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docs: Bluetooth: Add additional notes about LE Audio

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LE Audio sticks out in a few ways that's worth knowing
not only as a contributor, but also as an application
developer. These things have been documented, and may be
expanded upon later.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>
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Emil Gydesen 2024-03-28 20:33:40 +01:00 • committed by Carles Cufí
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doc/connectivity/bluetooth/bluetooth-le-audio-arch.rst
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@ -556,6 +556,48 @@ but can also reject the requests.
The choice of what the return type of the callbacks often depend on the specifications,
and how much control the role has in a given context.
Things worth knowing or considering when using LE Audio
=======================================================
This section describes a few tings to consider when contributing to or using LE Audio in Zephyr.
The things described by this section are not unique to Zephyr as they are defined by the
specifications.
Security requirements
---------------------
All LE Audio services require Security Level 2 but where the key must be 128-bit and derived via an
OOB method or via LE Secure connections.
There is no Core-spec defined way of reporting this in GATT,
as ATT does not have a specific error code for missing OOB method or LE Secure Connections
(although there is a way to report wrong key size).
In Zephyr we do not force the device to always use these, as a device that uses LE Audio may also
use other profiles and services that do not require such security.
We guard all access to services using a custom security check implemented in
:zephyr_file:`subsys/bluetooth/audio/audio.c`, where all LE Audio services must use the
internal `BT_AUDIO_CHRC` macro for proper security verification.
Access to the LTK for encrypted SIRKs in CSIS
---------------------------------------------
The Coordinated Set Identification Service (CSIS) may encrypt the SIRK (set identity resolving key).
The process of encrypting the SIRK requires the LTK as the encryption key,
which is typically not exposed to higher layer implementations such as CSIS.
This does not have any effect on the security though.
MTU requirements
----------------
The Basic Audio Profile (BAP) has a requirement that both sides shall support a minimum ATT_MTU of
at least 64 on the unenhanced ATT bearer or at least one enhanced ATT bearer.
The requirement comes from the preferred (or sometimes mandatory) use of GATT Write Without
Response, and where support for Write Long Characterstic Value is optional in most cases.
If a ASCS device supports values larger than the minimum ATT_MTU of 64 octets, then it shall supoort
Read long Characterstic Value by setting :kconfig:option:`CONFIG_BT_ATT_PREPARE_COUNT` to a
non-zero value.
LE Audio resources
##################