Constant MAX_FIRST_RSEQ 

pub const MAX_FIRST_RSEQ: u32 = _; // 2_147_483_647u32

The largest first sequence number the RFC allows.

§3: the first RSeq “MUST be between 1 and 2**31 - 1”. The ceiling is not decoration — the field is 32 bits and “RSeq numbers MUST NOT wrap around”, so starting in the lower half leaves 2^31 responses of headroom before the rule could be broken.