Dynamic-GET, Dynamic-DELETE |
          +----------------+-----------------------------------+

              Table 2: An authorization instance in the AIF
             Information Model With Dynamic Resource Creation

   For a method X, the presence of a Dynamic-X permission means that the
   subject holds permission to exercise the method X on resources that
   have been returned in a 2.01 (201 Created) response by a Location-
   indicating mechanism to a request that the subject made to the
   resource listed.  In the example shown in Table 2, POST operations on
   /a/make-coffee might return the location of a resource dynamically
   created on the coffee machine that allows GET to find out about the
   status of, and DELETE to cancel, the coffee-making operation.

   Since the use of the extension defined in this section can be
   detected by the mentioning of the Dynamic-X permissions, there is no
   need for another explicit switch between the basic and the model
   extended by dynamic resource creation; the extended model is always
   presumed once a Dynamic-X permission is present.

3.  Data Model

   Different data model specializations can be defined for the generic
   information model given above.

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   In this section, we will give the data model for simple REST
   authorization as per Section 2.1 and Section 2.3.  As discussed, in
   this case the object identifier is specialized as a text string
   giving a relative URI (URI-local-part as absolute path on the server
   serving as enforcement point).  The permission set is specialized to
   a single number REST-method-set by the following steps:

   *  The entries in the table that specify the same URI-local-part are
      merged into a single entry that specifies the union of the
      permission sets.

   *  The (non-dynamic) methods in the permission sets are converted
      into their CoAP method numbers, minus 1.

   *  Dynamic-X permissions are converted into what the number would
      have been for X, plus a Dynamic-Offset chosen as 32 (e.g., 35 is
      the number for Dynamic-DELETE as the number for DELETE is 3).

   *  The set of numbers is converted into a single number REST-method-
      set by taking two to the power of each (decremented) method number
      and computing the inclusive OR of the binary representations of
      all the power values.

   This data model could be interchanged in the JSON [RFC8259]
   representation given in Figure 3.

   [["/s/temp",1],["/a/led",5],["/dtls",2]]

       Figure 3: An authorization instance encoded in JSON (40 bytes)

   In Figure 4, a straightforward specification of the data model
   (including both the methods from [RFC7252] and the new ones from
   [RFC8132], identified by the method code minus 1) is shown in CDDL
   [RFC8610] [RFC9165]:

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   AIF-REST = AIF-Generic<local-path, REST-method-set>
   local-path = tstr   ; URI relative to enforcement point
   REST-method-set = uint .bits methods
   methods = &(
     GET: 0
     POST: 1
     PUT: 2
     DELETE: 3
     FETCH: 4
     PATCH: 5
     iPATCH: 6
     Dynamic-GET: 32; 0 .plus Dynamic-Offset
     Dynamic-POST: 33; 1 .plus Dynamic-Offset
     Dynamic-PUT: 34; 2 .plus Dynamic-Offset
     Dynamic-DELETE: 35; 3 .plus Dynamic-Offset
     Dynamic-FETCH: 36; 4 .plus Dynamic-Offset
     Dynamic-PATCH: 37; 5 .plus Dynamic-Offset
     Dynamic-iPATCH: 38; 6 .plus Dynamic-Offset
   )

                           Figure 4: AIF in CDDL

   For the information shown in Table 1 and Figure 3, a representation
   in CBOR [RFC8949] is given in Figure 5; again, several optimizations/
   improvements are possible.

   83                        # array(3)
      82                     # array(2)
         67                  # text(7)
            2f732f74656d70   # "/s/temp"
         01                  # unsigned(1)
      82                     # array(2)
         66                  # text(6)
            2f612f6c6564     # "/a/led"
         05                  # unsigned(5)
      82                     # array(2)
         65                  # text(5)
            2f64746c73       # "/dtls"
         02                  # unsigned(2)

       Figure 5: An authorization instance encoded in CBOR (28 bytes)

   Note that choosing 32 as Dynamic-Offset means that all future CoAP
   methods that can be registered can be represented both as themselves
   and in the Dynamic-X variant, but only the dynamic forms of methods 1
   to 21 are typically usable in a JSON form [RFC7493].

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4.  Media Types

   This specification defines media types for the generic information
   model, expressed in JSON (application/aif+json) or in CBOR
   (application/aif+cbor).  These media types have parameters for
   specifying Toid and Tperm; default values are the values "URI-local-
   part" for Toid and "REST-method-set" for Tperm, as per Section 3 of
   the present specification.

   A specification that wants to use Generic AIF with different Toid
   and/or Tperm is expected to request these as media type parameters
   (Section 5.2) and register a corresponding Content-Format
   (Section 5.3).

5.  IANA Considerations

   // RFC Ed.: throughout this section, please replace RFC XXXX with the
   // RFC number of this specification and remove this note.

5.1.  Media Types

   IANA is requested to add the following Media-Types to the "Media
   Types" registry.

   +==========+======================+=====================+
   | Name     | Template             | Reference           |
   +==========+======================+=====================+
   | aif+cbor | application/aif+cbor | RFC XXXX, Section 4 |
   +----------+----------------------+---------------------+
   | aif+json | application/aif+json | RFC XXXX, Section 4 |
   +----------+----------------------+---------------------+

                    Table 3: New Media Types

   For application/aif+cbor:

   Type name:  application
   Subtype name:  aif+cbor
   Required parameters:  N/A
   Optional parameters:
      *  Toid: the identifier for the object for which permissions are
         supplied.  A value from the media-type parameter sub-registry
         for Toid.  Default value: "URI-local-part" (RFC XXXX).

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      *  Tperm: the data type of a permission set for the object
         identified via a Toid.  A value from the media-type parameter
         sub-registry for Tperm.  Default value: "REST-method-set" (RFC
         XXXX).
   Encoding considerations:  binary (CBOR)
   Security considerations:  Section 6 of RFC XXXX
   Interoperability considerations:  none
   Published specification:  Section 4 of RFC XXXX
   Applications that use this media type:  Applications that need to
      convey structured authorization data for identified resources,
      conveying sets of permissions.
   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/cbor".  (At
      publication of RFC XXXX, there is no fragment identification
      syntax defined for "application/cbor".)
   Person & email address to contact for further information:  ACE WG
      mailing list (ace@ietf.org), or IETF Applications and Real-Time
      Area (art@ietf.org)
   Intended usage:  COMMON
   Restrictions on usage:  none
   Author/Change controller:  IETF
   Provisional registration:  no

   For application/aif+json:

   Type name:  application
   Subtype name:  aif+json
   Required parameters:  N/A
   Optional parameters:
      *  Toid: the identifier for the object for which permissions are
         supplied.  A value from the media-type parameter sub-registry
         for Toid.  Default value: "URI-local-part" (RFC XXXX).

      *  Tperm: the data type of a permission set for the object
         identified via a Toid.  A value from the media-type parameter
         sub-registry for Tperm.  Default value: "REST-method-set" (RFC
         XXXX).
   Encoding considerations:  binary (JSON is UTF-8-encoded text)
   Security considerations:  Section 6 of RFC XXXX
   Interoperability considerations:  none
   Published specification:  Section 4 of RFC XXXX
   Applications that use this media type:  Applications that need to
      convey structured authorization data for identified resources,
      conveying sets of permissions.
   Fragment identifier considerations:  The syntax and semantics of
      fragment identifiers is as specified for "application/json".  (At
      publication of RFC XXXX, there is no fragment identification
      syntax defined for "application/json".)

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   Person & email address to contact for further information:  ACE WG
      mailing list (ace@ietf.org), or IETF Applications and Real-Time
      Area (art@ietf.org)
   Intended usage:  COMMON
   Restrictions on usage:  none
   Author/Change controller:  IETF
   Provisional registration:  no

5.2.  Registries

   For the media types application/aif+cbor and application/aif+json,
   IANA is requested to create a sub-registry within
   [IANA.media-type-sub-parameters] for the two media-type parameters
   Toid and Tperm, populated with:

   +===========+=================+=====================+===========+
   | Parameter | name            | Description/        | Reference |
   |           |                 | Specification       |           |
   +===========+=================+=====================+===========+
   | Toid      | URI-local-part  | local-part of URI   | RFC XXXX  |
   +-----------+-----------------+---------------------+-----------+
   | Tperm     | REST-method-set | set of REST methods | RFC XXXX  |
   |           |                 | represented         |           |
   +-----------+-----------------+---------------------+-----------+

                   Table 4: New Media Type Parameters

   The registration policy is Specification required [RFC8126].  The
   designated expert will engage with the submitter to ascertain the
   requirements of this document are addressed:

   *  The specifications for Toid and Tperm need to realize the general
      ideas of unambiguous object identifiers and permission lists in
      the context where the AIF data item is intended to be used, and
      their structure needs to be usable with the intended media types
      (application/aif+cbor and application/aif+json) as identified in
      the specification.

   *  The parameter names need to conform to Section 4.3 of [RFC6838],
      but preferably are in [KebabCase] so they can easily be translated
      into names used in popular programming language APIs.

   The designated experts will develop further criteria and guidelines
   as needed.

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5.3.  Content-Format

   IANA is requested to register Content-Format numbers in the "CoAP
   Content-Formats" sub-registry, within the "Constrained RESTful
   Environments (CoRE) Parameters" Registry [IANA.core-parameters], as
   follows:

   +======================+================+======+===========+
   | Content-Type         | Content Coding | ID   | Reference |
   +======================+================+======+===========+
   | application/aif+cbor | -              | TBD1 | RFC XXXX  |
   +----------------------+----------------+------+-----------+
   | application/aif+json | -              | TBD2 | RFC XXXX  |
   +----------------------+----------------+------+-----------+

                   Table 5: New Content-Formats

   // RFC Ed.: please replace TBD1 and TBD2 with assigned IDs and remove
   this note.

   In the registry as defined by Section 12.3 of [RFC7252] at the time
   of writing, the column "Content-Type" is called "Media type" and the
   column "Content Coding" is called "Encoding".

   Note that applications that register Toid and Tperm values are
   encouraged to also register Content-Formats for the relevant
   combinations.

6.  Security Considerations

   The security considerations of [RFC7252] apply when AIF is used with
   CoAP, and, if complex formats such as URIs are used for Toid or
   Tperm, specifically Section 11.1 of [RFC7252].  Some wider issues are
   discussed in [RFC8576].

   When applying these formats, the referencing specification needs to
   be careful to:

   *  ensure that the cryptographic armor employed around this format
      fulfills the referencing specification's security objectives, and
      that the armor or some additional information included in it with
      the AIF data item (1) unambiguously identifies the subject to
      which the authorizations shall apply and (2) provides any context
      information needed to derive the identity of the object to which
      authorization is being granted from the object identifiers (such
      as, for the data models defined in the present specification, the
      scheme and authority information that is used to construct the
      full URI), and

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   *  ensure that the types used for Toid and Tperm provide the
      appropriate granularity and precision so that application
      requirements on the precision of the authorization information are
      fulfilled, and that all parties have the same understanding of
      each Toid/Tperm pair in terms of specified objects (resources) and
      operations on those.

   For the data formats, the security considerations of [RFC8259] and
   [RFC8949] apply.

   A plain implementation of AIF might implement just the basic REST
   model as per Section 2.1.  If it receives authorizations that include
   permissions that use the REST-specific Model With Dynamic Resource
   Creation Section 2.3, it needs to either reject the AIF data item
   entirely or act only on the permissions that it does understand.  In
   other words, the semantics underlying an allow-list as discussed
   above need to hold here as well.

   An implementation of the REST-specific Model With Dynamic Resource
   Creation Section 2.3 needs to carefully keep track of the dynamically
   created objects and the subjects to which the Dynamic-X permissions
   apply -- both on the server side to enforce the permissions and on
   the client side to know which permissions are available.

7.  References

7.1.  Normative References

   [I-D.ietf-httpbis-semantics]
              Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,
              <https://www.ietf.org/archive/id/draft-ietf-httpbis-
              semantics-19.txt>.

   [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>.

   [RFC3986]  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>.

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   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC9165]  Bormann, C., "Additional Control Operators for the Concise
              Data Definition Language (CDDL)", RFC 9165,
              DOI 10.17487/RFC9165, December 2021,
              <https://www.rfc-editor.org/info/rfc9165>.

7.2.  Informative References

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-46, 8 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-46.txt>.

   [IANA.core-parameters]
              IANA, "Constrained RESTful Environments (CoRE)
              Parameters",
              <https://www.iana.org/assignments/core-parameters>.

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   [IANA.media-type-sub-parameters]
              IANA, "MIME Media Type Sub-Parameter Registries",
              <https://www.iana.org/assignments/media-type-sub-
              parameters>.

   [KebabCase]
              "KebabCase", 29 August 2014,
              <http://wiki.c2.com/?KebabCase>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570,
              DOI 10.17487/RFC6570, March 2012,
              <https://www.rfc-editor.org/info/rfc6570>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7493]  Bray, T., Ed., "The I-JSON Message Format", RFC 7493,
              DOI 10.17487/RFC7493, March 2015,
              <https://www.rfc-editor.org/info/rfc7493>.

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8576]  Garcia-Morchon, O., Kumar, S., and M. Sethi, "Internet of
              Things (IoT) Security: State of the Art and Challenges",
              RFC 8576, DOI 10.17487/RFC8576, April 2019,
              <https://www.rfc-editor.org/info/rfc8576>.

   [RFC8881]  Noveck, D., Ed. and C. Lever, "Network File System (NFS)
              Version 4 Minor Version 1 Protocol", RFC 8881,
              DOI 10.17487/RFC8881, August 2020,
              <https://www.rfc-editor.org/info/rfc8881>.

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   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

Acknowledgements

   Jim Schaad, Francesca Palombini, Olaf Bergmann, Marco Tiloca, and
   Christian Amsüss provided comments that shaped the direction of this
   document.  Alexey Melnikov pointed out that there were gaps in the
   media type specifications, and Loganaden Velvindron provided a
   shepherd review with further comments.  Many thanks also to the IESG
   reviewers, which provided several small but significant observations.
   Benjamin Kaduk provided an extensive review as responsible Area
   Director, and indeed is responsible for much improvement in the
   document.

Author's Address

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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