This is a generic negative acknowledgement to an SMPP PDU submitted with an invalid message header. A generic_nack response is returned in the following cases:
Invalid command_length
If the receiving SMPP entity, on decoding an SMPP PDU, detects an invalid
command_length (either too short or too long), it should assume that the data is corrupt. In
such cases a generic_nack PDU must be returned to the message originator.
Unknown command_id
If an unknown or invalid command_id is received, a generic_nack PDU must also be
returned to the originator.