Uri Data Length and semantics

[schilit]

Hi @rgrover

BLE_API / services / URIBeacon2Service.h includes the definitions:

    static const size_t MAX_SIZEOF_SERVICE_DATA_PAYLOAD = 27;
    static const size_t MAX_SIZE_URI_DATA_CHAR_VALUE    = 48;

However Uri Data must be between 0 and 18 bytes in length.

URIBeacon2Service.h also treats Uri Data as unencoded but it is already encoded and should be treated as an opaque type.

[rgrover]

Thanks for pointing this out. I'll fix this shortly.

[rgrover]

Why is the encoded URI data restricted to 18 bytes when you can go up to 21?
ADV payload can be up to 31 bytes.
4 bytes for the service UUID: Type + length + 16-bit UUID
6 bytes of header for the URI data: Type + length + 16-bitUUID + flags + power

[schilit]

The case of the missing bytes!

See the Figure below. The missing bytes are from the AD Flags (AD Type 0x01).

Is mbed broadcasting flags? CSR firmware forces the flags in the ADV packet.

The thing is that some central devices care about them and some don't. We've seen versions of Android/iOS discard ADV packets without flags (though I'd have to dig up which).

Robin Hayden was going to propose that missing flags default to 0... which is reasonable but right now it doesn't seem to be widely adopted in scanners.

[rgrover]

Why is the service UUID duplicated?

[rgrover]

Hi, AD flags aren't needed. Mbed doesn't add them. Secondly, Advertisement payload can be up to 31 octets long. Lastly, you shouldn't be duplicating the Service UUID.

You should be able to add another 7 bytes to your max URI data length!!

[hugovincent]

@rgrover are you sure that's true for all versions of Android and iOS? If I recall correctly, Scott said these were added for compatibility with either iOS (something to do with detecting tags from a background process) or older versions of Android.

[rgrover]

I'm not sure about all version of Android/iOS. Even if we need to keep AD flags, there's some more room in the advertisement payload, and there's also some redundancy. You could certain get a few more URI octets out of the payload.

[schilit]

As mentioned above, AD Flags is necessary for some devices, else packets are filtered. We can revisit this and see exactly which devices.

Why is the service UUID duplicated?

I assume you mean why is there both a AD Service UUIDs and AD Service Data? AD Complete List of 16-Bit Service UUIDs is needed in order to background scanning on iOS.

[schilit]

31 - 3 (AD Flags) - 4 (AD Service UUIDs) - 4 (AD Service Data header) - 2 (Flags, TX Power) = 18