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Zorluk: ZorOSI Model Layers and Functions

A network engineer is troubleshooting a communication failure affecting a industrial telemetry client connected to a central monitoring server. Packet captures obtained at the server interface show that incoming IP datagrams containing UDP segments arrive intact with valid IP headers and checksums. However, the host operating system immediately responds to each datagram with an ICMP Destination Unreachable (Port Unreachable) message. At which layer of the OSI model is the demultiplexing process failing, preventing the payload from reaching the receiving application process?

  1. A
    Network Layer (Layer 3)
  2. Transport Layer (Layer 4)Cevap
  3. C
    Data Link Layer (Layer 2)
  4. D
    Session Layer (Layer 5)

Cevap

Transport Layer (Layer 4)
The correct answer identifies the Transport Layer (Layer 4). Port numbers reside at Layer 4 and are used to multiplex and demultiplex network streams to specific host application sockets. Receiving an ICMP Port Unreachable error confirms that Layer 2 framing and Layer 3 IP routing succeeded in delivering the frame to the host, but the Transport Layer stack could not map the destination UDP port to an active, listening application process.

Adım Adım Çözüm

1
Analyze the packet capture symptoms
IP datagrams reach the destination host with valid IP headers, but generate an ICMP Port Unreachable message from the host.
ICMP Port Unreachable is emitted when an endpoint receives a transport segment destined for a port number with no listening socket.
2
Map port addressing to the OSI Model
Port numbers (TCP/UDP) function as software addressing endpoints at Layer 4 (Transport Layer).
Layer 4 uses port numbers to demultiplex incoming network data to the correct application layer process.
3
Identify the OSI layer responsible for the failure
The demultiplexing failure occurs at Layer 4.
Because Layer 3 IP processing succeeded, the breakdown occurs during Layer 4 port matching.

Anahtar Kavram

OSI Layer 4 Transport Layer protocol demultiplexing using port numbers
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Konular

OSI Model Layers and Functions

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