Constant BATCH_END_FRAME_LEN 

pub const BATCH_END_FRAME_LEN: usize = _ /* 13usize */;

The frame of a batch terminator (TAG_BATCH_END or TAG_BATCH_END_DEGRADED), length prefix included: what a response stage pre-encodes and writes a request’s ID into (write_terminal_id) before it appends the terminator.