Constant DEFAULT_MTU_UPPER_BOUND 

pub const DEFAULT_MTU_UPPER_BOUND: u16 = 1452;

Default ceiling for path MTU discovery, in bytes.

quinn’s own default, which “stays within Ethernet’s MTU when using IPv4 and IPv6” (quinn-proto config/transport.rs): 1500 minus IPv6’s 40-byte header and UDP’s 8. Safe whichever family the path runs.