The IP protocol provides a connectionless packet delivery service. Delivery is not guaranteed; the protocol is unreliable, although a `best effort' is made to deliver packets. Still, packets can be lost, duplicated, corrupted, or arrive out of order, but IP does not care about such conditions. It is the responsibility of higher-layer protocols to provide a reliable, in-sequence data stream.
IP packets are called Internet datagrams . A datagram contains a header (the IP header ), which specifies the source and destination addresses, the packet length, a header checksum, and other information, possibly including some IP options for network control and debugging. The IP protocol defines the format of the packets, how they should be routed, and under what conditions they can be discarded.
Routing is done on each gateway by means of routing tables. These tables may be set up statically, or may be determined on-the-fly by means of special routing protocols1.1. The routing tables specify the next hop gateways associated with different network addresses; that is, which gateway a packet should be passed to in order to bring it closer to its destination. When the routing tables are dynamic, the route that packets take between a source and destination may vary over time according to traffic levels, availability and other factors. (This is the main reason why IP datagrams can arrive out of order.)
IP provides a limited facility for error reporting and problem diagnosis, by means of the Internet Control Message Protocol , or ICMP. ICMP messages are themselves encapsulated in IP packets and delivered using IP, so ICMP is also unreliable. Apart from error reporting, ICMP is also used in the well-known network diagnostic utilities ping and traceroute .