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CoAP Protocol Indication

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Author Christian Amsüss
Last updated 2022-05-30
Replaces draft-amsuess-core-transport-indication
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CoRE                                                           C. Amsüss
Internet-Draft                                               30 May 2022
Intended status: Standards Track                                        
Expires: 1 December 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 and provisions 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 (, 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 1 December 2022.

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Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
       1.1.1.  Using URIs to identify protocol endpoints . . . . . .   4
     1.2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Indicating alternative transports . . . . . . . . . . . . . .   6
     2.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.2.  Security context propagation  . . . . . . . . . . . . . .   8
     2.3.  Choice of transports  . . . . . . . . . . . . . . . . . .   8
     2.4.  Selection of a canonical origin . . . . . . . . . . . . .   9
     2.5.  Advertisement through a Resource Directory  . . . . . . .   9
   3.  Elision of Proxy-Scheme and Uri-Host  . . . . . . . . . . . .   9
     3.1.  Impact on caches  . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Using unique proxies securely . . . . . . . . . . . . . .  12
   4.  Third party proxy services  . . . . . . . . . . . . . . . . .  13
     4.1.  Generic proxy advertisements  . . . . . . . . . . . . . .  14
   5.  Client picked proxies . . . . . . . . . . . . . . . . . . . .  15
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  16
     6.1.  Security context propagation  . . . . . . . . . . . . . .  16
     6.2.  Traffic misdirection  . . . . . . . . . . . . . . . . . .  16
     6.3.  Protecting the proxy  . . . . . . . . . . . . . . . . . .  17
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Link Relation Types . . . . . . . . . . . . . . . . . . .  17
     7.2.  Resource Types  . . . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .  20
   Appendix B.  Related work and applicability to related fields . .  22
     B.1.  On HTTP . . . . . . . . . . . . . . . . . . . . . . . . .  22
     B.2.  Using DNS . . . . . . . . . . . . . . . . . . . . . . . .  22
     B.3.  Using names outside regular DNS . . . . . . . . . . . . .  23
     B.4.  Multipath TCP . . . . . . . . . . . . . . . . . . . . . .  24

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   Appendix C.  Open Questions / further ideas . . . . . . . . . . .  24
   Appendix D.  Acknowledgements . . . . . . . . . . . . . . . . . .  25
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  25

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-
   independent 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

   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252] and link format ([RFC6690] (or,
   equivalently, web links as described in [RFC8288]).

   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].

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      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" is implied in
      the link.

      The link relation has been occasionally used with different
      interpretations, which ascribe more meaning to the term than it
      has in its definition.  In particular,

      *  the "hosts" relation can not be inferred merely by two URIs
         having the same scheme, host and port (and vice versa), and

      *  the "hosts" relation on its own does not make any statement
         about the physical devices that hold the resource's

      [ TBD: The former could probably still be used without too many
      ill effects; but things might also get weird when 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.

1.1.1.  Using URIs to identify protocol endpoints

   The URI coap:// identifies a particular resource,
   possibly a "welcome" text.  It is, colloquially, also used to
   identify the combination of a host (identified through a name), the
   default port, and the CoAP method of sending requests to the host.

   For precision, this document uses the term "the transport address
   indicated by (a URI)" to refer to the host / port / protocol
   combination, but otherwise no big deal is made of it.

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   For the CoAP schemes (coap, coaps, coap+tcp, coaps+tcp, coap+ws,
   coaps+ws), URIs indicating a transport are always given with an empty
   path (which under their URI normalization rules is equivalent to a
   path containing a single slash).  For the coap and coap+tcp schemes,
   URIs with different host names can indicate the same transport as
   long as the names resolve to the same addresses.  For the other
   protocols, the given host name informs the name set in TLS's Server
   Name Indication (SNI) and/or the host sent in the "Host" header of
   the underlying HTTP request.

   If an update to this document extends the list, for new schemes it
   might be allowed to have paths, queries or fragment identifiers
   present in the URI indicating the transport address.  No guidance can
   be given here for these, as no realistic example is known.  (Note
   that while the coap+ws scheme does use the well-known path /.well-
   known/coap internally, that is used purely on the HTTP side, and not
   part of the CoAP URI, not even for indicating the transport address).

   URIs indicating a transport are especially useful when talking about
   proxies; this use is aligned with the way they are exprssed in the
   conventional environment variables http_proxy etc. [ cite ].
   Furthermore, URIs processing is widespread in CoAP systems, and when
   that changes (e.g. through the introduction of [I-D.ietf-core-href]),
   URIs indicating a transport will still be efficient to encode.  And
   last but not least, it lines up well with the colloquial identity
   mentioned above.  (An alternative would be using a dedicated naming
   scheme, say,, but that would
   needlessly introduce implementation complexity).

   Note that this mechanism can only used with proxies that use CoAP's
   native address indication mechanisms.  Proxies that perform URI
   mapping (as described in Section 5 of [RFC8075], especially using URI
   templates) are not supported in this document.

   [ TBD: Do we want to extend this to HTTP proxies?  Probably just not,
   and if so, only to those that can just take coap://... for a URI. ]

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.

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   *  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 their availability in advance.  Hosts whose transport's
   availability changes over time can utilize any suitable mechanism to
   keep client updated, such as placing a suitable Max-Age value on
   their resources or having them observable.

2.  Indicating alternative transports

   While CoAP can set the authority component of the requested URI in
   all requests (by means of Uri-Host and Uri-Port), setting 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 proxy, the device should then
   check whether it recognizes the name set in Uri-Host as one of its
   own (as it should if no Proxy-Scheme option accompanied it).  If the
   name is not recognized, it should reject the request with 5.05
   (Proxying Not Supported) -- unless, of course, it implements forward
   proxy functionality exceeding the same-host proxy.  If the name is
   recognized, it should process the request as it would process a
   request coming in on the given protocol (which, for many hosts, is
   the same as if the option were absent completely).

   A server can advertise a recommended proxy by serving a Web Link with
   the "has-proxy" relation to a URI indicating its transport address.
   In particular (and that is a typical case), it can indicate its own
   transport address on an alternative transport when implementing same-
   host proxy functionality.

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   The semantics of a link from S to P with relations has-proxy ("S has-
   proxy P", <P>;rel=has-proxy;anchor="S") are that for any resource R
   hosted on S ("S hosts R"), the proxy with the transport address
   indicated by P can be used to obtain R.

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:

   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: Discovery and follow-up request through a has-proxy

   Note that generating this discovery file needs to be dynamic based on
   its available addresses; only if queried using a link-local source
   address, the server 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 it contains a Proxy-Scheme option.

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   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

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 P if the proxy can provide suitable credentials.  (The hosting
   URI S is immaterial to these considerations).

   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.

   Some applications have requirements exceeding the requirements of a
   secure connection, e.g., (explicitly or implicitly) requiring that
   name resolution happen through a secure process and packets are only
   routed into networks where it trusts that they will not be
   intercepted on the path to the server.  Such applications need to
   extend their requirements to the source of the has-proxy statement; a
   sufficient (but maybe needlessly strict) requirement is to only
   follow has-proxy statements that are part of the same resource that
   advertises the link currently being followed.  Section Section 6.2
   adds further considerations.

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 to efficiently renew its
   justification for using the alternative transport.

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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.  For example,
   asking ?,sensor&proxy-links=tcp could give as a
   result: <coap://[2001:db8::1]/s>;,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

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   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 requests arriving at the proxy
   are treated the same no matter whether the scheme, authority and port
   of the link context are set in the Proxy-Scheme, Uri-Host and Uri-
   Port options, respectively, or whether all of them are absent.

   [ 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.

   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
   (for the latter, see Section 5).  Thus, cached responses to the
   canonical and any aliased URI are mutually interchangeable 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 -- on 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] via WebSockets over HTTPS
   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
   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:// 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.

3.1.  Impact on caches

   [ This section is written with the "there is implied URI aliasing"
   mindset; it should be possible to write it with the "compression"
   mindset as well (but there is no point in having both around in the
   document at this time).

   It is also slightly duplicating, but also more detailed than, the
   brief note on the topic in Section 5 ]

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   When a node that performs caching learns of a has-unique-proxy
   statement, it can utilize the information about the implied URI
   aliasing: Requests to resources hosted by S can be answered with
   cached entries from P (because by the rules of has-unique-proxy a
   request can be crafted that is sent to P for which a fresh response
   is available).  The inverse direction (serving resources whose URI
   "starts with" P from a cached request that was sent to S) is harder
   to serve because it additionaly requires a fresh statement that "S
   hosts R" for the matching resource R.

3.2.  Using unique proxies securely

   [ This section is work in progress, it is more a flow of
   considerations turning back on each other.  This is all made a bit
   trickier by not applying to OSCORE which is usually the author's go-
   to example, because OSCORE's requirements already preclude all these
   troubles. ]

   The use of unique proxies requires slightly more care in terms of

   No requirements are necessary on the client side; those of {#secctx-
   propagation} suffice.  (In particular, it is not necessary for the
   statement to originate from the original server unless that were
   already a requirement without the uniqueness property).

   The extra care is necessary on the side of servers that are
   commissioned with wide ranging authorization [ or is it? ]: These may
   now be tricked into serving a resource of which the client assumes a
   different name.  For example, if the desired resource is
   coaps://, and there exists a
   "home page" style service for employees with patterns of
   coaps+tcp://user-${username} at which they can store
   files, and the server operating that service is commissioned with a
   wild-card certificate "*", then a device that receives
   the (malicious) information <coaps+tcp://user->;rel=has-unique-proxy;anchor="coaps://high-" might use this statement to contact the
   transport address indicated by coaps+tcp:// and
   ask for /config (which, to the server, is indistinguishable from
   coaps+tcp:// and obtain a malicious

   In a non-unique proxy situation, the error would have been caught by
   the server, which would have seen the request for coaps://high- and refused to serve a request containing
   critical options it can not adaequately process.

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   In the unique proxy situation, ... [ TBD: now whose fault is it?  Can
   only be the client's ... because it looked at the wildcard
   certificate rather than whether the host-name it was narrowing it
   down to is authorized to speak for  The
   server (operator) can barely be blamed, for while the certificate is
   needlessly wide, to the server it did look precisely like a good
   request. ]

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.  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 6.2).

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

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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

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

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

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

   Res: 2.05 Content
   Content-Format: text/plain
   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

   The considerations of Section 6.2 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.  (A proxy limited to a single outbound protocol
   might in theory work as a unique proxy when using a transport in
   which the full default Uri-Host value is configured at setup time,
   but these are considered impractical and thus not assigned a resource
   type here.)

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   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

5.  Client picked proxies

   This section is purely informative, and serves to illustrate that the
   mechanisms introduced in this document do not hinder the continued
   use of existing 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 and resolves the given name
   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, may use that
   information in choosing the upstream transport, to correct the URI
   associated with a cached response, 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 instead, 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://

   A client that uses a forward proxy and learns of a different proxy
   advertised to access a particular resource will not change its
   behavior if its original proxy is part of its configuration.  If the
   forward proxy was only used out of necessity (e.g., to access a
   resource on the protocol not supported by the client) it can be
   practical for the client to use the advertised proxy instead.

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6.  Security considerations

6.1.  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.

6.2.  Traffic misdirection

   Accepting arbitrary proxies, even with security context propagation
   performed properly, would allow 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

      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.

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6.3.  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.  IANA considerations

7.1.  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

7.2.  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

   Description: Forward proxying services

   Reference: [ this document ]

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   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 ].

8.  References

8.1.  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,

8.2.  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.,

              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,

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              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-10, 7 March 2022,

              Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P. V.
              D. Stok, "Constrained RESTful Environments (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,

              Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
              in Progress, Internet-Draft, draft-tiloca-core-oscore-
              capable-proxies-02, 7 March 2022,

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

   [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,

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

   [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,

   [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 draft-amsuess-core-transport-indication-03:

   *  No changes (merely changing the name after WG adoption)

   Since -02 (mainly processing reviews from Marco and Klaus):

   *  Acknowledge that 'coap://hostname/' is not the proxy but a URI
      that (in a particular phrasing) is used to stand in for the
      proxy's address (while it regularly identifies a resurce on the

   *  Security: Referencing traffic misdirection already in the first
      security block.

   *  Security: Add (incomplete) considerations for unique-proxy case.

   *  Narrow down "unique" proxy semantics to those properties used by
      the client, allowing unique proxies to be co-hosted with forward

   *  "Client picked proxies" clarified to merely illustrate how this is
      compatible with them.

   *  Use of "hosts" relation sharpened.

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   *  Precision on how this does and does not consider changing

   *  "Related work" section demoted to appendix.

   *  Add note on DTLS session resumption.

   *  Variable renaming.

   *  Various editorial fixes.

   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

   *  Added concrete suggestion for RD lookup extension that provides

   *  Minor editorial and example changes.

   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

   *  Added IANA considerations

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   *  Added section on open questions

Appendix B.  Related work and applicability to related fields

B.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 and thus enable the "transport address indicated
   by a URI" concept.  Thus, there is no need for a distinction between
   protocol-id and scheme.

   To accommodate 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.

B.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),

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


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   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

B.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 5, a server using a cryptographic name as described in
   [I-D.amsuess-t2trg-rdlink] is discovered and used.

Req: to [ff02::fd]:5683 on UDP
Code: GET
Uri-Path: /.well-known/core
Uri-Query: rel=has-proxy
Uri-Query: 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 5: Obtaining a sensor value from a local device with a
                             global name

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B.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 (independently
   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
   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)?  ]

   [ TBD: Do we need a section analog to this that deals with (D)TLS
   session resumption in absence of SNI? ]

Appendix C.  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

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     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 correctness 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 D.  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.

   [ TBD: reviewers Marco Klaus ]

Author's Address

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   Christian Amsüss

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