Austria christian@amsuess.com

CoRE CoAP, bluetooth, gatt 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/. Source for this draft and an issue tracker can be found at https://gitlab.com/chrysn/coap-over-gatt/.

Introduction The Constrained Application Protocol (CoAP) 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 Protocol Support Profile of Bluetooth Low Energy (BLE), ) 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 . In that, it is similar to CoAP-over-WebSockets .

Procedural status [ This section will be removed before publication. ] The path of this document is currently not clear. It might attract interest in the CoRE working group, but might be easier to process as an indpenendent submission.

Appplication 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. 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.

Terminology

Protocol description

Requests and responses [ This section is not thought through or implemented yet, and could probably end up very different. ] 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 ), and no tokens. In the place of tokens, . 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 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 not need to keep the response readable after it has been read successfully. 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 ( Part F Section 3.2.9), practically limiting blockwise operation () 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. If a server provides multiple OC typed characteristics, parallel requests or observations are possible; otherwise, this transport is limited to a single pending request.

Addresses [ ... coap+bluetooth://00-11-22-33-44-55-66-77-88-99/.well-known/core ... ] Note that when using Web Bluetooth , neither the own nor the peer's address are known to the application. They may come up with an application-internal authority component (e. g. coap+bluetooth://id-SomeInternalIdentifier/.well-known/core), but must be aware that those can not be expressed towards anything outside the local stack.

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

IANA considerations

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

• URI Scheme: "coap+gatt"
• Description: CoAP over Bluetooth GATT (sharing the footnote of coap+tcp)
• Well-Known URI Support: yes, analogous to

Security considerations All data received over GATT is considered untrusted; secure communication can be achieved using OSCORE . Physical proximity can not be inferred from this means of communication.

References Normative References The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. This document updates the guidelines and recommendations, as well as the IANA registration processes, for the definition of Uniform Resource Identifier (URI) schemes. It obsoletes RFC 4395. Informative References Bluetooth Smart is the brand name for the Bluetooth low energy feature in the Bluetooth specification defined by the Bluetooth Special Interest Group. The standard Bluetooth radio has been widely implemented and available in mobile phones, notebook computers, audio headsets, and many other devices. The low-power version of Bluetooth is a specification that enables the use of this air interface with devices such as sensors, smart meters, appliances, etc. The low-power variant of Bluetooth has been standardized since revision 4.0 of the Bluetooth specifications, although version 4.1 or newer is required for IPv6. This document describes how IPv6 is transported over Bluetooth low energy using IPv6 over Low-power Wireless Personal Area Network (6LoWPAN) techniques. The Constrained Application Protocol (CoAP), although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of CoAP over UDP includes support for reliable delivery, simple congestion control, and flow control. Some environments benefit from the availability of CoAP carried over reliable transports such as TCP or Transport Layer Security (TLS). This document outlines the changes required to use CoAP over TCP, TLS, and WebSockets transports. It also formally updates RFC 7641 for use with these transports and RFC 7959 to enable the use of larger messages over a reliable transport. This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. The Constrained Application Protocol (CoAP) is a RESTful transfer protocol for constrained nodes and networks. Basic CoAP messages work well for small payloads from sensors and actuators; however, applications will need to transfer larger payloads occasionally -- for instance, for firmware updates. In contrast to HTTP, where TCP does the grunt work of segmenting and resequencing, CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS). These transports only offer fragmentation, which is even more problematic in constrained nodes and networks, limiting the maximum size of resource representations that can practically be transferred. Instead of relying on IP fragmentation, this specification extends basic CoAP with a pair of "Block" options for transferring multiple blocks of information from a resource representation in multiple request-response pairs. In many important cases, the Block options enable a server to be truly stateless: the server can handle each block transfer separately, with no need for a connection setup or other server-side memory of previous block transfers. Essentially, the Block options provide a minimal way to transfer larger representations in a block-wise fashion. A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, so there is an expectation that the Block options will be widely used in CoAP implementations. Therefore, this specification updates RFC 7252.