ESP_GATT_MAX_ATTR_LEN = 512 (IDFGH-16534)

[nebkat]

Description

The maximum attribute length as per the BLE GATT spec is 512 - see https://www.bluetooth.com/wp-content/uploads/Files/Specification/HTML/Core-54/out/en/host/attribute-protocol--att-.html#UUID-d24e0156-618e-728d-f19b-01349ced952b:~:text=The%20maximum%20length%20of%20an%20attribute%20value%20shall%20be%20512%20octets.

---

Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[github-actions]

	Messages
📖	🎉 Good Job! All checks are passing!

👋 Hello nebkat, we appreciate your contribution to this project!

---

📘 Please review the project's Contributions Guide for key guidelines on code, documentation, testing, and more.

---

🖊️ Please also make sure you have read and signed the Contributor License Agreement for this project.

---

Click to see more instructions ...

This automated output is generated by the PR linter DangerJS, which checks if your Pull Request meets the project's requirements and helps you fix potential issues.

DangerJS is triggered with each push event to a Pull Request and modify the contents of this comment.

Please consider the following:
- Danger mainly focuses on the PR structure and formatting and can't understand the meaning behind your code or changes.
- Danger is not a substitute for human code reviews; it's still important to request a code review from your colleagues.
- To manually retry these Danger checks, please navigate to the Actions tab and re-run last Danger workflow.

Review and merge process you can expect ...

We do welcome contributions in the form of bug reports, feature requests and pull requests via this public GitHub repository.

This GitHub project is public mirror of our internal git repository

1. An internal issue has been created for the PR, we assign it to the relevant engineer.
2. They review the PR and either approve it or ask you for changes or clarifications.
3. Once the GitHub PR is approved, we synchronize it into our internal git repository.
4. In the internal git repository we do the final review, collect approvals from core owners and make sure all the automated tests are passing.
- At this point we may do some adjustments to the proposed change, or extend it by adding tests or documentation.
5. If the change is approved and passes the tests it is merged into the default branch.
5. On next sync from the internal git repository merged change will appear in this public GitHub repository.

Generated by 🚫 dangerJS against f7db0d2

[esp-zhp]

We cannot change the max attribute length to 512, as it would cause other issues.

[esp-zhp]

we encountered a data loss issue, which we found was related to setting the max ATT length to 512. May I ask what impact setting it to 517 would have on you?

[nebkat]

@esp-zhp unless I have completely misunderstood the BLE spec I believe it is invalid for any attribute to contain a value length greater than 512, even if the MTU can in theory be up to 517.

If there is data loss occurring with a 512 max it is presumably because something is trying to send an attribute with length >512?

We must strictly adhere to the standard as otherwise another BLE stack may receive our packets and experience data loss on their end as they expect a max length of 512 bytes. Alternatively if this data loss is occurring when receiving data from another device, it is the fault of that device for sending a packet that is too large.

[esp-zhp]

For maximum compatibility on ESP, we retain data even when its length exceeds 512 bytes.
If ESP attempts to send data larger than 512 bytes, a warning will be provided to help ensure that the ESP32 sends data in accordance with the BLE specification. At the same time, the system is capable of handling cases where the data exceeds 512 bytes.