Request for Comments 791

Request for Comments 791
Title	Request for Comments 791
Series	RFC
Number	791
Status	Historic
Author	Jon Postel
Organization	Internet Engineering Task Force
Pub date	1981-09
Pages	24

Request for Comments 791

Introduction

Request for Comments 791 is a standards-track specification authored by Jon Postel and published under the aegis of the Internet Engineering Task Force and the Internet Assigned Numbers Authority in September 1981, defining the IPv4 datagram format used on the ARPANET, the early Internet backbone, and influencing later work at the National Science Foundation and within the Defense Advanced Research Projects Agency. The document codified packet header fields and processing rules used by implementations such as those developed at University of California, Los Angeles, Stanford University, and the Massachusetts Institute of Technology, and it informed protocol engineering activities at Xerox PARC, Bolt Beranek and Newman, and commercial vendors like DEC and IBM.

Background and Purpose

The specification originated during a period of rapid protocol design involving contributors from SRI International, BBN Technologies, RAND Corporation, Carnegie Mellon University, and researchers associated with the Internet Research Task Force. RFC 791 sought to replace and integrate prior efforts exemplified by earlier RFCs and to provide a common packet format to support experiments with routing algorithms such as those implemented by Routing Information Protocol proponents and research at MITRE Corporation. The purpose was to standardize how hosts and gateways exchange datagrams so that heterogeneous systems at institutions like NASA Ames Research Center and Bell Labs could interoperate, enabling deployments across networks overseen by bodies including the Department of Defense and academic consortia such as NSFNet.

Specification Overview

The document specifies the IPv4 datagram header fields, including version, header length, differentiated services predecessors influenced by work at Xerox PARC, type of service concepts evaluated at Bell Labs, total length, identification, flags, fragment offset, time to live, protocol, header checksum, source address, and destination address, which together define packet processing for routers implemented at BBN Technologies and hosts at UCLA. It defines fragmentation and reassembly behavior used by gateways developed at SRI International and influences the design of encapsulation methods considered by teams at IBM and DEC. The specification enumerates protocol numbers (for example, those later used by Transmission Control Protocol and User Datagram Protocol implementations developed at University of California, Berkeley) and prescribes checksum calculation methods that echo practices from ARPANET engineering. The formatting, field semantics, and error handling rules informed subsequent standardization efforts at the Internet Engineering Task Force and operational practices at network operators such as ANSP and research deployments at Lawrence Berkeley National Laboratory.

Security Considerations

RFC 791 discusses error handling and header checksum mechanisms but predates modern threat models analyzed by organizations like CERT Coordination Center and the National Institute of Standards and Technology, and thus it does not address later concerns such as source address spoofing exploited in attacks documented by CAIDA and mitigations proposed in subsequent standards like ingress filtering promoted by IETF working groups. The original specification’s fragmentation rules created opportunities exploited in later vulnerabilities observed by operators at Cloudflare and researchers at University of Michigan, prompting later advisories from US-CERT and protocol updates from the IETF and contributors such as Eric Rescorla and Van Jacobson.

Implementation and Deployment

Implementations of the RFC’s packet format were produced in kernel stacks at institutions including UCLA, UC Berkeley, MIT, and commercial systems from DEC and IBM, and were critical to the deployment of production networks like ARPANET, CSNet, and later NSFNet. Gateway and router implementations by BBN Technologies and experimental stacks at Carnegie Mellon University exercised fragmentation and TTL behaviors, while measurements by researchers at Lawrence Livermore National Laboratory and Los Alamos National Laboratory helped tune operational parameters. Vendor interoperability testing and conformance work occurred in venues such as IETF meetings and workshops hosted by Stanford University and Xerox PARC, and deployment experiences fed back into educational courses at MIT and UC Berkeley that trained generations of network engineers.

History and Revisions

Following its 1981 publication, the specification influenced later RFCs and standards bodies including the IETF working groups responsible for Internet Protocol evolution and updates such as RFCs addressing fragmentation, options, and security hardening contributed by authors at SRI International, BBN Technologies, and IBM. Work at NSFNet, commercial adoption by vendors like Sun Microsystems, and operational lessons from backbone operators led to refinements captured in later documents and errata tracked by the Internet Assigned Numbers Authority and discussed at IETF meetings attended by contributors from DARPA and academic institutions like Carnegie Mellon University and Stanford University. The legacy of this specification persists in contemporary IPv4 deployments and in the archival collections maintained by institutions such as UCLA and MIT.

Category:Internet Standards