Constant CONTROL_STREAM_TYPE 

const CONTROL_STREAM_TYPE: u64 = 0x2F00;

The unidirectional stream type that marks a control stream on the drafts control_plane_is_unidirectional names, and the SETUP message type on the same drafts. They are one number, 0x2F00.

A control stream therefore starts with the first field of a SETUP message and carries no separate stream header, which is why a control stream can be forwarded byte for byte onto a fresh unidirectional stream: the type varint the classifier read is the type varint the peer needs to read.

Encoded with the varint the draft uses — from draft-17 that is MoQT’s leading-ones form, in which 0x2F00 is the two bytes AF 00 — so the classifier decodes through DraftVersion rather than assuming a width.