4.1.3 Message 3

Node A sends Message 3 to the receiver, Node B. This message is one Data frame (section 2.2.2) in length, is marked as reliable, and will drop the first transmission.

 Message 3:  DATA FRAME
 Flags:      STA, EOM, RLY messageid: 2, sequence: 4, serial:0

If Message 3 is dropped, but the sender, Node A, still sends Message 4 (section 4.1.4) to the receiver, Node B, this would result in the following.

 Message 4:  DATA FRAME: 1
 Flags:      STA messageid: 3, sequence: 5, serial: 0
 Message 4:  DATA FRAME: 2
 Flags:      EOM messageid: 3, sequence: 6, serial: 1

This can trigger a NACK on the receiver, Node B.

The receiver, Node B, sends back a NACK to the sender, Node A.

 Flags:      NACK messageid: 2, sequence: 4,
             nNACK: 0, no NACK mask present, bytes received 8954

In response to the NACK, the sender, Node A, attempts a retransmission of Message 3 to Node B.

 Message 3:  DATA FRAME (retry)
 Flags:      STA, EOM, RLY messageid: 2, sequence: 4, serial: 1