Internet Layer - Purpose

Purpose

The internet layer has three (3) basic functions:

1) For outgoing packets, select the next-hop host (gateway) and transmit the packet to this host by passing it to the appropriate link layer implementation;

2) For incoming packets, capture packets and pass the packet payload up to the appropriate transport-layer protocol, if appropriate.

3) In addition it provides error detection and diagnostic capability.

In Version 4 of the Internet Protocol (IPv4), during both transmit and receive operations, IP is capable of automatic or intentional fragmentation or defragmentation of packets, based, for example, on the maximum transmission unit (MTU) of link elements. However, this feature has been dropped in IPv6, as the communications end points, the hosts, now have to perform path MTU discovery and assure that end-to-end transmissions don't exceed the maximum discovered.

In its operation, the internet layer is not responsible for reliable transmission. It provides only an unreliable service, and "best effort" delivery. This means that the network makes no guarantees about packets' proper arrival (see also Internet Protocol#Reliability). This was an important design principle and change from the previous protocols used on the early ARPANET. Since packet delivery across diverse networks is inherently an unreliable and failure-prone operation, the burden of providing reliability was placed with the end points of a communication path, i.e., the hosts, rather than on the network. This is one of the reasons of the resiliency of the Internet against individual link failures and its proven scalability.

The function of providing reliability of service is the duty of higher level protocols, such as the Transmission Control Protocol (TCP) in the transport layer.

In IPv4 (not IPv6), a checksum is used to protect the header of each datagram. The checksum ensures that the information in a received header is accurate, however, IP does not attempt to detect errors that may have occurred to the data in each packet.