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central_mtu_update

Q&A:

Question: What do I have to do to update my ATT MTU?

Answer: To get an ATT MTU of x octets, do the following:

1，Set CONFIG_BT_L2CAP_TX_MTU to at least x

2, Set CONFIG_BT_BUF_ACL_RX_SIZE to at least x + L2CAP headersize (4 octets)

3, Set CONFIG_BT_BUF_ACL_RX_SIZE to at least x + L2CAP header + SDU length field length (6 octets) if using CONFIG_BT_EATT

Ensure the remote side supports the same MTUs.

Question: I only want to send large packets. I don’t need to receive large ones. Do I still need to set CONFIG_BT_BUF_ACL_RX_SIZE ?

Answer: Yes. [1] The Bluetooth specification mandates a symmetric MTU for ATT.

Overview:

This sample demonstrates the exchange of MTU between two devices to allow a large notification to be sent.Using a larger ATT MTU helps in transmitting and receiving larger data packets, thereby improving throughput.

In order to send larger data packets, both the server and the client need to update their MTU and use the smaller value among the two. The MTU is a non-negative value, and although different values can be used for sending and receiving by both parties, the MTU value on the server side should be greater than or equal to the MTU on the client side.

It is important to note that in the HOST protocol stack, SDUs will not be fragmented unless dynamic channels of L2CAP are used.

Caution

If the data packet sent in the application is larger than the TX MTU value, it needs to be manually packetized and sent in multiple parts.

Diagram of the MTUs and their corresponding Kconfig symbols

Diagram of the MTUs and their corresponding Kconfig symbols

Building and Running

Example Location：examples/bluetooth/central_mtu_update

compile, burn, and more, see：Quick Start Guide

Running Result

Central:

I/bt_hci_core     [0.064] HCI: version 4.2 (0x08) revision 0x0709, manufacturer 0x070c
I/bt_hci_core     [0.072] LMP: version 4.2 (0x08) subver 0x0709
I/central_mtu_update [0.078] Bluetooth initialized
I/central_mtu_update [0.086] Scanning successfully started
I/central_mtu_update [2.080] Connected: 4A:C6:42:30:10:56 (random)
I/central_mtu_update [2.086] mtu_exchange: Current MTU = 23
I/central_mtu_update [2.092] mtu_exchange: Exchange MTU...
I/central_mtu_update [2.472] mtu_exchange_cb: MTU exchange successful (247)
I/central_mtu_update [2.572] Discover complete

References

Note

This example need to work together with periphal_mtu_update