Network Working Group                                      Jeffrey Mogul
Request for Comments: 922                    Computer Science Department
                                                     Stanford University
                                                            October 1984

       BROADCASTING INTERNET DATAGRAMS IN THE PRESENCE OF SUBNETS


Status of this Memo

   We propose simple rules for broadcasting Internet datagrams on local
   networks that support broadcast, for addressing broadcasts, and for
   how gateways should handle them.

   This RFC suggests a proposed protocol for the ARPA-Internet
   community, and requests discussion and suggestions for improvements.
   Distribution of this memo is unlimited.

Acknowledgement(—ª)

1. Introduction

   The use of broadcasts, especially on high-speed local area networks,
   is a good base for many applications.  Since broadcasting is not
   covered in the basic IP specification [12], there is no agreed-upon
   way to do it, and so protocol designers have not made use of it. (The
   issue has been touched upon before, e.g. [6], but has not been the
   subject of a standard.)

   We consider here only the case of unreliable, unsequenced, possibly
   duplicated datagram broadcasts (for a discussion of TCP broadcasting,
   see [10].) Even though unreliable and limited in length, datagram
   broadcasts are quite useful [1].

   We assume that the data link layer of the local network supports
   efficient broadcasting.  Most common local area networks do support
   broadcast; for example, Ethernet [7, 5], ChaosNet [9], token ring
   networks [2], etc.

   We do not assume, however, that broadcasts are reliably delivered.
   (One might consider providing a reliable datagram broadcast protocol
   as a layer above IP.) It is quite expensive to guarantee delivery of
   broadcasts; instead, what we assume is that a host will receive most
   of the broadcasts that are sent.  This is important to avoid
   excessive use of broadcasts; since every host on the network devotes
   at least some effort to every broadcast, they are costly.
   When a datagram is broadcast, it imposes a cost on every host that
   hears it.  Therefore, broadcasting should not be used
   indiscriminately, but rather only when it is the best solution to a
   problem.

(—ª)

8. References

   1.   David Reeves Boggs.  Internet Broadcasting.  Ph.D. Th., Stanford
        University, January 1982.

   2.   D.D. Clark, K.T. Pogran, and D.P. Reed.  "An Introduction to
        Local Area Networks".  Proc. IEEE 66, 11, pp1497-1516,
        November 1978.

   3.   Yogan Kantilal Dalal.  Broadcast Protocols in Packet Switched
        Computer Networks.  Ph.D. Th., Stanford University, April 1977.

   4.   Yogan K. Dalal and Robert M. Metcalfe.  "Reverse Path Forwarding
        of Broadcast Packets".  Comm. ACM 21, 12, pp1040-1048,
        December 1978.

   5.   The Ethernet, A Local Area Network: Data Link Layer and Physical
        Layer Specifications.  Version 1.0, Digital Equipment
        Corporation, Intel, Xerox, September 1980.

   6.   Robert Gurwitz and Robert Hinden.  IP - Local Area Network
        Addressing Issues.  IEN-212, BBN, September 1982.

   7.   R.M. Metcalfe and D.R. Boggs.  "Ethernet: Distributed Packet
        Switching for Local Computer Networks".  Comm. ACM 19, 7,
        pp395-404, July 1976.  Also CSL-75-7, Xerox Palo Alto Research
        Center, reprinted in CSL-80-2.

   8.   Jeffrey Mogul.  Internet Subnets.  RFC-917, Stanford University,
        October 1984.

   9.   David A. Moon.  Chaosnet.  A.I. Memo 628, Massachusetts
        Institute of Technology Artificial Intelligence Laboratory,
        June 1981.

   10.  William W. Plummer.  Internet Broadcast Protocols.  IEN-10, BBN,
        March 1977.

   11.  David Plummer.  An Ethernet Address Resolution Protocol.
        RFC-826, Symbolics, September 1982.

   12.  Jon Postel.  Internet Protocol.  RFC-791, ISI, September 1981.

   13.  David W. Wall.  Mechanisms for Broadcast and Selective
        Broadcast.  Ph.D. Th., Stanford University, June 1980.

   14.  David W. Wall and Susan S. Owicki.  Center-based Broadcasting.
        Computer Systems Lab Technical Report TR189, Stanford
        University, June 1980.