- 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.
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:
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