Link layer

Incomputer networking,thelink layeris the lowestlayerin theInternet protocol suite,the networking architecture of theInternet.The link layer is the group of methods andcommunications protocolsconfined to the link that a host is physically connected to. The link is the physical and logical network component used to interconnecthostsornodesin the network and alink protocolis a suite of methods and standards that operate only between adjacent network nodes of anetwork segment.

Despite the different semantics of layering between theInternet protocol suiteandOSI model,the link layer is sometimes described as a combination of the OSI'sdata link layer(layer 2) andphysical layer(layer 1).

The link layer is described inRFC1122andRFC1123.RFC 1122 considers local area network protocols such asEthernetand otherIEEE 802networks (e.g.Wi-Fi), and framing protocols such asPoint-to-Point Protocol(PPP) to belong to the link layer.

Definition in standards and textbooks

Local area networking standards such asEthernetandIEEE 802.3specifications use terminology from the seven-layerOSI modelrather 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 physicaldata transmissionand 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 termdata link layerinstead of link layer in a modified description of layering. In the predecessor to the TCP/IP model, theARPAnet Reference Model(RFC 908, 1982), aspects of the link layer are referred to by several poorly defined terms, such asnetwork-access layer,network-access protocol,as well asnetwork layer,while the next higher layer is calledinternetwork layer.In some modern textbooks,network-interface layer,host-to-network layerandnetwork-access layeroccur as synonyms either to the link layer or the data link layer, often including the physical layer.

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 are not routed to other networks. The link layer includes the protocols that define communication between local (on-link) network nodes which fulfill the purpose of maintaining link states between the local nodes, such as the local network topology, and that usually use protocols that are based on the framing of packets specific to the link types.

The core protocols specified by theInternet Engineering Task Force(IETF) in this layer are theAddress Resolution Protocol(ARP), theReverse Address Resolution Protocol(RARP), and theNeighbor Discovery Protocol(NDP), which is a facility delivering similar functionality as ARP forIPv6.

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 thephysical layerin theOpen Systems Interconnection(OSI) protocolstack. Although they are congruent to some degree in technical coverage of protocols, they are not 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 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, 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, and discover routers and neighboring hosts, i.e. functions that go well beyond network access.

IETF standards

• R. Coltun; D. Ferguson; J. Moy (July 2008). A. Lindem (ed.).OSPF for IPv6.Network Working Group.doi:10.17487/RFC5340.RFC5340.Proposed Standard.ObsoletesRFC2740.Updated byRFC6845,6860,8362,7503and9454
• R. Braden,ed. (October 1989).Requirements for Internet Hosts -- Communication Layers.Network Working Group.doi:10.17487/RFC1122.STD 3.RFC1122.Internet Standard 3.Updated byRFC1349,4379,5884,6093,6298,6633,6864,8029and9293.
• R. Braden,ed. (October 1989).Requirements for Internet Hosts -- Application and Support.Network Working Group.doi:10.17487/RFC1123.STD 3.RFC1123.Internet Standard 3.Updated byRFC1349,2181,5321,5966and7766.
• J. Postel;J. Reynolds(February 1988).A Standard for the Transmission of IP Datagrams over IEEE 802 Networks.Network Working Group.doi:10.17487/RFC1042.STD 43.RFC1042.Internet Standard 43.ObsoletesRFC948.
• C. Hornig (April 1984).A Standard for the Transmission of IP Datagrams over Ethernet Networks.Network Working Group.doi:10.17487/RFC0894.STD 41.RFC894.Internet Standard 41.
• S. J. Leffler; M. J. Karels (April 1984).Trailer Encapsulations.Network Working Group.doi:10.17487/RFC0893.RFC893.Status Unknown.
• David C. Plummer (November 1982).An Ethernet Address Resolution Protocol.Network Working Group.doi:10.17487/RFC0826.STD 37.RFC826.Internet Standard 37.Updated byRFC5227and5494.

See also

• Carrier-sense multiple access
• Carrier-sense multiple access with collision detection
• Network interface layer security

External links

• IEEE 802 Standards