CoAP Protocol Indication
draft-amsuess-core-transport-indication-01
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| Document | Type | Active Internet-Draft (individual) | |
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| Author | Christian Amsüss | ||
| Last updated | 2021-07-10 | ||
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draft-amsuess-core-transport-indication-01
CoRE C. Amsüss
Internet-Draft 10 July 2021
Intended status: Standards Track
Expires: 11 January 2022
CoAP Protocol Indication
draft-amsuess-core-transport-indication-01
Abstract
The Constrained Application Protocol (CoAP, [RFC7252]) is available
over different transports (UDP, DTLS, TCP, TLS, WebSockets), but
lacks a way to unify these addresses. This document provides
terminology based on Web Linking [RFC8288] to express alternative
transports available to a device, and to optimize exchanges using
these.
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/transport-indication.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 11 January 2022.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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 Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Indicating alternative transports . . . . . . . . . . . . . . 5
2.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Security context propagation . . . . . . . . . . . . . . 7
2.3. Choice of transports . . . . . . . . . . . . . . . . . . 7
2.4. Selection of a canonical origin . . . . . . . . . . . . . 8
2.5. Advertisement through a Resource Directory . . . . . . . 8
3. Elision of Proxy-Scheme and Uri-Host . . . . . . . . . . . . 9
4. Third party proxy services . . . . . . . . . . . . . . . . . 10
4.1. Generic proxy advertisements . . . . . . . . . . . . . . 11
5. Client picked proxies . . . . . . . . . . . . . . . . . . . . 11
6. Related work and applicability to related fields . . . . . . 12
6.1. On HTTP . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Using DNS . . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. Using names outside regular DNS . . . . . . . . . . . . . 13
7. Security considerations . . . . . . . . . . . . . . . . . . . 14
7.1. Security context propagation . . . . . . . . . . . . . . 14
7.2. Traffic misdirection . . . . . . . . . . . . . . . . . . 14
7.3. Protecting the proxy . . . . . . . . . . . . . . . . . . 15
7.4. Implementing proxies . . . . . . . . . . . . . . . . . . 15
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 16
8.1. Link Relation Types . . . . . . . . . . . . . . . . . . . 16
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
9.1. Normative References . . . . . . . . . . . . . . . . . . 16
9.2. Informative References . . . . . . . . . . . . . . . . . 16
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 18
Appendix B. Open Questions / further ideas . . . . . . . . . . . 19
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
The Constrained Application Protocol (CoAP) provides transports
mechanisms (UDP and DTLS since [RFC7252], TCP, TLS and WebSockets
since [RFC8323]), with some additional being used in LwM2M [lwm2m]
and even more being explored ([I-D.bormann-t2trg-slipmux],
[I-D.amsuess-core-coap-over-gatt]). These are mutually incompatible
on the wire, but CoAP implementations commonly support several of
them, and proxies can translate between them.
CoAP currently lacks a way to indicate which transports are available
for a given resource, and to indicate that a device is prepared to
serve as a proxy; this document solves both by introducing the "has-
proxy" terminology to Web Linking [RFC8288] that expresses the former
through the latter. The additional "has-unique-proxy" term is
introduced to negate any per-request overhead that would otherwise be
introduced in the course of this.
CoAP also lacks a unified scheme to label a resource in a transport-
indepenent way. This document does _not_ attempt to introduce any
new scheme here, or raise a scheme to be the canonical one. Instead,
each host can pick a canonical address for its resources, and
advertise other transports in addition.
1.1. Terminology
Same-host proxy A CoAP server that accepts forward proxy requests
(i.e., requests carrying the Proxy-Scheme option) exclusively for
URIs that it is the authoritative server for is defined as a
"same-host proxy".
The distinction between a same-host and any other proxy is only
relevant on a practical, server-implementation and illustrative
level; this specification does not use the distinction in
normative requirements, and clients need not make the distinction
at all.
hosts The verb "to host" is used here in the sense of the link
relation of the same name defined in [RFC6690].
For resources discovered via CoAP's discovery interface, a hosting
statement is typically provided by the defaults implied by
[RFC6690] where a link like "</sensor/temp>" is implied to have
the relation "hosts" and the anchor "/", such that a statement
"coap://hostname hosts coap://hostname/sensor/temp" can be
inferred.
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For many application it can make sense to assume that any resource
has a "host" relation leading from the root path of the server
without having performed that discovery explicitly.
[ TBD: The last paragraph could be a contentuous point; things
should still work without it, and maybe that's even better because
it precludes a dynamic resource created with one transport from
use with another transport unless explicitly cleared. ]
When talking of proxy requests, this document only talks of the
Proxy-Scheme option. Given that all URIs this is usable with can be
expressed in decomposed CoAP URIs, the need for using the Proxy-URI
option should never arise.
1.2. Goals
This document introduces provisions for the seamless use of different
transport mechanisms for CoAP. Combined, these provide:
* Enablement: Inform clients of the availability of other transports
of servers.
* No Aliasing: Any URI aliasing must be opt-in by the server. Any
defined mechanisms must allow applications to keep working on the
canonical URIs given by the server.
* Optimization: Do not incur per-request overhead from switching
protocls. This may depend on the server's willingness to create
aliased URIs.
* Proxy usability: All information provided must be usable by aware
proxies to reduce the need for duplicate cache entries.
* Proxy announcement: Allow third parties to announce that they
provide alternative transports to a host.
For all these functions, security policies must be described that
allow the client to use them as securely as the original transport.
This document will not concern itself with changes in transport
availability over time, neither in causing them ("Please take up your
TCP interface, I'm going to send a firmware update") nor in
advertising them (other than by the server putting suitable Max-Age
values on any of its statements).
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2. Indicating alternative transports
While CoAP can indicate the authority component of the requested URI
in all requests (by means of Uri-Host), indicating the scheme of a
requested URI (by means of Proxy-Scheme) makes the request implicitly
a proxy request. However, this needs to be of only little practical
concern: Any device can serve as a proxy for itself (a "same-host
proxy") by accepting requests that carry the Proxy-Scheme option. If
it is to be a well-behaved as a proxy, the device should then check
whether it recognizes the name indicated in Uri-Host as one of its
own [ TBD: Check whether 7252 makes this a stricter requirement ],
reject the request with 5.05 when it is not recognized, and otherwise
process it as it would process a request coming in on that protocol
(which, for many hosts, is the same as if the option were absent
completely).
A server can indicate support for same-host proxying (or any kind of
proxying, really) by serving a Web Link with the "has-proxy"
relation.
The semantics of a link from C to T with relations has-proxy ("C has-
proxy T", "<T>;rel=has-proxy;anchor="C"") are that for any resource R
hosted on C ("C hosts R"), T is can be used as a proxy to obtain R.
Note that HTTP and CoAP proxies are not located at a particular
resource, but at a host in general. Thus, a proxy URI "T" in these
protocols can not carry a path or query component. This is true even
for CoAP over WebSockets (which uses the concrete resource "/.well-
known/coap", but that can not expressed in "coap+ws" URI). Future
protocols for which CoAP proxying is defined may have expressible
path components.
2.1. Example
A constrained device at the address 2001:db8::1 that supports CoAP
over TCP in addition to CoAP can self-describe like this:
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Req: to [ff02::fd]:5683 on UDP
Code: GET
Uri-Path: /.well-known/core
Uri-Query: if=tag:example.com,sensor
Res: from [2001:db8::1]:5683
Content-Format: application/link-format
Payload:
</sensors/temp>;if="tag:example.com,sensor",
<coap+tcp://[2001:db8::1]>;rel=has-proxy;anchor="/"
Req: to [2001:db8::1]:5683 on TCP
Code: GET
Proxy-Scheme: coap
Uri-Path: /sensors/temp
Observe: 0
Res: 2.05 Content
Observe: 0
Payload:
39.1°C
Figure 1: Follow-up request through a has-proxy relation
Note that generating this discovery file needs to be dynamic based on
its available addresses; only if queried using a link-local source
address, it may also respond with a link-local address in the
authority component of the proxy URI.
Unless the device makes resources discoverable at
"coap+tcp://[2001:db8::1]/.well-known/core" or another discovery
mechanism, clients may not assume that
"coap+tcp://[2001:db8::1]/sensors/temp" is a valid resource (let
alone has any relation to the other resource on the same path). The
server advertising itself like this may reject any request on CoAP-
over-TCP unless they contain a Proxy-Scheme option.
Clients that want to access the device using CoAP-over-TCP would send
a request by connecting to 2001:db8::1 TCP port 5683 and sending a
GET with the options Proxy-Scheme: coap, no Uri-Host or -Port options
(utilizing their default values), and the Uri-Paths "sensors" and
"temp".
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2.2. Security context propagation
If the originally requested URI R or the application requirements
demand a security mechanism is used, the client MUST only use the
proxy T if the proxy can provide suitable credentials. (The hosting
URI C is immaterial to these considerations).
Credentials are usable if either:
* The credentials are good for the intended use of R.
For example, if the application uses the host name and a public
key infrastructure and R is "coap://example.com/" the proxy
accessed as "coap+tcp://[2001:db8::1]" still needs to provide a
certificate chain for the name example.com to one of the system's
trust anchors. If, on the other hand, the application is doing a
firmware update and requires any certificate from its configured
firmware update issuer, the proxy needs to provide such a firmware
update certificate.
* The credentials are suitable as a general trusted proxy for the
system.
This applies only to security mechsnisms that are terminated in
proxies (i.e. (D)TLS and not OSCORE).
For a client to trust a proxy to this extent, it must have
configured knowledge which proxies it may trust. Such
configuration is generally only possible if the application's
security selection is based on the host name (as the client's
intention to, as in the above example, obtain a firmware update,
can not be transported to the proxy).
This option is unlikely to be useful in same-host proxies, but
convenient in scenarios like in Section 4.
2.3. Choice of transports
It is up to the client whether to use an advertised proxy transport,
or (if multiple are provided) which to pick.
Links to proxies may be annotated with additional metadata that may
help guide such a choice; defining such metadata is out of scope for
this document.
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Clients MAY switch between advertised transports as long as the
document describing them is fresh; they may even do so per request.
(For example, they may perform individual requests using CoAP-over-
UDP, but choose CoAP-over-TCP for requests with large expected
responses).
2.4. Selection of a canonical origin
While a server is at liberty to provide the same resource
independently on different transports (i.e. to create aliases), it
may make sense for it to pick a single scheme and authority under
which it announces its resources. Using only one address helps
proxies keep their caches efficient, and makes it easier for clients
to avoid exploring the same server twice from different angles.
When there is a predominant scheme and authority through which an
existing service is discovered, it makes sense to use these for the
canonical addresses.
Otherwise, it is suggested to use the "coap" or "coaps" scheme (given
that these are the most basic and widespread ones), and the most
stable usable name the host has.
2.5. Advertisement through a Resource Directory
In the Resource Directory specification
[I-D.ietf-core-resource-directory], protocol negotiation was
anticipated to use multiple base values. This approach was abandoned
since then, as it would incur heavy URI aliasing.
Instead, devices can submit their has-proxy links to the Resource
Directory like all their other metadata.
A client performing resource lookup can ask the RD to provide
available (same-host-)proxies in a follow-up request by asking for
"?anchor=the-discovered-host&rel=has-proxy". The RD may also
volunteer that information during resource lookups even though the
has-proxy link itself does not match the search criteria.
[ It may be useful to define RD parameters for use with lookup here,
which'd guide which available proxies to include. ]
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3. Elision of Proxy-Scheme and Uri-Host
A CoAP server may publish and accept multiple URIs for the same
resource, for example when it accepts requests on different IP
addresses that do not carry a Uri-Host option, or when it accepts
requests both with and without the Uri-Host option carrying a
registered name. Likewise, the server may serve the same resources
on different transports. This makes for efficient requests (with no
Proxy-Scheme or Uri-Host option), but In general is discouraged
[aliases].
To make efficient requests possible without creating URI aliases that
propagate, the "has-unique-proxy" specialization of the has-proxy
relation is defined.
If a proxy is unique, it means that it unconditionally forwards to
the server indicated in the link context, even if the Proxy-Scheme
and Uri-Host options are elided.
[ The following two paragraphs are both true but follow different
approaches to explaining the observable and implementable behavior;
it may later be decided to focus on one or the other in this
document. ]
While this creates URI aliasing in the requests as they are sent over
the network, applications that discover a proxy this way should not
"think" in terms of these URIs, but retain the originally discovered
URIs (which, because Cool URIs Don't Change[cooluris], should be
long-term usable). They use the proxy for as long as they have fresh
knowledge of the has-(unique-)proxy statement.
In a way, advertising "has-unique-proxy" can be viewed as a
description of the link target in terms of SCHC
[I-D.ietf-lpwan-coap-static-context-hc]: In requests to that target,
the link source's scheme and host are implicitly present.
A client MAY use a unique-proxy like a proxy and still send the
Proxy-Scheme and Uri-Host option; such a client needs to recognize
both relation types, as relations of the has-unique-proxy type are a
specialization of has-proxy and typically don't carry the latter
(redundant) annotation. [ To be evaluated -- one one hand, supporting
it this way means that the server needs to identify all of its
addresses and reject others. Then again, is a server that (like many
now do) fully ignore any set Uri-Host correct at all? ]
Example:
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Req: to [ff02::fd]:5683 on UDP
Code: GET
Uri-Path: /.well-known/core
Uri-Query: if=tag:example.com,sensor
Res: from [2001:db8::1]:5683
Content-Format: application/link-format
Payload:
</sensors/>;if="tag:example.com,collection",
<coap+tcp://[2001:db8::1]>;rel=has-proxy;anchor="/"
Req: to [2001:db8::1]:5683 on TCP
Code: GET
Uri-Path: /sensors/
Res: 2.05 Content
Content-Format: application/link-format
Payload:
</sensors/temperature>;if="tag:example.com,sensor"
Figure 2: Follow-up request through a has-unique-proxy relation.
Compared to the last example, 5 bytes of scheme indication are
saved during the follow-up request.
It is noteworthy that when the URI reference "/sensors/temperature"
is resolved, the base URI is "coap://device0815.example.com" and not
its coap+ws counterpart -- as the request is implicitly forwarded
there, which both the client and the server are aware of. However,
this detail is of little practical importance: A simplistic client
that uses "coap+ws://device0815.proxy.rd.example.com" as a base URI
will still arrive at an identical follow-up request with no ill
effect, as long as it only uses the wrongly assembled URI for
dereferencing resources, the security context is the same, and it
does not (for example) pass it on to other devices.
4. Third party proxy services
A server that is aware of a suitable cross proxy may use the has-
proxy relation to advertise that proxy. If the protocol used towards
the proxy provides name indication (as CoAP over TLS or WebSockets
does), or by using a large number of addresses or ports, it can even
advertise a (more efficient) has-unique-proxy relation. This is
particularly interesting when the advertisements are made available
across transports, for example in a Resource Directory.
How the server can discover and trust such a proxy is out of scope
for this document, but generally involves the same kind of links.
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The proxy may advertise itself without the origin server's
involvement; in that case, the client needs to take additional care
(see Section 7.2).
Req: GET http://rd.example.com/rd-lookup?if=tag:example.com,sensor
Res:
Content-Format: application/link-format
Payload:
<coap://device0815.example.com/sensors/>;if="tag:example.com,collection",
<coap+wss://device0815.proxy.rd.example.com>;rel=has-unique-proxy;anchor="coap://device0815.example.com/"
Req: to device0815.proxy.rd.example.com on WebSocket
Host (indicated during upgrade): device0815.proxy.rd.example.com
Code: GET
Uri-Path: /sensors/
Res: 2.05 Content
Content-Format: application/link-format
Payload:
</sensors/temperature>;if="tag:example.com,sensor"
Figure 3: HTTP based discovery and CoAP-over-WS request to a CoAP
resource through a has-unique-proxy relation
4.1. Generic proxy advertisements
A third party proxy may advertise its availability to act as a proxy
for arbitrary CoAP requests.
[ TBD: Specify a mechanism for this; "<coap+ws://myself>;rel=has-
proxy;anchor="coap://*"" for all supported protocols appears to be an
obvious but wrong solution. ]
The considerations of Section 7.2 apply here.
5. Client picked proxies
When a resource is accessed through an "actual" proxy (i.e., a host
between the client and the server, which itself may have a same-host
proxy in addition to that), the proxy's choice of the upstream server
is originally (i.e., without the mechanisms of this document) either
configured (as in a "chain" of proxies) or determined by the request
URI (where a proxy picks CoAP over TCP for a request aimed at a
coap+tcp URI).
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A proxy that has learned, by active solicitation of the information
or by consulting links in its cache, that the requested URI is
available through a same-host proxy, or that has learned of
advertised URI aliasings, may use that information in choosing the
upstream transport, and to use responses obtained through one
transport to satisfy requests on another.
For example, if a host at coap://h1.example.com has advertised
"</res>,<coap+tcp://h1.example.com>;rel=has-proxy;anchor="/"", then a
proxy that has an active CoAP-over-TCP connection to h1.example.com
can forward an incoming request for coap://h1.example.com/res through
that CoAP-over-TCP connection with a suitable Proxy-Scheme on that
connection.
If the host had marked the proxy point as
"<coap+tcp://h1.example.com>;rel=has-unique-proxy", then the proxy
could elide the Proxy-Scheme and Uri-Host options, and would (from
the original CoAP caching rules) also be allowed to use any fresh
cache representation of coap+tcp://h1.example.com/res to satisfy
requests for coap://h1.example.com/res.
6. Related work and applicability to related fields
6.1. On HTTP
The mechanisms introduced here are similar to the Alt-Svc header of
[RFC7838] in that they do not create different application-visible
addresses, but provide dispatch through lower transport
implementations.
Unlike in HTTP, the variations of CoAP protocols each come with their
unique URI schemes. Thus, origin URIs can be used without
introducing a distrinction between protocol-id and scheme.
To accomodate the message size constraints typical of CoRE
environments, and accounting for the differences between HTTP headers
and CoAP options, information is delivered once at discovery time.
Using the has-proxy and has-unique-proxy with HTTP URIs as the
context is NOT RECOMMENDED; the HTTP provisions the Alt-Svc header
and ALPN are preferred.
6.2. Using DNS
As pointed out in [RFC7838], DNS can already serve some of the
applications of Alt-Svc and has-unique-proxy by providing different
CNAME records. These cover cases of multiple addresses, but not
different ports or protocols.
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While not specified for CoAP yet (and neither being specified here),
[ which is an open discussion point for CoRE -- should we? Here? In
a separte DNS-SD document? ]
DNS SRV records (possibly in combination with DNS Service Discovery
[RFC6763]) can provide records that could be considered equivalent to
has-unique-proxy relations. If "_coap._tcp", "_coaps._tcp",
"_coap._udp", "_coap+ws._tcp" etc. were defined with suitable
semantics, these can be equivalent:
``` _coap._udp.device.example.com SRV 0 0 device.example.com 61616
device.example.com AAAA 2001:db8::1
<coap://[2001:db8::1]>;rel=has-unique-
proxy;anchor="coap://device.example.com" ```
It would be up to such a specification to give details on what the
link's context is; unlike the link based discovery of this document,
it would either need to pick one distinguished context scheme for
which these records are looked up, or would introduce aliasing on its
own.
6.3. Using names outside regular DNS
Names that are resolved through different mechanisms than DNS, or
names which are defined within the scope of DNS but have no
universally valid answers to A/AAAA requests, can be advertised using
the relation types defined here and CoAP discovery.
In figure Figure 4, a server using a cryptographic name as described
in [I-D.amsuess-t2trg-rdlink] is discovered and used.
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Req: to [ff02::fd]:5683 on UDP
Code: GET
Uri-Path: /.well-known/core
Uri-Query: rel=has-proxy&anchor=coap://nbswy3dpo5xxe3denbswy3dpo5xxe3de.ab.rdlink.arpa
Res: from [2001:db8::1]:5683
Content-Format: application/link-format
Payload:
<coap://[2001:db8::1]>;rel=has-unique-proxy;anchor="coap://nbswy3dpo5xxe3denbswy3dpo5xxe3de.ab.rdlink.arpa"
Req: to [2001:db8::1]:5683 on TCP
Code: GET
OSCORE: ...
Uri-Path: /sensors/temp
Observe: 0
Res: 2.05 Content
OSCORE: ...
Observe: 0
Payload:
39.1°C
Figure 4: Obtaining a sensor value from a local device with a
global name
7. Security considerations
7.1. Security context propagation
Clients need to strictly enforce the rules of Section 2.2. Failure
to do so, in particular using a thusly announced proxy based on a
certificate that attests the proxy's name, would allow attackers to
circumvent the client's security expectation.
The option to accept credentials suitable for a general trusted proxy
is in place for (D)TLS protected scenarios, in which cross-protocol
end-to-end protection is not available. Whether a client will
recognize certificates for general trusted proxies at all depends on
the original proxy setup's security considerations (of [RFC7252]
Section 11.2 and [RFC2616] Section 15.7).
7.2. Traffic misdirection
Accepting arbitrary proxies, even with security context propagation
performed properly, would attackers to redirect traffic through
systems under their control. Not only does that impact availability,
it also allows an attacker to observe traffic patterns.
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This affects both OSCORE and (D)TLS, as neither protect the
participants' network addresses.
Other than the security context propagation rules, there are no hard
and general rules about when an advertised proxy is a suitable
candidate. Aspects for consideration are:
* When no direct connection is possible (e.g. because the resource
to be accessed is served as coap+tcp and TCP is not implemented in
the client, or because the resource's host is available on IPv6
while the client has no default IPv6 route), using a proxy is
necessary if complete service disruption is to be avoided.
While an adversary can cause such a situation (e.g. by
manipulating routing or DNS entries), such an adversary is usually
already in a position to observe traffic patterns.
* A proxy advertised by the device hosting the resource to be
accessed is less risky to use than one advertised by a third
party.
Note that in some applications, servers produce representations
based on unverified user input. In such cases, and more so when
multiple applications share a security context, the
advertisements' provenance may need to be considered.
7.3. Protecting the proxy
A widely published statement about a host's availability as a proxy
can cause many clients to attempt to use it.
This is mitigated in well-behaved clients by observing the rate
limits of [RFC7252], and by ceasing attempts to reach a proxy for the
Max-Age of received errors.
Operators can further limit ill-effects by ensuring that their client
systems do not needlessly use proxies advertised in an unsecured way,
and by providing own proxies when their clients need them.
7.4. Implementing proxies
Proxies that are trusted (i.e., that terminate (D)TLS connections and
have an own server certificate) need to consider the same aspects as
clients for their client-side interface as all other clients.
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Proxies can often process data from different security contexts.
When they do, care needs to be taken to not apply has-proxy
statements across security contexts. (This consideration is not
specific to proxies, but comes up more frequently there).
8. IANA considerations
8.1. Link Relation Types
IANA is asked to add two entries into the Link Relation Type Registry
last updated in [RFC8288]:
+==================+==================================+===========+
| Relation Name | Description | Reference |
+==================+==================================+===========+
| has-proxy | The link target can be used as a | RFCthis |
| | proxy to reach the link context. | |
+------------------+----------------------------------+-----------+
| has-unique-proxy | Like has-proxy, and using this | RFCthis |
| | proxy implies scheme and host of | |
| | the target. | |
+------------------+----------------------------------+-----------+
Table 1: New Link Relation types
9. References
9.1. Normative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://doi.org/10.17487/RFC7252>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://doi.org/10.17487/RFC8288>.
9.2. Informative References
[aliases] W3C, "Architecture of the World Wide Web, Section 2.3.1
URI aliases", n.d.,
<https://www.w3.org/TR/webarch/#uri-aliases>.
[cooluris] BL, T., "Cool URIs don't change", n.d.,
<https://www.w3.org/Provider/Style/URI>.
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[I-D.amsuess-core-coap-over-gatt]
Amsüss, C., "CoAP over GATT (Bluetooth Low Energy Generic
Attributes)", Work in Progress, Internet-Draft, draft-
amsuess-core-coap-over-gatt-01, 2 November 2020,
<https://datatracker.ietf.org/doc/html/draft-amsuess-core-
coap-over-gatt-01>.
[I-D.amsuess-t2trg-rdlink]
Amsüss, C., "rdlink: Robust distributed links to
constrained devices", Work in Progress, Internet-Draft,
draft-amsuess-t2trg-rdlink-01, 23 September 2019,
<https://datatracker.ietf.org/doc/html/draft-amsuess-
t2trg-rdlink-01>.
[I-D.bormann-t2trg-slipmux]
Bormann, C. and T. Kaupat, "Slipmux: Using an UART
interface for diagnostics, configuration, and packet
transfer", Work in Progress, Internet-Draft, draft-
bormann-t2trg-slipmux-03, 4 November 2019,
<https://datatracker.ietf.org/doc/html/draft-bormann-
t2trg-slipmux-03>.
[I-D.ietf-core-resource-directory]
Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P. V.
D. Stok, "CoRE Resource Directory", Work in Progress,
Internet-Draft, draft-ietf-core-resource-directory-28, 7
March 2021, <https://datatracker.ietf.org/doc/html/draft-
ietf-core-resource-directory-28>.
[I-D.ietf-lpwan-coap-static-context-hc]
Minaburo, A., Toutain, L., and R. Andreasen, "Static
Context Header Compression (SCHC) for the Constrained
Application Protocol (CoAP)", Work in Progress, Internet-
Draft, draft-ietf-lpwan-coap-static-context-hc-19, 8 March
2021, <https://datatracker.ietf.org/doc/html/draft-ietf-
lpwan-coap-static-context-hc-19>.
[I-D.silverajan-core-coap-protocol-negotiation]
Silverajan, B. and M. Ocak, "CoAP Protocol Negotiation",
Work in Progress, Internet-Draft, draft-silverajan-core-
coap-protocol-negotiation-09, 2 July 2018,
<https://datatracker.ietf.org/doc/html/draft-silverajan-
core-coap-protocol-negotiation-09>.
[lwm2m] OMA SpecWorks, "White Paper - Lightweight M2M 1.1", n.d.,
<https://omaspecworks.org/white-paper-lightweight-m2m-
1-1/>.
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[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616,
DOI 10.17487/RFC2616, June 1999,
<https://doi.org/10.17487/RFC2616>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://doi.org/10.17487/RFC6690>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://doi.org/10.17487/RFC6763>.
[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://doi.org/10.17487/RFC7838>.
[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://doi.org/10.17487/RFC8323>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://doi.org/10.17487/RFC8613>.
Appendix A. Change log
Since -00:
* Added introduction
* Added examples
* Added SCHC analogy
* Expanded security considerations
* Added guidance on choice of transport, and canonical addresses
* Added subsection on interaction with a Resource Directory
* Added comparisons with related work, including rdlink and DNS-SD
sketches
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* Added IANA considerations
* Added section on open questions
Appendix B. Open Questions / further ideas
* OSCORE interaction: [RFC8613] Section 4.1.3.2 requirements place
OSCORE use in a weird category between has-proxy and has-unique-
proxy (because if routing still works, the result will be
correct). Not sure how to write this down properly, or whether
it's actionable at all.
Possibly there is an inbetween category of "The host needs the
Uri-Host etc. when accessed through CoAP, but because the host
does not use the same OSCORE KID across different virtual hosts,
it's has-unique-proxy as soon as you talk OSCORE".
* Self-uniqueness:
A host that wants to indicate that it doesn't care about Uri-Host
can probably publish something like "</>;rel=has-unique-proxy" to
do so.
This'd help applications justify when they can elide the Uri-Host,
even when no different protocols are involved.
* Advertising under a stable name:
If a host wants to advertise its host name rather than its IP
address during multicast, how does it best do that?
Options, when answering from 2001:db8::1 to a request to ff02::fd
are:
<coap://myhostname/foo>,...,
<coap://[2001:db8::1]>;rel=has-unique-proxy;anchor="coap://myhostname"
which is verbose but formally clear, and
</foo>,...,
<coap://[2001:db8::1]>;rel=has-unique-proxy;anchor="coap://myhostname"
which is compatible with unaware clients, but its correctnes with
respect to canonical URIs needs to be argued by the client, in
this sequence
- understanding the has-unique-proxy line,
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- understanding that the request that went to 2001:db8::1 was
really a Proxy-Scheme/Uri-Host-elided version of a request to
coap://myhostname, and then
- processing any relative reference with this new base in mind.
(Not that it'd need to happen in software in that sequence, but
that's the sequence needed to understand how the "/foo" here is
really "coap://myhostname/foo").
Appendix C. Acknowledgements
This document heavily builds on concepts explored by Bill Silverajan
and Mert Ocak in [I-D.silverajan-core-coap-protocol-negotiation], and
together with Ines Robles and Klaus Hartke inside T2TRG.
Author's Address
Christian Amsüss
Hollandstr. 12/4
1020
Austria
Phone: +43-664-9790639
Email: christian@amsuess.com
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