Constant MAX_MTU_UPPER_BOUND 

pub const MAX_MTU_UPPER_BOUND: u16 = 1472;

The largest mtu_upper_bound this server accepts, in bytes.

Ethernet’s 1500 minus IPv4’s 20-byte header and UDP’s 8. Nothing on a standard internet path carries more than that, so a higher bound could only spend probes on sizes that cannot exist. The ceiling being above MAX_INITIAL_MTU is deliberate, not an inconsistency: discovery only ever reaches a size by probing it — a lost probe is retried, then abandoned, and is never treated as congestion — whereas initial_mtu is sent blind in the handshake with no recovery path (see MAX_INITIAL_MTU). An upper bound the path cannot carry costs a few PINGs; an initial the path cannot carry costs the server.