RE: SMP server FOTA speed

Amanda Hsieh — Wed, 24 Nov 2021 13:23:30 GMT

Hi,

According to the BLE Spec 5.2, vol. 3, Part F, section 3.2.9, it is not possible to have attributes larger than 512 bytes:

"3.2.9 Long attribute values
The longest attribute that can be sent in a single packet is (ATT_MTU-1) octets
in size. At a minimum, the Attribute Opcode is included in an Attribute PDU.
An attribute value may be defined to be larger than (ATT_MTU-1) octets in
size. These attributes are called long attributes.
To read the entire value of an attributes larger than (ATT_MTU-1) octets, the
ATT_READ_BLOB_REQ PDU is used. It is possible to read the first
(ATT_MTU-1) octets of a long attribute value using the ATT_READ_REQ PDU.
To write the entire value of an attribute larger than (ATT_MTU-3) octets, the
ATT_PREPARE_WRITE_REQ and ATT_EXECUTE_WRITE_REQ PDUs are
used. It is possible to write the first (ATT_MTU-3) octets of a long attribute
value using the ATT_WRITE_CMD PDU.
It is not possible to determine if an attribute value is longer than (ATT_MTU-3)
octets using this protocol. A higher layer specification will state that a given
attribute can have a maximum length larger than (ATT_MTU-3) octets.
The maximum length of an attribute value shall be 512 octets.
Note: The protection of an attribute value changing when reading the value
using multiple Attribute protocol PDUs is the responsibility of the higher layer."

You also could take look at this post for setting MTU to 512 bytes.

Regards,
Amanda