MagTek MCP - Manual (Page 35)

Section 4. Protocol
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Rule 3.7
A node can accept the connection reset by sending RESYNC response according to
Rule 3.3 .
Any outstanding unconfirmed I-frames shall be considered not transmitted successfully before
the RESYNC response is sent.
Data Transfer
Rule 4.1
A node can send user data in an I-frame at any time when the connection is established and there
is no outstanding, unconfirmed I-frame sent by this node to the destination node.
Rule 4.2
A node that sends an I-frame shall include its current send sequence number in the N(S) field of
the frame header and its current receive sequence number in the N(R) field.
Rule 4.3
A node that receives an I-frame without error shall send a response within the block wait
timeout. The response shall be sent regardless of whether the N(S) field of the received frame
matches the node’s current receive sequence number.
Rule 4.4
If the N(S) field of the received I-frame matches the node’s current receive sequence number, the
node shall increment its receive sequence number before sending a response and pass the
received data to the upper layer.
Rule 4.5
If the N(S) field of the received I-frame does not match the node’s current receive sequence
number, the node shall ignore the enclosed user data and retain its current receive sequence
number.
Rule 4.6
A valid response to an I-frame is sending an I-frame.