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CoAP over GATT (Bluetooth Low Energy Generic Attributes)
draft-amsuess-core-coap-over-gatt-02

Document Type Active Internet-Draft (individual)
Author Christian Amsüss
Last updated 2022-07-11
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draft-amsuess-core-coap-over-gatt-02
CoRE                                                           C. Amsüss
Internet-Draft                                              12 July 2022
Intended status: Standards Track                                        
Expires: 13 January 2023

        CoAP over GATT (Bluetooth Low Energy Generic Attributes)
                  draft-amsuess-core-coap-over-gatt-02

Abstract

   Interaction from computers and cell phones to constrained devices is
   limited by the different network technologies used, and by the
   available APIs.  This document describes a transport for the
   Constrained Application Protocol (CoAP) that uses Bluetooth GATT
   (Generic Attribute Profile) and its use cases.

Note to Readers

   Discussion of this document takes place on the CORE Working Group
   mailing list (core@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/core/
   (https://mailarchive.ietf.org/arch/browse/core/).

   Source for this draft and an issue tracker can be found at
   https://gitlab.com/chrysn/coap-over-gatt/ (https://gitlab.com/chrysn/
   coap-over-gatt/-/tree/master).

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 13 January 2023.

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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 (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
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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Application example . . . . . . . . . . . . . . . . . . .   3
     1.2.  Alternatives  . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol description  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Requests and responses  . . . . . . . . . . . . . . . . .   4
       3.1.1.  Development directions  . . . . . . . . . . . . . . .   6
     3.2.  Addresses . . . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Scheme-free alternative . . . . . . . . . . . . . . .   7
       3.2.2.  Use with persistent addresses . . . . . . . . . . . .   8
     3.3.  Compression and reinterpretation of non-CoAP
           characteristics . . . . . . . . . . . . . . . . . . . . .   8
   4.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Uniform Resource Identifier (URI) Schemes . . . . . . . .   8
   5.  Security considerations . . . . . . . . . . . . . . . . . . .   9
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] can be used
   with different network and transport technologies, for example UDP on
   6LoWPAN networks.

   Not all those network technologies are available at end user devices
   in the vicinity of the constrained devices, which inhibits direct
   communication and necessitates the use of gateway devices or cloud
   services.  In particular, 6LoWPAN is not available at all in typical
   end user devices, and while 6LoWPAN-over-BLE (IPSP, the Internet

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   Protocol Support Profile of Bluetooth Low Energy (BLE), [RFC7668])
   might be compatible from a radio point of view, many operating
   systems or platforms lack support for it, especially in a user-
   accessible way.

   As a workaround to access constrained CoAP devices from end user
   devices, this document describes a way encapsulate generic CoAP
   exchanges in Bluetooth GATT (Generic Attribute Profile).  This is
   explicitly not designed as means of communication between two devices
   in full control of themselves -- those should rather build an IP
   based network and transport CoAP as originally specified.  It is
   intended as a means for an application to escape the limitations of
   its environment, with a special focus on web applications that use
   the Web Bluetooth [webbluetooth].  In that, it is similar to CoAP-
   over-WebSockets [RFC8323].  GATT, which has read and write semantics,
   is not a perfect match for CoAP's request/response semantics; this
   specification bridges the gap in order to make CoAP transportable
   over what is sometimes the only available protocol.

1.1.  Application example

   Consider a network of home automation light bulbs and switches, which
   internally uses CoAP on a 6LoWPAN network and whose basic pairing
   configuration can be done without additional electronic devices.

   Without CoAP-over-GATT, an application that offers advanced
   configuration requires the use of a dedicated gateway device or a
   router that is equipped and configured to forward between the 6LoWPAN
   and the local network.  In practice, this is often delivered as a
   wired gateway device and a custom app.

   With CoAP-over-GATT, the light bulbs can advertise themselves via
   BLE, and the configuration application can run as a web site.  The
   user navigates to that web site, and it asks permission to contact
   the light bulbs using Web Bluetooth.  The web application can then
   exchange CoAP messages directly with the light bulb, and have it
   proxy requests to other devices connected in the 6LoWPAN network.

   For browsers that do not support Web Bluetooth, the same web
   application can be packaged into an native application consisting of
   a proxy process that forwards requests received via CoAP-over-
   WebSockets on the loopback interface to CoAP-over-GATT, and a browser
   view that runs the original web application in a configuration to use
   WebSockets rather than CoAP-over-GATT.

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   That connection is no replacement when remote control of the system
   is desired (in which case, again, a router is required that
   translates 6LoWPAN to the rest of the network), but suffices for many
   commissioning tasks.

1.2.  Alternatives

   Several approaches were considered, but considered unsuitable for the
   intended use cases:

   *  CoAP over 6LoWPAN over BLE: While this is the natural choice for
      transporting CoAP over BLE, it is unavailable on typical end user
      devices.  There is no clear path toward how that would be
      integrated in platforms like Android or iOS, and even if it were,
      creating a network connection to a nearby device from within an
      application might not be possible (if how WLAN networks are
      managed is any indication).

   *  GoldenGate [goldengate]: This introduces significant network
      overhead, and burdens the end user device application with
      shipping a full network stack that is executed in a position where
      it can not integrate fully with the operating system's network
      stack.

      Moreover, this places a retransmission layer on top of a reliable
      transport (GATT), duplicating effort and possibly aggravating
      congestion situations.

   *  CoAP over UDP over SLIP over GATT UART [nefzger]: This is similar
      to the GoldenGate approach, but built on the GATT UART provided
      with Nordic Semiconductor's libraries.

      This shares the network stack duplication and retransmission
      concerns of GoldenGate.

   *  slipmux [I-D.bormann-t2trg-slipmux] over BLE GATT UART service:
      This is similar to the previous item; the stack duplication
      concern is addressed, but retransmissions are still active atop of
      a service that already provides reliability.

2.  Terminology

3.  Protocol description

3.1.  Requests and responses

   [ This section is not thought through or implemented yet, and could
   probably end up very different. ]

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   CoAP-over-GATT uses individual GATT Characteristics to model a
   reliable request-response mechanism.  Therefore, it has no message
   types or message IDs (in which it resembles CoAP-over-TCP [RFC8323]),
   and no tokens.  In the place of tokens, different Bluetooth
   characteristics (comparable to open ports in IP based networks) can
   be used.  All messages use GATT to ensure reliable transmission.

   A GATT server announces service of UUID 8df804b7-3300-496d-9dfa-
   f8fb40a236bc (abbreviated US in this document), with one or more
   characteristics of UUID 2a58fc3f-3c62-4ecc-8167-d66d4d9410c2
   (abbreviated UC).

   [ Right now, this only supports requests from the GATT client to the
   GATT server; role reversal might be added later. ]

   A client can start a CoAP request by writing to the UC characteristic
   a sequence composed of a single code byte, any options encoded in the
   option format of [RFC7252] Section 3.1, optionally followed by a
   payload marker and the request payload.

   After the successful write, the client can read the response back
   from the server on the same characteristic.  The client may need to
   attempt reading the characteristic several times until the response
   is ready, and may subscribe to indications to get notifiied when the
   response is ready.

   The server does needs to keep the response readable after it has been
   read, for the server can not know whether the read was completed by
   the client.

   If the request and initial response establish an observation, the
   client may keep reading; the server may keep the latest notification
   available indefinitely (especially if it turns out that "has been
   read successfully" is hard to determine) or make it readable only
   once for each new state.

   Once the client writes a new request to a UC characteristic, any
   later reads pertain to that request, and any observation previously
   established is cancelled implicitly.

   Attribute values are limited to 512 Bytes ([bluetooth52] Part F
   Section 3.2.9), practically limiting blockwise operation ([RFC7959])
   to size exponents to 4 (resulting in a block size of 256 byte).  Even
   smaller messages might enhance the transfer efficiency when they
   avoid fragmentation at the L2CAP level.

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   If a server provides multiple OC typed characteristics, parallel
   requests or observations are possible; otherwise, this transport is
   limited to a single pending request.

3.1.1.  Development directions

   Three major concerns may need addressing in future iterations of this
   protocol:

   *  Role reversal.

      This may be implemented by adding a GATT server to the central, or
      by multiplexing requests and responses onto a single read and
      write channel.

   *  Response reliability.

      When multiple responses are sent to a request (e.g. when using
      [I-D.tiloca-core-groupcomm-proxy], or more generally
      [I-D.bormann-core-responses]) of which all need to be delivered,
      or if role reversal is implemented by multiplexing, the GATT
      server needs to know when a message has been read; the GATT
      mechanisms do not provide that information.

      Previously, this was not deemed relevant, as for the original non-
      traditional responses, observation notifications [RFC7641], only
      eventual consistency is relevant.

      One option is to replace reads with write-with-response
      operations, and to introduce a flag that marks previously read
      messages as received.  This is essentially building a 1-bit
      message ID mechanism.  (No longer IDs are necessary, because
      messages on GATT are not reordered on the network).

   *  Fragmentation.  If the current approach of requiring devices to
      support large MTU sizes turns out to be impractical, or if GATT
      level fragmentation vastly outperforms CoAP fragmentation, it may
      be necessary to use composite reads and writes on GATT.

      Care has to be taken to use only operations supported by
      [webbluetooth]: that API does not expose reads with offsets.

      Offset based fragmentation may also be incompatible with the
      write-with-response approach suggested for reliability.

   *  Concurrent requests.  If a multiplexing approach is chosen for
      role reversal, the current setup of multiple characteristics for
      multiple requests may become obsolete.

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      A possible solution is to re-introduce tokens, in a message format
      similar to that of CoAP-over-WebSockets [RFC8323].

3.2.  Addresses

   The URI scheme associated with CoAP over GATT is "coap+gatt".  The
   default value of Uri-Host is the MAC address of the CoAP server, in
   hexadecimal encoding, with the dash character ("-") separating the
   bytes. [ Some bikeshedding is expected on these details. ]

   User information and port are always absent with this scheme.

   Assembling the URI of a request for the discovery resource of a BLE
   device with the MAC address 00:11:22:33:44:55 would thus be
   assembled, under the rules of Section 6.4 of [RFC7252], to
   coap+gatt://00-11-22-33-44-55/.well-known/core.

   Locally defined host or service name registries may be used to create
   names that are more suitable for human interaction.  For DNS, which
   is widely used for this purpose, no record types are registered that
   map to Bluetooth MAC addresses at the time of writing.

   Note that on some platforms (e.g.  Web Bluetooth [webbluetooth]), the
   peer's or the own address may not be known application.  They may
   come up with an application-internal registered name component (e. g.
   coap+gatt://id-SomeInternalIdentifier/.well-known/core), but must be
   aware that those can not be expressed towards anything outside the
   local stack -- the same way they would avoid using IPv6 zone
   identifiers or URIs whose host name is localhost.

3.2.1.  Scheme-free alternative

   As an alternative to the abovementioned scheme, a zone in .arpa could
   be registered to use addresses like

   coap://001122334455.ble.arpa/.well-known/core

   where the .ble.arpa address do not resolve to any IP addresses.

   [ Accepting this will require a .arpa registering IANA consideration
   to replace the URI one. ]

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3.2.2.  Use with persistent addresses

   When services are meant to provide long-lived and universally usable
   URIs, addresses based on MAC addresses can be impractical, because
   they fluctuate on hardware changes.  (Moreover, privacy mechanisms on
   the device or the platform can render them unusable even before
   hardware changes).

   In the absence of a usable host or service name registry,
   implementers may opt for non-GATT addresses right away.
   [I-D.ietf-core-transport-indication] provides the means to advertise
   a different canonical address, and to announce availability of that
   advertised service on the present transport, CoAP-over-GATT.  If the
   device is not generally reachable, the canonical address might also
   be unreachable (see [I-D.ietf-core-transport-indication] section
   "Unreachable canonical origin address").

   When long-lived addresses circumvent privacy preserving measures,
   considerations concering the tracking of devices [ are TBD along the
   lines of "don't make it discoverable to unauthorized sources, and in
   case of doubt let the peer show its credentials first" ].

3.3.  Compression and reinterpretation of non-CoAP characteristics

   The use of SCHC is being evaluated in combination with CoAP-over-
   GATT; the device can use the characteristic UUID to announce the
   static context used.

   Together with non-traditional response forms
   ([I-D.bormann-core-responses] and contexts that expand, say, a
   numeric value 0x1234 to a message like

   2.05 Content Response-For: GET /temperature Content-Format:
   application/senml+cbor Payload (in JSON-ish equivalent): [ {1 /* unit
   */: "K", 2 /* value */: 0x1234} ]

   This enables a different use case than dealing with limited
   environments: Accessing BLE devices via CoAP without application
   specific gateways.  Any required information about the application
   can be expressed in the SCHC context.

4.  IANA considerations

4.1.  Uniform Resource Identifier (URI) Schemes

   IANA is asked to enter a new scheme into the "Uniform Resource
   Identifier (URI) Schemes" registry set up in [RFC7595]:

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   *  URI Scheme: "coap+gatt"

   *  Description: CoAP over Bluetooth GATT (sharing the footnote of
      coap+tcp)

   *  Well-Known URI Support: yes, analogous to [RFC7252]

5.  Security considerations

   All data received over GATT is considered untrusted; secure
   communication can be achieved using OSCORE [RFC8613].

   Physical proximity can not be inferred from this means of
   communication.

6.  References

6.1.  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/info/rfc7252>.

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <https://www.rfc-editor.org/info/rfc7595>.

6.2.  Informative References

   [RFC7668]  Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "IPv6 over BLUETOOTH(R) Low
              Energy", RFC 7668, DOI 10.17487/RFC7668, October 2015,
              <https://www.rfc-editor.org/info/rfc7668>.

   [webbluetooth]
              Grant, R. and O. Ruiz-Henríquez, "Web Bluetooth", 24
              February 2020,
              <https://webbluetoothcg.github.io/web-bluetooth/>.

   [goldengate]
              Fitbit, Inc, "Golden Gate", 2020,
              <https://fitbit.github.io/golden-gate/>.

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   [nefzger]  Matthias Nefzger, "Talk CoAP to me – IoT over Bluetooth
              Low Energy", 1 March 2021,
              <https://www.maibornwolff.de/en/blog/talk-coap-me-iot-
              over-bluetooth-low-energy>.

   [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/info/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/info/rfc8613>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [bluetooth52]
              "Bluetooth Core Specification v5.2", 31 December 2019,
              <https://www.bluetooth.org/docman/handlers/
              downloaddoc.ashx?doc_id=478726>.

   [I-D.bormann-t2trg-slipmux]
              Bormann, C. and T. Kaupat, "Slipmux: Using an UART
              interface for diagnostics, configuration, and packet
              transfer", Work in Progress, Internet-Draft, draft-
              bormann-t2trg-slipmux-03, 4 November 2019,
              <https://www.ietf.org/archive/id/draft-bormann-t2trg-
              slipmux-03.txt>.

   [I-D.tiloca-core-groupcomm-proxy]
              Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
              Communication", Work in Progress, Internet-Draft, draft-
              tiloca-core-groupcomm-proxy-06, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-tiloca-core-
              groupcomm-proxy-06.txt>.

   [I-D.bormann-core-responses]
              Bormann, C. and C. Amsüss, "CoAP: Non-traditional response
              forms", Work in Progress, Internet-Draft, draft-bormann-
              core-responses-01, 3 February 2022,
              <https://www.ietf.org/archive/id/draft-bormann-core-
              responses-01.txt>.

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   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [I-D.ietf-core-transport-indication]
              Amsüss, C., "CoAP Protocol Indication", Work in Progress,
              Internet-Draft, draft-ietf-core-transport-indication-01,
              11 July 2022, <https://www.ietf.org/archive/id/draft-ietf-
              core-transport-indication-01.txt>.

Appendix A.  Change log

   Since -01:

   *  Point out (possibly conflicting) development directions.

   *  Describe URI scheme more completely, including persistent
      addresses.

   *  Aim for standards track.

   *  Describe rejeced alternative approaches.

   Since -00:

   *  Add note on SCHC possibilities.

Author's Address

   Christian Amsüss
   Austria
   Email: christian@amsuess.com

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