Link Layer Protocols
The Link Layer in the TCP/IP model is a descriptive realm of networking protocols that operate only on the local network segment (link) that a host is connected to. Such protocol packets do not 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 (ARP), 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.
IS-IS (RFC 1142) is another link-state routing protocol that fits into this layer when considering TCP/IP model, however it was developed within the OSI reference stack (where it is a layer-3 protocol). It is not an Internet standard.
The link layer also contains all hardware specific interface methods, such as Ethernet and other IEEE 802 encapsulation schemes (see References).
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