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CoAP Protocol Indication

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Author Christian Amsüss
Last updated 2021-07-10
Replaced by draft-ietf-core-transport-indication
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CoRE                                                           C. Amsüss
Internet-Draft                                              10 July 2021
Intended status: Standards Track                                        
Expires: 11 January 2022

                        CoAP Protocol Indication


   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

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 (, which is
   archived at

   Source for this draft and an issue tracker can be found at

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

   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 (
   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

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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

   A server can indicate support for same-host proxying (or any kind of
   proxying, really) by serving a Web Link with the "has-proxy"

   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

   Res: from [2001:db8::1]:5683
   Content-Format: application/link-format

   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

          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

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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://" the proxy
      accessed as "coap+tcp://[2001:db8::1]" still needs to provide a
      certificate chain for the name 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

      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

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

   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? ]


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   Req: to [ff02::fd]:5683 on UDP
   Code: GET
   Uri-Path: /.well-known/core

   Res: from [2001:db8::1]:5683
   Content-Format: application/link-format

   Req: to [2001:db8::1]:5683 on TCP
   Code: GET
   Uri-Path: /sensors/

   Res: 2.05 Content
   Content-Format: application/link-format

      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://" 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://" 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,sensor

Content-Format: application/link-format

Req: to on WebSocket
Host (indicated during upgrade):
Code: GET
Uri-Path: /sensors/

Res: 2.05 Content
Content-Format: application/link-format

  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:// has advertised
   "</res>,<coap+tcp://>;rel=has-proxy;anchor="/"", then a
   proxy that has an active CoAP-over-TCP connection to
   can forward an incoming request for coap:// through
   that CoAP-over-TCP connection with a suitable Proxy-Scheme on that

   If the host had marked the proxy point as
   "<coap+tcp://>;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:// to satisfy
   requests for coap://

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

   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:

   ``` SRV 0 0 61616 AAAA 2001:db8::1

   proxy;anchor="coap://" ```

   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

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://

Res: from [2001:db8::1]:5683
Content-Format: application/link-format

Req: to [2001:db8::1]:5683 on TCP
Code: GET
Uri-Path: /sensors/temp
Observe: 0

Res: 2.05 Content
Observe: 0

    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

      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,

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

9.2.  Informative References

   [aliases]  W3C, "Architecture of the World Wide Web, Section 2.3.1
              URI aliases", n.d.,

   [cooluris] BL, T., "Cool URIs don't change", n.d.,

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              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,

              Amsüss, C., "rdlink: Robust distributed links to
              constrained devices", Work in Progress, Internet-Draft,
              draft-amsuess-t2trg-rdlink-01, 23 September 2019,

              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,

              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, <

              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, <

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

   [lwm2m]    OMA SpecWorks, "White Paper - Lightweight M2M 1.1", n.d.,

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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,

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

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

   [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,

   [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,

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

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   *  Added IANA considerations

   *  Added section on open questions

Appendix B.  Open Questions / further ideas

  *  OSCORE interaction: [RFC8613] Section 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


     which is verbose but formally clear, and


     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

   Phone: +43-664-9790639

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