End-to-End connectivity is provided from Last-to-Last in:

Concept

In the OSI reference model each layer is defined by the scope of the delivery it is responsible for, and three scopes must be kept apart. Node-to-node delivery moves a unit between two directly attached devices over a single link. Host-to-host delivery moves a unit from the source machine to the destination machine across a routed path. Process-to-process delivery moves a whole message from the sending application to the receiving application, and it is this widest scope that the term end-to-end names in OSI terminology.

Applying it here

The stem asks which of the layers offered owns the scope that spans the whole path, from the last device at one end to the last device at the other. Match each layer offered to its scope:

1. Data link layer: node-to-node. It delivers a frame between two directly attached nodes over one physical link, so its responsibility ends at the far end of that link.

2. Network layer: host-to-host. It delivers a packet from the source machine to the destination machine across interconnected networks and chooses the route, so its delivery does span the path; but its addresses identify machines only, it never identifies which application on the destination should receive the data, and it offers no delivery guarantee.

3. Transport layer: process-to-process. Using port numbers it delivers the message from the sending application to the receiving application, and it runs only in the two end systems, so its sequencing, acknowledgement and flow control are held between those endpoints themselves.

The widest of these three scopes, the genuinely end-to-end one, belongs to the transport layer.

Cross-check

RFC 1122 states that the transport layer provides end-to-end communication services for applications, and RFC 1208 defines the OSI transport layer as responsible for reliable end-to-end transfer between end systems. TCP shows this concretely: sequence numbers, acknowledgements and the flow-control window are maintained only at the two endpoints, while every router in between reads no further than the network-layer header.

How the other layers differ

• Network layer: host-to-host delivery of packets, with routing and logical addressing; its addresses name machines, not the applications running on them.

• Session layer: sits above an already-established end-to-end connection and manages the dialogue itself, that is setup, synchronisation checkpoints and teardown.

• Data link layer: node-to-node delivery of frames over one link, with physical (MAC) addressing and error detection on that hop.

Note on the wording

The original paper's phrase Last-to-Last is the examiner's way of saying from the last device at one end to the last device at the other, that is, between the two end systems.