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The PDU is passed to layer N−1, where it is known as the service data unit (SDU). At layer N−1 the SDU is concatenated with a header, a footer, or both, producing a layer N−1 PDU. It is then passed to layer N−2. The process continues until reaching the lowermost level, from which the data is transmitted to the receiving device.
This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. [2] The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer.
This layer, presentation Layer and application layer are combined in TCP/IP model. 9P Distributed file system protocol developed originally as part of Plan 9; ADSP AppleTalk Data Stream Protocol; ASP AppleTalk Session Protocol; H.245 Call Control Protocol for Multimedia Communications; iSNS Internet Storage Name Service
Such protocols belong to the link layer. Internet-layer protocols use IP-based packets. A common design aspect in the internet layer is the robustness principle: "Be liberal in what you accept, and conservative in what you send" [1] as a misbehaving host can deny Internet service to many other users.
Despite the different semantics of layering between the Internet protocol suite and OSI model, the link layer is sometimes described as a combination of the OSI's data link layer (layer 2) and physical layer (layer 1). The link layer is described in RFC 1122 and RFC 1123.
OSI model Layer Protocol data unit (PDU) Function [3]; Host layers 7 Application: Data: High-level protocols such as for resource sharing or remote file access, e.g. HTTP. 6
Protocol data units of the OSI model are: [1] The Layer 4: transport layer PDU is the segment or the datagram. The Layer 3: network layer PDU is the packet. The Layer 2: data link layer PDU is the frame. The Layer 1: physical layer PDU is the bit or, more generally, symbol.
An application layer is an abstraction layer that specifies the shared communication protocols and interface methods used by hosts in a communications network. [1] An application layer abstraction is specified in both the Internet Protocol Suite (TCP/IP) and the OSI model . [ 2 ]