The link layer offers services to the network layer that allow delivery of packets from source to destination point to point (hosts, routers, etc.). In effect the link layer ensures (or guarantees) that the network traffic is delivered by controlling the flow of data whilst correcting and communicating errors. The link layer does this by framing data and allowing full duplex communication.
The Internet Protocol (IP) offers similar services as although it cannot guarantee delivery of data it can detect framed data errors. The Transmission Control Protocol (TCP) offers similar services as it too guarantees delivery of framed data by controlling the flow and can therefore detect delivery/data errors via full duplex communication.