In the OSI model, end-to-end host-to-host connectivity is provided by the:

Concept

In the OSI reference model each layer adds a distinct service. The layer that establishes a logical connection between the two communicating end systems — so that a complete message is delivered from the source host/process to the destination host/process — is defined as the end-to-end (host-to-host) layer. Intermediate routers do not participate in this layer; only the two end hosts do.

Application

"End-to-end host-to-host connectivity" is exactly the service signature of OSI Layer 4. It establishes a logical conversation directly between the two end hosts and uses port (service-access-point) numbers to hand data to the correct application/process. Depending on the protocol it adds segmentation/reassembly, multiplexing, flow control, and (with a connection-oriented protocol such as TCP) reliability and ordered delivery. Hence the service named in the stem maps to Layer 4.

Contrast

• network layer: gives logical addressing and routing/path selection hop by hop so packets can cross networks; its scope is host-to-router-to-host forwarding, not a single logical end-to-end conversation.

• session layer: opens, synchronises and closes dialogs/sessions between applications; it relies on the transport service below it rather than providing it.

• transport layer: the only layer whose defining job is the direct end-to-end, host-to-host (and process-to-process) delivery service the stem describes.

Result: the end-to-end host-to-host connectivity service is provided by the transport layer.