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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

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 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

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 of all other gateways which are to receive broadcasts.  If the
 recipient gateway was serving a network without a capacity to
 broadcast it could forward the messages directly to one or more
 designated hosts on its network but, again, it would require that
 tables be kept in the gateway.  Putting this sort of function into
 gateways was rejected for a number of reasons: (a) it would require
 extensions to the gateway control protocol to allow updating the
 lists gateways would have to maintain, (b) since not all messages
 (e.g., LAN address- resolution messages) need be forwarded, the need
 to control forwarding should be under the control of higher levels of
 the protocol than may be available to the gateways, (c) Cronus could
 be put into environments where the gateways may be provided by
 alternative vendors who may not implement broadcast propagation, (d)
 as a part of the underlying network, gateways are likely to be
 controlled by a different agency from that controlling the
 configuration of a Cronus system, adding bureaucratic complexity to
 reconfiguration.

 Another idea which was rejected was to put broadcast functionality
 into the Cronus kernel.  The Cronus kernel is a process which runs on
 each host participating in Cronus, and has the task of routing all
 messages passed between Cronus processes.  The Cronus kernel is the
 only program in the Cronus system which directly uses broadcast
 capability (other parts of Cronus communicate using mechanisms
 provided by the kernel).  We could either entirely remove the Cronus
 kernel's dependence on broadcast, or add a mechanism for emulating
 broadcast using serially-transmitted messages when the underlying
 network does not provide a broadcast facility itself.  Either
 solution requires all Cronus kernel processes to know the addresses
 of all other participants in a Cronus system, which we view as an
 undesirable limit on configuration flexibility.  Also, this solution
 would be Cronus-specific, while the broadcast-repeater solution is
 applicable to other broadcast-based protocols.

5. Implementation

 The broadcast repeater is implemented as two separate processes - the
 forwarder and the repeater.  The forwarder process waits for
 broadcast UDP packets to come across its local network which match
 one or more specific port numbers (or destination addresses).  When
 such a packet is found, it is encapsulated in a forwarder-repeater
 message sent to a repeater process on a foreign network.  The
 repeater then relays the forwarded packet onto its LAN using that
 network's link-level broadcast address in the packet's destination
 field, but preserving the source address from the original packet.

 When the forwarder process starts for the first time it reads a

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RFC 947 June 1985 Multi-network Broadcasting within the Internet

 configuration file.  This file specifies the addresses of repeater
 processes, and selects which packets should be forwarded to each
 repeater process (different repeaters may select different sets of
 UDP packets).  The forwarder attempts to establish a TCP connection
 to each repeater listed in the configuration file.  If a TCP link to
 a repeater fails, the forwarder will periodically retry connecting to
 it.  Non-repeater hosts may also be listed in the configuration file.
 For these hosts the forwarder will simply replace the destination
 broadcast address in the UDP packet with the host's address and send
 this new datagram directly to the non-repeater host.

 If a repeater and a forwarder co-exist on the same LAN a problem may
 arise if the forwarder picks up packets which have been rebroadcast
 by the repeater.  As a precaution against rebroadcast of forwarded
 packets ("feedback" or "ringing"), the forwarder does not connect to
 any repeaters listed in its configuration file which are on the same
 network as the forwarder itself.  Also, to avoid a broadcast loop
 involving two LANs, each with a forwarder talking to a repeater on
 the other LAN, forwarders do not forward packets whose source address
 is not on the forwarder's LAN.

6. Experience

 To date, the broadcast repeater has been implemented on the VAX
 running 4.2 BSD UNIX operating system with BBN's networking software
 and has proven to work quite well for our purposes.  Our current
 configuration includes two Ethernets which are physically separated
 by two other LANs.  For the past few months the broadcast repeater
 has successfully extended our broadcast domain to include both
 Ethernets even though messages between the two networks must pass
 through at least two gateways.  We were forced to add a special
 capability to the BBN TCP/IP implementation which allows privileged
 processes to send out IP packets with another host's source address.

 The repeater imposes a fair amount of overhead on the shared hosts
 that currently support it due to the necessity of waking the
 forwarder process on all UDP packets which arrive at the host, since
 the decision to reject a packet is made by user-level software,
 rather than in the network protocol drivers.  One solution to this
 problem would be to implement the packet filtering in the system
 kernel (leaving the configuration management and rebroadcast
 mechanism in user code) as has been done by Stanford/CMU in a UNIX
 packet filter they have developed.  As an alternative we are planning
 to rehost the implementation of the repeater function as a
 specialized network service provided by a microcomputer based

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RFC 947 June 1985 Multi-network Broadcasting within the Internet

 real-time system which is already part of our Cronus configuration.
 Such a machine is better suited to the task since scheduling overhead
 is much less for them than it is on a multi-user timesharing system.

7. Reference

 [1]  "Cronus, A Distributed Operating System: Phase 1 Final Report",
      R. Schantz, R. Thomas, R. Gurwitz, G. Bono, M. Dean,
      K. Lebowitz, K.  Schroder, M. Barrow and R. Sands, Technical
      Report No. 5885, Bolt Beranek and Newman, Inc., January 1985.
      The Cronus project is supported by the Rome Air Development
      Center.

8. Editors Note

 Also see RFCs 919 and 940 on this topic.

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