CoRE                                                           C. Amsüss
Internet-Draft                                           2 November 2020
Intended status: Experimental
Expires: 6 May 2021

                   CoRE Resource Directory Extensions


   A collection of extensions to the Resource Directory
   [I-D.ietf-core-resource-directory] that can stand on their own, and
   have no clear future in specification yet.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 6 May 2021.

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   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Reverse Proxy requests  . . . . . . . . . . . . . . . . . . .   3
     2.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Registration  . . . . . . . . . . . . . . . . . . . . . .   3
       2.2.1.  Registration updates  . . . . . . . . . . . . . . . .   4
       2.2.2.  Proxy behavior  . . . . . . . . . . . . . . . . . . .   4
       2.2.3.  On-Demand proxying  . . . . . . . . . . . . . . . . .   5
       2.2.4.  Multiple upstreams  . . . . . . . . . . . . . . . . .   5
       2.2.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .   5
       2.2.6.  Notes on stability and maturity . . . . . . . . . . .   7
       2.2.7.  Security considerations . . . . . . . . . . . . . . .   7
   3.  Infinite lifetime . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Lookup across link relations  . . . . . . . . . . . . . . . .   8
     4.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Lifetime Age  . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Zone identifier introspection . . . . . . . . . . . . . . . .   9
     6.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Proxying multicast requests . . . . . . . . . . . . . . . . .  10
     7.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Opportunistic RD  . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Applications  . . . . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   This document pools some extensions to the Resource Directory
   [I-D.ietf-core-resource-directory] 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.

   [ This document is being developed at
   resource-directory-extensions (
   directory-extensions). ]

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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).

   [I-D.ietf-core-resource-directory] 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 clients that want to
   conceal their network address from its clients.

2.1.  Discovery

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

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

   The RD MAY mint several alternative Registration Base URIs using
   different protocols to make the proxied content available;
   [I-D.silverajan-core-coap-protocol-negotiation] can be used to
   advertise them.

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   The registrant is not informed of the chosen public name by the RD.

   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 ([I-D.ietf-core-resource-directory]
   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, it keeps 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.

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.

   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.

   Note that transports can not be switched in a registration update, as
   the protocol is part of the registration resource.

2.2.2.  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

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2.2.3.  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.

   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 ].

2.2.4.  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.

2.2.5.  Examples  Registration through a firewall

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   Req from [2001:db8:42::9876]:5683:
   POST coap://

   Req from
   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:

   Req: GET coap://

   Res: 2.05 Content

   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.  Registration from a browser context

   Req: POST coaps+ws://

   Res: 2.04 Created
   Location: /reg/123

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

   Req: GET coap://

   Res: 2.05 Content

   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

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2.2.6.  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. ]

2.2.7.  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.

   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.

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.

   Infinite lifetimes SHOULD only be used by commissioning tools, or for
   proxy registrations over stateful connections.

3.1.  Example

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

   Req: GET coaps+ws://*

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

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   it could register like that:

   Req: POST coaps+ws://

   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

4.  Lookup across link relations

   Resource lookup occasionally needs execute multiple queries to follow

   An RD server (or any other server that supports [RFC6690] compatible
   lookup), can announce support for following links in resource lookups
   by announcing support for the TBD3 interface type on its resource

   A client can the query that server to not only provide the matched
   links, but also links that are reachable over relations given in
   "follow" query parameters.

4.1.  Example

   Assume a node presents the following data in its <.well-known/core>
   resource (and submitted the same to the RD):


   A lookup client can, in one query, find the temperature sensor and
   its relevant metadata:

Req: GET /rd-lookup/res?rt=example.temperature&follow=calibration-protocol&follow=describedby


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   [ There is a better example (
   issues/120#issuecomment-407997786) in an earlier stage of
   [I-D.tiloca-core-oscore-discovery] ]

   Given the likelihood of a CoRAL based successor to [RFC6690], this
   lookup variant might easily be superseeded by a CoRAL FETCH format;
   it might look like this there:

   Req: FETCH /reef-lookup
   Content-Format: application/template-coral+cbor
   #using core = <...>
   #using reef = <...>
   reef:content ?x {
     core:rt "example.temperature"
     calibration-protocol ?y {
       core:describedby ?z

   Res: 2.01 Content
   Content-Format: aplication/coral+cbor
   reef:content <coap://node1/temp> {
       core:rt "example.temperature"
       calibration-protocol <coap://node1/t-prot> {
         core:describedby <>

5.  Lifetime Age

   This extension is described in [I-D.amsuess-core-rd-replication]
   Section 5.2.

   The "provenance" extension in Section 5.1 of the same document should
   probably be expressed differently to avoid using non-target link

6.  Zone identifier introspection

   The 'split-horizon' mechanism introduced in
   [I-D.ietf-core-resource-directory] (-19) (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

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   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.

6.1.  Example

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

Res: 2.05 Content

7.  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 destribes the difficulties in
   more detail.

   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.

   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.

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   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.

7.1.  Example

   Req: GET coap+ws://

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

   Req: GET coap+ws://
   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

   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"

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

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]"

8.  Opportunistic RD

   An application that wants to advertise its resources in Resource
   Directory can find itself in a network that has no RD deployed.  It
   may be able to start an RD on its own to fill in that gap until an
   explicitly configured one gets installed.

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   This bears the risk of having competing RDs on the same network,
   where resources registered at one can not be discovered on the other.
   To mitigate that, such Opportunistic Resource Directories should
   follow those steps:

   *  The RD chooses its own Opportunistic Capability value.  That
      integer number is an estimate of number of target attributes it
      expects to be able to store, where in absence of better estimates
      one would assume that a registration contains 16 links, and each
      links contains three target attributes each with an eight byte key
      and a 16 byte value.

      The Opportunistic Capability value is advertised as a TBD5-cap=
      target attribute on the registration resource.

   *  The RD chooses its own Opportunistic Tie-Break value.  That
      integer number needs no other properties than being likely to be
      different even for two instances of the same device being started;
      numeric forms of MAC addresses or random numbers make good

      The Opportunistic Capability value is advertised as a TBD5-tie=
      target attribute on the registration resource.

   *  The Opportunistic RD, before advertising its resources, performs
      RD discovery itself, using at least all the discovery paths it may
      become discoverable on itself.

   *  If the Opportunistic RD finds no other RD, or if the RD it finds
      is less capable than itself, it can start advertising itself as a
      Resource Directory.

      An RD is called more capable than another if its TBD5-cap value is
      greater than the other's, or if its TBD5-tie value is greater than
      the other's if the former results in a tie.  Absent or unparsable
      attributes are considered greater than any present attribute.

      In case an RD observes a tie even after evaluating the tie
      breaker, it may change its Opportunistic Tie-Break value if that
      was picked randomly, or reevaluate its life choices if it uses its
      own MAC address.

   *  A running Opportunistic RD needs to perform discovery for other
      RDs repeatedly.  If it discovers a more capable RD, it stops
      advertising its own resources.  It should continue to serve lookup
      requests, but refuse any new registration or registration updates
      (which will trigger the registrant endpoints to look for a new

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      An inactive Opportunistic RD will be notified of the highter
      capability RD's shutdown by the expiry of whatever it may be
      started to advertise that was now advertised there; see below for
      possible improvements.

   *  An RD that discovers an Opportunistic RD of lower capability may
      speed up the transition process by (not mutually exclusive) two

      -  It can register its own (registration) resource(s) into the
         lower capability directory.  That RD can take that as having
         discovered a higher capability RD and stop advertising.

      -  It can expose resources and registrations of the lower
         capability directory using the methods described in

   *  An Opportunistic RD that yields to a more capable RD may ease the
      transition by attempting to register its active registrations at
      the more capable RD, taking the role of a CT.  The lifetimes
      picked for those must not exceed the remaining lifetime of its
      registrations, and it must not renew those registrations.

   Future iterations of this document may want to cut down on the
   possibilities listed above.

   Some ideas are around for making the shutdown of a commissioned or
   otherwise high-capability RD more graceful, but they still have some

   *  Setting a commissioned or high-capability RD's Capability to zero
      in preparation of the shutdown may create loops in any distributed

   *  Asking the lower capability RD to register its registration
      resource (even though not otherwise advertised) at the higher
      capability RD still creates a situation where clients may find two
      RDs running simultaneously, and we can't expect clients to make
      any decisions based on TBD5 values.

   *  Asking the higher capability RD to register its registration
      resource with the lower capability RD contradicts the current
      recommendation for the passive Opportunistic RD to not accept
      registrations / renewals.  Also, the deployed RD may not know that
      Opportunistic RDs are a thing.

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   *  Advertising an almost-but-not-quite rt= value on passive
      registration resources may be an option, but needs to be thought
      through thoroughly.

   Installations of Opportunistic RDs are at special risk of resource
   exhaustion because they are not sized with their actual deployment in
   mind, but rely on defaults set by the application that starts the RD.
   Opportunistic RDs should only be started if the application's
   administrator can be informed in a timely fashion when the RD's
   resources are nearing exhaustion; guidance towards installing a more
   capable RD on the network should be provided in that case.

8.1.  Applications

   *  Group managers using [I-D.tiloca-core-oscore-discovery] can ship
      its own low-priority Opportunistic RD to announce its join
      resources.  This provides benefits over announcing them on
      multicast discovery if the network can efficiently route requests
      to the All CoAP Resource Directories multicast address (so group
      members get a response back from an early focused request to all
      RDs rather than falling back to multicasting All CoAP Nodes for
      "?rt=osc.j&..."), or if discovery of the group's multicast address
      is used.

   *  Administrative tools that try to provide a broad overview of a
      network's CoAP devices could offer to open an Opportunistic RD if
      they find no active RD on the network (but should ask the user in
      interactive scenarios).

      That allows them to see devices that newly join the network
      quickly (by observing their own or the found RD), rather than
      relying periodic multicasts.

9.  References

9.1.  Normative References

              Amsuess, C., "Resource Directory Replication", Work in
              Progress, Internet-Draft, draft-amsuess-core-rd-
              replication-02, 11 March 2019, <

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              Amsuess, C., Shelby, Z., Koster, M., Bormann, C., and P.
              Stok, "CoRE Resource Directory", Work in Progress,
              Internet-Draft, draft-ietf-core-resource-directory-26, 2
              November 2020, <

   [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, <>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

9.2.  Informative References

              Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation",
              Work in Progress, Internet-Draft, draft-silverajan-core-
              coap-protocol-negotiation-09, 2 July 2018,

              Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
              Groups with the CoRE Resource Directory", Work in
              Progress, Internet-Draft, draft-tiloca-core-oscore-
              discovery-07, 2 November 2020, <

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,

   [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,

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,

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   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,

   [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,

   [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,

Appendix A.  Change log

   Since -03:

   *  Added interaction with PvD (Provisioning Domains)

   Since -02:

   *  Added abstract

   *  Added example of CoRAL FETCH to Lookup across link relations

   Since -01:

   *  Added section on Opportunistic RDs

   Since -00:

   *  Add multicast proxy usage pattern

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

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

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

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Internet-Draft     CoRE Resource Directory Extensions      November 2020

Appendix B.  Acknowledgements

   [ Reviews from: Jaime Jimenez ]

Author's Address

   Christian Amsüss
   Hollandstr. 12/4

   Phone: +43-664-9790639

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