Constant BOOTSTRAP_MESSAGE_LEN_PREFIX_MAX 

pub const BOOTSTRAP_MESSAGE_LEN_PREFIX_MAX: u32 = MAX_BOOTSTRAP_MESSAGE_FROM_SERVER_SIZE; // 300_000_000u32

Bound the bootstrap message length prefix is encoded against, in both directions.

This is part of the wire format, not a policy knob: to_be_bytes_min derives the width of the prefix from the bound it is given, and the frame header is fixed-width, so lowering this narrows the header and leaves any peer built against the wider one blocked forever on bytes that never arrive. It stays pinned to the server-to-client bound, which is the encoding that has been on the wire since before MAIN.5.0. The tighter per-direction caps are enforced as value checks on the decoded length instead of by the encoding width.