We will deal with the link and physical layers together, since they are usually designed in close coordination. We will only touch upon physical layer aspects in this course.
What are the main function of the link layer? To transport an IP datagram between two IP hops. Links can be between end host and first hop router, or between two IP hops in the core. Different link layer technologies are united by IP. Link layers run on many different physical media. Examples: Ethernet over copper or fiber, wireless, long distance fiber links.
The link layer is implemented in software + hardware. Some L2 functionality is in software (in device drivers in the OS for example), rest of the L2 and PHY functionality is usually in hardawre. The link layer hardware consists of the network interface card (NIC) at the end hosts, and intermediate L2 network elements like switches.
Link layers forwarding happens using Layer-2 or MAC addresses, which are 48 bits long. MAC addresses are unique across all adapters (a manufacturer gets a prefix, and ensures uniqueness within the adapters of the organization). A special broadcast MAC address is the address with all 1s - ff:ff:ff:ff:ff:ff. Link layer headers have the source and destination MAC addresses. The network adapter filters out non-broadcast frames that are not destined to its MAC address, except when configured to be in a special “promiscuous” mode.
When forwarding an IP datagram, the IP layer gives the next hop IP address. The link layer then maps the next hop IP address to the MAC address of the next hop IP router, places this destination MAC in the link layer header and forwards the packet. How does a node know the MAC address of the first hop IP router? Using Address Resolution Protocol (ARP).
How does ARP work? When A is communicating with a host B (say, its first hop IP router) for the first time, A sends a broadcast packet requesting to know the MAC address of B. Then the destination B replies with its MAC address to A. B also stores a mapping between A’s IP and MAC, as it may need to communicate with A in the future. All other nodes which have mappings for A and B also refresh them. The ARP mappings are valid for a certain period and timeout. The ARP mappings are stored as part of the forwarding table itself - map from prefix to next hop IP, interface / link, and MAC address of next hop.
The notion of a “broadcast domain”: a set of nodes you can reach via a link layer broadcast. It is required that your next IP hop is within your broadcast domain, so that it can reply to your ARP query and enable communication.
Note that one IP “link” need not be one physical link. There can be several physical nodes along this layer-2 path that forward the packet towards the destination MAC address / next IP hop.
Example: consider a simple two-hop network A to B to C. When A has a datagram to send to C, the source and destination IP addresses in the IP datagram are always that of A and C. Then A learns that the next hop IP is B, figures out the MAC address of B via ARP, and places the MAC addresses of A and B as source and destination MAC addresses in the link layer header. When packet reaches B, B looks up its forwarding table, realizes that the next hop is C, and modifies the link layer header to have MAC addresses of B and C as source and destination MAC addresses (B learns of C’s MAC address via ARP again). This is how the IP datagram finally reaches C. Note that the source and destination IP in the packet is always that of A and C, but the source and destination MACs keep changing with every IP hop.
What are the main functions of the link layer for every transmission?
Framing. Place an IP datagram (or any other network protocol packet) in a frame. Framing usually involves adding a special start symbol or preamble to denote start of frame, and an optional end-of-frame marker at the end of the frame. A frame is a unit of transmission at the link layer. A network datagram may or may not fit in one link layer frame. For example, ethernet frames are big enough to hold TCP segments if MSS is sized properly. On the other hand, some optical links use much smaller frame sizes, so need to partition datagrams.
What if preamble occurs within the data part of a frame? Either escape it (break the pattern with a special symbol) or use fixed size frames, so you know where to look for preambles. Different link layer protocols use different frame formats. The only requirement is that an IP datagram should be transported.
Error detection, correction, recovery. Link layers use techniques like parity bits, checksums, or Cyclic Redundancy Check (CRC) to detect bit errors in received frames. Ocassionally, error correcting codes can also be used to recover from bit errors. More wired link layers use only error detection (e.g., Ethernet uses 32 bit CRC). Wireless link/physical layers often use error correction because bit errors are more prominent.
Some link layers also retransmit frames (stop-and-wait or sliding window) to recover from bit errors. Ethernet does not do retransmissions at the link layer, as packet errors are rare.
Link access: Some link layers like point-to-point have only one sender at a time on a link. However, some link layers like Ethernet and wireless are shared broadcast media, and require protocols to arbitrate access to the medium.