How to use a existing channel ssh connect to another remote host?

I want to connect to another remote host through a springboard machine that is already connected.
My code as follows:

let tcp = TcpStream::connect("xx").unwrap();
let mut sess = Session::new().unwrap();
sess.set_tcp_stream(tcp);
sess.handshake().unwrap();
sess.userauth_password("user", "passwd").unwrap();
let mut channel = sess.channel_session().unwrap();
channel.request_pty(
  "xterm",
   None,
   Some((80, 24, 0, 0)),).unwrap();
channel.shell().unwrap();
channel.write(b"ssh xx@xx \n").unwrap();
//channel.write(b"yes \n").unwrap();
channel.write(b"passwd\n").unwrap();
channel.write(b"ls\n").unwrap()

It doesn't work, would someone help me. Thank you in advance

Start by reviewing the reported Rust calls to Session, channel_session, request_pty, shell, and write in the ssh2-rs documentation and examples. Reproduce the nested SSH attempt and determine whether the reported behavior is a library bug or an unsupported usage pattern; document the result and a verified resolution.