Multicast

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Multicast

• Matthew Wolf
• College of Computing
• Georgia Institute of Technology

www.cercs.gatech.edu
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Overview

1. Why Multicast? A users perspective.
2. The ABCs of Multicast some important acronyms
   and what they mean.

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I. Why Multicast?

• The last time Bill Gates gave a web-cast speech,
  how many simultaneous connections did they need
  to use?
• OK, I dont know, but it was lots.

• Multicast supports a group communication model.
• Everyone whos interested in the web-cast is a
  group ? only one outgoing video feed.
• Huge reductions in bandwidth!

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Example Access Grid

• The Access Grid
• http//www.accessgrid.org
• A large-group teleconferencing facility
• The human interactions interface to grid
  computing
• Core middleware with support for for multimedia
  streams, interfaces to grid data, and data
  visualizations
• Summary The futuristic Internet2 application you
  can use to justify whatever upgrades you want.

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Access Grid (cont)

• Realities Mainly video audio right now, along
  with distributed PowerPoint.
• This isnt a bad thing Bandwidth consumption
  can hit 45 Mb on big conferences just with this
  set.
• 90 nodes, 4 or more video streams audio per
  node

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A Day in the Lobby
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A Typical Day in the Lobby
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Multicast to the Rescue

• Multicast tools (vic rat) make the large-scale
  collaboration possible
• End-users only need to know a multicast address
  to send to, which defines the group.
• This is handled through a web interface
• The networking hardware manages getting the data
  to everyone else in the group.

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II. The ABCs of Multicast
Flood Prune
Messages automatically go everywhere, except
where excluded. Router builds table from prune
messages.
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PIM Protocol Independent Multicast

• PIM uses the unicast routing tables rather than
  building its own hence independent
• PIM-DM (dense mode)
• Uses the Flood and Prune idea
• PIM-SM (sparse mode)
• Messages only go where explicitly requested
• Client host uses IGMP (Internet group management
  protocol) to signal interest in multicast group
  to the last-hop router.

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PIM-SM
MSDP
Sparse Mode
RP
IGMP
Client specifies interest with IGMP. The RP
(rendezvous point) acts as a clearing house for
requests within a domain. MSDP allows RPs to
talk between domains.
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Routing

• MSDP (multicast source discovery protocol) allows
  peered Rendezvous Points to share group
  memberships.
• You still need a routing table to tell you how to
  get from one zone to another
• mBGP (multiprotocol Border Gateway Protocol)
  provides the solution
• Allows for different unicast multicast routes
• ISM (Internet Standard Multicast) is based on
  these protocols

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Problems with ISM

• Provides a good service model, but
• Lots of state gets held in the routers
• Discovery, updated delivery lists, etc
• Makes unintentional DoS attacks easier
• 100MB host talking through a 10MB hub.
• CS networking class projects...

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PIM-SSM

• SSM (Source-Specific Multicast) extends the IGMP
  message format.
• Application submits a (Source, Group) pair
• Router only builds tree to specified source
• IGMP v3 is required to support this
• V3 will add the ability to explicitly include or
  exclude a source (when it gets here)

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Discovery of Resources

• Finding the proper multicast address for
  particular content can be difficult
• With SSM, you need a multicast address and a
  (list of) source(s).
• SDAP (session directory announcement protocol)
  and SDP (session description protocol) give you
  tools to announce and describe your multicast
  group.
• Access Grid Virtual Venue is an example of an
  http-based discovery method.

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Miscellaneous Future Developments

• BGMP (border gateway multicast protocol) is a
  next generation replacement for MSDP
• Lacks the single point of failure a whole
  domain acts as the root of the broadcast tree,
  not just a single RP.

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For More Information

• Internet2 WG at http//www.internet2.edu/multica
  st
• Two upcoming events
• 1st I2 Multicast Hands-On Workshop. (In Eugene,
  most likely 19-21 June.)
• An "Ask the Experts" In-depth session the
  afternoon of 31 July as part of the next
  NLANR/Interent2 Techs Workshop in Boulder.