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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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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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.
