Link Layer - Relation To OSI Model

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 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 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 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.