Function forward_undefined_bidi_streams 

async fn forward_undefined_bidi_streams(
    source: &Transport,
    dest: &Transport,
    side: ProxySide,
    ctx: &ForwardCtx,
) -> Result<(), ProxyError>

Forward, unread, the bidirectional streams a draft does not define: every one after the control stream on drafts 07-15. The relay answers them as its draft says.