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CoRE Resource Directory Extensions
draft-amsuess-core-resource-directory-extensions-11

Document Type Active Internet-Draft (individual)
Author Christian Amsüss
Last updated 2024-11-06
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draft-amsuess-core-resource-directory-extensions-11
CoRE                                                           C. Amsüss
Internet-Draft                                           7 November 2024
Intended status: Experimental                                           
Expires: 11 May 2025

                   CoRE Resource Directory Extensions
          draft-amsuess-core-resource-directory-extensions-11

Abstract

   A collection of extensions to the Resource Directory [rfc9176] that
   can stand on their own, and have no clear future in specification
   yet.

Discussion Venues

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

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (core@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://gitlab.com/chrysn/resource-directory-extensions.

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 May 2025.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Reverse Proxy requests  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Registration  . . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Registration updates  . . . . . . . . . . . . . . . .   4
     2.3.  Proxy behavior  . . . . . . . . . . . . . . . . . . . . .   5
       2.3.1.  Limitations from using a reverse proxy  . . . . . . .   5
     2.4.  On-Demand proxying  . . . . . . . . . . . . . . . . . . .   5
     2.5.  Multiple upstreams  . . . . . . . . . . . . . . . . . . .   6
     2.6.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   6
       2.6.1.  Registration through a firewall . . . . . . . . . . .   6
       2.6.2.  Registration from a browser context . . . . . . . . .   7
     2.7.  Notes on stability and maturity . . . . . . . . . . . . .   7
     2.8.  Security considerations . . . . . . . . . . . . . . . . .   7
     2.9.  Alternatives to be explored . . . . . . . . . . . . . . .   8
   3.  Infinite lifetime . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Limited lifetimes . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Zone identifier introspection . . . . . . . . . . . . . . . .  11
     5.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Proxying multicast requests . . . . . . . . . . . . . . . . .  11
     6.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Registrations that update DNS records . . . . . . . . . . . .  13
   8.  Propagating server generated registration information . . . .  13
   9.  Combining simple registration with EDHOC and ACE  . . . . . .  14
     9.1.  Generic EDHOC in reverse flow . . . . . . . . . . . . . .  14
     9.2.  ACE roles . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.3.  ACE EDHOC profile . . . . . . . . . . . . . . . . . . . .  15
     9.4.  ACE OSCORE profile  . . . . . . . . . . . . . . . . . . .  16
     9.5.  ACE OSCORE profile without ACE  . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Attic  . . . . . . . . . . . . . . . . . . . . . . .  20
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .  21
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  23
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  23

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1.  Introduction

   This document pools some extensions to the Resource Directory
   [rfc9176] that might be useful but have no place in the original
   document.

   They might become individual documents for IETF submission, simple
   registrations in the RD Parameter Registry at IANA, or grow into a
   shape where they can be submitted as a collection of tools.

   At its current state, this draft is a collection of ideas.

2.  Reverse Proxy requests

   When a registrant registers at a Resource Directory, it might not
   have a suitable address it can use as a base address.  Typical
   reasons include being inside a NAT without control over port
   forwarding, or only being able to open outgoing connections (as
   program running inside a web browser utilizing CoAP over WebSocket
   [RFC8323] might be).

   [rfc9176] suggests (in the Cellular M2M use case) that proxy access
   to such endpoints can be provided, it gives no concrete mechanism to
   do that; this is such a mechanism.

   This mechanism is intended to be a last-resort option to provide
   connectivity.  Where possible, direct connections are preferred.
   Before registering for proxying, the registrant should attempt to
   obtain a publicly available port, for example using PCP ([RFC6887]).

   The same mechanism can also be employed by registrants that want to
   conceal their network address from its clients.

   A deployed application where this is implicitly done is LwM2M
   [citation missing].  Notable differences are that the protocol used
   between the client and the proxying RD is not CoAP but application
   specific, and that the RD (depending on some configuration) eagerly
   populates its proxy caches by sending requests and starting
   observations at registration time.

2.1.  Discovery

   An RD that provides proxying functionality advertises it by
   announcing the additional resource type "TBD1" on its directory
   resource.

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2.2.  Registration

   A client passes the "proxy=yes" or "proxy=ondemand" query parameter
   in addition to (but typically instead of) a "base" query parameter.

   A server that receives a "proxy=yes" query parameter in a
   registration (or receives "proxy=ondemand" and decides it needs to
   proxy) MUST come up with a "Proxy URL" on which it accepts requests,
   and which it uses as a Registration Base URI for lookups on the
   present registration.

   The Proxy URL SHOULD have no path component, as acting as a reverse
   proxy in such a scenario means that any relative references in all
   representations that are proxied must be recognized and possibly
   rewritten.

   The RD MAY accept connections also on alternative Registration Base
   URIs using different protocols; it can advertise them using the
   mechanisms of [I-D.ietf-core-transport-indication].

   The registrant is not informed of the chosen public name by the RD.
   (Section 8 discusses means how to change that).

   This mechanism is applicable to all transports that can be used to
   register.  If proxying is active, the restrictions on when the base
   parameter needs to be present ([rfc9176] Registration template) are
   relaxed: The base parameter may also be absent if the connection
   originates from an ephemeral port, as long as the underlying protocol
   supports role reversal, and link-local IPv6 addresses may be used
   without any concerns of expressibility.

   If the client uses the role reversal rule relaxation, both it and the
   server keep that connection open for as long as it wants to be
   reachable.  When the connection terminates, the RD SHOULD treat the
   registration as having timed out (even if its lifetime has not been
   exceeded) and MAY eventually remove the registration.  It is yet to
   be decided whether the RD's announced ability to do proxying should
   imply that infinite lifetimes are necessarily supported for such
   registrations; at least, it is RECOMMENDED.

2.2.1.  Registration updates

   The "proxy" query parameter can not be changed or repeated in a
   registration update; RD servers MUST answer 4.00 Bad Request to any
   registration update that has a "proxy" query parameter.

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   As always, registration updates can explicitly or implicitly update
   the Registration Base URI.  In proxied registrations, those changes
   are not propagated to lookup, but do change the forwarding address of
   the proxy.

   For example, if a registration is established over TCP, an update can
   come along in a new TCP connection.  Starting then, proxied requests
   are forwarded along that new connection.

2.3.  Proxy behavior

   The RD operates as a reverse-proxy as described in [RFC7252]
   Section 5.7.3 at the announced Proxy URL(s), where it decides based
   on the requested host and port to which registrant endpoint to
   forward the request.

   The address the incoming request are forwarded to is the base address
   of the registration.  If an explicit "base" paremter is given, the RD
   will forward requests to that location.  Otherwise, it forwards to
   the registration's source address (which is the implied base
   parameter).

   When an implicit base is used, the requests forwarded by the RD to
   the EP contain no Uri-Host option.  EPs that want to run multiple
   parallel registrations (especially gateway-like devices) can either
   open up separate connections, or use an additional (to-be-specified)
   mechanism to set the "virtual host name" for that registration in a
   separate argument.

2.3.1.  Limitations from using a reverse proxy

   The registrant requesting the reverse proxying needs to ensure that
   all services it provides are compatible with being operated behind a
   reverse proxy with an unknown name.  In particular, this rules out
   all applications that refer to local resources by a full URI (as
   opposed to relative references without scheme and host).
   Applications behind a reverse proxy can not use role reversal.

   Some of these limitations can be mitigated if the application knows
   its advertised address.  The mechanisms of Section 8 might be used to
   change that.

2.4.  On-Demand proxying

   If an endpoint is deployed in an unknown network, it might not know
   whether it is behind a NAT that would require it to configure an
   explicit base address, and ask the RD to assist by proxying if
   necessary by registering with the "proxy=ondemand" query parameter.

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   A server receiving that SHOULD use a different IP address to try to
   access the registrant's .well-known/core file using a GET request
   under the Registration Base URI.  If that succeeds, it may assume
   that no NAT is present, and ignore the proxying request.  Otherwise,
   it configures proxying as if "proxy=yes" were requested.

   Note that this is only a heuristic [ and not tested in deployments
   yet ].

2.5.  Multiple upstreams

   When a proxying RD is operating behind a router that has uplinks with
   multiple provisioning domains (see [RFC7556]) or a similar setup, it
   MAY mint multiple addresses that are reachable on the respective
   provisioning domains.  When possible, it is preferred to keep the
   number of URIs handed out low (avoiding URI aliasing); this can be
   achieved by announcing both the proxy's public addresses under the
   same wildcard name.

   If RDs are announced by the uplinks using RDAO, the proxy may use the
   methods of [I-D.amsuess-core-rd-replication] to distribute its
   registrations to all the announced upstream RDs.

   In such setups, the router can forward the upstream RDs using the PvD
   option ([RFC8801]) to PvD-aware hosts and only announce the local RD
   to PvD-unaware ones (which then forwards their registrations).  It
   can be expected that PvD-aware endpoints are capable of registering
   with multiple RDs simultaneously.

2.6.  Examples

2.6.1.  Registration through a firewall

   Req from [2001:db8:42::9876]:5683:
   POST coap://rd.example.net/rd?ep=node9876&proxy=ondemand
   </some-resource>;rt="example.x"

   Req from other-address.rd.example.net:
   GET coap://[2001:db8:42::9876]/.well-known/core

   Request blocked by stateful firewall around [2001:db8:42::]

   RD decides that proxying is necessary

   Res: 2.04 Created
   Location: /reg/abcd

   Later, lookup of that registration might say:

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   Req: GET coap://rd.example.net/lookup/res?rt=example.x

   Res: 2.05 Content
   <coap://node987.rd.example.net/some-resource>;rt="example.x

   A request to that resource will end up at an IP address of the RD,
   which will forward it using its the IP and port on which the
   registrant had registered as source port, thus reaching the
   registrant through the stateful firewall.

2.6.2.  Registration from a browser context

   Req: POST coaps+ws://rd.example.net/rd?ep=node1234&proxy=yes
   </gyroscope>;rt="core.s"

   Res: 2.04 Created
   Location: /reg/123

   The gyroscope can now not only be looked up in the RD, but also be
   reached:

   Req: GET coap://rd.example.net/lookup/res?rt=core.s

   Res: 2.05 Content
   <coap://[2001:db8:1::1]:10123/gyroscope>;rt="core.s"

   In this example, the RD has chosen to do port-based rather than host-
   based virtual hosting and announces its literal IP address as that
   allows clients to not send the lengthy Uri-Host option with all
   requests.

2.7.  Notes on stability and maturity

   Using this with UDP can be quite fragile; the author only draws on
   own experience that this can work across cell-phone NATs and does not
   claim that this will work over generic firewalls.

   [ It may make sense to have the example as TCP right away. ]

2.8.  Security considerations

   An RD MAY impose additional restrictions on which endpoints can
   register for proxying, and thus respond 4.01 Unauthorized to request
   that would pass had they not requested proxying.

   Attackers could do third party registrations with an attacked
   device's address as base URI, though the RD would probably not
   amplify any attacks in that case.

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   The RD MUST NOT reveal the address at which it reaches the registrant
   except for adaequately authenticated and authorized debugging
   purposes, as that address could reveal sensitive location data the
   registrant may wish to hide by using a proxy.

   Usual caveats for proxies apply.

2.9.  Alternatives to be explored

   With the mechanisms of [I-D.ietf-core-transport-indication], an RD
   could also operate as a forward proxy, and indicate its availability
   for that purpose in a has-proxy link it creates on its own, and which
   it makes discoverable through its lookup interfaces.

   How a registrant opts in to that behavior, how it selects a suitable
   public address (using the base attribute is tempting, but conflicts
   with the currently prescribed proxy behavior) and for which scenarios
   this is preferable is a topic being explored.

   As with the reverse proxy address, the registrant is not informed of
   the public addresses (though again, Section 8 can be used to change
   that).  Knowing these addresses can be relevant when the endpoint
   advertises its services out of band (e.g. by showing a QR code or
   exposing links through NFC), but also when the mechanism of
   [I-D.ietf-core-transport-indication] Appendix D is used.

3.  Infinite lifetime

   An RD can indicate support for infinite lifetimes by adding the
   resoruce type "TBD2" to its list of resource types.

   A registrant that wishes to keep its registration alive indefinitely
   can set the lifetime value as "lt=inf".

   Registrations with infinite lifetimes never time out on their own.

   Registrations whose base is explicit are not "soft state"; the
   registrant can expect the RD to persist the registrations across
   network changes, reboots, softare updates and that like.

   Registrations with an implicit base are kept active by the RD for as
   long as they are reachable; the RD should make reasonable efforts to
   persist them, but will remove them when they are unavailable
   temporarily, which may happen outside of the RD's control.

   Typical use cases for infinite life times are:

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   *  Commissioning tools (CTs) that do not return to the deployment
      site, and thus can not refresh the soft state

   *  Proxy registrations whose lifetime is limited by a connection that
      is kept alive.  Once the connection terminates even temporarily,
      the RD has no means of restoring the connection, and removes the
      registration.  (Exceptionally, the RD can make such an attempt if
      the registration happened over a role-reversed TCP connection).

   *  Simple registrations.  For those, the RD keeps the registration
      active as long as it can obtain a Valid response for the .well-
      known/core document on the endpoint, checked at intervals guided
      by its Max-Age.

3.1.  Example

   Had the example of Section 2.6.2 discovered support for infinite
   lifetimes during lookup like this:

   Req: GET coaps+ws://rd.example.net/.well-known/core?rt=core.rd*

   Res: 2.05 Content
   </rd>;rt="core.rd TBD1 TBD2";ct=40

   it could register like that:

   Req: POST coaps+ws://rd.example.net/rd?ep=node1234&proxy=yes&lt=inf
   </gyroscope>;rt="core.s"

   Res: 2.04 Created
   Location: /reg/123

   and never need to update the registration for as long as the
   websocket connection is open.

   (When it gets terminated, it could try renewing the registration, but
   needs to be prepared for the RD to already have removed the original
   registration.)

4.  Limited lifetimes

   Even if an RD supports infinite lifetimes, it may not accept them
   from just any registrant.  Even more, an RD may have policies in
   place that require a certain frequency of updates and thus place an
   upper limit on lt lower than the technical limit of 136 years.

   This document does not define any means of communicating lifetime
   limits, but explores a few options:

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   *  Administrative channels.

      An RD that sees registrations with unreasonably long lifetimes can
      flag them for its operator to take further measures.

      While sounding tediously manual, this captures the observation
      that different components are configured in a softly incompatible
      way, and need operator intervention (because if there were
      automatic means, they obviously failed).

   *  General advertisement of preferred lifetimes.

      When the limitations on the lifetimes are not from authorization
      but from general setup, an RD could advertise that property in a
      to-be-created link target attribute of its registration resource.
      Different attributes could express preference or hard limits.

      This information is also available easily for registrants, which
      may then heed the advice if supported, and may notify their
      operators that they just started spending more resources than they
      were configured to.

      It is also available to tools that provision endpoints with their
      RD address (and parameters), as they can use the same lookup
      interface.

   *  Per-registration information.

      For soft limits, the RD can offer the endpoint additional metadata
      if it queries them post-registration.  That query can use the
      endpoint lookup interface, or the extension of Section 8.  This
      may require additional round-trips on the part of endpoint.

   *  Hard limits informed by error codes.

      An RD can reject registrations with overly long lifetimes if the
      endpoint is not authorized to use such long lifetimes with a 4.01
      Unauthorized error.  The mechanisms of [RFC9290], with a to-be-
      defined error detail on the permissible lifetime, can be used to
      propagate information back to then endpoint.

      This behavior is explicitly NOT RECOMMENDED, because devices may
      crucially depend on the RD's services -- this rejection may even
      be the reason why the device is not configured with the new
      settings that would contain a shorter lifetime.

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5.  Zone identifier introspection

   The 'split-horizon' mechanism of [rfc9176] (that registrations with
   link-local bases can only be read from the zone they registered on)
   reduces the usability of the endpoint lookup interface for debugging
   purposes.

   To allow an administrator to read out the "show-zone-id" query
   parameter for endpoint and resource lookup is introduced.

   A Resource Directory that understands this parameter MUST NOT limit
   lookup results to registrations from the lookup's zone, and MUST use
   [RFC6874] zone identifiers to annotate which zone those registrations
   are valid on.

   The RD MUST limit such requests to authenticated and authorized
   debugging requests, as registrants may rely on the RD to keep their
   presence secret from other links.

5.1.  Example

Req: GET /rd-lookup/ep?show-zone-id&et=printer

Res: 2.05 Content
</reg/1>;base="coap://[2001:db8::1]";et=printer;ep="bigprinter",
</reg/2>;base="coap://[fe80::99%wlan0]";et=printer;ep="localprinter-1234",
</reg/3>;base="coap://[fe80::99%eth2]";et=printer;ep="localprinter-5678",

6.  Proxying multicast requests

   Multicast requests are hard to forward at a proxy: Even if a media
   type is used in which multiple responses can be aggregated
   transparently, the proxy can not reliably know when all responses
   have come in.  [RFC7390] Section 2.9 describes the difficulties in
   more detail.

   Note that [I-D.tiloca-core-groupcomm-proxy] provides a mechanism that
   _does_ allow the forwarding of multicast requests.  It is yet to be
   determined what the respective pros and cons are.  Conversely, that
   lookup mechanism may also serve as an alternative to resource lookup
   on an RD.

   A proxy MAY expose an interface compatible with the RD lookup
   interface, which SHOULD be advertised by a link to it that indicates
   the resource types core.rd-lookup-res and TBD4.

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   The proxy sends multicast requests to All CoAP Nodes ([RFC7252]
   Section 12.8) requesting their .well-known/core files either eagerly
   (ie. in regular intervals independent of queries) or on demand (in
   which case it SHOULD limit the results by applying [RFC6690] query
   filtering; if it has received multiple query parameters it should
   forward the one it deems most likely to limit the results, as .well-
   known/core only supports a single query parameter).

   In comparison to classical RD operation, this RD behaves roughly as
   if it had received a simple registration with a All CoAP Nodes
   address as the source address, if such behavior were specified.  The
   individual registrations that result from this neither have an
   explicit registration resource nor an explicit endpoint name; given
   that the endpoint lookup interface is not present on such proxies,
   neither can be queried.

   Clients that would intend to do run a multicast discovery operation
   behind the proxy can then instead query that resource lookup
   interface.  They SHOULD use observation on lookups, as an on-demand
   implementation MAY return the first result before others have
   arrived, or MAY even return an empty link set immediately.

6.1.  Example

   Req: GET coap+ws://gateway.example.com/.well-known/core?rt=TBD4

   Res: 2.05 Content
   </discover>;rt="core.rd-lookup-res TBD4";ct=40

   Req: GET coap+ws://gateway.example.com/discover?rt=core.s
   Observe: 0

   Res: 2.05 Content
   Observe: 0
   Content-Format: 40
   (empty payload)

   At the same time, the proxy sends out multicast requests on its
   interfaces:

   Req: GET coap://ff05::fd/.well-known/core?rt=core.s

   Res (from [2001:db8::1]:5683): 2.05 Content
   </temp>;ct="0 112";rt="core.s"

   Res (from [2001:db8::2]:5683): 2.05 Content
   </light>;ct="0 112";rt="core.s"

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   upon receipt of which it sends out a notification to the websocket
   client:

Res: 2.05 Content
Observe: 1
Content-Format: 40
<coap://[2001:db8::1]/temp>;ct="0 112";rt="core.s";anchor="coap://[2001:db8::1]",
<coap://[2001:db8::2]/light>;ct="0 112";rt="core.s";anchro="coap://[2001:db8::2]"

7.  Registrations that update DNS records

   An RD that is provisioned with means to update a DNS zone and that
   has a known mapping from registrants to host names could use
   registrations to populate DNS records from registration base
   addresses.

   When combined with Section 2, these records point to the RD's built-
   in proxy rather than to the base address.

   This mechanism is not described in further detail yet as it does not
   interact well yet with how the base registration attribute interacts
   with the proxy announcements of [I-D.ietf-core-transport-indication].

8.  Propagating server generated registration information

   The RD can populate some data into the registration: The RD may pick
   the sector and endpoint name based on the endpoint's credentials, or
   (as introduced in this documents) reverse proxy names and soft
   lifetime limits can be added.

   With the exception of sector and endpoint name, the registrant can
   query those properties through the endpoint lookup interface.
   However, this is cumbersome as it requires it to use both the
   registration and the lookup interface.

   The architecture of [I-D.ietf-core-coap-pubsub] offers a different
   architectural setup: Applied to the RD, the registration would
   generate both a registration metadata resource (at which the
   registrant can set or query its registration's metadata) and a
   registration link resource (which contains all the links the
   registrant provides).  Such a setup would make it easier for
   registrants to query or update registration metadata, including
   querying for an implicitly assigned endpoint name or sector.

   Extending the RD specification to allow this style of operation would
   be possible without altering its client facing interfaces.
   Alternatively, using a new media type for operations on the
   registration resource and/or the FETCH and PATCH methods would enable

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   such operations in a less intrusive way.  While it would be tempting
   to add an Accept option to the registration request to solicit
   immediate information on the registration that was just created, the
   Accept option's criticality would render this incompatible with
   existing servers.  The option can still be set if the new content
   format is advertised by the RD.

   Without any media type suggested so far, this is what a registration
   could look like if the RD advertised that it provided content format
   TBD6 on the registration interface:

   Req from [2001:db8::1]:5683:
   POST coap://rd.example.net/rd
   Accept: TBD6
   Payload:
   </some-resource>;rt="example.x"

   Res: 2.04 Created
   Location: /reg/abcd
   Content-Format: TBD6
   Payload:
   Soft lifetime limit 3600, please update your registration in time.
   Forward proxy services are offered at coaps+ws://rd.example.net and
   coaps+tcp://rd.example.net.

9.  Combining simple registration with EDHOC and ACE

   For very constrained devices, starting a simple registration may be
   the only occasion at which they use the CoAP client role.  If they
   exclusively send piggybacked responses (Section 5.2.1 of [RFC7252])
   and handle only idempotent requests, they can completely avoid the
   need for handling retransmissions.

   This section presents some patterns in which an endpoint can register
   securely without implementing more CoAP features.

9.1.  Generic EDHOC in reverse flow

   When such a endpoints uses EDHOC [I-D.ietf-lake-edhoc], it can follow
   this flow:

   The endpoints requests simple registration with its RD.  This request
   is unencrypted.  Both for privacy reasons and to reduce configuration
   effort, the endpoints elides the ep registration parameter -- it will
   be established during authentication anyway.  The endpoint may set
   the No-Response option defined in [RFC7967], given it is not relying
   on the response but waiting for the OSCORE context to be established.

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   TBD: Can we establish the correctness of the parameters somewhere
   later?  (It could if it repeated any parameters in EAD3, at least as
   an empty item indicating that it's a simple registration).

   The RD initiates an EDHOC exchange as part of its query for the
   endpoints's /.well-known/core resource.  As the endpoints has the
   more vulnerable identity, EDHOC is performed in reverse message flow.
   After the RD has received message 3, it sends the GET request for the
   endpoints's /.well-known/core resource to complete the registration
   and responds to the original unencrypted registration.

   TBD: Can the endpoints obtain confirmation that it is now registered?
   (It could, if the EAD3 item above is critical: then, the presence of
   OSCORE traffic with that key material implies acceptance of that
   EAD3.)

9.2.  ACE roles

   In an ACE context ([RFC9200]), the endpoints is the Resource Server
   (RS), and the RD is the client (C).  This is aligned with the
   endpoints' capabilities: The RD is in a position to talk to the ACE
   Authorization Server (AS) and obtain a token to enable communication
   with the endpoints, and the endpoints does not need to perform any
   additional communication.

   The token's audience may be "any endpoint eligible for RD
   registration" or a particular endpoint.  Its subject is the RD.  Its
   scope is to read the /.well-known/core resource.

   In some scenarios it may make sense to operate the AS and the RD in a
   single system, in which case communication between those parties is
   cut short.

9.3.  ACE EDHOC profile

   When the ACE EDHOC profile [I-D.ietf-ace-edhoc-oscore-profile] is
   used, the RD needs to upload its token to the endpoints's authz-info
   endpoint (which is a term from ACE, using "endpoint" for a resource
   path and not for a host as the RD specification does) before
   executing EDHOC, because sending a token is only supported in EDHOC
   messages 1 and 3 (whereas the RD sends message 2).  The posted token
   needs to be issued for the audience group of all eligible endpoints,
   as the RD does not know the identity of the endpoint at this stage.
   When the endpoint sends its credentials, the RD will know from
   matching the endpoint's credentials against the rs_cnf2 and aud2 list
   it obtained with the token (defined in Section 3.2 of
   [I-D.ietf-ace-workflow-and-params]) which endpoint is being
   registered.

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   Both parties can use kid as their ID_CRED_x to keep messages small.
   The endpoint receives the full ID_CRED of the RD as part of the
   signed token; the RD can look up the ID_CRED of the endpoint in its
   rs_cnf2 data.

   Hypothetically, if a token were permitted to be sent in message 2,
   the RS could do that, and save the extra round-trip for POSTing the
   token.

   Alternatively, the RD could initiate EDHOC in forward message flow.
   By the time it receives the endpoint's credentials (eg. kid) in
   message 2, it can ask the AS for a token suitable for that particular
   endpoint, or find a suitable token in a list it has obtained earlier
   (TBD: how?).  This workflow has the downside of revealing the
   endpoint's ID_CRED_x to active attackers.  This may be acceptable,
   especially when the endpoint is only sending a kid, and more so if
   the AS has a means of updating that ID.

9.4.  ACE OSCORE profile

   In the ACE OSCORE profile [RFC9203], a token is used that contains
   symmetric key material.

   The message flow is similar to EDHOC and OSCORE: The endpoint sends
   an unencrypted registration request, but the endpoint needs to
   publicly reveal its identity by sending the ep registration
   parameter.

   Then, the RD can obtain a token for this particular endpoint.  Like
   in ACE EDHOC, it POSTs it to the endpoint's authz-info endpoint; in
   addition, it sends a random nonce and receives one in the response.

   Without any further steps, it can then derive an OSCORE context from
   the token and the nonces, and send an OSCORE request for the
   endpoint's /.well-known/core resource.

   This mode of operation is only recommended if the endpoint already
   makes its identity public for other reasons.

9.5.  ACE OSCORE profile without ACE

   When assigning the reversed ACE roles to the participants, there is a
   mode of operation that enables the ACE OSCORE profile while
   preserving privacy of the endpoint:

   If the endpoint is provisioned with a public key of the AS in
   addition to the symmetric material it shares with it in the ACE
   OSCORE profile, the device can generate a token containing a secret

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   key, symmstrically encrypt (or MAC it) for the AS, and asymmetrically
   encrypt it for the AS (eg. using Direct Key Agreement).  It then
   initiates the ACE OSCORE profile with the RD, which needs to
   introspect the token at the AS to obtain the secret key material
   within.

   This token's roles are different: Its subject is an endpoint, its
   audience is the RD, and its scope is to register with a particular
   endpoint name.  The AS verifies during introspection whether the
   endpoint is actually eligible to do this.

   It is unsure whether this whole process provides complexity benefits
   over the EDHOC based workflow, given that it does necessitate an
   asymmetric operation.

   (Note that while it is well possible to perform ACE OSCORE profile
   without the asymmetrical step, for example by just symmetrically
   encrypting the token created by the endpoint, by provisioning the
   endpoint with a single token response containing a token encrypted to
   the RS, or with one containing an abbreviated token which the RS can
   introspect at the AS, this adds little compared to Section 9.4,
   because the endpoint's initial message will always contain identical
   parts that allow identification.  The endpoint creating a random
   token and encrypting it symmetrically to the AS is almost viable and
   privacy preserving, but decrypting the token at the AS without any
   information identifying the symmetric ke would scale badly.)

10.  References

10.1.  Normative References

   [I-D.amsuess-core-rd-replication]
              Amsüss, C., "Resource Directory Replication", Work in
              Progress, Internet-Draft, draft-amsuess-core-rd-
              replication-02, 11 March 2019,
              <https://datatracker.ietf.org/doc/html/draft-amsuess-core-
              rd-replication-02>.

   [RFC6874]  Carpenter, B., Cheshire, S., and R. Hinden, "Representing
              IPv6 Zone Identifiers in Address Literals and Uniform
              Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
              February 2013, <https://www.rfc-editor.org/rfc/rfc6874>.

   [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/rfc/rfc7252>.

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   [rfc9176]  Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
              P. van der Stok, "Constrained RESTful Environments (CoRE)
              Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
              2022, <https://www.rfc-editor.org/rfc/rfc9176>.

10.2.  Informative References

   [I-D.ietf-ace-edhoc-oscore-profile]
              Selander, G., Mattsson, J. P., Tiloca, M., and R. Höglund,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC) and Object
              Security for Constrained Environments (OSCORE) Profile for
              Authentication and Authorization for Constrained
              Environments (ACE)", Work in Progress, Internet-Draft,
              draft-ietf-ace-edhoc-oscore-profile-06, 21 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ace-
              edhoc-oscore-profile-06>.

   [I-D.ietf-ace-workflow-and-params]
              Tiloca, M. and G. Selander, "Alternative Workflow and
              OAuth Parameters for the Authentication and Authorization
              for Constrained Environments (ACE) Framework", Work in
              Progress, Internet-Draft, draft-ietf-ace-workflow-and-
              params-03, 21 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ace-
              workflow-and-params-03>.

   [I-D.ietf-core-coap-pubsub]
              Jimenez, J., Koster, M., and A. Keränen, "A publish-
              subscribe architecture for the Constrained Application
              Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
              ietf-core-coap-pubsub-15, 21 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coap-pubsub-15>.

   [I-D.ietf-core-coral]
              Amsüss, C. and T. Fossati, "The Constrained RESTful
              Application Language (CoRAL)", Work in Progress, Internet-
              Draft, draft-ietf-core-coral-06, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coral-06>.

   [I-D.ietf-core-transport-indication]
              Amsüss, C. and M. S. Lenders, "CoAP Transport Indication",
              Work in Progress, Internet-Draft, draft-ietf-core-
              transport-indication-07, 21 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              transport-indication-07>.

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   [I-D.ietf-lake-edhoc]
              Selander, G., Mattsson, J. P., and F. Palombini,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC)", Work in
              Progress, Internet-Draft, draft-ietf-lake-edhoc-23, 22
              January 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-lake-edhoc-23>.

   [I-D.tiloca-core-groupcomm-proxy]
              Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
              Communication", Work in Progress, Internet-Draft, draft-
              tiloca-core-groupcomm-proxy-09, 31 August 2023,
              <https://datatracker.ietf.org/doc/html/draft-tiloca-core-
              groupcomm-proxy-09>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/rfc/rfc6690>.

   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,
              <https://www.rfc-editor.org/rfc/rfc6887>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/rfc/rfc7390>.

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
              <https://www.rfc-editor.org/rfc/rfc7556>.

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/rfc/rfc7967>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/rfc/rfc8323>.

   [RFC8801]  Pfister, P., Vyncke, É., Pauly, T., Schinazi, D., and W.
              Shao, "Discovering Provisioning Domain Names and Data",
              RFC 8801, DOI 10.17487/RFC8801, July 2020,
              <https://www.rfc-editor.org/rfc/rfc8801>.

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   [RFC9200]  Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments Using the OAuth 2.0 Framework
              (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9200>.

   [RFC9203]  Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "The Object Security for Constrained RESTful Environments
              (OSCORE) Profile of the Authentication and Authorization
              for Constrained Environments (ACE) Framework", RFC 9203,
              DOI 10.17487/RFC9203, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9203>.

   [RFC9290]  Fossati, T. and C. Bormann, "Concise Problem Details for
              Constrained Application Protocol (CoAP) APIs", RFC 9290,
              DOI 10.17487/RFC9290, October 2022,
              <https://www.rfc-editor.org/rfc/rfc9290>.

Appendix A.  Attic

   Several extensions to the RD have been proposed in earlier versions
   of this document and were removed; this section summarizes them,
   lists where to look up the latest version, and gives reasons for
   their removal:

   *  Opportunistic RD (until -10)

      Describes how moderately capable devices can automatically
      configure and advertise themselves as an RD while no
      administratively configured RD is present.

      Removed due to large complexity and lack of real use cases.

   *  Lifetime age (until -10)

      References Section 5.2 of [I-D.amsuess-core-rd-replication] to
      allow administrators to see how much of a registration's lifetime
      has expired.

      Removed in favor of more generic provenance mechanisms described
      in Section 5.1 of [I-D.amsuess-core-rd-replication], and for lack
      of use cases.

   *  Lookup across link relations (until -10)

      Describes how a lookup may be combined ahead of time with requests
      for following more link relations.

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      Removed in favor of utilizing [I-D.ietf-core-coral] FETCH
      requests.

Appendix B.  Change log

   Since -10:

   *  Describe implications of simple registration with infinite
      lifetime.

   *  Point out applicability of No-Response option.

   Since -09:

   *  Added section on use with EDHOC and ACE security

   *  Moved Opportunistic RD, Lifetime age and Lookup across link
      relations into the newly created attic.

   Since -08:

   *  Add section on propagating server generated information.

   *  Reference transport-indication appendix as one reason why
      propagation can be relevant.

   Since -07:

   *  Update references.

   Since -06:

   *  Add sketch for DNS updates.

   *  Add sketch for forward proxying.

   *  Fix erroneous section numbers.

   Since -05:

   *  Add section on Limited Lifetimes.

   *  Point out limitations to applications that use reverse proxying.

   *  Minor reference and bugfix updates.

   Since -04:

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   *  Minor adjustments:

      -  Mention LwM2M and how it is already doing RD proxying.

      -  Tie proxying in with infinite lifetimes.

      -  Remove note on not being able to switch protocols: RDs that
         support some future protocol negotiation can do that.

      -  Point out that there is no Uri-Host from the RD proxy to the
         EP, but there could be.

      -  Infinite lifetimes: Take up CTs more explicitly from RD
         discussion.

      -  Start exploring interactions with groupcomm-proxy.

   Since -03:

   *  Added interaction with PvD (Provisioning Domains)

   Since -02:

   *  Added abstract

   *  Added example of CoRAL FETCH to Lookup across link relations
      section

   Since -01:

   *  Added section on Opportunistic RDs

   Since -00:

   *  Add multicast proxy usage pattern

   *  ondemand proxying: Probing queries must be sent from a different
      address

   *  proxying: Point to RFC7252 to describe how the actual proxying
      happens

   *  proxying: Describe this as a last-resort options and suggest
      attempting PCP first

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Appendix C.  Acknowledgements

   [ Reviews from: Jaime Jimenez ]

   Section 4 was inspired by Ben Kaduk's comments from reviewing
   [rfc9176].  Marco Tiloca pointed out the usefulness of the No-
   Response option in simple registration.

Author's Address

   Christian Amsüss
   Email: christian@amsuess.com

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