Because IP datagrams are an Open Systems Interconnection (OSI) Network Layer entity, IP datagrams must be encapsulated with a Data Link Layer header and trailer before being sent on the physical medium. The Data Link Layer header and trailer provide the following services:
• Delimitation. Frames at the Data Link Layer must be distinguished from each other. For each frame, the start and end of the frame are indicated, and the frame’s payload is distinguished from the Data Link Layer header and trailer.
• Protocol identification. Because many organizations use multiple protocol suites such as TCP/IP or AppleTalk, the protocols must be distinguished from each other.
• Addressing. For shared-access LAN technologies such as Ethernet, the source node and destination node must be identified.
• Bit-level integrity. To detect bit-level errors in the entire frame received by the hardware, a bit-level integrity check in the form of a checksum is needed. The checksum is computed by the source node and included in the frame header or trailer. The destination recalculates the checksum and checks it against the included checksum. If the checksums match, the frame is considered free of bit-level errors. If the checksums do not match, the frame is silently discarded. This frame checksum is in addition to the checksums provided by upper layer protocols such as IP or TCP.
The particular way a network type (such as Ethernet or Token Ring) encapsulates data to be transmitted is called a frame format. The frame format corresponds to the information placed on the frame at the Logical Link Control (LLC) and Media Access Control (MAC) sublayers of the OSI Data Link Layer, and the frame format manifests itself as a header and trailer. If multiple frame formats exist for a given network type (such as Ethernet), the frame formats represent different header and trailer structures and are, therefore, incompatible with each other. In other words, all the nodes on the same network segment (bounded by routers) must use the same frame format to communicate.
Source of Information : Microsoft Press Windows Server 2008 TCP IP Protocols and Services
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