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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 . 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.

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. 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.

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 : 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 : 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 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.

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, 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 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 ( 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.

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 , or more generally ) 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 , 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 : 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. A possible solution is to re-introduce tokens, in a message format similar to that of CoAP-over-WebSockets .

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 , 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 ), 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.

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. ]

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. 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 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" ].

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 ( 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.

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. Universitaet Bremen TZI Lobaro UG Many research and maker platforms for Internet of Things experimentation offer a serial interface. This is often used for programming, diagnostic output, as well as a crude command interface ("AT interface"). Alternatively, it is often used with SLIP (RFC1055) to transfer IP packets only. The present report describes how to use a single serial interface for diagnostics, configuration commands and state readback, as well as packet transfer. RISE AB IoTconsultancy.nl This document specifies the operations performed by a proxy, when using the Constrained Application Protocol (CoAP) in group communication scenarios. Such a proxy processes a single request sent by a client over unicast, and distributes the request over IP multicast to a group of servers. Then, the proxy collects the individual responses from those servers and relays those responses back to the client, in a way that allows the client to distinguish the responses and their origin servers through embedded addressing information. This document updates RFC7252 with respect to caching of response messages at proxies. Universität Bremen TZI In CoAP as defined by RFC 7252, responses are always unicast back to a client that posed a request. The present memo describes two forms of responses that go beyond that model. These descriptions are not intended as advocacy for adopting these approaches immediately, they are provided to point out potential avenues for development that would have to be carefully evaluated. The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server. 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.

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.