Internet DRAFT - draft-amsuess-core-coap-over-gatt
draft-amsuess-core-coap-over-gatt
CoRE C. Amsüss
Internet-Draft 14 March 2023
Intended status: Standards Track
Expires: 15 September 2023
CoAP over GATT (Bluetooth Low Energy Generic Attributes)
draft-amsuess-core-coap-over-gatt-03
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 15 September 2023.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Application example . . . . . . . . . . . . . . . . . . . 3
1.2. Alternatives . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Protocol description . . . . . . . . . . . . . . . . . . . . 5
3.1. Boundary conditions: GATT properties . . . . . . . . . . 5
3.2. Requests and responses . . . . . . . . . . . . . . . . . 6
3.2.1. Message sub-layer . . . . . . . . . . . . . . . . . . 6
3.2.2. Using the message sub-layer . . . . . . . . . . . . . 7
3.2.3. Message deduplication . . . . . . . . . . . . . . . . 8
3.2.4. Requests and responses . . . . . . . . . . . . . . . 8
3.2.5. Fragmentation . . . . . . . . . . . . . . . . . . . . 8
3.2.6. Multiple characteristics . . . . . . . . . . . . . . 9
3.2.7. Development directions . . . . . . . . . . . . . . . 9
3.3. Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1. Scheme-free alternative . . . . . . . . . . . . . . . 10
3.3.2. Use with persistent addresses . . . . . . . . . . . . 10
3.4. Compression and reinterpretation of non-CoAP
characteristics . . . . . . . . . . . . . . . . . . . . . 11
3.5. Additional use of advertisements . . . . . . . . . . . . 11
4. IANA considerations . . . . . . . . . . . . . . . . . . . . . 12
4.1. Uniform Resource Identifier (URI) Schemes . . . . . . . . 12
5. Security considerations . . . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Change log . . . . . . . . . . . . . . . . . . . . . 14
Appendix B. Other ToDos . . . . . . . . . . . . . . . . . . . . 15
Appendix C. -03 sketch . . . . . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
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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
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.
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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.
1.2. Alternatives
Several approaches were considered, but considered unsuitable for the
intended use cases:
* CoAP over 6LoWPAN over BLE (BLE IPSP): 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 partially
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 some reliability.
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2. Terminology
3. Protocol description
3.1. Boundary conditions: GATT properties
[ This section may be shortened in later iterations, but is kept
around while the protocol is being developed to easily fix mistakes
made from wrong assumptions. ]
CoAP-over-GATT has different properties than UDP transported over the
Internet:
* Messages sent by one party are received by the other party in the
order in which they are sent. There is no re-ordering.
(There is also a total order on messages sent by any party, but
that property is not useful because it's often not accessible
through the Bluetooth stacks.)
* There is limited reliabiliy built into the protocol.
Data transmissions initiated by the data source can be unreliable
("write without response", "notify") or reliable ("write with
response", "indicate").
The caveat with their relability is that acknowledgements are sent
by the BLE stack, without consulting with the application. (This
is not only done for simplicity but also for power efficiency:
There is only a short time window in which the data source is
listening for confirmations). Thus, these confirmations can not
serve to acknowledge that the a CoAP request contained in the
event was read, understood and is being processed.
The reliability mechanisms are still useful, though: Both "write"
and "notify"/"indicate" update the GATT characteristic's state,
and while a slow application may miss data when sent in fast
succession, it is reasonable to expect from the BLE stack to
deliver the last data to the application when no more data is
sent.
* Reads and writes may be subtly confused: When a characteristic is
written to, and it is read before the BLE server application has
had time to interact with its BLE stack, the written value may be
echoed back at read time.
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This is likely not problematic when "notify"/"indicate" is used
instead of polling reads, but it seems prudent to take
precautions.
3.2. Requests and responses
CoAP-over-GATT uses a GATT Characteristics to transport requst and
response messages. Similar CoAP-over-UDP it offers both reliable and
unreliable transfer and message deduplication, but as GATT's
properties (see Section 3.1) differ from UDP's, it uses a different
serialization and a different kind of message IDs.
Tokens are used like with other CoAP transports, and allow keeping
multiple requests active at the same time.
A GATT server announces service of UUID 8df804b7-3300-496d-9dfa-
f8fb40a236bc (abbreviated US in this document), with one or more
characteristics of UUID 3d4190a8-f322-4ff8-93fa-8d7bed520333
(abbreviated UC) through BLE advertisements from a BLE peripheral
(typically a constrained device), which are discovered by a BLE
central (typically an end user device). The server and client roles
of CoAP and GATT are independent of each other: either BLE
participant can send requests in a CoAP client role.
3.2.1. Message sub-layer
At its CoAP-over-GATT characteristic, each party maintains a single
bit Message ID (initialized at 1 when a connection is created), and
the last Message ID sent by the peer (initialized at 0 when a
connection is created).
Messages are serialized as GATT values. The GATT client sends a
message by writing it to the characteristic (reliably using the
"write with response" or unreliably using "write without response"
operation); the GATT server sends them reliably using an "indicate"
or unreliably "notify" event. The serialization format is the same
for all, and illustrated in Figure 1:
0 1 2 3 4 8 16 varying
+---+---+---+---+-------+-------+-------+---------+----+---------+
| R | M | C | A | TKL | Code | Token | Options | ff | Payload |
+---+---+---+---+-------+-------+-------+---------+----+---------+
Figure 1: Components of a message
* a single message description byte, compose of 4 bits R (Role), M
(Message ID), C (Confirm) and A (Acknowledge ID), followed by 4
bits of token length (TKL).
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* Code, token, options, payload marker and payload as in [RFC7252].
Unlike there, there is no 16-bit Message ID field (a similar role
is taken by bits M and A), and in empty messages, the code is not
sent.
The bits are set as follows:
* The R bit is always set to 0 by the GATT server, and to 1 by the
GATT client.
* The Message ID bit is always set to the current Message ID of the
sender.
* The Confirm bit is set if the sender asks the peer to acknowledge
that the message has been noted.
* The Acknowledge ID is always set to the peer's last sent Message
ID that had the Confirm bit set.
When receiving a message with the C bit set, the recipient MUST
eventually send a response message with radio reliability.
3.2.2. Using the message sub-layer
[ This section reflects ongoing experimentation with the above
serialization format and rules. Senders may use other patterns as
long as they do not stall their peer by not sending any messages
after the Confirm bit was set. ]
To send a message unreliably in terms of CoAP transmission, a sender
sets its latest Message ID in the M bit, sets C to 0, and populates
the remaining bits per the rules above. It then sends the message
unreliably on the radio (it may be sent reliably, especially when the
peer set the C bit before). After a CoAP-unreliable message, the
sender may send more CoAP-unreliable messages. It should avoid
sending multiple messages in the same connection event.
To send a message reliably in terms of CoAP transmission, a sender
sets its latest Message ID in the M bit, sets C to 1, and populates
the remaining bits per the rules above. It thens ends the message
reliably on the radio (it may send unreliably if a message is
expected from the peer soon, but then needs to be prepared to send
the same message again). After sending that message, the sender does
not send any other message until a message is received with A equal
to the sent message's M bit. The sender may need to send the very
same message again if no earlier transmission of the message happened
reliably. [ Do we need to give timing guidance here? Probably not,
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because it only happens if there is some expectation in the first
place. ] The sender may cancel the transmission by sending an empty
message with the same M and C bits, or by sending different message
with these bits (which are then all unreliable transmissions).
When receiving a message with the C bit set, it is up to the
recipient when to send the radio-reliable message. If it is expected
that a radio-reliable message will be sent soon, it is permissible
and useful to send unrelated unreliable messages that already account
for the set C bit in their A bit.
3.2.3. Message deduplication
CoAP-over-GATT participants MUST ignore a message arriving at a
characteristic if it is identical to the one received previously in
the same connection. (The first message is never ignored).
Note that it is not possible to send two identical consecutive
messages unreliably. When sending identical requests, the sender may
vary the token. Sending identical responses generally is rarely
significant, even with the generalized [I-D.bormann-core-responses],
because the mechanism to make responses "non-matching" in that
document's terminology typically incurs variation. When it does not,
but the repetition is still significant, sending the messages
reliably becomes necessary.
3.2.4. Requests and responses
CoAP requests and responses are built on the message sub-layer as
they are in [RFC7252]: requests are sent with a token chosen by the
CoAP client, and the CoAP server sends a response with the same
token.
Responses and message-layer acknowledgments can happen in the same
message. Unlike in [RFC7252], there is no association between a
request and its message ID: Any message may serve as an
acknowledgement; it is always only the token that matches requests to
responses.
3.2.5. Fragmentation
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. [ TBD: Verify: ]
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3.2.6. Multiple characteristics
If a server provides multiple OC typed characteristics, multiple
messages can be sent without waiting for individual confirmation.
This is similar to using RFC7252 with NSTART > 1, and may be used by
the GATT client if the GATT server lists multiple UC characteristics.
The GATT server can send messages only through characteristics on
which the GATT client enabled "indicate" or "notify"; if the GATT
client does not support multiple characteristics, it will just pick
any and only enable them on that one.
Each characteristic has its independent message ID bits. All
characteristics of a service share a single token space, and
responses need not necessarily be sent on the characteristic the
request was sent on.
The use of muliple characteristics is primarily practical when large
amounts of data are to be transferred. These transfers can utilize
much of BLE's bandwidth because they make it easy to send much data
within a single BLE connection event.
3.2.7. Development directions
* Is there any good reason to allow read operations?
A GATT client that is waiting for a Confirm bit to be acknowledged
might attempt a Read (for the case that the confirmation arrived
in an unreliable message), but might just as well perform the last
write again.
Reading would be more efficient (because it can happen without
application intervention, and no data is sent), but the added
complexity might not be worth the enhancements.
* 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.
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3.3. 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.
3.3.1. 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. ]
3.3.2. 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).
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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" ].
3.4. 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.
3.5. Additional use of advertisements
In the current specification, advertisements are used to indicate
that CoAP-over-GATT is being used.
Two more uses of them are being considered:
* Some resource metadata might already be transported in
advertisements.
These would need to be compact (in the order of magnitude of 10
bytes or less), and could contain data otherwise only discovered
by querying the .well-known/core resource, or (hashes of) AS and
audience values for ACE to facilitate connection creation with a
device known by its managed identity.
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* Advertisements could contain broadcast CoAP messages.
Given that these non-traditional responses can not have embedded
requests (as defined in [I-D.bormann-core-responses]) due to size
contraints, a mechanism such as
[I-D.ietf-core-observe-multicast-notifications] could be used to
distribute some consensus request.
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]:
* 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. Acknowledgements
TBD
in particular Hauke / and process his input
7. References
7.1. 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/rfc/rfc7252>.
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[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/rfc/rfc7595>.
7.2. 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/rfc/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/>.
[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/rfc/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/rfc/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/rfc/rfc7959>.
[bluetooth52]
"Bluetooth Core Specification v5.2", 31 December 2019,
<https://www.bluetooth.org/docman/handlers/
downloaddoc.ashx?doc_id=478726>.
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[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://datatracker.ietf.org/doc/html/draft-bormann-
t2trg-slipmux-03>.
[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://datatracker.ietf.org/doc/html/draft-bormann-core-
responses-01>.
[I-D.ietf-core-transport-indication]
Amsüss, C., "CoAP Protocol Indication", Work in Progress,
Internet-Draft, draft-ietf-core-transport-indication-02,
13 March 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-transport-indication-02>.
[I-D.ietf-core-observe-multicast-notifications]
Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
"Observe Notifications as CoAP Multicast Responses", Work
in Progress, Internet-Draft, draft-ietf-core-observe-
multicast-notifications-05, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
observe-multicast-notifications-05>.
Appendix A. Change log
Since -02:
* Message format extended by a leading byte, the option to have a
token. This enables role reversal and concurrent requests.
* The UC identifier was changed to reflect the incompatible change
in protocol.
* A section on used BLE properties was added.
* A section providing outlook on other data for advertisements was
added.
Since -01:
* Point out (possibly conflicting) development directions.
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* Describe URI scheme more completely, including persistent
addresses.
* Aim for standards track.
* Describe rejeced alternative approaches.
Since -00:
* Add note on SCHC possibilities.
Appendix B. Other ToDos
* Future proofness?
- https://www.ti.com/lit/ml/slyp707/slyp707.pdf mentiones EATT
for interleaving ATT -- looks like it's just for multiple
applications, can we be one application?
- Same source: MTU can change over time. (Hopefully issue
https://gitlab.com/chrysn/coap-over-
gatt/-/issues/1#note_518745553 (https://gitlab.com/chrysn/coap-
over-gatt/-/issues/1#note_518745553) handles that
transparently) -- queued writes? long writes?
- GATT caching seems not to be harmful.
Appendix C. -03 sketch
* single prefix byte
- "reliable" is always only radio-reliable, doesn't mean ready-
to-process
- unprocessed: assume (demand) latest msg is reliably processed
- tkl nibble
- 1-bit msgid
o NONish msgs don't change msgid
both radio-reliable and radio-unreliable msgs can be NONish
o CONish msgs do change msgid, and block characteristic in
that direction until ACKed
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CONish are best sent radio-reliably, and that reliability
plus guaranteed processing of latest state means that no
GATT level retransmissions need to be done (no timer mgmt:
it'll work eventually, although timer would detect broken /
thread-crashed peer)
- 1-bit "please ACK" (aka "ready for changing MID?"), which
places onus on peer to send a radio-reliable msg back
ACKs can end cycle of tell-me-its-done b/c they're radio-
reliable, eventually processed, and if the ACKer later sends
more (esp. non-empty) msgs, they'd still ACK
- bits "is-ack, ack-what"? that'd fill up the initial byte.
o can do with single-ack bit? it wouldn't flip until ACK
requested...
- need bit for who-sent to recognize own state? (that'd only be
needed if read is done) and would excede init'l byte
- ~NSTART > 1 (precisely: >1 un-acked CON in flight, indep of
reqs): "trunking" via multiple chars
- ACKs as write or notify: these may stall if got lost; party may
do them if it knows it'll send radio-relible msg anyway. (then,
may be elided altogether, but sending them allows the original
sender to send more if we're lucky and it gets through)
- minimal subset: echo this-or-that-bit, always send new msgid
and ask for next OK and block
bits:
Party YourMID PleaseAck MyMid | 4 TKL | token* | msg
Author's Address
Christian Amsüss
Austria
Email: christian@amsuess.com
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