This parameter specifies a YYMUX server
to be used as an upstream proxy server of this DeleGate.
The connection established to the YYMUX server, to be used to
tunnel and multiplex TCP/IP connections over it, is retained
as if it is persistent.
If the tunnel is broken before the disconnection of the tunneled
TCP/IP connections, for example when the host of this DeleGate
changed its IP-address, the YYMUX tunnel is reconnected and
restored based on the resumption protocol of the YYMUX protocol.