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2022 cyber threat trends

by Yee Wei Law - Wednesday, 7 June 2023, 1:03 PM
 

Every year, major cybersecurity firms release their report on the trends of cyber threats/attacks they observe during that year.

Some also release their forecast of cybersecurity trends for the next year.

Examples of reports for 2022 are provided in the list of references.

A summary of the trends observed in these reports is provided here, with additional commentary on how some of these attacks happened.

Among the most impactful trends identified [Che22, Cro22b, Man22a, Spl22] are (in no particular order):

  • State-sponsored cyber armies, advanced persistent threats (APTs) and cyberwarfare are on the rise.

    Mandiant has an extensive report [Man22a] on the activities of various threat groups in 2022.

    Threat groups from the “big four” — Russia, China, Iran, North Korea — are expected to be highly active in 2023, using destructive attacks, information operations, financial threats and more [Man22b].

  • Ransomware has been occupying news headlines, and nobody should be a stranger to this escalating threat anymore.

    It is not even farfetched to expect ransomware to be used to attack space systems [Pet22].

  • Macro viruses have existed ever since Microsoft Office started supporting macros.

    They do not seem to be going away.

    Documents containing malicious macros are called “maldoc”, and Emotet (🖱 for details), one of the world’s most prevalent malware (see Fig. 1), has been hailed as the “unofficial king of maldoc usage” [Che22]

Fig. 1: Global ranking of multipurpose malware families in terms of percentage of corporate networks attacked by each malware family [Che22, p. 34].
Fig. 2: Global ranking of cryptomining malware (presumably) in terms of number of infections [Che22, Figure 28].
  • Mobile malware has in recent years started exploiting zero-click vulnerabilities [Jin21], posing tremendous risks to unpatched devices.

  • Cloud-based services are increasingly abused by malicious actors in the course of computer network operations, a trend that is likely to continue in the foreseeable future as more businesses seek hybrid work environments [Cro22b].

    Common cloud attack vectors include cloud vulnerability (e.g., CVE-2021-21972) exploitation, credential theft, cloud service provider abuse, use of cloud services for malware hosting and command & control (C2), and the exploitation of misconfigured Docker containers [Cro22b].

  • In 2021, compromise of cyber supply chain accounted for 17% of intrusions, compared to less than 1% in 2020 [Man22a].

    Furthermore, 86% of these compromises were related to the SolarWinds breach and the SUNBURST malware (trojanised digitally signed component of the SolarWinds Orion software framework that contains a backdoor that communicates via HTTP with third-party servers).

    Watch news report by ABC:

  • An increasing number of malware has cryptocurrency mining (“cryptomining” for short) capabilities [Che22], since these capabilities are readily available in the public domain.

    For example, XMRig is available on GitHub, and is the most popular cryptominer (see Fig. 2).

  • The highest-profile example of attack on cryptocurrency is undoubtedly the “FTX hack”, which allegedly stole USD 415 million from the FTX exchange.

    The attacker had been using crypto laundering services like ChipMixer to launder the stolen funds.

References

[Che22] Check Point, Cyber Attack Trends: Check Point’s 2022 Mid-Year Report, 2022. Available at https://www.checkpoint.com/downloads/resources/cyber-attack-trends-report-mid-year-2022.pdf.
[Cro22b] CrowdStrike, 2022 Global Threat Report, 2022. Available at https://go.crowdstrike.com/global-threat-report-2022.html.
[Jin21] M. Jin, Analyzing The ForcedEntry Zero-Click iPhone Exploit Used By Pegasus, Exploits & Vulnerabilities, September 2021. Available at https://www.trendmicro.com/en_au/research/21/i/analyzing-pegasus-spywares-zero-click-iphone-exploit-forcedentry.html.
[Man22a] Mandiant, M-Trends 2022: Mandiant Special Report, April 2022. Available at https://www.mandiant.com/m-trends.
[Man22b] Mandiant, Mandiant Cyber Security Forecast 2023, November 2022. Available at https://www.mandiant.com/resources/reports/2022/mandiant-security-forecast-2023-predictions.
[Pet22] V. Petkauskas, The ingredients for ransomware attack in space are here - interview, editorial, March 2022. Available at https://cybernews.com/editorial/the-ingredients-for-ransomware-attack-in-space-are-here-interview/.
[Spl22] Splunk, Top 50 Cybersecurity Threats, 2022. Available at https://www.splunk.com/content/dam/splunk2/en_us/gated/ebooks/top-50-cybersecurity-threats.pdf.
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Abstract interpretation

by Yee Wei Law - Wednesday, 17 May 2023, 10:01 AM
 

High-level overview

Abstract interpretation, first formalised by Cousot and Cousot in 1977 [CC77], executes a program on an abstract machine to determine the properties of interest [vJ11, p. 1255].

The level of detail in the abstract machine specification typically determines the accuracy of the results.

At one end of the spectrum is an abstract machine that faithfully represents the concrete semantics of the language.

Abstract interpretation on such a machine 👆 would correspond to a concrete execution on the real machine.

Typical abstract interpretation analyses, however, do not require detailed semantics.

A bit of theory

Fundamentally, the correctness problem of a program is undecidable, so approximation is necessary [Cou01, Abstract].

The purpose of abstract interpretation is to formalise this idea 👆 of approximation; see Fig. 1.

Formally, abstract interpretation is founded in the theory for approximating sets and set operations [Cou01, Sec. 2].

The semantics of a program can be defined as the solution to a fixpoint (short for fixed point) equation.

Thus, an essential role of abstract interpretation is providing constructive and effective methods for fixpoint approximation and checking by abstraction.

By observing computations at different levels of abstraction (trace semantics → relational semantics → denotational semantics → weakest precondition semantics → Hoare logics), fixpoints can be approximated.

Fig. 1: In abstract interpretation, the program analyzer computes an approximate semantics of the program [Cou01, Fig. 12].

The generator generates equations/constraints, the solution to which is a computer representation of the program semantics.

The solver solves these equations/constraints.

The diagnoser checks the solutions with respect to the specification and outputs “yes”, “no” or “unknown”.

References

[Cou01] P. Cousot, Abstract Interpretation Based Formal Methods and Future Challenges, pp. 138–156, Springer Berlin Heidelberg, Berlin, Heidelberg, 2001. https://doi.org/10.1007/3-540-44577-3_10.
[CC77] P. Cousot and R. Cousot, Abstract interpretation: A unified lattice model for static analysis of programs by construction or approximation of fixpoints, in Proceedings of the 4th ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages, POPL ’77, Association for Computing Machinery, 1977, pp. 238–252. https://doi.org/10.1145/512950.512973.
[NNH99] F. Nielson, H. R. Nielson, and C. Hankin, Principles of Program Analysis, Springer Berlin, Heidelberg, 1999. https://doi.org/10.1007/978-3-662-03811-6.
[vJ11] H. C. van Tilborg and S. Jajodia (eds.), Encyclopedia of Cryptography and Security, Springer, Boston, MA, 2011. https://doi.org/10.1007/978-1-4419-5906-5.
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Address space layout randomisation (ASLR)

by Yee Wei Law - Tuesday, 16 May 2023, 11:04 AM
 

Address space layout randomisation (ASLR) refers to the randomisation of the locations of the key components of an executable — stack, heap, libraries and memory-mapped files, executable code — in the address space; to mitigate remote code execution and other attacks targeting CWE-787 and CWE-125.

Watch Dr. Yeongjin Jang’s lecture on ASLR:

ASLR targets memory corruption vulnerabilities such as buffer overflows, which typically occur when data get written to memory that is smaller than the size of the data — a common programming error when using a memory-unsafe language like C.

By introducing uncertainty into the locations of the shellcode (or any other attack code), ASLR hinders exploitations of memory corruption vulnerabilities.

There are many ways ASLR can be done, and different operating systems do it differently.

There are three dimensions to be considered [MGRR19]:

When

This is about when the entropy used for ASLR is generated (see Table 1):

  • Per-deployment: The application is randomised when it is installed in the system.

    This limited form of randomisation, called pre-linking, enables randomisation on systems that do not support position-independent code (PIC) or relocatable code.

  • Per-boot: Randomisation is done whenever the system boots.

    Useful for systems whose shared libraries are not compiled as PIC.

  • Per-exec: Randomisation is done whenever a new executable image is loaded into memory.

    This pre-process randomisation is triggered by the exec() system call but not the fork() system call.

  • Per-fork: Randomisation is done whenever a new process is created/forked.

  • Per-object: Every object is randomised when it is created.

    ⚠ Objects that are at a constant distance away from another already mapped object are not considered to be randomised on a per-object basis, even if the reference object is randomised, because knowledge of one object’s location compromises the location of another.

What

This is about what to randomise.

In the extreme, if a single object (e.g., the executable) is not randomised, ASLR is considered to be broken.

An “aggressive” form of ASLR is when the randomisation is applied to the logical elements contained in the memory objects: processor instructions, blocks of code, functions, and even the data structures of the application; see Table 1.

In this case 👆, the compiler, linker and loader are required to play an active role.

This form of ASLR is not known to be in use.

How

Table 1 lists the main strategies:

  • Partial-VM vs full-VM: In the former case, a subset of the virtual memory space is divided into non-overlapping ranges or zones, and each zone contains one or more objects.

    In the latter case, randomisation applies to the whole virtual memory space.

    Full-VM randomisation does not honour the order of the main areas (i.e., exec, heap, libs, stack, ...), but it is not known to be in use.

  • Isolated-object vs correlated-object: In the former case, every object is randomly mapped with respect to any other.

    In the latter case, the position of an object is calculated as a function of the position of another object and a random value.

  • For details on the other strategies in Table 1, please refer to [MGRR19, pp. 5-6].
Table 1: Summary of the three dimensions of ASLR [MGRR19, Table 1]. vDSO = virtual dynamic shared object; VM = virtual memory.

References

[MGRR19] H. Marco-Gisbert and I. Ripoll Ripoll, Address space layout randomization next generation, Applied Sciences 9 no. 14 (2019). https://doi.org/10.3390/app9142928.
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Advanced persistent threat (APT)

by Yee Wei Law - Thursday, 15 June 2023, 2:54 PM
 

Advanced persistent threat (APT, see Definition 1) has been occupying the attention of many cybersecurity firms.

Definition 1: Advanced persistent threat (APT) [NIS11, Appendix B]

An adversary that possesses sophisticated levels of expertise and significant resources which allow it to create opportunities to achieve its objectives using multiple attack vectors (e.g., cyber, physical, and deception).

These objectives typically include establishing and extending footholds within the information technology infrastructure of the targeted organisations for purposes of exfiltrating information, undermining or impeding critical aspects of a mission, program, or organization; or positioning itself to carry out these objectives in the future.

The advanced persistent threat: 1️⃣ pursues its objectives repeatedly over an extended period of time; 2️⃣ adapts to defenders’ efforts to resist it; and 3️⃣ is determined to maintain the level of interaction needed to execute its objectives.

Since APT groups are characterised by sophistication, persistence and resourcefulness, they are challenging to counter. Lists of APT groups are being actively maintained, e.g., by MITRE and Mandiant.

References

[NIS11] NIST, Managing Information Security Risk: Organization, Mission, and Information System View, NIST Special Publication 800-39, March 2011. Available at https://nvlpubs.nist.gov/nistpubs/legacy/sp/nistspecialpublication800-39.pdf.
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Application security testing

by Yee Wei Law - Sunday, 4 June 2023, 9:42 AM
 

Application security testing can be static or dynamic.

Static application security testing (SAST) is a specialisation of static (code/program) analysis.

Watch an introduction to static code analysis on LinkedIn Learning:

Static analysis from Developing Secure Software by Jungwoo Ryoo

Exploring tools for static analysis from Developing Secure Software by Jungwoo Ryoo

Definitions and history:

  • Static analysis is the processes of extracting semantic information about a program at compile time [Lan92, Sec. 1].
  • Classic example: The live-variables problem, where a variable is live at a statement iff on some execution, is used/accessed after is executed without being redefined [Lan92, Sec. 1; NNH99, p. 49]. This is also an example of data flow analysis (discussed below).
  • In security, static analysis is supposed to classify the potential behaviour of a program as malicious or benign without executing it [vJ11, p. 754].
  • Static analysis algorithms historically have come from compiler research and implementations [vJ11, p. 1254], evolving from intraprocedural analysis to interprocedural analysis [Lan92, Sec. 1].
  • In the 1970s, two main frameworks were established: 1️⃣ data flow analysis, 2️⃣ abstract interpretation. Nowadays, we also have 3️⃣ model checking, and 4️⃣ type checking.

Challenges:

  • Fundamentally, no tool can determine whether a program terminates due to the uncomputability of the halting problem, as we learn from basic complexity theory.

    The halting problem aside, finding an exact solution to typical static analysis questions is almost always undecidable [vJ11, p. 1254].

  • As codebases grow, static analysis tools take longer to parse and traverse code because they generally operate over all possible branches of execution in a program [Tho21].

    Furthermore, static analyses are inherently computationally expensive — often quadratic, sometimes even cubic — in terms of required space or time [Tho21].

    Consequently, static analysis tools are under constant pressure to be more efficient.

  • In general, there exist code obfuscation techniques that can defeat static analysis [MKK07], rendering 1️⃣ static analysis better at finding bugs in benign programs than detecting malware, and 2️⃣ the complementary role of dynamic analysis indispensable.

  • Besides obfuscation, malware often employ polymorphism such as the Shikata Ga Nai polymorphic encoding scheme [MRC19] to deter static analysis [vJ11, p. 754].

Tools:

  • Static analysis is especially important for C/C++ code.

    Among the open-source static analysis tools for C/C++ code, the Clang Static Analyzer and Frama-C offer robust detect rates [ACC+17].

  • For common programming languages, see OWASP’s extensive catalog of static analysis tools.
  • Regardless of the tool used, it helps to write code that facilitates checking, e.g., by adopting Holzman’s power-of-ten rules [Hol06] for writing safety-critical code.

Dynamic application security testing (DAST) is a specialisation of dynamic (code/program) analysis.

Watch an introduction to dynamic code analysis on LinkedIn Learning:

Dynamic analysis from Developing Secure Software by Jungwoo Ryoo

Dynamic analysis tools from Developing Secure Software by Jungwoo Ryoo

Definitions and history:

  • Dynamic analysis refers to the broad class of techniques that make inferences about a program by observing its runtime execution behaviour [vJ11, p. 365].

  • An example of dynamic analysis is fuzz testing, which is the execution of a program-under-test (PUT) using input(s) sampled from an input space (the “fuzz input space”) that “protrudes” the expected input space of the PUT, to test if the PUT violates a correctness policy [MHH+21, Definition 1].
  • Another example of dynamic analysis is taint analysis, also called information flow tracking, which is the tracking of “tainted” data throughout a system while a program manipulating this data is executed [ESKK08].
  • Better than static analysis, dynamic analysis is robust to malware polymorphism, including low-level obfuscations that can thwart disassembly [vJ11, p. 755].
  • Applications: software debugging, software profiling and host-based intrusion detection [vJ11, p. 366].

Challenges:

  • Dynamic analysis tools are typically competent on checking soundness (analysis results are consistent with the actual behaviour of the program), but not so much on completeness (analysis can infer all behaviours of interest of the program).

    In general, static analysis often suffers from a high false-positive rate, whereas dynamic analysis is limited in coverage.

  • There exist anti-emulation techniques that check for certain low-level processor features (e.g., undocumented instructions) or timings, enabling determination of whether the execution environment is an emulation [vJ11, p. 755].

    In case of an emulation, the malware can terminate execution without performing any malicious action and risking detection.

Tools:

  • Among the open-source dynamic analysis tools for C/C++ code, Valgrind [NS07] is likely the best known.
  • For common programming languages, an extensive catalog of dynamic analysis tools can be found on GitHub.

References

[ACC+17] A. Arusoaie, S. Ciobâca, V. Craciun, D. Gavrilut, and D. Lucanu, A Comparison of Open-Source Static Analysis Tools for Vulnerability Detection in C/C++ Code, in 2017 19th International Symposium on Symbolic and Numeric Algorithms for Scientific Computing (SYNASC), 2017, pp. 161–168. https://doi.org/10.1109/SYNASC.2017.00035.
[ESKK08] M. Egele, T. Scholte, E. Kirda, and C. Kruegel, A survey on automated dynamic malware-analysis techniques and tools, ACM Comput. Surv. 44 no. 2 (2008). https://doi.org/10.1145/2089125.2089126.
[Hol06] G. J. Holzmann, The power of 10: rules for developing safety-critical code, Computer 39 no. 6 (2006), 95–99. https://doi.org/10.1109/MC.2006.212.
[Lan92] W. Landi, Undecidability of static analysis, ACM Lett. Program. Lang. Syst. 1 no. 4 (1992), 323 – 337. https://doi.org/10.1145/161494.161501.
[MHH+21] V. J. Manès, H. Han, C. Han, S. K. Cha, M. Egele, E. J. Schwartz, and M. Woo, The art, science, and engineering of fuzzing: A survey, IEEE Transactions on Software Engineering 47 no. 11 (2021), 2312–2331. https://doi.org/10.1109/TSE.2019.2946563.
[MRC19] S. Miller, E. Reese, and N. Carr, Shikata Ga Nai encoder still going strong, Mandiant threat research, 2019. Available at https://www.mandiant.com/resources/blog/shikata-ga-nai-encoder-still-going-strong.
[MKK07] A. Moser, C. Kruegel, and E. Kirda, Limits of static analysis for malware detection, in Twenty-Third Annual Computer Security Applications Conference (ACSAC 2007), 2007, pp. 421–430. https://doi.org/10.1109/ACSAC.2007.21.
[NS07] N. Nethercote and J. Seward, Valgrind: A framework for heavyweight dynamic binary instrumentation, SIGPLAN Not. 42 no. 6 (2007), 89 – 100. https://doi.org/10.1145/1273442.1250746.
[NNH99] F. Nielson, H. R. Nielson, and C. Hankin, Principles of Program Analysis, Springer Berlin, Heidelberg, 1999. https://doi.org/10.1007/978-3-662-03811-6.
[SKK+16] S. Schrittwieser, S. Katzenbeisser, J. Kinder, G. Merzdovnik, and E. Weippl, Protecting software through obfuscation: Can it keep pace with progress in code analysis?, ACM Comput. Surv. 49 no. 1 (2016). https://doi.org/10.1145/2886012.
[Tho21] P. Thomson, Static analysis: An introduction: The fundamental challenge of software engineering is one of complexity, Queue 19 no. 4 (2021), 29 – 41. https://doi.org/10.1145/3487019.3487021.
[vJ11] H. C. van Tilborg and S. Jajodia (eds.), Encyclopedia of Cryptography and Security, Springer, Boston, MA, 2011. https://doi.org/10.1007/978-1-4419-5906-5.
[ZWCX22] X. Zhu, S. Wen, S. Camtepe, and Y. Xiang, Fuzzing: A survey for roadmap, ACM Comput. Surv. 54 no. 11s (2022). https://doi.org/10.1145/3512345.
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Asymmetric-key cryptography

by Yee Wei Law - Friday, 26 May 2023, 11:00 PM
 
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Authenticated encryption with associated data (AEAD)

by Yee Wei Law - Saturday, 19 August 2023, 5:18 PM
 

See 👇 attachment (coming soon) or the latest source on Overleaf.

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Block ciphers and their modes of operation

by Yee Wei Law - Tuesday, 17 October 2023, 12:06 PM
 
See 👇 attachment or the latest source on Overleaf, for block ciphers and their modes of operation.
PDF document l_bciphers.pdf
PDF document l_bcmodes.pdf
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Bundle Protocol

by Yee Wei Law - Friday, 8 March 2024, 9:07 AM
 

The original purpose of the delay-tolerant networking (DTN) protocols was to provide space communications scenarios with network-layer functionality similar to that provided by IP-based networks on Earth.

Since space communication scenarios cannot be supported by the terrestrial IP protocol suite, a new solution had to be developed.

The CCSDS Bundle Protocol (BP), based on Bundle Protocol version 6 as defined in RFC 5050 [SB07] and RFC 6260 [Bur11], is meant to provide 1️⃣ basic network-layer functionality, and 2️⃣ storage capability to enable networking benefits even in the presence of delays, disconnections, and rate mismatches [IEH+19].

⚠

The latest version of the Bundle Protocol is version 7, as defined in RFC 9171 [BFB22], but this version has yet to be adopted by CCSDS.

For providing integrity and confidentiality services for BP bundles (see Table 1), Bundle Protocol Security (BPSec) is defined in RFC 9172 [BM22].

More concretely, the BP provides network-layer services to applications through these capabilities [CCS15, Secs. 1.1 and 2.1]:

  • custody transfer;
  • coping with intermittent connectivity;
  • taking advantage of scheduled, predicted or opportunistic connectivity (in addition to continuous connectivity);
  • notional data accountability with built-in status reporting;
  • late binding of names to addresses.

When used in conjunction with the Bundle Security Protocol, as defined in RFC 6257 [FWSL11], the BP also provides:

  • hop-by-hop sender authentication; as well as
  • end-to-end data integrity and confidentiality.
Table 1: Bundle Protocol definitions [CCS15, SB07].
Term Definition
Bundle A protocol data unit (PDU) comprising a sequence of two or more blocks of data.
Bundle node An entity that can send and/or receive bundles.
Bundle protocol agent (BPA) A node component that offers the BP services and executes the procedures of the BP.
Convergence layer adapter (CLA)

An adapter that sends and receives bundles on behalf of a BPA.

This is necessary for interoperation with existing Internet protocols; see Fig. 1.
Fig. 1: A sample configuration with the BP and a CLA running over a transport protocol on the left, and over a data link layer on the right [CCS15, Figure 2-1]. The CLA B labelled “CL B” on the right could for instance be the interface to the Licklider Transmission Protocol (LTP) with the “Link B1” representing LTP running over one of the Space Data Link Protocols.

The BP is such an important protocol several open-source implementations exist:

References

[BM22] E. J. Birrane and K. McKeever, Bundle Protocol Security (BPSec), RFC 9172, January 2022. https://doi.org/10.17487/RFC9172.
[Bur11] S. Burleigh, Compressed Bundle Header Encoding (CBHE), RFC 6260, May 2011. https://doi.org/10.17487/RFC6260.
[BFB22] S. Burleigh, K. Fall, and E. J. Birrane, Bundle Protocol Version 7, RFC 9171, January 2022. https://doi.org/10.17487/RFC9171.
[CCS15] CCSDS, CCSDS Bundle Protocol Specification, Recommended Standard CCSDS 734.2-B-1, The Consultative Committee for Space Data Systems, September 2015. Available at https://public.ccsds.org/Pubs/734x2b1.pdf.
[FWSL11] S. Farrell, H. Weiss, S. Symington, and P. Lovell, Bundle Security Protocol Specification, RFC 6257, May 2011. https://doi.org/10.17487/RFC6257.
[IEH+19] D. Israel, B. Edwards, J. Hayes, W. Knopf, A. Robles, and L. Braatz, The Benefits of Delay/Disruption Tolerant Networking (DTN) for Future NASA Science Missions, in 70th International Astronautical Congress (IAC), October 2019. Available at https://ntrs.nasa.gov/citations/20190032313.
[SB07] K. Scott and S. Burleigh, Bundle protocol specification, RFC 5050, November 2007. Available at https://datatracker.ietf.org/doc/rfc5050/.

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CCSDS File Delivery Protocol

by Yee Wei Law - Sunday, 21 May 2023, 3:44 PM
 

The CCSDS File Delivery Protocol (CFDP) is standardised in

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):

Class 1 [CCS20, Sec. 7.2]

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.

Class 2 [CCS20, Sec. 7.3]

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.,

References

[CCS20] 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.

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