Correct Answer: A. Transport
The Transport layer, Layer 4 of the Open Systems Interconnection (OSI) model, is responsible for end-to-end communication between applications running on network devices.
Its primary functions include data segmentation, transport-layer addressing, multiplexing, flow control, and, when TCP is used, reliable delivery.
The Transport layer uses two principal protocols:
Transmission Control Protocol (TCP): A connection-oriented protocol that divides application data into segments. TCP headers contain source and destination port numbers, sequence numbers, acknowledgment numbers, and other control information. TCP supports reliable, ordered delivery through acknowledgments and retransmission mechanisms.
User Datagram Protocol (UDP): A connectionless protocol that transmits datagrams with minimal overhead. UDP headers contain source and destination port numbers, length, and checksum fields.
Port numbers identify the sending and receiving application endpoints. For example, HTTPS commonly uses TCP destination port 443.
Why the other options are incorrect:
B. Data Link: Layer 2 handles Ethernet frames, MAC addressing, and local network delivery.
C. Session: Layer 5 manages communication sessions between applications.
D. Network: Layer 3 handles logical IP addressing and routing packets between networks.
Therefore, Option A is correct because segmentation and transport-layer port addressing are Layer 4 functions.
[References:, Cisco Certified Support Technician Networking (100-150) — Standards and Concepts, OSI Reference Model., Cisco Networking Academy — Transport Layer Role and Services., Cisco Networking Academy — TCP and UDP Header Fields, Segmentation and Port Numbers., ==========]
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