Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 28
Connectionless, Best–Effort Delivery Service

The IP layer provides the entry into the delivery system used to transport data across the Internet. Usually, when anyone hears the name IP, he or she automatically thinks of the networks connected together through devices commonly known as routers, which connect multiple subnetworks together. It is true the IP performs these tasks, but the IP protocol performs many other tasks, as mentioned previously. The IP protocol runs in all the participating network stations that are attached to subnetworks so that they may submit their packets to routers or directly to other devices on the same network. It resides between the datalink layer and the transport layer. IP also provides for connectionless data delivery between nodes on an IP network.

The primary goal of IP is to provide the basic algorithm for transfer of data to and from a network. In order to achieve this, it implements two functions: addressing and fragmentation. It provides a connectionless delivery service for the upper–layer protocols. This means that IP does not set up a session (a virtual link) between the transmitting station and the receiving station prior to submitting the data to the receiving station. It encapsulates the data handed to it and delivers it on a best–effort basis. IP does not inform the sender or receiver of the status of the packet; it merely attempts to deliver the packet and will not make up for the faults encountered in this attempt. This means that if the datalink fails or incurs a recoverable error, the IP layer will not inform anyone. It tried to deliver (addressed) a message and failed. It is up to the upper–layer protocols (TCP, or even the application itself) to perform error recovery. For example, if your application is using TCP as its transport layer protocol, TCP will time–out for that transmission and will resend the data. If the application is using UDP as its transport, then it is up to the application to perform error recovery procedures.

IP submits a properly formatted data packet to the destination station and does not expect a status response. Because IP is a connectionless protocol, IP may receive and deliver the data (data sent to the transport layer in the receiving station) in the wrong order from which it was sent, or it may duplicate the data. Again, it is up to the higher–layer protocols (layer 4 and above) to provide error recovery procedures. IP is part of the network delivery system. It accepts data and formats it for transmission to the datalink layer. (Remember, the datalink layer provides the access methods to transmit and receive data from the attached cable plant.) IP also retrieves data from the datalink and presents it to the requesting upper layer.