Internet Guide Logo

Link Layer of the Internet protocol suite (TCP/IP)

Last Edit: 11/07/17

The Internet is a network of networks that interconnect using the protocols of the Internet protocol suite (tcp/ip). The Internet protocol suite features four layers, and the link layer is the lowest of these four layers (application > transport > internet > link). The lower the layer, the closer the layer is to the physical transmission of data. The lowest three layers of the Internet protocol suite have been termed the 'communication protocol layers' (rfc-1122): highlighting their role in transporting data (in small blocks termed datagrams and packets) from host to host, whereas the application layer are the protocols that provide the services for the networks (email, web, voice communication etc).

The protocols of the link layer are local in their scope: they are designed to handle data communication for the specific link a host is connected to. The internet layer, and the internet protocol (ip), is the layer with a broader scope, facilitating the transportation of data across networks. After application layer data is encapsulated into Internet Protocol (IP) datagrams (packets), these IP datagrams are then encapsulated into a link layer frame for each local 'hop' as it is transported across networks. While the IP datagrams remains the same across the networks it is transported, it is 'wrapped' and 'unwrapped' for each local data link: this is why the link layer is defined as local.

The link layer features many types of communication protocols, some of which are listed below (the critical protocols defined by IETF are in bold). RFC 1122 states there are a wide range of link layers protocols that are needed to correspond to the many different types of networks that comprise the Internet.

  1. Address Resolution Protocol (ARP)
  2. Media Access Control (MAC)
    1. Digital Subscriber Line
    2. Ethernet
    3. Fiber Distributed Data Interface (FDDI)
    4. Integrated Services for Digital Network (ISDN)
  3. Layer 2 Tunneling Protocol (L2TP)
  4. Neighbor Discovery Protocol (NDP)
  5. Open Shortest Path First (OSPF)
  6. Point-to-Point Protocol (PPP)
  7. Reverse Address Resolution Protocol (RARP)

Due to link layer protocols being closely linked to electronics, many of the protocols of the Internet's link layer are also Institute of Electrical and Electronics Engineers (IEEE) standards.