Hans Erikson

M6ONE: The Multicast Backbone

The first thing many researchers like me do when they come to work is read their email. The sec- ond 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 ses- sion from an interesting conference

or a software demonstration for the Swedish prime minister.

It all started in March 1992 when the first audiocast on the Internet took place from the Internet Engi- nrering Task Force (IETF) meeting in San Diego. At that event 20 sites listened to the audiocast. Two years later, at the IETF meeting in Scattle 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 commu- nity now takes it for grantrd 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 Cor- tee (as will be described later) as well as a late Saturday night feature movie WAX or the Dmovely sf Televmon

Among the Bee.~ by David Blair.

As soon as some crucial tools ex- isted, the usage just exploded. Many people started using MBone for con- ferences, weather maps, research experiments, to follow the Space Shuttle, for rxample. At thr Swedish Institute of Computer Science (SICS) we saw our contribution to the Swed- ish University Network SUNET, in- crease from 26GB per month in Feb- ruary 1993 to 69GB per month in March 1993. This was mainly due to multicast tmff,c as SICS at that time was the major connection point be-

tween the U.S. and Europe in MBone.

MBone has also (in)directIy been

the cause of severe problam m the NSFnet backbone, saturation of major international links rendering

them useless as well as sites being completely disconnected due to Internet Connrction Management Protocol (ICMP) responses flooding the networks. We will expand on this later in this article.

Multicasting Background When we talk about MBone we somr- times mean thr virtual network that implements multicasting, sometimes we refer to the applications that run

on top of MBone (vat, nv, ivs, for ex- ample), and often we mea” every- thing. We will come back to the appli- cations that are in use on MBone later in this article, but for now we will con- centrate on the MBone proper, the multicasting virtual network.

First let us define what is meant by the different types of “casting.” The usual way packets are sent on the Intrrnet is unicasting, that is, one host is sending to another specific sin- gle host. Broadcasting is when one

host sends to all hosts on the same subnet. Normally, the routers be- tween 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 (In- terface Protocol or IP) it started with the work of Steve Deering of Xerox PARC when he developed multicast at the IP level [3]. The IP address space is divided into different classes.

A” 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 des- ignating network and hosts. Class A addresses have one byte for the net- work and three for host, B addresses have two bytes for each, and class C addresses have three bytes for the

n+rwork and one for the hoar. .Io dif- ferentiate between the classes, star, with 0, I 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 “lo” with a range of 128.0.0.0 to 191.255.255.255, and class C starts with “I IO” with a range of 192.0.0.0 to 223.255.255.255. No[ all addresses are available for host addresses, however, as some are de- fined for specific uses (e.g., broadcasr addresses). Class D is indicated by “I 110” at the start, giving an ad-

dress range of 224.0.0.0 t”

239.255.255.255. This class has been reserved for multicast addresses.

When a host wishes tojoin a muh- cast group, that is, get packets with a specific multicast address, the host issues a” Internet Group Manage- ment Protocol (IGMP) request. The multicast router for that subnet will then inform the other routers so that such packets will get to this subner and eventually be placed on the local-

area network (IAN) where the host is connected. Frequently, the local router will poll the hosts on the IAN if they are still listening to the multi- cast 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 a” address and it is up to the receivers to join that group (i.e., multicast address). An- other style of multicasting is where the sender specifies who should re- ceive the multicast. This gives more

control over the distribution, but one drawback is that it does not scale well. Having thousands of receivers is al- most impossible to handle this way. This second style of multicasting has been used in ST-2 [6, 81.

MBone Today As previously mentioned, MBonc 1s a virtual network running on “top” of