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CoAP Protocol Indication
draft-amsuess-core-transport-indication-02

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Document	Type	This is an older version of an Internet-Draft whose latest revision state is "Replaced".
	Author	Christian Amsüss
	Last updated	2021-10-25
	Replaced by	draft-ietf-core-transport-indication, draft-ietf-core-transport-indication
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draft-amsuess-core-transport-indication-02

CoRE C. Amsüss
Internet-Draft 25 October 2021
Intended status: Standards Track
Expires: 28 April 2022

CoAP Protocol Indication
draft-amsuess-core-transport-indication-02

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 28 April 2022.

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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 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 . . . . . . . . . . . . . 7
2.5. Advertisement through a Resource Directory . . . . . . . 8
3. Elision of Proxy-Scheme and Uri-Host . . . . . . . . . . . . 8
4. Third party proxy services . . . . . . . . . . . . . . . . . 10
4.1. Generic proxy advertisements . . . . . . . . . . . . . . 11
5. Client picked proxies . . . . . . . . . . . . . . . . . . . . 12
6. Related work and applicability to related fields . . . . . . 13
6.1. On HTTP . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Using DNS . . . . . . . . . . . . . . . . . . . . . . . . 13
6.3. Using names outside regular DNS . . . . . . . . . . . . . 14
6.4. Multipath TCP . . . . . . . . . . . . . . . . . . . . . . 15
7. Security considerations . . . . . . . . . . . . . . . . . . . 16
7.1. Security context propagation . . . . . . . . . . . . . . 16
7.2. Traffic misdirection . . . . . . . . . . . . . . . . . . 16
7.3. Protecting the proxy . . . . . . . . . . . . . . . . . . 17
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 17
8.1. Link Relation Types . . . . . . . . . . . . . . . . . . . 17
8.2. Resource Types . . . . . . . . . . . . . . . . . . . . . 17
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 20
Appendix B. Open Questions / further ideas . . . . . . . . . . . 21
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 22

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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 or application 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 also 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. The Proxy-URI option is still equivalent
to the decomposed options, and can be used if the server supports it.

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
protocols. 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 (as it should if no Proxy-Scheme option accompanied it), 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 has the path "/", and no query component or fragment
identifier. This is true even for CoAP over WebSockets (which uses
the concrete resource /.well-known/coap, but that is not expressed in
"coap+ws" URI).

Future protocols for which CoAP proxying is defined may use more
components. As this is designed primarily for CoAP, proxies that
perform URI mapping (as described in Section 5 of [RFC8075],
especially using URI templates) are not supported in this document.

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: Discovery and 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
is equivalent 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).

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.

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.

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). When the describing document approaches expiry, the
client can use the representation's ETag renew their justification
for using the alternative transport.

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.

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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. For example,
asking ?if=tag:example.com,sensor&proxy-links=tcp could give as a
result: <coap://[2001:db8::1]/s>;rt=tag:example.com,sensor,<coap+tcp:
//[2001:db8::1]/>;rel=has-proxy;anchor="coap://[2001:db8::1]/" ]

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. ]

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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.

While applications retain knowledge of the originally requested URI
(even if it is not expressed in full on the wire), the original URI
is not accessible to caches both within the host and on the network
(the latter see Section 5). Thus, cached responses to the canonical
and any aliased URI are mutually interchangable as long as both the
response and the proxy statement are fresh.

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",
<coaps+ws://[2001:db8::1]>;rel=has-unique-proxy;anchor="/"

Req: to [2001:db8::1] via WebSockets over HTTPS
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
coaps+ws counterpart -- as the request is still for that URI, which
both the client and the server are aware of. However, this detail is
of little practical importance: A simplistic client that uses
coaps+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, the state is kept no
longer than the has-unique-proxy statement is fresh, and it does not
(for example) pass the URI 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.

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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. In
particular, a server may obtain a link to a third party proxy from an
administrator as part of its configuration.

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. This use is not directly related to the
protocol indication in other parts of this document, but sufficiently
similar to warrant being described in the same document.

The resource type "TBDcore.proxy" can be used to describe such a
proxy. The link target attribute "proxy-schemes" can be used to
indicate the scheme(s) supported by the proxy, separated by the space
character.

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Req: GET coap://[fe80::1]/.well-known/core?rt=TBDcore.proxy

Res:
Content-Format: application/link-format
Payload:
<>;rt=TBDcore.proxy;proxy-schemes="coap coap+tcp coap+ws http"

Req: to [fe80::1] via CoAP
Code: GET
Proxy-Scheme: http
Uri-Host: example.com
Uri-Path: /motd
Accept: text/plain

Res: 2.05 Content
Content-Format: text/plain
Payload:
On Monday, October 25th 2021, there is no special message of the day.

Figure 4: A CoAP client discovers that its border router can also
serve as a proxy, and uses that to access a resource on an HTTP
server.

The considerations of Section 7.2 apply here.

A generic advertised proxy is always a forward proxy, and can not be
advertised as a "unique" proxy as it would lack information about
where to forward.

The use of a generic proxy can be limited to a set of devices that
have permission to use it. Clients can be allowed by their network
address if they can be verified, or by using explicit client
authentication using the methods of
[I-D.tiloca-core-oscore-capable-proxies].

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).

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 (possibly same-host) proxy, or that has learned

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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 of 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.

While not specified for CoAP yet (and neither being specified here),

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[ which is an open discussion point for CoRE -- should we? Here? In
a separate 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 5, 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+tcp://[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 5: Obtaining a sensor value from a local device with a
global name

6.4. Multipath TCP

When CoAP-over-TCP is used over Multipath TCP and no Uri-Host option
is sent, the implicit assumption is that there is aliasing between
URIs containing any of the endpoints' addresses.

As these are negotiated within MPTCP, this works independently of
this document's mechanisms. As long as all the server's addresses
are equally reachable, there is no need to advertise multiple
addresses that can later be discovered through MPTCP anyway. When
advertisements are long-lived and there is no single more stable
address, several available addresses can be advertised
(indepependently of whether MPTCP is involved or not). If a client
uses an address that is merely a proxy address (and not a unique
proxy address), but during MPTCP finds out that the network location
being accessed is actually an MPTCP alternative address of the used
one, the client MAY forego sending of the Proxy-Scheme and Uri-Path
option.

[ This follows from multiple addresses being valid for that TCP
connection; at some point we may want to say something about what
that means for the default value of the Uri-Host option -- maybe

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something like "has the default value of any of the associated
addresses, but the server may only enable MPTCP if there is implicit
aliasing between all of them" (similar to OSCORE's statement)? ]

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.

When security is terminated at proxies (as is in DTLS and TLS), a
third party proxy can usually not satisfy this requirement; these
transports are limited to same-host proxies.

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.

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.

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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.

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]:

Table 1: New Link Relation types

8.2. Resource Types

IANA is asked to add an entry into the "Resource Type (rt=) Link
Target Attribute Values" registry under the Constrained RESTful
Environments (CoRE) Parameters:

[ The RFC Editor is asked to replace any occurrence of TBDcore.proxy
with the actually registered attribute value. ]

Attribute Value: core.proxy

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Description: Forward proxying services

Reference: [ this document ]

Notes: The schemes for which the proxy is usable may be indicated
using the proxy-schemes target attribute as per Section 4.1 of [ this
document ].

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://www.rfc-editor.org/rfc/rfc7252>.

[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/rfc/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>.

[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-

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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>.

[I-D.tiloca-core-oscore-capable-proxies]
Tiloca, M. and R. Hoeglund, "OSCORE-capable Proxies", Work
in Progress, Internet-Draft, draft-tiloca-core-oscore-
capable-proxies-00, 12 July 2021,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-capable-proxies-00>.

[lwm2m] OMA SpecWorks, "White Paper – Lightweight M2M 1.1", n.d.,
<https://omaspecworks.org/white-paper-lightweight-m2m-
1-1/>.

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

[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/rfc/rfc6763>.

[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/rfc/rfc7838>.

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[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/rfc/rfc8075>.

[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>.

[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://www.rfc-editor.org/rfc/rfc8613>.

Appendix A. Change log

Since -01:

* Removed suggestion for generally trusted proxies; now stating that
with (D)TLS, "a third party proxy can usually not satisfy [the
security context propagation requirement]".

* State more clearly that valid cache entries for resources aliased
through has-unique-proxy can be used.

* Added considerations for Multipath TCP.

* Added concrete suggestion and example for advertisement of general
proxies.

* Added concrete suggestion for RD lookup extension that provides
proxies.

* Minor editorial and example changes.

Since -00:

* Added introduction

* Added examples

* Added SCHC analogy

* Expanded security considerations

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* 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

* 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"

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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,

- 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).

If CoRAL is used during discovery, a base directive or reverse
relation to has-unique-proxy would make this easier.

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
Austria

Email: christian@amsuess.com

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