Constant MAX_FRAME_BYTES 

pub const MAX_FRAME_BYTES: usize = _ /* 16_777_216usize */;

Largest message this listener accepts, in bytes.

A sender that never writes <EB> would otherwise grow the buffer forever. 16 MiB is far above any real HL7 v2 message a large one with embedded documents runs to a few hundred KiB so this rejects only a broken or hostile sender.