OpenBSD Implementation
TCP ISN Randomisation

Constraints
32 bits
Must not repeat too soon
Should be gap of at least 32768 between numbers

15 bits (16-30) from nonrepeating random number generator
Bottom 15 bit from arc4random()
bit 15 is 0 to ensure a gap of 32768
Rekeys after 7200 seconds or 30000 isn's
Toggle most significant bit to avoid reuse