Use the LOOP CIRCUIT, LOOP EXECUTOR, LOOP LINE, and LOOP NODE
 commands to test specified components in the network.  Each
 command causes test blocks of data to be transmitted over the
 specified components.  The parameters can be entered in any order
 and are optional, except in the case of broadcast circuit loopback
 tests which require a parameter to define the loopback destination.

 LOOP    CIRCUIT circuit-id    (parameters ...)
         EXECUTOR
         LINE    line-id
         NODE    node-id