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# 1.2.4.5 IPv4 Module

IP is the workhorse protocol of the TCP/IP protocol suite. All TCP, UDP, and ICMP data gets transmitted as IP datagrams. IP provides an unreliable, connectionless datagram delivery service.

IP provides a best effort service. When something goes wrong, such as a router temporarily running out of buffers, IP has a simple error handling algorithm: throw away the datagram and try to send an ICMP message back to the source. Any required reliability must be provided by the upper layers (e.g., TCP).

The term connectionless means that IP does not maintain any state information about successive datagrams. Each datagram is handled independently from all other datagrams. This also means that IP datagrams can get delivered out of order. If a source sends two consecutive datagrams (first A, and then B) to the same destination, each is routed independently and can take different routes, with B arriving before A.