Internet DRAFT - draft-amsuess-core-resource-directory-extensions
draft-amsuess-core-resource-directory-extensions
CoRE C. Amsüss
Internet-Draft 4 March 2024
Intended status: Experimental
Expires: 5 September 2024
CoRE Resource Directory Extensions
draft-amsuess-core-resource-directory-extensions-10
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 5 September 2024.
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
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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 . . . . . . . . . . . . . . . . 10
5.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Proxying multicast requests . . . . . . . . . . . . . . . . . 11
6.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Registrations that update DNS records . . . . . . . . . . . . 12
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 . . . . . . . . . . . . . . . . . 17
Appendix A. Attic . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 20
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22
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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. Unlike regular
registrations, they are not "soft state"; the registrant can expect
the RD to persist the registrations across network changes, reboots,
softare updates and that like.
Typical use cases for infinite life times are:
* 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
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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<=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:
* 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.
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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.
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.
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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.
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.
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"
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.
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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
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:
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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.
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.
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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.
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.
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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
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.
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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>.
[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
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[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-03, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
edhoc-oscore-profile-03>.
[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-00, 2 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
workflow-and-params-00>.
[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-13, 20 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-13>.
[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., "CoAP Protocol Indication", Work in Progress,
Internet-Draft, draft-ietf-core-transport-indication-03,
23 October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-transport-indication-03>.
[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>.
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[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>.
[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>.
[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>.
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[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.
Removed in favor of utilizing [I-D.ietf-core-coral] FETCH
requests.
Appendix B. Change log
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:
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* 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:
* 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)
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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
Appendix C. Acknowledgements
[ Reviews from: Jaime Jimenez ]
Section 4 was inspired by Ben Kaduk's comments from reviewing
[rfc9176].
Author's Address
Christian Amsüss
Email: christian@amsuess.com
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