ESA Document - Emits - ESA

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HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 203 of 422
These protocols establish a framework for interoperability by providing standard
communication, navigation, and timing services. In addition, these services include strategies to
recover gracefully from communication interruptions and interference while ensuring backward
compatibility with previous missions from previous phases of exploration.
CFDP (CCSDS File Delivery Protocol) is a new international standard, built on the familiar
Consultative Committee for Space Data Systems (CCSDS [RD40]) space data communication
protocols, developed to meet a comprehensive set of deep-space file transfer requirements as
stated by a number of space agencies including NASA, ESA, NASDA, CNES, and
BNSC/DERA.
In addition, CFDP will serve as a prototype for the future Interplanetary Internet (IPN [RD42])
as envisioned by the IPN Study team: it encompasses a subset of the anticipated functionality of
the IPN, and it implements several key IPN design concepts including store-and-forward
operation with deferred transmission and concurrent transactions.
CFDP [RD28] allows an automatic, reliable file transfer between spacecraft and ground (in both
directions) designed to support the operation of spacecraft by means of file transfer and remote
file system management.
Its embedded transport layer provides applications the capability of transferring their data
products end-to-end across the entire space link with two optional transmission modes: reliable
or unreliable.
In reliable mode the data loss is automatically detected and retransmission of the lost data is
performed automatically.
In unreliable mode, data are transferred in a “best effort” way over an unidirectional link.
Furthermore, CFDP provides four different selective retransmission strategies for negative
acknowledgement. Its capabilities include:
• Reliable/Unreliable copying a file from the filestore of one entity (protocol engine,
located in a spacecraft or ground control centre) to that of another entity
• Reliable/Unreliable transmission of arbitrary small messages, defined by the user, in the
metadata accompanying a file
• Reliable/Unreliable transmission of file system management commands to be executed
automatically at a remote entity – typically at a spacecraft – upon complete reception of a
file
• Store-and-forward mechanism allowing an end-to-end transfers that can span multiple
CFDP waypoint nodes in case source and destination entities are not in direct view.
CFDP is designed to offer these capabilities even across interplanetary distances, where data
errors, data loss and out-of-sequence delivery may occur; minimising the return path overhead of
the protocol for optimised performances. As such, it must function despite extremely long data
propagation delays (measured in minutes or hours, rather than in milliseconds as in terrestrial
networks) and frequent, lengthy interruptions in connectivity. Unlike TCP/IP, the transport layer
embedded in CFDP requires no handshaking and is datagram-and transaction based to deal with
space link characteristics (e.g. long RTLT and non-persistent links). Additionally, CFDP is
adaptable to fit the proximity link as well. Metadata associated with each transaction describes
the data transfer including data processing once the file arrives.