CCSDS 727.0-B-5 [CCS20], with CCSDS 720.1-G-4 [CCS21a] and CCSDS 720.2-G-4 [CCS21b] providing further explanatory information.
CFDP has existed for decades, and it is intended to enable packet delivery services in space (space-to-ground, ground-to-space, and space-to-space) environments [CCS20, Sec. 1.1].
CFDP defines 1️⃣ a protocol suitable for the transmission of files to and from spacecraft data storage, and 2️⃣ file management services to allow control over the storage medium [CCS20, Sec. 2.1].
CFDP assumes a virtual filestore and associated services that an implementation must map to the capabilities of the actual underlying filestore used [CCS20, Sec. 1.1].
File transfers can be either unrealiable (class 1) or reliable (class 2):
All file segments are transferred without the possibility of retransmission.
End of file (EOF) is not acknowledged by the receiver.
When the flag Closure Requested is set, the receiver is required to send a Finished PDU upon receiving all file segments (or when the request is cancelled), but the sender does not need to acknowledge the Finished PDU.
The Closure Requested flag is useful when the underlying communication protocol is reliable.
The receiver is required to acknowledge the EOF PDU and the sender has to acknowledge the Finished PDU.
Sending a PDU that requires acknowledgment triggers a timer.
When the timer expires and if the acknowledgment has not been received, the relevant file segment PDU is resent.
This repeats until the ACK Timer Expiration Counter [CCS21b, Sec. 4.4] reaches a predefined maximum value.
Finally, if the counter has reached its maximum value and the acknowledgment has still not been received, a fault condition is triggered which may cause the transfer to be abandoned, canceled or suspended.
The receiver can also indicate missing metadata or data by sending NAK PDUs.
Fig. 2 shows a sample class-2 Copy File transaction between two entities.
Fig. 2: A sample Copy File transaction where an NAK is sent once the (N+1)th File Data PDU is found missing [CCS20, Sec. 3.3]. Additionally, the source user replies with an ACK PDU upon receiving a Finished PDU from the destination user.
Fig. 3 summarises 1️⃣the operation primitives and PDUs in CFDP, as well as 2️⃣ the relationships of these primitives and PDUs to the operational process from initiation through termination.
Fig. 3: An operations view of CFDP: a summary of CFDP operation primitives, PDUs and their relationships to the operational process [CCS21b, Figure 2-1].
Watch an introduction to the CFDP on YouTube:
There are several open-source implementations of CFDP, e.g.,
CCSDS, CCSDS File Delivery Protocol (CFDP), Recommended Standard CCSDS 727.0-B-5, The Consultative Committee for Space Data Systems, July 2020. Available at https://public.ccsds.org/Pubs/727x0b5.pdf.
[CCS21a]
CCSDS, CCSDS File Delivery Protocol (CFDP) — Part 1: Introduction and Overview, Informational Report CCSDS 720.1-G-4, The Consultative Committee for Space Data Systems, May 2021. Available at https://public.ccsds.org/Pubs/720x1g4.pdf.
[CCS21b]
CCSDS, CCSDS File Delivery Protocol (CFDP) — Part 2: Implementers Guide, Informational Report CCSDS 720.2-G-4, The Consultative Committee for Space Data Systems, May 2021. Available at https://public.ccsds.org/Pubs/720x2g4.pdf.
