One approach to reliable transport that can tolerate extremely long and variable round-trip latency is reflected in the design of CFDP. CCSDS(Consultative Committee for Space Data Systems) File Delivery Protocol (CFDP) can operate in either acknowledged (reliable) or unacknowledged mode; in acknowledged mode, lost or corrupted data are automatically re-transmitted. CFDP’s design includes a number of measures adopted to enable robust operation of its ARQ system in high-latency environments:
- Because the time required to establish a connection might exceed the duration of a communication opportunity, there is no connection protocol; communication parameters are managed.
- Because round-trip latency may far exceed the time required to transmit a given file, CFDP never waits for acknowledgment of one transmission before beginning another. Therefore, the re-transmitted data for one file may arrive long after the originally transmitted data for a subsequently issued file, so CFDP must attach a common transaction identifier to all messages pertaining to a given file transmission.
- Because a large number of file transmissions may concurrently be in various stages of transmission, re-transmission buffers typically must be retained in nonvolatile storage; this can help prevent catastrophic communications failure in the event of an unplanned power cycle at either the sender or the receiver.
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