data link layer is divided in two sublayers.. LLC and MAC. LLC is at the top of MAC. ethernet comes under MAC layer, as it evolved to provide improved MAC over the then existing technologies like token ring, token bus, etc. sliding window protocols like stop and wait arq, GBN arq,SR arq comes under LLC sublayer, and it is there to provide flow control to DLL. we can see it as TCP of transport layer and IP of network layer, as how one is at the top of other when one is connection oriented and other is connectionless.