Link Layer

Link Layer is a term used in the general classification (layering) of internetworking methods used in the TCP/IP suite of protocols used for the Internet (RFC 1122). It is the group of methods, protocols, and specifications that is closest to the physical network components used to connect hosts or nodes in the network, which it implicitly includes. In general it is the suite of methods that operate only on the link between adjacent network nodes of a Local area network segment or a wide area network. This layer is therefore often referred to as a lower level layer.

Link Layer protocols

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) is now 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).

Relation to OSI model

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 (OSI) protocol stack. Although they are congruent to some degree, they are not identical. The Link Layer in TCP/IP is still wider in scope. It is a different concept. 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 is not a principal design criterion and in general 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".

See also

Layers defined in the OSI model of networking that relate to the Link Layer:
* Data link layer
* Physical layer
* Network layer

References

* RFC 1122, "Requirements for Internet Hosts -- Communication Layers," IETF, R. Braden (Editor), October 1989
* RFC 1123, "Requirements for Internet Hosts -- Application and Support," IETF, R. Braden (Editor), October 1989
* RFC 893, "Trailer Encapsulations," S. Leffler and M. Karels, April 1984
* RFC 826, "An Ethernet Address Resolution Protocol," D. Plummer, November 1982
* RFC 894, "A Standard for the Transmission of IP Datagrams over Ethernet Networks," C. Hornig, April 1984
* RFC 1042, "A Standard for the Transmission of IP Datagrams over IEEE 802 Networks," J. Postel and J. Reynolds, February 1988
* RFC 2740, "OSPF for IPv6", R. Coltun, et al., December 1999
* [http://standards.ieee.org/getieee802/portfolio.html IEEE 802 Standards]