Constant MAX_CONTROL_PAYLOAD 

const MAX_CONTROL_PAYLOAD: usize = _; // 1_048_576usize

A declared control-message payload length above which ControlFrameWalker stops believing what it is reading.

Not a protocol limit and not enforced on anything — the bytes are forwarded either way. It is a sanity bound on the walker’s own arithmetic: drafts 11 and later cap a control payload at 65535 by framing it in sixteen bits, and drafts 07-10 frame it as a varint that can say 2^62 but never does. A length that large is not a large message, it is a length field read at the wrong offset — most likely because the session’s draft guess is wrong for the moq-00 cohort, where the framing style changed at draft 11.

Without the bound the walker would count down through that number for the rest of the session, hold every injection, and claim at teardown that a message was half-written. With it, the walker says it does not know where the boundaries are, which is the truth and which suppresses both.