Examkin

Question

Difficulty: MediumEncapsulation and De-encapsulation

A server receives an incoming network transmission where an HTTP request payload is encapsulated within a TCP segment, an IPv4 packet, and an Ethernet frame. As the destination host's operating system stack performs de-encapsulation at the Transport layer (Layer 4), which header field is inspected to deliver the inner payload to the correct server process?

  1. Destination Port NumberAnswer
  2. B
    Protocol field
  3. C
    EtherType field
  4. D
    Source Port Number

Answer

The destination port number in the Transport layer header is inspected by the receiving network stack to direct the payload to the appropriate listening application service.
During the de-encapsulation process, each layer of the OSI model inspects its corresponding header to determine how to route the inner Protocol Data Unit (PDU) upward. At the Transport layer (Layer 4), TCP or UDP headers contain a Destination Port field. The host OS reads this destination port to direct the segment's data payload to the exact application service bound to that port.

Step-by-Step Solution

1
Analyze the incoming data flow during de-encapsulation.
The physical network interface accepts the electrical/optical signals and strips the Layer 2 Ethernet header after checking the frame header and EtherType field.
The EtherType field notifies Layer 2 that the encapsulated payload belongs to Network layer IPv4.
2
Evaluate Network layer (Layer 3) processing.
The IP protocol stack inspects the destination IP address and the Protocol field within the IPv4 header.
The Protocol field specifies that the payload should be handed off to TCP at Layer 4.
3
Evaluate Transport layer (Layer 4) de-encapsulation.
The TCP protocol stack receives the segment and inspects the Destination Port Number in the TCP header.
The destination port number acts as the address identifier for the specific application socket listening on the server.

Key Concept

De-encapsulation and Layer 4 Port Multiplexing
Rate this question

Topics

Encapsulation and De-encapsulation

More questions from this topic