ISC2 Systems Security Certified Practitioner (SSCP) Practice Question
During a packet-capture review, you notice source and destination port numbers in a TCP header that allow multiple client sessions to reach a web service on an EC2 instance. To document your findings, you must identify the OSI layer where these port numbers are defined and the corresponding layer in the TCP/IP model. Which mapping is correct?
Session layer in the OSI model and the Application layer in the TCP/IP model
Transport layer in the OSI model and the Transport layer in the TCP/IP model
Presentation layer in the OSI model and the Internet layer in the TCP/IP model
Network layer in the OSI model and the Transport layer in the TCP/IP model
Port numbers are carried in the headers of TCP and UDP segments. Both protocols provide end-to-end communication services such as segmentation, flow control, and error recovery, functions that belong to the Transport layer (Layer 4) of the OSI model. In the four-layer TCP/IP model, these same responsibilities reside in the Transport layer (also called the Host-to-Host layer). Therefore, the correct mapping pairs the OSI Transport layer with the TCP/IP Transport layer. The other options are incorrect because:
The OSI Session layer manages inter-host dialogues but does not define port numbers, and it maps to the Application layer group in TCP/IP.
The OSI Network layer handles logical addressing and routing (IP), not ports; its TCP/IP counterpart is the Internet layer.
The OSI Presentation layer is responsible for syntax and data representation, and has no direct one-to-one mapping to the TCP/IP Internet layer.
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ISC2 Systems Security Certified Practitioner (SSCP)
Network and Communication Security
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