Within the OSI model, which layer provides true end-to-end connectivity between host applications, including sequencing, flow control, and retransmission for reliable delivery?

Introduction / Context:
The OSI reference model separates concerns across seven layers. Only one of these layers is responsible for end-to-end reliability between processes on different hosts. Knowing this separation clarifies which protocols handle routing versus which guarantee delivery semantics to applications.

Given Data / Assumptions:

• “End-to-end connectivity” means from source host application to destination host application.
• Reliability implies sequencing, acknowledgments, and retransmission.
• We are mapping responsibilities to a specific OSI layer.

Concept / Approach:
The Transport layer (Layer 4) sits between the Network layer and the Session layer, providing services such as segmentation, reassembly, sequencing, flow control, and error recovery. The Network layer (Layer 3) provides logical addressing and routing but is typically best effort. Data Link (Layer 2) provides hop-to-hop framing and error detection on a single link, not end-to-end. Session focuses on dialog control and checkpoints rather than raw data reliability.

Step-by-Step Solution:
Identify that reliability and sequencing are required end-to-end.Map these functions to OSI Layer 4 (Transport).Exclude Layer 3 (routing), Layer 2 (local link), and Layer 5 (session coordination).

Verification / Alternative check:
In the TCP/IP suite, TCP is the Transport-layer analogue that implements exactly these guarantees through sequence numbers, ACKs, windows, and timers.

Why Other Options Are Wrong:
Network layer: Routes packets; does not guarantee reliable delivery.

Data Link layer: Local reliability per link only.

Session layer: Dialog and synchronization, not data transport reliability.

None of the above: Incorrect because Transport is correct.

Common Pitfalls:
Confusing hop-by-hop error handling (Layer 2) with end-to-end reliability (Layer 4). Applications experience the Transport service, not the raw network path.

Final Answer:
Transport layer