The International Standards Organization (ISO) has defined a standard model for describing communications across a network, called the OSI model, for Open Systems Interconnect (reference model).

The OSI model is seven layered monster. It does not have to be taken literally - it might not be natural to separate all of these parts in every single program - but it is useful as a way of discussing the logically distinct parts of network communication. The layers are described as follows.

At the lowest level, the sending of data between two machine takes place by manipulating voltages along wires. This means we need a device driver for the signaller, and something to receive the data at the other end - a way of converting the signals into bytes; then we need a way of structuring the data so that they make sense. Each of these elements is achieved by a different level of abstraction.
 

1. Physical layer
   This is the problem of sending a signal along a wire, amplifying it if it gets weak, removing noise etc. If the type of cable changes (we might want to reflect signals off a satellite or use fibre optics) we need to convert one kind of signal into another. Each type of transmission might have its own accepted ways of sending data (i.e. protocols).
   
   
2. Data link layer
   This is a layer of checking which makes sure that what as sent from one end of a cable to the other actually arrived. This is sometimes called handshaking.
   
   
3. Network layer
   This is the layer of software which remembers which machines are talking to other machines. It establishes connections and handles the delivery of data by manipulating the physical layer. The network layer needs to know something about addresses - i.e. where the data are going, since data might flow along many cables and conections to arrive where they are going.
   
   
4. Transport layer
   The transport layer builds `packets' or `datagrams' so that the network layer knows what is data and how to get the data to their destination. Because many machines could be talking on the same network all at the same time, data are broken up into short `bursts'. Only one machine can talk over a cable at a time so we must have sharing. It is easy to share if the signals are sent in short bursts. This is analogous to the sharing of CPU time by use of time-slices.
   
   
5. Session layer
   This is the part of a host's operating system which helps a user program to set up a connection. This is typically done with sockets or the RPC.
   
   
6. Presentation layer
   How are the data to be sent by the sender and interpreted by the receiver, so that there is no doubt about their contents? This is the role played by the external data representation (XDR) in the RPC system.
   
   
7. Application layer
   The program which wants to send data.

OSPF         Open Shortest Path First

It is an Internet Engineering Task Force (IETF) standard link-state routing protocol used for routing IP. OSPF is a more sophisticated routing protocol than RIP, offering faster routing algorithm convergence. The service's OSPF implementation is a result of collaborative effort between Microsoft and Bay Networks, a leading provider of internetworking systems.

Developed in response to the inability of RIP to serve large, heterogeneous internetworks, OSPF is a link-state protocol based on the Shortest Path First (SPF) algorithm. This algorithm computes the shortest path between one source node and the other nodes in the network. Various industry sources indicate that about 35% to 40% of the routed networks in place today make use of OSPF and this number is growing.

Instead of exchanging distances to destinations like RIP routers do, OSPF routers maintain a "map" of the network that is updated after any change in the network topology. This map, called the link-state database, is used to compute the network routes, which must be computed again after any change in the topology. From this computation, the router derives the next hop for the destination, that is, the next router to which the data should be sent and the link that should be used for reaching this next router. Network changes are propagated or flooded across the entire network to ensure that each copy of the database is accurate at all times.

Because OSPF routers keep an overview of the network from the perspective of any router, some of the problems that are inherent in RIP (such as loops) are eliminated.