Constant MAX_FRAME_BYTES 

const MAX_FRAME_BYTES: u32 = _; // 67_108_864u32

The largest frame that will be written or believed on read.

This is not a tuning knob, it is a bound on trust. len is four bytes read out of a file that may have been corrupted, and without a ceiling a garbage value asks for an allocation of up to 4 GiB before the CRC that would have caught it has been checked. 64 MiB is four times the default ingest.max_request_bytes, so it cannot refuse a frame the ingest path would have accepted.