Constant RELAY_BUF_SIZE 

pub const RELAY_BUF_SIZE: usize = _; // 16_384usize

Smallest window read_buf is ever offered on the target → client relay.

The threshold half of the pair below: a block is kept in use until fewer than this many bytes are left in it, so every read still has at least 16 KiB of room. It is also the size of the first allocation a tunnel makes, so a tunnel that never carries a byte costs 16 KiB rather than a whole block.