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Difficulty: MediumEncapsulation and De-encapsulation

A host operating system receives an incoming transmission containing web application traffic. As the destination network stack processes the protocol data unit (PDU) through de-encapsulation, which of the following operations occur specifically at Layer 3 (Network Layer) before the payload is passed up to Layer 4? (Select TWO.)

  1. The destination IP address in the packet header is verified against the receiving host's configured address.Answer
  2. The Protocol field within the IP header is inspected to identify which Transport layer protocol should receive the encapsulated payload.Answer
  3. C
    The destination port number in the header is read to determine which specific application service thread should handle the data.
  4. D
    The Frame Check Sequence (FCS) trailer is recalculated to verify bit-level integrity across the physical transmission medium.

Answer

The correct Layer 3 operations are verifying the destination IP address against the local host address and inspecting the IP header Protocol field to determine the target Layer 4 protocol handler.
During de-encapsulation at Layer 3 (Network Layer), the network stack inspects the IP header to verify that the packet's destination IP matches the host interface address. It also checks the IP header's Protocol field (such as 6 for TCP or 17 for UDP) to determine which Layer 4 protocol module should receive the stripped payload.

Step-by-Step Solution

1
Analyze Layer 3 IP header processing during de-encapsulation.
Identify that Layer 3 processes the IPv4 or IPv6 header containing source/destination IP addresses and protocol demultiplexing information.
De-encapsulation strips headers bottom-up; Layer 3 processes IP header fields before stripping them to yield the Layer 4 segment.
2
Evaluate destination addressing at Layer 3.
Verifying destination IP address ensures the host is the intended recipient.
If the destination IP does not match the receiving host or allowed multicast/broadcast, the packet is dropped.
3
Determine how Layer 3 hands off data to Layer 4.
Reading the Protocol field in the IP header tells Layer 3 which protocol (e.g., TCP or UDP) receives the payload.
Demultiplexing at Layer 3 relies on the IP Protocol field to pass the payload up to Layer 4.

Key Concept

Layer 3 De-encapsulation and Header Demultiplexing
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Topics

Encapsulation and De-encapsulation

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