MTU exchange procedure

RE: MTU exchange procedure

Amanda Hsieh — Fri, 29 May 2020 07:36:05 GMT

Thanks for your feedback. I will report to the internal. 🙂

-Amanda H.

RE: MTU exchange procedure

alemv — Thu, 28 May 2020 13:45:55 GMT

In my opinion, your interpretation of the "The ATT_EXCHANGE_MTU_REQ PDU is used by the client to inform the server" as "the paragraph does not explicitly prohibit the server from doing the same" is very controversial.
Because:
1. The ATT_EXCHANGE_MTU_REQ PDU has "Client Rx MTU" parameter, not "Rx MTU" or "Server Rx MTU";
2. If the GATT server sends the MTU request, it may cause problems:
- with Windows 10, see this message
- with Linux (Python + BluePY + Bluez), see this thread

So, in my opinion, the modifying on_connected_evt() function in nrf_ble_gatt.c like in this message is not a good alternative. A preprocessor macro or other choice for a programmer would be better.

Anyway, thank you for your time and clarification of your position.

RE: MTU exchange procedure

Amanda Hsieh — Thu, 28 May 2020 11:16:51 GMT

Hi,

[quote user="alemv"]The ATT_EXCHANGE_MTU_REQ PDU is used by the client to inform the server of the client’s maximum receive MTU size and request the server to respond with its maximum receive MTU size.[/quote]

The spec. indicates that MTU exchange is only used by the Client, but the paragraph does not explicitly prohibit the server from doing the same. BT spec is usually not this vague in its description. Both Central and Peripheral devices can issue an MTU exchange request, regardless of the GATT role. Our SoftDevice will handle the request so as to be in accordance with BT spec. Please see this similar question.

-Amanda H.

RE: MTU exchange procedure

alemv — Wed, 27 May 2020 12:17:31 GMT

The topic you were referring to does inform nothing about ATT_EXCHANGE_MTU_REQ.

It seems you do not understand my question. I will try to explain in steps:
- select for example BLE Blinky Application (<InstallFolder>\examples\ble_peripheral\ble_app_blinky);
- increase NRF_SDH_BLE_GATT_MAX_MTU_SIZE in sdk_config.h:
#define NRF_SDH_BLE_GATT_MAX_MTU_SIZE 50;
- build and run example;
- establish BLE connection.

The roles of the BLE Blinky Application in this connection are:
- LL slave
- GAP peripheral
- GATT server

So the question is the same: Why does the GATT server (BLE Blinky Application) send an ATT_EXCHANGE_MTU_REQ request at all?

BLUETOOTH CORE SPECIFICATION Version 5.2 | Vol 3, Part F
3.4.2 MTU exchange
3.4.2.1 ATT_EXCHANGE_MTU_REQ
The ATT_EXCHANGE_MTU_REQ PDU is used by the client to inform the server of the client’s maximum receive MTU size and request the server to respond with its maximum receive MTU size.

Please do not redirect me to the threads not related to the question.

RE: MTU exchange procedure

Amanda Hsieh — Wed, 27 May 2020 10:28:05 GMT

Hi,

[quote user="alemv"]Why does the GATT Server (any peripheral example) send an ATT_EXCHANGE_MTU_REQ request at all?[/quote]

As the central and peripheral are the GAP roles of the device, while the server and client are the GATT roles of the device. Better explained in this thread.

-Amanda H.

RE: MTU exchange procedure

alemv — Tue, 26 May 2020 09:57:41 GMT

You did not answer my question.
Your link "Data Length Update Procedure" is not related to the question at all (this is a procedure for LL PDU).
The "GATTC ATT_MTU Exchange" link shows the correct procedure for the GATT Client, the "GATTS ATT_MTU Exchange" link - for the GATT Server.
But the question was different: Why does the GATT Server (any peripheral example) send an ATT_EXCHANGE_MTU_REQ request at all?
Bluetooth Core Specification indicates that "The ATT_EXCHANGE_MTU_REQ PDU is used by the client to inform the server of the client’s maximum receive MTU size and request the server to respond with its maximum receive MTU size".

RE: MTU exchange procedure

Amanda Hsieh — Tue, 26 May 2020 08:15:00 GMT

Hi,

See GATTS ATT_MTU Exchange, GATTC ATT_MTU Exchange, and Data Length Update Procedure.

-Amanda H.