Q & A on Data Link and Transport Protocols Q: I found a very high similarities between Data Link/Transport Protocols, both of them use the same sliding window technique for example. A: Transport protocols are end-to-end, while data link protocols are machine-to-machine. Furthermore, if you compare TCP to typcial data-link protocol, you will find vast differences (e.g., in the windowing mechanisms). Q: My questions are: 1: why does reliability/flow control implemented in two different layers? A: Reliabaility at each layer is an old idea; TCP/IP does *not* require reliable lower layers. Q: 2: does people usually use both of them at the same time? A: Not any more; see above. Q: 3: If they are exclusively used, how do people usually select between them? A: End-to-end reliability is required; hence, one would choose to place reliabilty at the transport layer.