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BLE notifications get silently truncated when the negotiated MTU is too small

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#28 1 comment 0 reactions 0 assignees View on GitHub

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A pull request for this has already been merged.

  • #52 by @Tichael — merged

Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
flutter, rust
Domain
embedded-iot, mobile-dev

Research direction

Start with the BLE TX task in boards/esp/src/ble/transport.rs around lines 638 and 666, then check how trouble-host handles notifications in gatt::assemble and how the mobile app handles the requested MTU in mobile-app/lib/providers/ble_connection_provider.dart. The issue suggests waiting for the MTU exchange event and checking each payload against the negotiated MTU; done means oversized messages are never silently truncated and a small MTU is logged on both sides.

Written by the indexing model from the issue text.

Description

Problem
BLE messages aren't fragmented: every message must fit in one ATT packet, at most ble.payload.max_message_bytes (244 bytes, from an ATT MTU of 247). The firmware never checks the MTU that was actually negotiated:

  • After the link is encrypted, the TX task sleeps 500 ms and assumes the MTU exchange is done by then (boards/esp/src/ble/transport.rs:638). Nothing confirms it.
  • trouble-host 0.6 trims a notification to the connection's ATT MTU without returning an error (gatt::assemble in trouble-host-0.6.0/src/gatt.rs:644). So notify(...) at transport.rs:666 reports success while the app receives a cut-off protobuf it can't decode.
  • The app requests kBleAttMtu (mobile-app/lib/providers/ble_connection_provider.dart:219). When it gets less, it only logs a warning.

When it happens
The MTU exchange takes longer than 500 ms, or the phone negotiates less than 247. Some Android phones do; iOS picks its own MTU, which matters if iOS is ever supported. State updates near the worst-case size are then lost silently.

Ideas

  • Wait for the MTU-exchange event instead of sleeping.
  • Before each notify, compare the payload with conn.raw().att_mtu() - 3 and drop or log anything that doesn't fit, rather than sending a truncated message.
  • Longer term: BLE fragmentation (https://github.com/OpenCarControls/contracts/issues/5) removes the limit.

Done when
No message is ever sent truncated, and an MTU that's too small is visible in the logs on both sides.

Dominant language
Rust
Stars
1
Forks
0
Avg merge
10h 56m
Merged PRs (30d)
15

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