Protocol Frames
Protocol Frames
The C++ PacketCodec treats each decoded buffer as exactly one frame. Stream splitting belongs to the transport/session boundary.
flowchart TD
Bytes[Incoming bytes] --> Type{Leading byte}
Type -->|STX| App[Application frame]
Type -->|SOH| Progress[Progress frame]
Type -->|ACK or NAK| Ack[Handshake confirmation]
App --> LRC[Validate LRC]
LRC --> Payload[Parse command payload]
Progress --> Display[Emit progress event]
Ack --> Session[Advance retransmit state]
Application frame
STX payload ETX LRC
Progress frame
SOH 20-char-message EOT
ACK and NAK
The terminal can acknowledge a frame or request retransmission. The session retransmits according to the configured retry count and the command safety policy.
Retransmit is not business retry
ACK/NAK retransmission happens inside one physical exchange. It is different from replaying a payment after a disconnected transaction.
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