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Difficulty: Very hardOSI Model Layers and Functions

A network engineer is troubleshooting an enterprise application connectivity issue using a packet analyzer. The capture indicates that the host successfully establishes a lower-level transport connection via a TCP three-way handshake. Following segment reassembly at Layer 4, the data stream is handed up the stack. However, the connection drops because the host fails to negotiate character encoding syntax and payload encryption protocols required to interpret the message structure. At which layer of the Open Systems Interconnection (OSI) model does this specific negotiation failure occur, and what general term describes the Protocol Data Unit (PDU) at this stage?

  1. Presentation Layer (Layer 6); the PDU is referred to as DataAnswer
  2. B
    Session Layer (Layer 5); the PDU is referred to as Segment
  3. C
    Application Layer (Layer 7); the PDU is referred to as Packet
  4. D
    Transport Layer (Layer 4); the PDU is referred to as Frame

Answer

The negotiation failure occurs at the Presentation Layer (Layer 6), where the PDU is generically referred to as Data.
The correct answer identifies the Presentation Layer (Layer 6) as responsible for data representation, encryption/decryption, compression, and syntax conversion. Upper OSI layers (Layers 5 through 7) process the payload referred to as Data.

Step-by-Step Solution

1
Analyze the scenario description to identify the failing network operation.
The failure occurs during character encoding syntax negotiation and payload encryption formatting after TCP segment reassembly.
Character set translation (e.g., ASCII/EBCDIC), data compression, and cryptographic formatting (e.g., TLS syntax/presentation handling) are core functions of the OSI Presentation Layer.
2
Determine the corresponding OSI model layer for syntax, formatting, and encryption.
Layer 6 (Presentation Layer) is identified as the layer responsible.
Layer 6 acts as the data translator for the network, ensuring payload structures are formatted correctly for consumption by Layer 7.
3
Identify the proper Protocol Data Unit (PDU) nomenclature at this stage of processing.
At Layers 5, 6, and 7, the encapsulated payload is classified generically as Data.
Specific PDU naming conventions apply to lower layers (Layer 2 = Frame, Layer 3 = Packet, Layer 4 = Segment), while upper layers process unified Data.

Key Concept

OSI Layer 6 (Presentation Layer) functions and PDU classification
Estimated Time:2m 0s
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Topics

OSI Model Layers and Functions

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