答案:【计分规则】: In order to transfer data packets from the source to the destination, each layer of the source OSI model must communicate with the peer layer of the destination, which is called peer layer communication.(2 points) In this process, the protocols of each layer exchange information between peers, which becomes protocol data unit (PDU). Located in each communication layer of the source computer, the PDU for that layer is used to communicate with the peer layer of the destination computer.(2 points)