Link layer
Despite the different semantics of layering in TCP/IP and OSI, the link layer is often described as a combination of the data link layer and the physical layer (layer 1) in the Open Systems Interconnection OSI model. However, TCP/IP's layers 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 Point-to-Point Protocol belong to the link layer.
LAN standards such as Ethernet and IEEE 802 specifications use terminology from the seven-layer OSI model rather than the TCP/IP reference 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, don't discuss hardware issues and physical data transmission and set no standards for those aspects, other than broadly including them as link-layer components. Some textbook authors have supported the interpretation that physical data transmission aspects are part of the link layer. That position will be held in the rest of this article. Others assumed that physical data transmission standards aren't considered as communication protocols, and aren't 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, 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.
The Link Layer in the TCP/IP model is a descriptive realm of networking protocols that operate only on the local network segment that a host is connected to. Such protocol packets don't get routed to other networks subnetworks.
The core protocols specified by the Internet Engineering Task Force to be placed into this layer are the Address Resolution Protocol, its cousin, the Reverse Address Resolution Protocol (RARP), and the Neighbor Discovery Protocol (NDP), which is a facility delivering similar functionality as ARP for IPv6. Since the advent of IPv6, Open Shortest Path First (OSPF) can be considered to operate on the link level as well, although the IPv4 version of the protocol was considered at the Internet layer.
The link layer also contains all hardware specific interface methods, such as Ethernet and other IEEE 802 encapsulation schemes.
The link layer of the TCP/IP model is often compared directly with the combination of the data link layer and the physical layer in the Open Systems Interconnection protocol stack. Although they are congruent to some degree in technical coverage of protocols, they aren't identical. The link layer in TCP/IP is still wider in scope and in principle a different concept and terminology of classification. This may be observed when certain protocols, such as the Address Resolution Protocol (ARP), which is confined to the link layer in the TCP/IP model, is often said to fit between OSI's data link layer and the network layer. In general, direct or strict comparisons should be avoided, because the layering in TCP/IP isn't a principal design criterion and in general is considered to be "harmful" (RFC 3439).
Another term sometimes encountered, "network-access layer", tries to suggest the closeness of this layer to the physical network. However, this use is misleading and non-standard, since the link layer implies functions that are wider in scope than just network access. Important link-layer protocols are used to probe the topology of the local network, discover routers and neighboring hosts, i.e. functions that go well beyond network access.
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