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Multicast Networking
By Dr Kim Chin
Australian Catholic University,
North Sydney,
New South Wales,
Australia
1
Outline

What is Multicast?
–
Unicast vs Broadcast vs Multicast
Why Multicast?
 IP Multicast Model
 MBone and its growth

 Multicast Trees
– Some Logical Multicast Trees generated when the
MBone experienced an explosive growth
2
Outline (continue…)



IP Multicast deployment status
Factors that hinder deployment
Beyond IP multicast
–
–

Application layer multicast
Overlay multicast
Summary and the future of Multicast
3
What is multicast?

a receiver-based concept

is the sending of messages from one-to-many
receivers or many-to-many receivers

is a subset of broadcast
4
Broadcast vs Unicast

Broadcast
–

sends data to everyone whether they want them
or not
Unicast
–
–
the common way of transmitting data across the
Internet
sending a single copy of the message to anyone
who requests it
5
UC Berkeley
Multimedia Seminar
Network node
Data packets replicated by this node
Audio packet
law
(California)
Network
Network
edgar
(Washington)
tove
(Maryland)
erlang
(Massachusetts)
alps
(Georgia)
float
(Virginia)
cedar
(Texas)
ursa
anhur
(Germany)
(Sweden)
6
Problem with Unicast

The Internet has traditionally been built to send
information to one person (or computer) at a
time--unicast

consider sending information simultaneously to
more than one person
–
e.g. e-mail message to which you've attached a
graphics file
 unicast--to one person
 multicast--to many at the
same time
7
Why Multicast?
 “The unicast delivery paradigm predominant in
today’s Internet does not scale to support the
widespread use of video.” [Edwards et. al.]
 “Multicast is becoming important because it
enables desired applications to scale, ...
Widespread use of these applications would be
virtually impossible without the scaling provided
by multicast services” [Miller]
– “push” services of PointCast -- traffic clogged up
networks
8
Why Multicast? (continue 1…)

Large-scale events:
–
–
(1998), latest version of Microsoft explorer-meltdown
landing of Pathfinder on Mars--”surfers” overwhelm
NASA’s Web site
9
Why Multicast? (continue 2…)
–
–
September 11 tragedy, at Northern University,
CNN was rebroadcast using multicast on the
Internet--over 2000 viewers
“users on multicast-enabled networks were able
to watch real-time video accounts throughout
the entire day.” [Edwards et al.]
10
Video Transmission in Unicast & Multicast
Networks (from http://www.mcclellanconsulting.com/)
11
Multicast Applications
Multimedia
Data-only








Video
Video conferencing
Internet audio
Multimedia events
Stock quotes
News feeds
Whiteboarding
Interactive gaming
 Replication:
Video and Web servers
Kiosks
 Content delivery
Intranet and Internet
 Data delivery
Server-server
Server-desktop
 Database replication
 Software distribution
12
The IP Multicast Model
Deering’s standard multicast model for IP
networks is as follows:
 IP-style semantics: source sends UDP/IP packets
–
–
–
–
no need to register
no schedule transmission
send at any time
best effort
13
The IP Multicast Model
(continue...)

Open groups:
–
–
–
–
multicast address known by sender only
need not know group membership
need not be a member of the multicast group they
send packets to
group can have any number of sources
14
The IP Multicast Model
(continue...)

Dynamic group:
–
–
–
–
members can join and leave a group
no need to register
no need to synchronize
no need to negotiate with any centralized group
management entity
15
16
17
The IP Multicast backbone
(MBone)

The “virtual network backbone” which joins
together the multicast-capable portion of the
Internet was the MBone.
–

Originated from DARTNet (DARPA Research
Testbed network)
the success of the weekly DARTNet meetings
resulted in extending the multicast
infrastructure:
18
MBone (continue…)
–
multicast routing function provided by mrouted
a
daemon process--received encapsulated multicast
packets-- incoming interface outgoing interface
–
tunnels (multicast routers at the edge of a
multicast-capable portion of the Internet)
 each
tunnel connected two end-points via one logical
link--crossed several Internet routers
–
–
multicast packet received at a tunnel endpoint-broadcast on a local network
DVMRP (Distance Vector Routing Protocol)
19
20
21
Multicast Trees
multicast routing was a controlled form of
flooding
 no pruning initially -- pruning was deployed
several years later
 broadcast & prune used to create multicast
trees
 reverse shortest path tree --rooted the source

22
Multicast Trees (continue 1…)
4 steps to creating a multicast tree:
 source broadcasts packets on its local network
–
–

an attached router receives the packets
sends them through all outgoing interfaces
router receiving a packet performs a RPF
(Reverse Path Forwarding) check
–
router checks incoming interface (packet received)
is used as the outgoing interface to reach the
source
23
Multicast Trees (continue 2…)

When a packet reaches a router (leaf router) with
attached hosts
–
–
–
leaf router checks if there are known group members
on its attached subnets
IGMP queries issued periodically to discover group
members
leaf router:
 does
nothing if there are group members
 no group members--leaf router sends a prune towards the
source on the RPF interface
24
Multicast Trees (continue 3…)

Prune packets sent back to the source
–
routers create prune state for the interface
receiving the prune
25
anhur
(Sweden)
World Radio Network
(Washington D.C)
zen
(Missouri)
cedar
(Texas)
law
(California)
tove
(Maryland)
collage
(California)
willow
(Arizona)
erlang
(Massachusetts)
ursa
(Germany)
alps
(Georgia)
float
(Virginia)
Logical Multicast Tree (WRN July 13th, 1995). It is adapted from Yajnik et al. The bold lines
provide connections between the “backbone” routers. The thin lines are branches of the tree on
the edge of the network.
•
is the backbone router and
•
is the local LAN router.
26
27
erlang
(Massachusetts)
ocarina
(Kentucky)
pax
(France)
cedar
(Texas)
anhur
(Sweden)
World Radio Network
(Washington DC)
alps
(Georgia)
tove
(Maryland)
•
•
Logical Multicast Tree with WRN (December 18th, 1995).
are the backbone routers and
are local LAN routers. The bold lines are the routes which connect some selected intermediate
MBone routers between hosts and the WRN root .
28
29
Logical Multicast Tree (April 19th, 1996). It is adapted
from Yajnik et al.
spiff
(Sweden)
erlang
(Massachusetts)
ursa
(Germany)
cedar
(Texas)
float
(Virginia)
Radio Free Vat
(California)
pax
(France)
artemis
(France)
edgar
(Washington)
excalibur
(California)
bagpipe
(Kentucky)
tove
(Maryland)
30
31
lupus
erlang
(Germany)
(Massachusetts)
tove
(Maryland)
cedar
spiff
(Texas)
(Sweden)
law
float
(California)
(Virginia)
edgar
(California)
Radio Free Vat (California)
ganef
excalibur
(California)
(California)
Logical Multicast Tree (May 8th, 1996). It is adapted from Yajnik et al. The bold lines represent
the connections between backbone routers. All other lines are branches of the tree and they are
on the edge of the network, leading to the receiving hosts.  is the backbone router and  is the
local LAN router.
32
Deployment Status
Deployment has been very slow even though
multicast is an old concept by Internet
standards—compare with WWW & HTTP
 Jan 1992—the MBone, did not exist—in
1995, it made up 20% of all the Internet data
bytes at one research lab—40% at another and
more than 50% at yet another---but traffic
overshadowed by Web soon after.
33
Deployment Status (continue…)
the Web was quite unknown until late 1992 -then a “stunning pattern of growth set in”: “a
research site’s Web traffic began to double every
6 weeks, and continued to do so for 2 full
years!!!” by 1994, Web traffic wholly dominated
the site’s activities [Paxson,1996].
 Web traffic has overshadowed MBone traffic
ever since

34
Factors that hinder deployment

multicast lacks the “killer” applications
–





most popular multimedia applications access the content via
multicast then falling over to unicast
lack of scalable inter-domain routing protocol
state scalability issue with a large number of groups
how to charge for Multicast services—lack of
appropriate pricing model
loss of revenues from unicast bandwidth
issue of end-to-end connectivity
35
Factors that hinder deployment
(continue…)
can the Internet ever have prime-time television
quality video?
 lack of test tools for trouble-shooting
 Denial of Service attacks—ASM (Any-SourceMulticast)
 lack of security support
 multicast protocols are complex and may break
the unicast network

36
Beyond IP Layer Multicast

Application layer multicast
 group
membership, tree construction, data
forwarding controlled by end hosts, thus requiring
no support from intermediate nodes such routers

Overlay multicast
–
Multicast functionalities supported by some
additionally deployed intermediate nodes
forming an overlay network
37
Taken from [Lao et al, 2005]
38
Application Layer Multicast

ALMI (Application Level Multicast
Infrastructure) [Pendarakis et al]
–
–
–
provides a multicast middleware which is
implemented above the socket layer
scales for a large number of groups with
number of members small
independent of multicast support in routers
39
ALMI

An ALMI session consists of a session
controller and multiple members
–
–
–
a session controller is in a location where it is
easily accessible by members
session members are organized into a multicast
tree
unicast connection between 2 members is
represented by a link
40
Application Layer Multicast
Approach
Multicast related features are implemented
at end hosts
 Data packets are transmitted between endhosts via unicast and replicated at end hosts
 Can be deployed with ease because it does
not require infrastructure support from
intermediate nodes [Lao et al]

41
Disadvantages of Application-layer
Multicast



Not scalable to support large multicast groups due
to its rather low bandwidth efficiency
Tree maintenance at end hosts causes heavy
control overhead
Difficult for ISPs to have an effective profitmaking service model because group membership
and multicast trees managed at end hosts—hard to
have member access control as well as knowledge
of a group’s bandwidth usage [Lao et al]
42
Overlay Multicast

Two-tier Overlay Multicast (TOM) [Lao et al]
–
–
A key feature is its backbone service domain,
MSON (Multicast Service Overlay Network),
consisting of service nodes or proxies strategically
deployed by MSON provider (ISP)
The design of MSON relies on well-defined
relationship between:
 the
MSON provider;
 the network service provider; and
 the group coordinators
43
Summary

Multicast is a more efficient mode of
transmitting packets, especially video
packets, than unicast and broadcast
–
Microsoft supports Multicast project in China
44
Summary (continue 1…)
–
“Microsoft to “fully support” and “co-operate”
with IP-Set-Top-Box manufacturer World
Multicast China, on first ever direct to home IP
Multicast service. This test to take place in the
city of Shaoxing China beginning in early
2005.” [http://www.ipmulticast.com/]
45
Summary (continue 2…)
–
“Any time you are dealing with an emerging
technology that has infinite potential you are
going to have interested parties, our secure
reliable IP multicast technology is the toolbox
of missing links that have thus far hindered
Inter-domain IP Multicast from proliferation.
Now this test will change all of that. “
46
Summary (continue 3…)
–
“IP Multicast is the only technology built-into
the current inter-domain routing infrastructure
that addresses mass media on the Internet.We
have dubbed our technology “Infinicast”
because of its ability to support an infinite
number of users from just one stream of video
or audio.”[World Multicast Technology
inventor Ian A. Stewart ]
47
The file size required to represent about one minute of media
(taken from ASTD’s source for E-learning)
48
Summary (continue 4…)
IP Multicast Model
 IP Multicast Routing Protocols
 The MBone and its growth
 Multicast trees
 IP multicast deployment status
 Factors hindering IP multicast deployment

49
Summary (continue 5…)

Beyond IP multicast
–
–
Application layer multicast
Overlay multicast
50
References
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

B. M. Edwards, L. A. Giuliano and B. R. Wright. Interdomain Multicast Routing: Practical
Juniper Networks and Cisco Systems Solutions. Addison Wesley Professional, April 2002.
C. K. Miller. Multicast Networking and Applications. Addison-Wesley Longman, Inc.,
Massachussetts, January 1999.
K.C. Almeroth. The Evolution of Multicast. (www.stardust.com)
Developing Media for Low Bandwidth. (ASTD’s Source for E-Learning)
V. Paxson. Why We Don’t Know How To Simulate The Internet. In Proceedings of the 1997
Winter Simulation Conference, Atlanta, GA, 1997.
M. Yajnik, J. Kurose and D. Towsley. Packet Loss Correlation in the Mbone Multicast
Network. UMASS CMPSCI Technical Report # 96-32.
http://www.ipmulticast.com/
L. Lao et al. A Comparative Study of Multicast Protocols: Top, Bottom, or in the Middle?
Technical Report TR040054 (2005) Computer Science Department UCLA.
L. Lao et al. A Scalable Overlay Multicast Architecture for Large-Scale Applications.
Technical Report TR040008 (2004) Computer Science Department UCLA.
D. Pendarakis et al. ALMI: An Application Level Multicast Infrastructure. WUCS-11-17
Department of Computer Science, Washington University.
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