CoRE Resource Directory Extensions
draft-amsuess-core-resource-directory-extensions-08
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Christian Amsüss | ||
| Last updated | 2023-03-13 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
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| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
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draft-amsuess-core-resource-directory-extensions-08
CoRE C. Amsüss
Internet-Draft 13 March 2023
Intended status: Experimental
Expires: 14 September 2023
CoRE Resource Directory Extensions
draft-amsuess-core-resource-directory-extensions-08
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 14 September 2023.
Copyright Notice
Copyright (c) 2023 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Limited lifetimes . . . . . . . . . . . . . . . . . . . . . . 9
5. Lookup across link relations . . . . . . . . . . . . . . . . 10
5.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Lifetime Age . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Zone identifier introspection . . . . . . . . . . . . . . . . 12
7.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Proxying multicast requests . . . . . . . . . . . . . . . . . 13
8.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 14
9. Opportunistic RD . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Applications . . . . . . . . . . . . . . . . . . . . . . 17
10. Registrations that update DNS records . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 19
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 21
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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.
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.
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.
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
3.1. Example
Had the example of Section 2.6.2 discovered support for infinite
lifetimes during lookup like this:
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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.
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.
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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 (where the endpoint would see additional
metadata to its own registration resource) or special media types
for GETting the registration resource itself. Either way, this
requires 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. Lookup across link relations
Resource lookup occasionally needs execute multiple queries to follow
links.
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
lookup.
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.
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5.1. Example
Assume a node presents the following data in its <.well-known/core>
resource (and submitted the same to the RD):
</temp>;if="core.s";rt="example.temperature",
</t-prot>;rel="calibration-protocol";anchor="/temp",
<http://vendor.example.com/temp9000>;rel="describedby";anchor="/temp",
</hum>;if="core.s";rt="example.humidity",
</h-prot>;rel="calibration-protocol";anchor="/hum",
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
<coap://node1/temp>;if="core.s";rt="example.temperature";anchor="coap://node1",
<coap://node1/t-prot>;rel="calibration-protocol";anchor="coap://node1/temp",
<http://vendor.example.com/temp9000>;rel="describedby";anchor="coap://node1/temp",
[ There is a better example (https://github.com/ace-wg/ace-oauth/
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:
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Req: FETCH /reef-lookup
Content-Format: application/template-coral+cbor
Payload:
#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
Payload:
reef:content <coap://node1/temp> {
core:rt "example.temperature"
calibration-protocol <coap://node1/t-prot> {
core:describedby <http://vendor.example.com/temp9000>
}
}
6. 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
attributes.
7. 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.
7.1. 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",
8. 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.
8.1. 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]"
9. 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
candidates.
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
RD).
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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
ways
- 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
[I-D.amsuess-core-rd-replication].
* 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
problems
* Setting a commissioned or high-capability RD's Capability to zero
in preparation of the shutdown may create loops in any distributed
lookups.
* 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.
9.1. 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.
10. 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].
11. References
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11.1. 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>.
11.2. Informative References
[I-D.ietf-core-transport-indication]
Amsüss, C., "CoAP Protocol Indication", Work in Progress,
Internet-Draft, draft-ietf-core-transport-indication-01,
11 July 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-transport-indication-01>.
[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-08, 28 February 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
groupcomm-proxy-08>.
[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsüss, C., and P. Van der Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-13, 8 March 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-discovery-13>.
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[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>.
[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. Change log
Since -07:
* Update references.
Since -06:
* Add sketch for DNS updates.
* Add sketch for forward proxying.
* Fix erroneous section numbers.
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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)
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
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* 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 B. 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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