Multi-network broadcasting within the Internet
RFC 947

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Network Working Group                                       Ken Lebowitz
Request for Comments: 947                                  David Mankins
                                                        BBN Laboratories
                                                               June 1985

             Multi-network Broadcasting within the Internet

1. Status of this Memo

   This RFC describes the extension of a network's broadcast domain to
   include more than one physical network through the use of a broadcast
   packet repeater.

   The following paper will present the problem of multi-network
   broadcasting and our motivation for solving this problem which is in
   the context of developing a distributed operating system.  We discuss
   different solutions to extending a broadcast domain and why we chose
   the one that has been implemented.  In addition, there is information
   on the implementation itself and some notes on its performance.

   It is hoped that the ideas presented here will help people in the
   Internet who have applications which make use of broadcasting and
   have come up against the limitation of only being able to broadcast
   within a single network.

   The information presented here is accurate as of the date of
   publication but specific details, particularly those regarding our
   implementation, may change in the future.  Distribution of this memo
   is unlimited.

2. The Problem

   Communication between hosts on separate networks has been addressed
   largely through the use of Internet protocols and gateways. One
   aspect of internetwork communication that hasn't been solved in the
   Internet is extending broadcasting to encompass two or more networks.
   Broadcasting is an efficient way to send information to many hosts
   while only having to transmit a single packet.  Many of the current
   local area network (LAN) architectures directly support a broadcast
   mechanism.  Unfortunately, this broadcast mechanism has a shortcoming
   when it is used in networking environments which include multiple
   LANs connected by gateways such as in the DARPA Internet.  This
   shortcoming is that broadcasted packets are only received by hosts on
   the physical network on which the packet was broadcast.  As a result,
   any application which takes advantage of LAN broadcasting can only
   broadcast to those hosts on its physical network.

   We took advantage of broadcasting in developing the Cronus
   Distributed Operating System [1].  Cronus provides services and
   communication to processes distributed among a variety of different

Lebowitz & Mankins                                              [Page 1]



RFC 947                                                        June 1985
Multi-network Broadcasting within the Internet

   types of computer systems.  Cronus is built around logical clusters
   of hosts connected to one or more high-speed LANs.  Communication in
   Cronus is built upon the TCP and UDP protocols.  Cronus makes use of
   broadcasting for dynamically locating resources on other hosts and
   collecting status information from a collection of servers.  Since
   Cronus's broadcast capabilities are not intended to be limited to the
   boundaries of a single LAN, we needed to find some way to extend our
   broadcasting domain to include hosts on distant LANs in order to
   experiment with clusters that span several physical networks.  Cronus
   predominantly uses broadcasting to communicate with a subset of the
   hosts that actually receive the broadcasted message.  A multicast
   mechanism would be more appropriate, but was unavailable in some of
   our network implementations, so we chose broadcast for the initial
   implementation of Cronus utilities.

3. Our Solution

   The technique we chose to experiment with the multi-network
   broadcasting problem can be described as a "broadcast repeater".  A
   broadcast repeater is a mechanism which transparently relays
   broadcast packets from one LAN to another, and may also forward
   broadcast packets to hosts on a network which doesn't support
   broadcasting at the link-level.  This mechanism provides flexibility
   while still taking advantage of the convenience of LAN broadcasts.

   Our broadcast repeater is a process on a network host which listens
   for broadcast packets.  These packets are picked up and
   retransmitted, using a simple repeater-to-repeater protocol, to one
   or more repeaters that are connected to distant LANs.  The repeater
   on the receiving end will rebroadcast the packet on its LAN,
   retaining the original packet's source address.  The broadcast
   repeater can be made very intelligent in its selection of messages to
   be forwarded.  We currently have the repeater forward only broadcast
   messages sent using the UDP ports used by Cronus, but messages may be
   selected using any field in the UDP or IP headers, or all IP-level
   broadcast messages may be forwarded.