Link Layer (Internet Protocol Suite)

The link layer is the lowest layer in the Internet Protocol Suite (commonly known as "TCP/IP"), the networking architecture of the Internet (RFC 1122, RFC 1123). It is the group of methods or protocols that only operate on a host's link. The link is the physical and logical network component used to interconnect hosts or nodes in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a Local area network segment or a wide area network connection.
Despite the different semantics of layering in TCP/IP and OSI, the link layer is often described as a combination of the data link layer (layer 2) and the physical layer (layer 1) in the Open Systems Interconnection (OSI) protocol stack. However, TCP/IP's layers are descriptions of operating scopes (application, host-to-host, network, link) and not detailed prescriptions of operating procedures, data semantics, or networking technologies.

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) 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

Functions supported

The link layer includes the following functionality:

Logical link-local networking methods
Encapsulation of IP packets into frames
Frame synchronization
Error detection with removal of erroneous packets (Used in LANs and point-to-point fiber optical cables)
Logical link control (LLC) sublayer: (Used in modem protocols and wireless protocols)
Node-to-node error detection and automatic repeat request of erroneous packets
Node-to-node flow control
Forward error correction (however much more common at the physical layer)
Media access control (MAC) sublayer:
Multiple access protocols, for example with collision detection or avoidance
Physical addressing (MAC addressing)
LAN switching (packet switching) including MAC filtering and spanning tree protocol
Store-and-forward switching or cut-through switching
Data packet queueing or scheduling algorithms
Help protocols not encapsulated in IP packets:
Some routing protocols
IP address to/from physical address resolution protocols
Physical networking aspects: (It is disputable if and to which extent these are included)
Bit-by-bit or symbol-by-symbol delivery
Net bit rate
Digital modulation
Line coding
Carrier sense and collision detection utilized by some level 2 multiple-access protocols
Circuit switching, establishing circuit switched connections
Multiplexing
Forward error correction codes
Serial or parallel communication
Bit synchronization
Start-stop signalling and flow control in asynchronous serial communication
Signal strength (voltage and power levels), impedance
Forward error correction, bit-interleaving and other channel coding
Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals
Mechanical specification of electrical connectors and cables, for example maximum cable length
Electrical specification of transmission line signal level and impedance
Radio interface, including electromagnetic spectrum frequency allocation and specification of signal  strength, analog bandwidth, etc.
Specifications for IR over optical fiber or a wireless IR communication link