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Difficulty: MediumEncapsulation and De-encapsulation

A workstation user initiates a connection to a remote network resource. As the outgoing data travels down the local OSI stack, it transitions from the Application layer to the Transport layer (Layer 4). Which Protocol Data Unit (PDU) is constructed at Layer 4, and what primary header information is appended during this encapsulation step?

  1. A Segment (or Datagram), with source and destination port numbers added to the header.Answer
  2. B
    A Packet, with source and destination IP addresses added to the header.
  3. C
    A Frame, with source and destination MAC addresses added to the header.
  4. D
    A Segment, with hardware MAC addresses and IP protocol identifiers added to the header.

Answer

At the Transport layer (Layer 4), data is encapsulated into a Segment (or Datagram) by appending a header containing source and destination port numbers.
During outbound encapsulation, when data moves down to the Transport layer (Layer 4), the operating system protocol stack wraps the payload into a Segment (when using TCP) or a Datagram (when using UDP). This process attaches a Layer 4 header containing source and destination port numbers to direct traffic to the correct application process on the receiving endpoint.

Step-by-Step Solution

1
Identify the OSI layer referenced in the scenario.
The scenario specifies data passing into Layer 4 (Transport layer).
Encapsulation naming conventions and header contents depend on the active layer.
2
Determine the PDU name used at Layer 4.
Layer 4 constructs Segments (TCP) or Datagrams (UDP).
Data units at upper layers are termed Application Data, while Layer 3 uses Packets and Layer 2 uses Frames.
3
Determine the specific addressing header added at Layer 4.
Transport layer headers encapsulate data with source and destination port numbers.
Port numbers enable process-to-process communication and multiplexing across network connections.

Key Concept

OSI Layer 4 Transport Encapsulation and PDU Identification
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Topics

Encapsulation and De-encapsulation

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