The Maintenance Operations Protocol (MOP) module is located in the
    Application layer described by the Digital Network Architecture.
    MOP has a direct connection with the Data Link layer; thus, for
    certain functions, MOP can bypass the higher layers in the DNA
    protocol tower.  This is useful for nodes which do not (yet) have
    all the higher layers of DNA protocol towers installed. Functions
    provided by the MOP module include down-line loading, up-line
    dumping, and communications testing.