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characteristic larger than 253 bytes freezes the onRead callback #912

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jingoro2112 opened this issue Mar 13, 2025 · 5 comments · May be fixed by #913
Open

characteristic larger than 253 bytes freezes the onRead callback #912

jingoro2112 opened this issue Mar 13, 2025 · 5 comments · May be fixed by #913

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@jingoro2112
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from standard characteristic read example:

void onRead( NimBLECharacteristic* characteristic, NimBLEConnInfo& connInfo ) override
{
	const uint8_t buf[256] = {0};
	characteristic->setValue( buf, 253 ); // for values of 253 or less, it works as expected, getting a callback for each read request
	characteristic->setValue( buf, 254 ); // onRead() is never called again, despite the client making and receiving a read request
}

Just stumbled across this, It is possible I'm doing something wrong? but there seems to be something magical happening at the 253-4 byte boundary, I suspect an int8 being used with some kind of two-byte header? I have not looked at the source yet, spent too many hours already zeroing in on this problem.
@jingoro2112
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this is on an esp32 S3 I should have mentioned. I tried with two separate GATT handlers (on the linux/raspberry pi side) and same result.

I swapped out NimBLE for BLEDevice (ArduinoBLE) and the problem went away so it seems certainly related to NimBLE

As a side note I get about 50% more throughput with the ArduinoBLE implementation.

@h2zero
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h2zero commented Mar 14, 2025 •
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Thanks for reporting this. The issue appears to be that the client is using a normal read rather than a long read. There is an unfortunate situation with the NimBLE stack itself where the onRead callback would be called successively for the same read operation when a characteristic value is longer than the MTU -3, I have tried to handle this here via detecting a long read PDU so that the application callback is only called once, if the client instead send normal read commands and the characteristic value is more than the MTU - 3 then the callback will not be called. Stuck between a rock and a hard place with this one sadly, my suggestion is to increase the MTU or if possible, use long read commands.

As a side note I get about 50% more throughput with the ArduinoBLE implementation.

This is likely due to the MTU being half the size, increasing it to 517 should fix it.

@jingoro2112
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jingoro2112 commented Mar 14, 2025 via email

@jingoro2112
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follow-up: upping the MTU NimBLEDevice::setMTU( 512 ); does fix both the throughput and read issues, thx again for the attention.

I will leave it to you to close/defer this issue since there seems to be an underlying tension.

@h2zero h2zero linked a pull request Mar 14, 2025 that will close this issue
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@h2zero
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h2zero commented Mar 14, 2025

Thanks for confirming, I have opened #913 to resolve this properly.

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Bugfix - Characteristic onRead callback not called. h2zero/NimBLE-Arduino
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@jingoro2112 @h2zero