RFC 791 — Internet Protocol (excerpt)

This document describes the Internet Protocol (IP), a protocol for use in interconnected systems of packet-switched computer communication networks. The internet protocol is specifically limited in scope to provide the functions necessary to deliver a package of bits (an internet datagram) from a source to a destination over an interconnected system of networks.

The internet protocol implements two basic functions: addressing and fragmentation. The internet modules use the addresses carried in the internet header to transmit internet datagrams toward their destinations. The selection of a path for transmission is called routing.

A datagram may have to traverse networks whose maximum packet size is smaller than the datagram. To handle this, the internet protocol provides a fragmentation mechanism. Fragments of an internet datagram are submitted to the next module independently. Fragments may be further fragmented. Fragments of a datagram are transmitted independently and may take different routes to the destination.

The internet protocol treats each internet datagram as an independent entity unrelated to any other internet datagram. There are no connections or logical circuits (virtual or otherwise). The internet protocol uses four key mechanisms in providing its service: Type of Service, Time to Live, Options, and Header Checksum.

Type of Service is used to indicate the quality of the service desired. Time to Live is an indication of an upper bound on the lifetime of an internet datagram. If the Time to Live is exceeded, the datagram is destroyed. The Options provide for control functions needed or useful in some situations but unnecessary for the most common communications. The Header Checksum provides a verification that the information used in processing internet datagrams has been transmitted correctly.

The official assignment of internet addresses is administered by the Internet Assigned Numbers Authority. Addresses are 32-bit quantities. The model of operation is that an internet module resides in each host engaged in internet communication and in each gateway that interconnects networks.

This excerpt is included as a public-domain retrieval document for a RAG demo. It is not a complete RFC.