Internet2
Multicast Workshop
Columbia University, New York, NY
December 2005
Engineering Workshops
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Acknowledgements
Greg Shepherd
Beau Williamson
Marshall Eubanks
Bill Nickless
Caren Litvanyi
Patrick Dorn
Leonard Giuliano
Alan Crosswell
Debbie Fligor
Mitch Kutzko
Matt Davy
Yul Pyun
Stig Venaas
University of Oregon
Columbia University
NYSERNet
Cisco Systems
Juniper Networks
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• Introductions
Preliminaries
– Instructors
– Students
• Who has Juniper experience?
• What's in the binder?
– Slides, with space for notes.
– Labs. For labs 2-5, there are separate "answers" docs
which are not included in the binder. These are
distributed after, or sometimes during, the labs (they're
labs, not tests). If you get stuck, please ask!
– References: Cisco/JUNOS multicast commands, JUNOS
RIB groups, JUNOS CLI. For use during the labs.
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Preliminaries
• Finding things at MITC
– Meals, snacks, and coffee
– Restrooms
– Etc.
• Workshop schedule
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Contents
•
•
•
•
Overview
Multicast on the LAN
Source-Specific Multicast (SSM)
Any-Source Multicast (ASM)
– Intra-domain ASM
– Inter-domain ASM
• Troubleshooting Methodology
• Best Current Practices; Future of Multicast
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Overview
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The Basic Idea
Instead of sending a separate copy
of the data for each recipient, the source
sends the data only once, and routers
along the way to the destinations
make copies as needed.
Unicast does mass mailings;
multicast does chain letters.
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Unicast vs. Multicast
Unicast
Multicast
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The MBone
• The original multicast network was called the MBone. It used
a simple routing protocol called DVMRP (Distance Vector
Multicast Routing Protocol).
• As there were only isolated subnetworks that wanted to deal
with DVMRP, the old MBone used tunnels to get multicast
traffic between DVMRP subnetworks.
– i.e., the multicast traffic was hidden and sent between the
subnetworks via unicast.
• This mechanism was simple, but required manual
administration and absolutely could not scale to the entire
Internet.
• Worse, DVMRP requires substantial routing traffic behind
the scenes and this grew with the size of the MBone.
– Thus, the legend grew that multicast was a
bandwidth hog.
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Multicast Grows Up
• Starting about 1997, the building blocks for a multicast-enabled
Internet were put into place.
–An efficient modern multicast routing protocol, Protocol
Independent Multicast - Sparse Mode (PIM-SM), was
deployed. (PIM also has Dense and Sparse-Dense modes, but
these are not widely used.)
–The mechanisms for multicast peering were established, using
an extension to BGP called Multiprotocol BGP (MBGP), and
peering became routine.
–The service model was split into:
• a one-to-many (or “broadcast”) part:
a many-to-many part (e.g., for videoconferencing):
Any-Source Multicast (ASM), and
• Source-Specific Multicast (SSM).
• By 2001, these had completely replaced the old MBone.
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What capabilities does
IP Multicast provide ?
• Cost-efficient distribution of data
• Timely distribution of data
• Robust distribution of data
“Data” here could be
– Files
– Streamed Audio or Video
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Cost Efficient Data Distribution
• This is the core of the streaming case.
– Unicast streaming is just too expensive.
– This is true either on the commodity Internet
or on the Intranet.
– Multicast is especially compelling for video.
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Timely Distribution of Data
• This is the argument for multicast in
financial services.
• In unicast, it takes time to send packets
separately to each receiver.
– Even if cost is not a problem, time may be.
– Example: A DS3 would take 2 days to
distribute a 100 megabyte file to 10,000
desktops. With multicast, this would take 18
seconds.
– Multicasting is compelling here if timely
distribution is important.
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Robust Distribution of Data
• In some streaming or data distribution cases,
the problem is handling sudden large increases
in load.
• Multicast was designed to handle sudden large
increases in load.
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Case Study: 9/11/2001
Internet News “Melt-down”:
Web Site Performance 9:00 AM to 10:00 AM
Site
% Users able to access
ABCNews.com
0%
CNN.com
0%
NYTimes.com
0%
USAToday.com
18 %
MSNBC.com
22 %
(source: Keynote’s Business Performance / Interactive Week 9/17/2001)
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Internet News Performance on 9/11/2001
• Of course, the “melt-down” was caused by the
incredible demand for news after the attacks.
• Unicast streaming web sites suffered similar
problems, at least from anecdotal evidence
• By contrast, multicast performed well
– Large increase in traffic
– Roughly 1 Gigabit per second saved at peak
– Over time, the multicast peering mesh degraded,
but this was NOT due to increased traffic
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Eyewitness Accounts
We had a large plasma screen in the iLabs [at Networld+Interop] intended to demonstrate
high rate HDTV over I2. We came in Tuesday morning and were preparing for the first day
of the show when word came in about the initial plane crash into the towers. Our I2 Lead,
Roy Hockett was able to switch the stream to a CNN broadcast from UMich. We began
attracting exhibitors to the display even before the showfloor opened. Once the attendees
were on the floor, the crowd had grown to well over a hundred.
By this point, three things had happened. The crowds around the one display had grown so
large as to constitute a fire hazard, all the major news web sites had completely melted
down, and CNN was being multicast from several sources. We then started loading
multicast tools on every PC in the NOC, from the one driving the large video wall to people's
individual laptops. By 10:30 (about half an hour after the floor opened) we had at least 3
large displays as well as a number of normal monitors turned out towards the plexiglass
walls.
Soon after, we had a good number of exhibitors come and ask how to get "the CNN viewer
software.”
— Jim Martin, <jrm@nortelnetworks.com>
More than 1,000 copies of StreamPlayerII, our multicast MPEG viewer, were downloaded or
handed out on disk between 9/11 and 9/12. We normally average 20 to 100 per day.
— Rich Mavrogeanes <rmavro@vbrick.com>
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Viewership
Sudden increase in Multicast
traffic of at least 1000 group
members
– Mostly viewing VBrick’s
television broadcast
– Measured viewership >830
– But each measured point
could have many individual
viewers since they multicast
locally
BANDWIDTH SAVED: in
excess of 1 Gbps vs. unicast
Crowds viewing the 9/11 multicasts at Networld+Interop
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How is Multicast Being Used Today?
•Network Video!
•Netcast Events, TV over IP,
Distance Learning, Collaboration
•See https://mail.internet2.edu/wws/arc/bigvideo/
•Other applications
•Some examples follow...
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Video: Netcast Events
• Technical events
– IETF, NANOG: see videolab.uoregon.edu
– Joint Techs: http://jointtechs.es.net/vancouver2005/netcast.html
• Scientific events
– Undersea exploration with Robert Ballard: www.explorethesea.com
• Performance events
– Digital Video Transport Service
(http://apps.internet2.edu/dvts.html) provides relatively cheap &
painless high-quality network video, and is increasingly popular
for a wide variety of uses.
– DVTS over multicast is ideal for netcasting performance events.
– DancingQ performance event:
http://arts.internet2.edu/dancingq.html
http://people.internet2.edu/%7Ebdr/CometDVIP/DanceQ.html
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Video: TV over IP
• DV Guide: http://db.arts.usf.edu/dvguide/
• ResearchChannel:
www.researchchannel.org/projects/i2wg/prj_multi.asp
• Northwestern University
– Cable TV via campus networks:
http://www.tss.northwestern.edu/nutv/helpguide/
– C-SPAN over Abilene:
http://www.i2-multicast.northwestern.edu/
• Several other campuses (Cornell, Columbia, Duke...) have
TV-over-IP projects, or are considering them
• Open Student Television Network
– www.ostn.tv
– "a national initiative of student television stations working together
to build a digital channel of student television shows"
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Video: TV over IP
• Set-top boxes are available from several
vendors, e.g.:
–
–
–
–
–
www.vbrick.com/products/EtherneTV-stb.asp
www.aminocom.com
www.i3micro.com
www.2wire.com
www.bastinc.com
• Some corporations, particularly in the financial
sector, pay big bucks to have cable news
multicast on their intranets.
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Video: Distance Learning
• University of Hawaii uses multicast in its
Hawaii Interactive Television Service
– http://www.hawaii.edu/dl/general/
– Two-way interactive video and audio to all UH
campuses and education centers
– Each classroom can view and converse with at
least two other sites, and listen to additional sites
– Each campus can receive and transmit multiple
classes simultaneously
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Video: Distance Learning
U. of Hawaii
Interactive TV
Locations and
Staff Sites
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Video: Collaboration
Access Grid:
• www.accessgrid.org: "The Access Grid® is an ensemble
of resources...used to support group-to-group
interactions across the Grid."
• survey of AG multicast issues:
www.andrewpatrick.ca/multicast-survey/
• Access Grid via VRVS:
http://www.vrvs.org/Documentation/VAG/
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Other Applications
• While it seems clear that the killer app for
multicast will involve video, there are other
things you can do with it...
– radio: www.onthei.com, www.kexp.org
– file distribution (a popular intranet ASM application)
– NNTP
• Please keep us informed about your current and
planned applications!
– See https://mail.internet2.edu/wws/arc/wg-multicast
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Essential Multicast Terminology
IP source = IP unicast addr
Ethernet source = MAC addr
source
IP destination = IP multicast addr
Ethernet dest = MAC addr
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receivers
e.g., video server
Multicast stream
source = origin of multicast stream
multicast address = an IP address in the Class D range (224.0.0.0 – 239.255.255.255), used to
refer to multiple recipients. A multicast address is also called a multicast group
or channel.
multicast stream = stream of IP packets with multicast address for IP destination address. All
multicast uses UDP packets.
receiver(s) = recipient(s) of multicast stream
tree = the path taken by multicast data. Routing loops are not allowed, so there is always a
unique series of branches between the root of the tree and the receivers.
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(S,G) notation
• For every multicast source there must be
two pieces of information: the source IP
address, S, and the group address, G.
– These correspond to the sender and receiver
addresses in unicast.
– This is generally expressed as (S,G).
– Also commonly used is (*,G) - every source
for a particular group.
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Essential Multicast Protocols
Receivers
Delivery tree
Membership reports
Senders
Multicast Routing
Protocol (e.g. PIM-SM)
•
•
Group Management
Protocol (e.g. IGMP)
Group Management Protocol - enables hosts to dynamically join/leave multicast
groups. Receivers send group membership reports to the nearest router.
Multicast Routing Protocol - enables routers to build a delivery tree between the
sender(s) and receivers of a multicast group.
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Multicast Building Blocks
• The SENDERS send without worrying about receivers
– Packets are sent to a multicast address (RFC 1700)
– This is in the class D range
(224.0.0.0 - 239.255.255.255)
• The RECEIVERS inform the routers what they want to
receive
– done via Internet Group Management Protocol
(IGMP), version 2 (RFC 2236) or later
• The routers make sure the STREAMS make it to the
correct receiving networks.
– Multicast routing protocol: PIM-SM
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Multicast Protocol Summary
• Essential Protocols
– PIM-SM - Protocol Independent Multicast - Sparse Mode
is used to propagate forwarding state between routers.
– IGMP - Internet Group Management Protocol is used by
hosts and routers to tell each other about group
membership.
• Other Protocols (much more on these later in the
workshop)
– MBGP - Multiprotocol Border Gateway Protocol is used
to exchange routing information for inter-domain
reverse-path forwarding (RPF) checking.
– MSDP - Multicast Source Discovery Protocol is used to
exchange active-source information.
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Multicast Addressing
• IPv4 Multicast Group Addresses
– 224.0.0.0–239.255.255.255
– Class D Address Space
• High order bits of 1st Octet = “1110”
– Source sends to group address
– Receivers receive traffic sent to group address
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CIDR Address Notation
• The multicast address block is 224.0.0.0 to
239.255.255.255
• It is cumbersome to refer to address blocks in the above
fashion. Address blocks are usually described using
“CIDR notation”
– This specifies the start of a block, and the number of bits
THAT ARE FIXED.
• In this shorthand, the multicast address space can be described
as 224.0.0.0/4 or, even more simply, as 224/4. The fixed part of
the address is referred to as the prefix, and this block would be
pronounced "two twenty four slash four."
– Note that the LARGER the number after the slash, the
LONGER the prefix and the SMALLER the address block.
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Multicast Addressing
• RFC 3171
• http://www.iana.org/assignments/multicast-addresses
• Examples:
– 224.0.0.0 - 224.0.0.255 (224.0.0/24) - reserved & not forwarded
•
•
•
•
•
•
•
•
•
224.0.0.1 - All local hosts
224.0.0.2 - All local routers
224.0.0.4 - DVMRP
224.0.0.5 - OSPF
224.0.0.6 - Designated Router OSPF
224.0.0.9 - RIP2
224.0.0.13 - PIM
224.0.0.18 - VRRP
224.0.0.22 – IGMP
– 232.0.0.0 - 232.255.255.255 (232/8) - SSM
– 239.0.0.0 - 239.255.255.255 (239/8) - Administrative Scoping
• “Ordinary” multicasts don’t have to request a multicast address
from IANA.
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Scoping
• TTL value defines scope and limits distribution
– IP multicast packet must have TTL > interface TTL or it is
discarded
– No longer recommended as a reliable scoping mechanism
• Administratively Scoped Addresses – RFC 2365
– 239.0.0.0–239.255.255.255
– Private address space
• Similar to RFC 1918 unicast addresses
• Not used for global Internet traffic
• Used to limit “scope” of multicast traffic
• Same addresses may be in used in different sub-networks
for different multicast sessions
– Examples
• Site-local scope: 239.253.0.0/16
• Organization-local scope: 239.192.0.0/14
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Multicast Address Allocation
• For a long time, this was a sore spot. There was no
way to claim or register a Multicast Class D address
like unicast address blocks can be registered.
– For temporary teleconferences, this is not such a
problem, but it does not fit well into a broadcast model.
• Now, there are two solutions:
– For SSM, addresses don’t matter, as the broadcast
address is really unique as long as the (S,G) pair is
unique.
– For ASM, there is “GLOP”.
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Multicast Addressing
GLOP addresses
– Provides globally available private Class D
space
– 233.x.x/24 per AS number
– RFC 2770
How?
– Insert the 16-bit AS number into the
middle two octets of the 233/8
– Online GLOP calculator:
www.shepfarm.com/multicast/glop.html
– If you have an AS, you have multicast
addresses.
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Expanding Multicast
Address Assignment
• GLOP based address assignment has
worked well.
– Every organization gets the same amount of
space, a /24.
• What if you need more?
– There is an (as yet unused) mechanism for
requesting more GLOP space: RFC 3138.
– Is this unused because of lack of demand, or
because the mechanism is not fully
implemented?
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Prefix-based Multicast
Address Assignment
• Dave Thaler of Microsoft has proposed prefix-based
assignment.
– draft-ietf-mboned-ipv4-uni-based-mcast-02.txt
• The idea is that every unicast address assignment you have
is mapped into a multicast address range.
– Take one of the unused multicast /8's
– For a /N unicast assignment, the multicast address block
becomes
• [/8] [/N][24 - N bits of available addresses]
– So, a /24 provides a /32; a /16 provides a /24; a /8 provides
a /16
• This would complement GLOP by giving larger organizations
more addresses.
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