4.1.2 Message 2

Node A sends Message 2 to the receiver, Node B. This message is one Data frame (section 2.2.2) in length and is not marked as reliable.

 Message 2:  DATA FRAME
 Flags:      STA, EOM messageid: 1, sequence: 3, serial: 0

It is possible that there can be an ACK by the receiver for a frame that is not marked as reliable; this is at the discretion of the implementation. However, sending an ACK allows the sender, Node A, to manage resources more efficiently.