TFTP: REQUEST: Unexpected ACK
This protocol anomaly is an unexpected ACK sent from a client. Normally, ACKs are sent by the client when the client attempts to read a file from the server and respond to the DATA messages sent by the server. This can indicate a tunneling attempt.
References
srx-branch-12.3
srx-19.3
srx-branch-19.3
vsrx3bsd-19.2
srx-branch-19.4
vsrx-19.4
mx-12.3
mx-19.4
vmx-19.4
mx-19.3
vsrx3bsd-19.4
srx-19.4
vsrx-12.3
vmx-19.3
vsrx-19.2
srx-12.3