Link layer

In computer networking, the link layer is the lowest layer in the , commonly known as TCP/IP, the networking architecture of the Internet. It is described in RFC 1122 and RFC 1123. The link layer is the group of methods and that only operate on the link that a host is physically connected to. The link is the physical and logical network component used to interconnect hosts or nodes in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a local area network segment or a wide area network connection.

Despite the different semantics of layering in TCP/IP and OSI, the link layer is sometimes described as a combination of the data link layer (layer 2) and the physical layer (layer 1) in the OSI model. However, the layers of TCP/IP are descriptions of operating scopes (application, host-to-host, network, link) and not detailed prescriptions of operating procedures, data semantics, or networking technologies.

RFC 1122 exemplifies that local area network protocols such as Ethernet and IEEE 802, and framing protocols such as (PPP) belong to the link layer.

Local area networking standards such as Ethernet and IEEE 802 specifications use terminology from the seven-layer OSI model rather than the TCP/IP model. The TCP/IP model in general does not consider physical specifications, rather it assumes a working network infrastructure that can deliver media level frames on the link. Therefore, RFC 1122 and RFC 1123, the definition of the TCP/IP model, do not discuss hardware issues and physical data transmission and set no standards for those aspects. Some textbook authors have supported the interpretation that physical data transmission aspects are part of the link layer. Others assumed that physical data transmission standards are not considered communication protocols, and are not part of the TCP/IP model. These authors assume a hardware layer or physical layer below the link layer, and several of them adopt the OSI term data link layer instead of link layer in a modified description of layering. In the predecessor to the TCP/IP model, the ARPAnet Reference Model (RFC 908, 1982), aspects of the link layer are referred to by several poorly defined terms, such as network-access layer, network-access protocol, as well as network layer, while the next higher layer is called internetwork layer. In some modern text books, network-interface layer, host-to-network layer and network-access layer occur as synonyms either to the link layer or the data link layer, often including the physical layer.

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