RFC 791

RFC 791. Published in September 1981 by the Internet Engineering Task Force, this document defines the Internet Protocol (IP), the fundamental communications protocol for the Internet protocol suite. It establishes the core rules for addressing hosts and routing datagrams across interconnected networks, forming the principal component of the Internet layer. This specification, developed primarily by the Information Sciences Institute at the University of Southern California, succeeded RFC 760 and became a cornerstone of modern digital networking.

Overview

RFC 791 was created as part of the DARPA research program to standardize the protocols used on the ARPANET, the precursor to the global Internet. The document formally describes the Internet Protocol version 4 (IPv4), which provides a best-effort, connectionless datagram delivery service. This design was crucial for creating a resilient, decentralized network capable of surviving partial failures, a key requirement during the Cold War. The protocol's architecture allows diverse physical networks, from Ethernet to X.25 networks, to be interconnected into a single logical network.

Technical specifications

The protocol operates at the Internet layer, sitting above various link layer technologies like Token Ring and below transport protocols such as the Transmission Control Protocol. Its primary functions include defining IP addresses, performing IP fragmentation and reassembly, and handling the Time to Live (TTL) field to prevent indefinite packet looping. The specification details algorithms for checksum calculation to detect header corruption and rules for processing Type of Service fields for rudimentary quality of service. It was designed to work in conjunction with other core standards like RFC 792 for the Internet Control Message Protocol.

Packet structure

An IP datagram header is at least 20 octets long and contains several critical fields. The Version field identifies IPv4, while the Internet Header Length specifies the header size. Key fields include the Total Length of the datagram, a 16-bit Identification number for fragmentation, and Flags controlling fragmentation. The Fragment Offset field enables reassembly of fragmented packets, and the Header Checksum provides error detection. The header also contains the source and destination IP address fields, each 32 bits long, defining the logical address of the sending and receiving hosts.

Addressing and routing

The protocol uses a 32-bit IP address space, typically represented in dotted-decimal notation, to uniquely identify interfaces on a network. Addressing supports a classful network design, dividing the space into classes like Class A for large networks and Class C for smaller ones. Routing is based on these addresses, where gateways (early routers) examine the destination address and forward datagrams using local routing tables. The design facilitates internetworking between heterogeneous networks managed by different organizations, such as the National Science Foundation Network and various university networks.

Legacy and impact

RFC 791 and the IPv4 protocol it defined became the foundation for the explosive growth of the global Internet throughout the 1980s and 1990s. Its widespread adoption by companies like IBM, Digital Equipment Corporation, and later Microsoft cemented it as a universal standard. The exhaustion of the IPv4 address space led to the development of its successor, IPv6, defined in RFC 2460. Despite this, IPv4 remains extensively used today, and the architectural principles from RFC 791 continue to influence modern network design, including protocols developed by the Internet Engineering Task Force and implemented in operating systems like BSD and Microsoft Windows.

Category:Internet Standards Category:Internet protocols Category:Request for Comments