cfdp

Overview ¶

Example: Downlinking a file with CFDP

The other examples move packets. This one moves a file, which is a different problem: a file has a name, a length, a checksum, and it is not delivered until every byte of it has arrived.

Spacecraft Side:
  1. A science image sits in the on-board filestore
  2. A CFDP Sender turns it into Metadata, File Data and EOF PDUs
  3. Each PDU rides inside a Space Packet

Link:
  One File Data PDU is dropped, the way a real pass loses frames

Ground Side:
  1. A CFDP Receiver reassembles the file
  2. It notices the gap and asks for it by offset
  3. The sender fills it, the receiver checksums, and both sides finish

Class 2 (acknowledged) mode is what makes the recovery possible. Class 1 would have delivered a file with a hole in it and told nobody.