OSI reference model — which layer provides acknowledgments, sequencing, and flow control? Identify the OSI layer responsible for reliable delivery features such as positive ACKs, sequence numbering, and window-based flow control.

Introduction / Context:
To troubleshoot end-to-end connectivity, you must map symptoms to the correct OSI layer. Reliability mechanisms—acknowledgments, sequencing, and flow control—determine whether segments arrive intact, in order, and at a manageable rate. These functions are central to TCP and defined at the Transport layer in the OSI model.

Given Data / Assumptions:

• We refer to the classic OSI 7-layer model.
• Examples include TCP features like ACKs, sequence numbers, and sliding windows.
• We distinguish per-hop behaviors (Layers 1–3) from end-to-end behaviors (Layer 4 and above).

Concept / Approach:
Layer 4 (Transport) provides end-to-end communication services for applications. TCP, a Transport protocol, implements reliable transport using acknowledgments, sequence/acknowledgment numbers, and flow control via sliding windows and receive buffers. UDP, also at Layer 4, omits these features to provide low-latency, connectionless delivery.

Step-by-Step Solution:
Associate ACKs and sequencing with TCP behavior.Map TCP to OSI Layer 4 (Transport).Conclude that the responsible OSI layer is Layer 4.

Verification / Alternative check:
Packet captures show TCP headers containing sequence and acknowledgment numbers and window sizes—these fields are absent from Layer 3 IP headers.

Why Other Options Are Wrong:

• A: Layer 2 ensures reliable delivery on a local segment (for example, frame checks) but not end-to-end sequencing.
• B: Layer 3 handles routing and logical addressing (IP), not ACK/sequence/flow control.
• D: Layer 7 is application logic, relying on services from lower layers.
• E: Layer 1 is purely physical signaling.

Common Pitfalls:
Attributing “reliability” generically to Layer 3; while some link technologies retransmit at Layer 2, true end-to-end reliability belongs to Layer 4.

Final Answer:
Layer 4 (Transport)