The first thing many researchers like me do when they come to work is read
their email. The second thing on my list is to check what is on the MBone-the
Multicast Backbone, which is a virtual network on “top” of the Internet
providing a multicasting facility to the Internet. There might be video
from the Space Shuttle, a seminar from Xerox, a plenary session from an
interesting conference or a software demonstration for the Swedish prime
It all started in March 1992 when the first audiocast on the Internet
took place from the Internet Engineering Task Force (IETF) meeting in San
Diego. At that event 20 sites listened to the audiocast. Two years later,
at the IETF meeting in Seattle about 567 hosts in 15 countries tuned in
to the two parallel broadcasting channels (audio and video) and also talked
back (audio) and joined the discussions! The networking community now takes
it for granted that the IETF meetings will be distributed via MBone. MBone
has also been used to distribute experimental data from a robot at the
bottom of the Sea of Cortez (as will be described later) as well as a late
Saturday night feature movie WAX or the Discovery of Television Among
the Bees by David Blair.
As soon as some crucial tools existed, the usage just exploded.
Many people started using MBone for conferences, weather maps, research
experiments, to follow the Space Shuttle, for example. At the Swedish Institute
of Computer Science (SICS) we saw our contribution to the Swedish University
Network SUNET, increase from 26GB per month in February 1993 to 69GB per
month in March 1993. This was mainly due to multicast traffic as SICS at
that time was the major connection point between the U.S. and Europe in
MBone has also (in)directly been the cause of severe problems
in the NSFnet backbone, saturation of major international links rendering
them useless as well as sites being completely disconnected due to Internet
Connection Management Protocol (ICMP) responses flooding the networks.
We will expand on this later in this article.
When we talk about MBone we sometimes mean the virtual network that implements
multicasting, sometimes we refer to the applications that run on top of
MBone (vat, nv, ivs, for example), and often we mean everything. We will
come back to the applications that are in use on MBone later in this article,
but for now we will concentrate on the MBone proper, the multicasting virtual
First let us define what is meant by the different types ot “casting.”
The usual way packets are sent on the Internet is unicasting, that is,
one host is sending to another specific single host. Broadcasting is when
one host sends to all hosts on the same subnet. Normally, the routers between
one subnet and another subnet will not let broadcast packets pass through.
Multicasting is when one host sends to a group of hosts.
On the link level (e.g., Ethernet) multicasting has been defined
for some time. On the network level (Interface Protocol or IP) it started
with the work of Steve Deering of Xerox PARC when he developed multicast
at the IP level . The IP address space is divided into different classes.
An IP address is four bytes and the address classes A, B and C divide the
addresses into a network part and a host part. The difference between the
classes is the balance between bits designating network and hosts. Class
A addresses have one byte for the network and three for host, B addresses
have two bytes for each, and class C addresses have three bytes for the
network and one for the host. To differentiate between the classes, start
with 0, 1 or 2 bits that are set followed by a zero bit. Class A addresses
start with binary “0” and are in the range 0.0.0.0 to 127.255.255.255,
class B starts with “10” with a range of 184.108.40.206 to 220.127.116.11,
and class C starts with “110” with a range of 192.0.0.0 to 18.104.22.168.
Not all addresses are available tor host addresses, however, as some are
defined for specific uses (e.g., broadcast addresses). Class D is indicated
by “1110” at the start, giving an address range of 22.214.171.124 to 126.96.36.199.
This class has been reserved for multicast addresses.
When a host wishes to join a multicast group, that is, get packets
with a specific multicast address, the host issues an Internet Group Management
Protocol (IGMP) request. The multicast router for that subnet will then
inform the other routers so that such packets will get to this subnet and
eventually be placed on the localarea network (LAN) where the host is connected.
Frequently, the local router will poll the hosts on the LAN if they are
still listening to the multicast group. If not, no more such packets will
be placed onto the LAN. When doing multicasting utilizing MBone, the sender
does not know who will receive the packets. The sender just sends to an
address and it is up to the receivers to join that group (i.e., multicast
address). Another style of multicasting is where the sender specifies who
should receive the multicast. This gives more control over the distribution,
but one drawback is that it does not scale well. Having thousands of receivers
is almost impossible to handle this way. This second style of multicasting
has been used in ST-2 [6, 8].
As previously mentioned, MBone is a virtual network running on “top” of
the Internet. MBone is composed of networks (islands) that support
multicast. On each of these islands, there is a host that is running the
multicast routing demon. The mrouted’s are connected with one another via
In Figure 1, we have three islands of MBone. Each island consists
of a local network connecting a number of client hosts (“C”) and
one host running mrouted (“M”) The mrouted-hosts are linked with point-to-point
tunnels. The thick tunnels are the primary feeds with the thin tunnel as
Basically, a multicast packet will be sent from one client who
puts the packet on the local subnet. The packet will be picked up hy the
mrouted for that subnet. The mrouted will consult its routing tables and
decide onto which tunnels the packet ought to be placed. At the other end
of the tunnel is another mrouted that will receive the multicast packet.
It will also examine its routing tables and decide if the packet should
be forwarded onto any other tunnels. The mrouted will also check if there
is any client on its subnet that has subscribed to that group (multicast
address) and if so, put it onto the subnet to be picked up by the client.
When sending the multicast packet through the tunnel, the multicast packets
must be repacked. There are two methods of doing this, adding the Loose
Source and Record Route (LSRR) IP option and encapsulation. The first implementations
of mrouted used the LSRR IP option. Mrouted modified the multicast datagram
coming from a client by appending an IP LSRR option where the multicast
address was placed. The IP destination address was set to the (unicast)
address of the mrouted on the other side of the tunnel. There have been
some problems with this approach (as will be described later) that prompted
the implementation of encapsulation. In this method the original multicast
datagram will be put into the data part of a normal IP datagram that is
addressed to the mrouted on the other side of the tunnel.
The receiving mrouted will strip off the encapsulation and forward
the datagram appropriately. Both these methods are available in the current
Each tunnel has a metric and a threshold. The metric is used for
routing and the threshold to limit the distribution scope for multicast
The metric specifies a routing cost that is used in the
Distance Vector Multicasting Routing Protocol (DVMRP). To implement the
primary and backup tunnels in Figure 1, the metrics could have been specified
as 1 for the thick tunnels and 3 for the thin tunnel. When M1 gets a multicast
packet from one of its clients, it will compute the cheapest path to each
of the other M’s. The tunnel M1-M3 has a cost of 3, whereas the cost via
the other tunnels is (1 + 1) 2. Hence, the tunnel M1-M3 is normally not
used. However, if any of the other tunnels breaks, the backup M1-M3 will
be used. However, since DVMRP is slow on propagating changes in network
topology, rapid changes will be a problem.
The threshold is the minimum time-to-live (TTL) that a
multicast datagram needs to be forwarded onto a given tunnel. When sent
to the network by a client, each multicast packet is assigned a specific
TTL. For each mrouted the packets pass, the TTL will be decremented by
1. If a packet’s remaining TTL is lower than the threshold of the tunnel
that DVMRP wants to send the packet onto, the packet is dropped. With that
mechanism we can limit the scope for a multicast transmission.
In the beginning there was no pruning of the multicast
tree. That is, every multicast datagram is sent to every mrouted in MBone
if it passes the threshold limit. The only pruning is done at the leaf
subnets, where the local mrouted will put a datagram onto the local network
only if there is a client host that has joined a particular multicast group/address.
This is called truncated broadcast. As the MBone grew, problems surfaced
which we will discuss later. These problems prompted work on proper pruning
of the multicast tree as well as work on other techniques for multicasting
[1, 5, 9]. Pruning as implemented in the MBone today works roughly like
this: If a mrouted gets a multicast packet for which it has no receiving
clients or tunnels to forward it to, it will drop the packet but also send
a signal upstream that it does not want packets with that address. The
upstream mrouted will notice this and stop sending packets that way. If
the downstream mrouted gets a client that joins that pruned multicast group,
it will signal its upstream neighbours that it wants these packets again.
Regularly the information will be flushed and packets will flow to every
corner of MBone until pushed back again.
There is no “network provider” of the MBone. In the spirit of the Internet,
MBone is loosely coordinated via a mailing list. When end users want to
connect to MBone, they are encouraged to contact their network provider.
If that network provider is not participating in MBone and for some reason
does not want to, a tunnel can be arranged to another point in MBone.
From time to time, there have been major overhauls of the topology
as MBone has grown. Usually this has been prompted by an upcoming IETF
meeting. These meetings put a big strain on MBone. The IETF multicast traffic
has been about 100 to 300Kb per second with spikes up to 500Kb per second.
Since MBone was set up, a number of wide-ranging applications have surfaced.
We have seen the astronauts repairing the Hubble telescope, listened to
seminars and seen cars come and go at the Bolt, Beranek, and Newman parking
lot in Boston. I will give an overview of some of the events that have
used MBone in some way. But first I will mention some of the most popular
tools for using the MBone. This list is by no means complete as new applications
For audio we have vat (visual audio tool) by Steve McCanne
and Van Jacobsen of Lawrence Berkeley Laboratory. The nevot (network
voice terminal) by Henning Schulzrinne of AT&T/Bell Laboratories is
another audio tool.
Video tools are ivs (inria videoconferencing system) by
Thierry Thurletti of INRIA in Sophia Antipolis, France and nv (network
video) by Ron Frederick of Xerox PARC.
Wb (white board) by McCanne and Jacobsen provides a shared
drawing space and is especially useful for presentations over the MBone.
Wb can import slides in PostScript and the speaker can make small annotations
during the lecture.
Figure 2 depicts the sd (session directory) by McCanne
and Jacobsen. Sd offers a convenient way of announcing “sessions” that
will take place on the MBone. When creating a session, you specify the
multicast address (an unused address is suggested by sd) and the various
tools that are used. Other people can then just click “Open” and sd will
start all the necessary tools with appropriate parameters.
When this snapshot was taken, the SIGGRAPH conference was taking
place. As a special event at that conference, children were invited to
talk with people on the MBone. This event is highlighted in the sd snapshot.
Going up in the list we have Radio Free Vat. This is the MBone “radio”
station where anyone on the MBone can be the “disk jockey.” Next up is
MBone Audio, which is the common chat channel of the MBone. Everyone is
free to join and start a discussion about any subject. Because MBone spans
about 16 time zones, Not everyone is at their workstation when you ask
“Is there anybody out there?” , but there is always someone out there!
The Global Mapping Satellite (GMS) sessions are pictures from a satellite
above Hawaii. The pictures (composite, infrared or visual spectra) are
sent out using imm (Image Multicast Client) by Winston Dang of the
University of Hawaii. Second to the top is the Bellcore WindowNet. If you
tuned in to this session, you would see the outlook from a window from
Bellcore. At the top we have not a session, but a plea. As audio and video
consumes a fair amount of bandwidth and MBone is global, rebroadcasting
your favourite local radio station onto MBone will put a hard strain on
many networks. We will come back to this problem later in this article.
Not shown in this particular snapshot, but a frequent and very
popular guest on MBone are the Space Shuttle missions. The NASA select
cable channel is broadcast onto the MBone during the flights. The pictures
of the astronauts travel a long way and traverse many different technologies
before appearing on the screen of your workstation. But it works!
Figure 1. MBone topology – islands, tunnels, mrouted
Figure 2. sd – session directory
A different type of event was mentioned earlier, the 1993 JASON
Project . Woods Hole Oceanographic Institution provided software for
Sun and Silicon Graphics workstations so anyone on the MBone could follow
three underwater vehicles on their tours in the Sea of Cortez. Position
data and some pictures were continuously distributed over the MBone. Beside
being interesting for scientists in other fields, it was very valuable
for oceanographic researchers to follow the experiments in real time and
give feedback immediately.
The multimedia conference control (mmcc) by Eve Schooler
of University of Southern California (USC)/ Information Sciences Institute
(ISI) goes beyond this simple support given by sd. We will include more
about this when discussing the MMUSIC protocol.
The popular Mosaic package from the National Center for Supercomputer
Applications (NCSA) is being enhanced by people at University of Oslo.
The idea is to use Mosaic for lectures and let the speaker multicast control
information to the Mosaic programs used by the students.
We also have the media-on-demand server created by Anders Klemets
of Royal Institute of Technology in Stockholm, Sweden, which offers unicasted
replays of sessions that have been multicasted on the MBone.
This is merely a snapshot of some of the developments taking place
in the MBone community. New ideas surface often and implementations follow
All traffic in MBone uses User Datagram Protocol (UDP) rather than the
usual Transport Control Protocol (TCP). TCP provides a point-to-point connection-oriented
reliable byte stream protocol, whereas UDP is just a transport-level envelope
around an IP packet with almost no control whatsoever. One reason for not
using TCP is that the reliability and flow control mechanisms are not suitable
for live audiocasting, for example. Occasional loss of an audio packet
(as when using UDP) is usually acceptable, whereas the delay for retransmission
(when using TCP) is not acceptable in a interactive conference. Also, TCP
does not easily lend itself to multicasting. One problem that must be resolved
is that UDP packets may be duplicated and reordered (beside being dropped)
when transmitted over the Internet.
On top of UDP most MBone applications use the Real-Time Protocol
(RTP) developed by the Audio-Video Transport Working Group within the IETF.
Each RTP packet is stamped with timing and sequencing information. With
appropriate buffering at the receiving hosts, this allows the applications
to achieve continuous playback in spite of varying network delays.
Each form ot media can be encoded and compressed in several ways.
Audio is usually encoded in PCM (Pulse Code Modulation) at 8KHz with 8-bit
resolution giving 64Kb per second bandwidth for audio. Including packet
overhead it raises to about 75Kb per second. By using Groupe Special Mobile
(GSM), a cellular phone standard, one can get down to about 18Kb per second
Video is more demanding. The ivs tool uses the CCITG (Consultative
Committee of International Telephone and Telegraph) standard H.261 
whereas the nv tool uses a unique compression scheme. It is possible to
limit the amount of bandwidth that should be produced in both tools. The
usual bandwidth setting is 128Kb per second. How this translates into quality
depends on the kind of scene that is captured.
During the lifetime of MBone, a fair number of problems have been encountered.
Some are inherent to multicasting in general, and some are more specific
to the current implementation of MBone.
A number of problems that have surfaced during operations of MBone
will be discussed in this section. Some problems have a direct bearing
on the MBone implementation, other problems have been discovered recently
during the use of MBone.
Currently there are three more-or-less permanent sessions going on in MBone.
There is one audio and video channel for free-for-all use and there is
Radio Free Vat. In addition to the IETF meetings, which are transmitted
three times per year, several major conferences and workshops are being
transmitted onto the net, such as JENC93 and some IETF working group meetings.
We have also seen President Clinton and Vice-president Gore on MBone, and
we have already mentioned the JASON project and the Space Shuttle.
MBone in its present form should be viewed as one single resource.
Only in a few places can it handle more than one video channel together
with audio. The IETF tries to make two video and four audio channels but
does not always accomplish this, even if the best “networkers” in the Internet
put in their best efforts. So far, we have not had any major collisions
of major events. The collisions that have occurred have been resolved after
some brief discussions. Essentially it is a first-announce-first-serve
scheduling. As MBone increases in popularity, one can expect more collisions
and the pressure for a particular slot will increase.
Some of the success of MBone is dependent on the “courtesy” of
TCP. When someone starts sending audio onto a fully loaded Internet link,
it will cause packet losses for many of the connections that are running
on that link. They are usually TCP connections and they will back off when
packet losses occur. UDP-based audio does not have any such mechanism and
will effectively take the bandwidth it needs.
On several occasions end users have started a video session with
a high time-to-live (TTL) and subsequently swamped the network with a continuous
stream of 300 to 500Kb per second. These users have not been malicious.
Sometimes the program has just been started with “-ttl 116″ instead of”-ttl
16″ with the effect that it reaches most parts of the MBone instead of
just the local part. At other times, the users have not really been aware
of what “256Kb per second” really is netwise. Very few links in the Internet
can handle that load without severely disturbing normal traffic. Usually
after the mistake has been pointed out, the users have stopped their transmissions.
The Problem is that with the new video and audio applications the mistakes
have severe consequences and with multicasting in MBone, the consequences
are spread globally. It will take some time before the user community gets
a feel for how much bandwidth video and audio takes. Existing applications
like ftp can also use a lot of bandwidth, but the backoff mechanism of
TCP ensures a fail split of resources, which a UDP-based application does
Lacking a fine-grained resource allocation mechanism, a way to
put a limit on the bandwidth usage of a tunnel could be very helpful. That
would make many network providers a lot less nervous about letting multicast
Thresholds on Tunnels
Lacking multicast tree pruning, the only way to limit the scope of a multicast
datagram is by using thresholds. If a datagram has a TTL greater than the
threshold, it will be forwarded onto the tunnel. Thresholds range between
0 and 255. The threshold levels chosen on the tunnel tries reflect both
a geographic partitioning (e.g., keeping a local conference local) and
a choice of traffic (e.g., restricting video more than audio). But expressing
two dimensions of choices in one metric always introduces some tradeoffs.
The guidelines establish that traffic within one site should be
sent with a ttl of 16, within one “community” 32, and global traffic should
have 127. The IETF transmission plan is shown in Table 1.
The table says that if you only want get audio channel 1 with
the GSM compression, your tunnel should have a threshold of 224.
The threshold mechanism is a very coarse method to limit traffic.
With the current IETF plan, there is no way you can use your 256Kb per
second link to join the session that is broadcast on channel 1. To get
PCM audio 1 you will open up for both GSM audio channels, giving a total
of ~105Kb per second. To get Video 1 you will also get the PCM audio 2
summing it up to ~310Kb per second for video and audio from channel 1.
Table 1. Time-to-live (TTL) and thresholds from the Internet
Engineering Task Force
Traffic type TTL ~kb per second threshold
GSM audio 1 255 15 224 GSM audio 2 223 15 192 PCM audio 1 191 75 160 PCM
audio 2 159 75 128 Video 1 127 130 96 Video 2 95 130 64 local event audio
63 =>250 32 local event video 31 =>250 1 When true pruning gets widely
deployed in MBone it will be possible to get only what you ask for.
Some mrouted hosts have a fair number of tunnels. The top ones had 11 tunnels
(early June 1993). However, not all of them are primary tunnels. For example,
hydra.sics.se (a SPARC 1+) has 10 tunnels, but only 5 of them are primary.
The aggregate traffic from an IETF meeting roughly generates one packet
every 4ms. Looking at how much time it takes to forward a multicast packet
we find that a SPARC 1+ needs ~1.0ms and a SPARC 10 needs ~0.6ms. This
suggests that hydra.sics.se is saturated during IETF sessions. This will
show up not only as dropped packets, but also as dropped tunnels. As hydra
will be busy, in kernel forwarding of multicast packets, the mrouted will
not get any cycles to run its business. Eventually, the peers at the other
ends of the tunnels will think that hydra is dead and hence they will reroute
their packets. Soon after the load has gone down on hydra, mrouted will
get some cpu cycles and talk to its peers and the overloading will start
A related problem is saturation of the local Ethernets. FIX-West
had 15 tunnels during the IETF meeting in March 1994. With an IETF load
of ~500Kb per second it results in ~6.5Mb per second pushed over the FIX-West
Ethernet. Even without the MBone traffic, that Ethernet is already busy.
Some mrouted operators use the cpu overload as a means to limit the impact
on the local network. That is, if you have a SPARC 1+ as a mrouted host,
it will not push more than 1,000 packets onto the net, probably much less.
This is a dangerous practice unless you are the only entry point to a part
of MBone. As stated previously, when your tunnel gets declared dead, MBone
will choose another route if possible until your mrouted gets its breath
back. This results in heavy route flapping, which becomes a global problem.
If you are the only path to take, traffic will just stop for a while, which
is a local problem only.
MBone Tunnels vs. Internet Links
Tunnels are set up along the links of the underlying real network. But
when a link fails and the underlying network does a rerouting, the tunnels
stay and become less optimally placed. As an example, traffic from Sweden
to the United Kingdom (UK) would normally go via the tunnel from Stockholm
to Washington over a T1 link and then take the tunnel Washington to London
over another T1 link. When the first link malfunctioned, traffic was rerouted
via Amsterdam and London and then to the U.S. The multicast traffic from
Sweden to the UK ended up going Stockholm to London to Washington and then
back to London. Eventually, we made a manual reroute of tunnels. Putting
multicast routing into the routers enables the multicast routing to follow
the unicast routing. There is a proposal for an extension to the OSPF (Open
Shortest Path First) routing protocol to also incorporate multicasting
MBone as a Bug Trigger
During the IETF meeting in Washington, D.C. in November 1992, there were
problems with the NSFnet which were due to the multicast traffic coming
from the IETF meeting. At that time the tunnels were all using the lose
source route option (LSRR). In modern router technology, packets are handled
by the interface cards as much as possible. Packets with IP options, however,
are usually forwarded to the main cpu for handling. At that time, the IETF
meeting was generating two audio channels of ~75Kb per second and two video
channels of ~130Kb per second. This was about 400 packets per second that
was sent from the IETF site to the mrouted at Cornell. That mrouted in
turn, fed a number of tunnels so the traffic from Cornell onto ENSS133
at Ithaca was above 1,000 packets per second that had to be handled by
the main cpu. Adding to that, a great number of ICMP-unreachable messages
were generated. The cpu was having a difficult time when regular routing
updates were added. This lead to routing timeouts when other networks had
problems due to excessive MBone traffic. A number of actions were taken
to fix these problems. One of the immediate actions was the disabling of
one video channel to lessen he load. In the aftermath, some inefficiencies
of routing updates were fixed, for example, EGP (External Gateway Protocol)-derived
routes will now be aggregated into a single BGP (Border Gateway Protocol)
update message. The most important change in MBone was the use of true
encapsulation instead of LSRR option for the tunneling.
During the packet video workshop at MCNC, Van Jacobson observed
a phenomenon in which it seemed that routing updates severely impacted
the audio transmissions. The congestive loss rate was about 0.5% but every
30 seconds he observed huge losses (50% to 85%) for about 3 seconds. Jacobson
concluded that it was due to the LSRR option processing competing with
routing updates. Not only does this affect MBone traffic, but also other
traffic such as pings and traceroutes.
Many hosts and routers do not handle multicast traffic properly.
Often they respond by sending an ICMP redirect or network unreachable.
These responses are not in accordance with the IP specifications. This
is usually not a problem until we have several such hosts reacting with
ICMPs to a number of audio streams of about 50 packets per second. The
any network tends to get flooded with ICMPs. It has happened that a site
was disconnected from MBone due to a “screaming” router. Over time, this
problem has diminished as router vendors update their software. Also, with
the new encapsulation tunnels, the ICMPs will be sent to the last tunnel
endpoint, not the entire route back to the original sender.
Multicasting can be a dangerous beast, but it also carries the promise
of very useful applications. As an indication of this MBone usage is increasing
very rapidly. As a probably unintentional side effect, it has also brought
out some bugs in some routers and hosts in the Internet. Before MBone can
be provided as a regular widespread service, some issues have to be addressed.
Some of these issues are still difficult research issues, like
resource control and real-time traffic control. Other work is directed
toward better management hooks and tools and incorporating multicasting
in the Internet routers. Maybe there are better technologies for multicasting
than those currently used in the MBone? The IDMR (Inter-Domain Multicast
Routing) working group in IETF is working on this.
MBone has enabled a lot of applications. One problem when starting
the applications is the question of what addresses should be used. Picking
one randomly will be fine for quite a while, but eventually when MBone
gets more crowded some mechanism has to be put in place for allocation
of multicast addresses and port numbers.
As MBone is today, the sender has no control or implicit knowledge
of who is listening out there. A receiver can just “tune in,” like a radio.
Some applications would want some kind of information about who is listening,
for example by asking MBone which hosts are currently in a particular multicast
group. There are mechanisms in some applications for end-to-end control
of who is listening (i.e., encryption) but there is so far no common architecture
for this. When the going gets rough and a lot of packets are dropped, some
applications would be helped by some feedback on the actual performance
of the network. A video application could for example, stop sending raw
HDTV data when only 2% make it to the receivers and instead start sending
slow-scan, heavily compessed pictures.
We look forward to the next round of developments as the MBone
continues to evolve.
Ballardie, A., Francis, P., Crowcroft, J. Core-based trees – An architecture
for scalable inter-domain multicast routing. ACM SIGcomm 93, pp.
Consultative Committee of International Telephone and Telegraph (CCIIT)
Deering, S. Host extensions for IP multicasting. RFC1112,
Maffei, A. Remote science over the MBone duling the 1993 JASON project.
Woods Hole Oceanographic Institution, Woods Hole, Mass.
Moy, J. Multicast routing extensions for OSPF. Commun. ACM 37, 8
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About the Author:
HANS ERIKSSON is a researcher at the Swedish Institute of Computer Science
(SICS), located in Kista, outside Stockholm. Current research interests
include computer networking and distributed multimedia applications. Author’s
Present Address: Swedish Institute of Computer Science, Box 1263, 164
28, Kista, Sweden; email: email@example.com
Permission to copy without fee all or part of this material is granted
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requires a fee and/or specific permission.
©ACM Communications of the ACM 37, 8 (August 1994), pp. 54-60.
Hypertext byGraeme Glen, firstname.lastname@example.org, 23-Mar-95