I am taking about the 5 layer model. It is well documented that TCP is a reliable service and IP is best-effort delivery and hence unreliable (ref RFC 792 – TCP). Also, the data link layer is 'reliable' because of its ARQ mechanism guaranteeing sequential delivery of packets (ref).
I cannot make sense of sending packets of reliable service (TCP) on a lower unreliable service (IP) which in turn sends packets on a layer which provides reliability (Link Layer). Why can't we do away with reliability at link layer as IP is unreliable?
Best Answer
The issue is that you're operating under the notion that each layer is in and of itself a separate, autonomous entity. Understand that IP is not the only packet delivery protocol in existence, it just happens to be the most common one. Also understand that these "layers" are simply abstraction tools - the take away is that each "layer" is dependent on the one beneath it for some functions, and the layers further down the stack can hand off certain duties (ie reliable data transmission) to the layers above them when appropriate to keep overhead low and performance high. Each lower layer encapsulates data sent from the layers above it on its way down to the wire, and the lower layers decapsulate the data on the way back up the stack to the application.
The paper you linked about the data link layer even states that while its delivery mechanisms are intended to be reliable, they are still best effort, and there is an assumption made that higher layer protocols (ie TCP) will handle retransmission if necessary.