RFC 947 (rfc947) - Page 2 of 5

Multi-network broadcasting within the Internet

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.

4. Alternatives to the Broadcast Repeater

   We explored a few alternatives before deciding on our technique to
   forward broadcast messages.  One of these methods was to put
   additional functions into the Internet gateways.  Gateways could
   listen at the link-level for broadcast packets and relay the packets
   to one or more gateways on distant LANs.  These gateways could then
   transmit the same packet onto their networks using the local
   network's link-level broadcast capability, if one is available.  All
   gateways participating in this scheme would have to maintain tables


Lebowitz & Mankins