Constant RECV_WINDOW_BYTES 

pub const RECV_WINDOW_BYTES: u64 = _; // 1_048_576u64

The default receiver reorder/replay window, in pad bytes. A cover whose offset is older than newest_end - RECV_WINDOW_BYTES is refused: it is either a replay of long-consumed pad or so far out of order the window can no longer prove it was not already accepted. Forward jumps (a dropped cover skipped) are always fine — they consume fresh pad, never reused pad.