Høst 09

Application Layer
The application layer is where the applications reside, which is the end systems of a network. It is this layer that is closest to the user and offers a service in the form of an API. Communication between two different end systems in the application layer is done through sockets. The sockets are again bound to the host address and a port to identify the application (or process) that is communicating.

Transport Layer
The transport provides process-to-process communication. The imperative services provided by this layer is integrity checking (e.g. check summing) and multiplexing/de- multiplexing (translation process-to-process to host-to-host and vice versa respectively). Other services such as reliable data transfer, in-order packet delivery, and security can also be given here. The most common protocols are TCP and UDP.

Network Layer
The network layer is responsible for routing and forwarding on the network and host-to-host delivery of datagrams. Many of the same services as the transport layer can be given here, but for the most common protocol, IP, they are not. IP is described as a best-effort protocol; No guarantees.

Link Layer
The link layers main responsibility is to link nodes on the network together. This is done by providing device-to-device (or node-to-node) communication. The services provided here are framing, half- and full-duplex (one- and two-way communication respectively), flow control, and/or error detection. General protocols include Ethernet, Wi-Fi, and such, and are addresses by the devices MAC-address.

Physical Layer
Physical Layer is the lowest layer in the Internet Protocol stack. It transfers the frames created in the link layer, bit-by-bit, between the nodes. The physical transfer is in example done through a twisted-copper cable, fiber, or air.