CISSP practice question 8 of 35

Which one of the following operates at the session, transport, or network layer of the Open System Interconnection (OSI) model?

Choose your answer, then check it against the explanation and the full breakdown below.

Domain Communication and Network Security

Which one of the following operates at the session, transport, or network layer of the Open System Interconnection (OSI) model?

Answer options

Correct Answer:

D

Explanation

A Cyclic Redundancy Check (CRC) is a type of error-detecting code used to detect accidental changes to data. While CRCs are most prominently used at the Data Link Layer (Layer 2) in the Frame Check Sequence (FCS), the fundamental principle of using a calculated value for integrity checking is also a core function of the Network and Transport layers. The IPv4 header (Network Layer 3) and the TCP/UDP headers (Transport Layer 4) both contain checksum fields for error detection. Among the choices provided, CRC is the only option that represents this type of low-level data integrity mechanism embedded within network protocol data units at the specified layers.

References

1. Kurose

J. F.

& Ross

K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson.

Page 231

Section 3.4.2

"The Internet's Transport-Layer Protocols": The text describes the UDP segment structure

stating

"The checksum is used by the receiving host to check whether errors have been introduced into the segment... The checksum is also calculated over a few of the fields in the IP header." This confirms checksum functionality at the Transport Layer (Layer 4).

Page 347

Section 4.4.1

"The IP Datagram Format": The text details the IPv4 datagram format

including the "Header checksum" field

which is used "to aid a router in detecting bit errors in a received IP datagram." This confirms checksum functionality at the Network Layer (Layer 3).

2. Postel

J. (1981

September). RFC 791: Internet Protocol. Internet Engineering Task Force (IETF).

Section 3.1

"Format": This RFC defines the IPv4 header. It specifies the "Header Checksum" field and its purpose: "A checksum on the header only. Since some header fields change (e.g.

time to live)

this is recomputed and verified at each point that the internet header is processed." This directly places an integrity-checking mechanism at the Network Layer (Layer 3).

3. Postel

J. (1981

September). RFC 793: Transmission Control Protocol. Internet Engineering Task Force (IETF).

Section 3.1

"Format": This RFC defines the TCP header

which includes a "Checksum" field. The description states

"The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header and text... This is a required field." This places an integrity-checking mechanism at the Transport Layer (Layer 4).

4. Stallings

W. (2017). Data and Computer Communications (10th ed.). Pearson Education.

Chapter 6

Section 6.2

"Error Detection": This section provides a detailed explanation of CRC

noting its power and common implementation. It states

"The CRC is one of the most common and most powerful of the error-detecting codes... The CRC is commonly used in data link control protocols

such as HDLC." While highlighting its primary use at Layer 2

the chapter's context on error detection is applicable to the checksums used in Layers 3 and 4

which serve the same purpose.

Question 8 of 35

Question breakdown

Tests: Map security mechanisms to OSI model layers
Difficulty: Hard

Key idea: Error-detection codes such as CRCs and checksums are built into protocol data units at lower OSI layers, unlike administrative or storage controls.

How to reach the answer

The clue is 'operates at the session, transport, or network layer', so look for a protocol-embedded mechanism.
Data at rest encryption applies to stored data, not to data in transit through the stack.
Configuration management is a process, and integrity checking software (such as file integrity monitoring) runs at the application or host level.
CRC and checksum-style error detection is embedded in protocol headers and trailers, which makes it the only network-stack mechanism listed.

Each option

AIncorrect. Data at rest encryption protects stored data and is not an OSI transmission-layer function.
BIncorrect. Configuration management is an administrative process, not a protocol-layer mechanism.
CIncorrect. Integrity checking software operates on files at the host or application level.
DCorrect. CRC-style error detection is part of protocol data units, and related checksums appear in network- and transport-layer headers.

Common trap: Choosing integrity checking software because 'integrity' sounds like a protocol function, even though it is a host-level tool.

In practice: Network engineers rely on frame CRC and IP/TCP checksums to detect corruption, but these codes provide no protection against deliberate tampering.

Remember: CRCs and checksums are lower-layer protocol mechanisms for accidental-error detection; host tools and processes are not.

Drafted with AI assistance and checked against the answer key.

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