An Introduction to the MBone (in 5 minutes)

Mark Handley

UCL/ISI

http://www.cs.ucl.ac.uk/staff/M.Handley/

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What is the MBone?

The MBone is the Internet's Multicast Backbone

IP Multicast is efficient many-to-many communication.

It's not everywhere yet (but it will be...)

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The Service Model

IP Multicast has a very simple service model:

Senders just send to the "group"
Receivers express an interest in the "group"
Routers conspire to deliver traffic from the senders to the receivers

What's this got to do with reality?

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Multicast Addresses

Multicast addresses are simply IP class-D addresses. For example, this session is being multicast to 224.2.132.54.

Senders and receivers must somehow choose the same class D address.
A sender sends its traffic to this address.
Receivers announce that they wish to receive traffic destined for this address.

- The receiver's host conveys this interest to its local router using IGMP
What the local router depends on the multicast routing protocol.

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Multicast Routing

There are several multicast routing protocols in use:

DVMRP: Distance Vector Multicast Routing Protocol (Xerox, 3COM)
PIM: Protocol Independant Multicast (Cisco)
CBT: Core Based Trees (UCL, Bay Networks)
MOSPF: Multicast OSPF

From an application point of view, it doesn't matter which is in use.

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What does multicast routing do?

On demand, it produces a distribution tree from each sender to all the receivers.

DVMRP and MOSPF build a shortest path tree from each sender.
CBT builds a shared tree that all senders use
PIM does both (first builds a shared tree, then optimises it)

An important factor here is that normally the very first packet a sender sends will reach all the receivers!

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Shortest Path Trees

So for the black source, its distribution tree might be:

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Another Shortest Path Tree

And for another source sending to the same group:

Both sources can send to the same group simultaneously

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Very pretty, but what's it useful for?

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Very pretty, but what's it useful for?

Perhaps for everything!

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Really, what's it useful for?

Multimedia Distribution - shortest path trees give minimal delay.
Multimedia Distribution - just "tune in" - you don't need to know the senders are in advance.
Multimedia Conferencing - don't need to explicitly manage senders or receivers.
Scalable Distribution - multicast trees are are repaired locally so can scale extremely large.
Efficient data distribution - multicast trees are generally more optimal than application based trees.
Resource Location - send a request and it gets to anyone who might be able to answer

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What's this got to do with the Web?

The Web is growing to support real Multimedia data

The Web community should learn from 4 years of MBone experimentation.
Caching of real-time media flows is as vitally important as it is of non-real-time documents.
Otherwise, if you think the net's in a bad state now....

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The Web needs to better support existing media

Use multicast for data location or replicated data
Use multicast for finding appropriate caches
Use multicast for cache revocation
Perhaps use multicast for actual data transfer (one request pre-loads other caches)

But don't forget the lessons of TCP congestion control

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We should change the world

The MBone gives us a whole new way of looking at data distribution, but there are still hard problems to be solved.

HTTP is dead.

The future of the internet will be determined by our choice of its successors.