Layer 3 - The network layer

The basic service of the network layer is to provide the transparent transfer of datagrams between the transport layers at the two nodes. This layer is also responsible for finding the right intermediate nodes that might be required to send data to the destination node, if the destination node is not on the same network as the source node. This layer also breaks down datagrams into smaller fragments, if the underlying datalink layer is not capable of handling the datagram that is offered to the network layer for transport on the network.

A fundamental concept in the OSI stack is that data should be passed to a higher layer at the receiving node as it was handed over to the lower layers by the transmitting peer. As an example, the TCP layer passes TCP segments to the IP layer, and the IP layer might use the services of the lower layers, leading to fragment packets on the way to the destination, but when the IP layer passes the data to the TCP layer at the receiving node, the data should be in the form of TCP segments that were handed down to the IP layer at the transmitting end. To ensure this transparent transfer of datagrams to the receiving node TCP layer, the network layer at the receiving node reassembles all the fragments of a single datagram before handing it over to the transport layer.

The OSI model describes both connection-oriented and connectionless modes of the OSI network layer.

Connection- oriented and connectionless modes are used to describe the readiness of the communicating nodes before the process of actual data transfer between the two nodes. In the connection-oriented mode, a connection is established between the source and the destination, and a path is defined along the network through which actual data transfer would happen. A telephone call is a typical example of this mode, where you cannot talk until a connection has been established between the calling number and the called number.

In the connectionless mode of data transfer, the transmitting node just sends the data on the network without first establishing a connection, or verifying whether the receiving end is ready to accept data, or even if the receiving node is up or not. In this mode, there is no connection or path established between the source and the destination, and data generally flows in a hop by hop manner, with a decision being taken on the best path towards the destination at every hop. Since, data is sent without any validation of the receiving node status, there is no acknowledgement of data in a connectionless mode of data transfer. This is unlike the connection-oriented mode, where the path is defined the moment a connection is established, and all data flows along that path, with the data transfer being acknowledged between the two communicating nodes.

Since data packets in a connection-oriented mode follow a fixed path to the destination, the packets arrive in the same sequence at the receiver in which they were transmitted. On the other hand, packets in the case of a connectionless network might reach the receiver out of sequence if the packets are routed on different links on the network, as decisions are taken at every hop.

The OSI standard defined the network layer to provide both modes. However, most of the services were implemented in practice as the connectionless mode at layer 3, and the connection-oriented aspects were left to layer 4. We will discuss this further during our discussion on TCP/IP.

Some of the protocols that operate at the network layer are AppleTalk, DDP, IP, IPX, CLNP, IS-IS, and so on.