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Multicast. Matthew Wolf College of Computing Georgia Institute of Technology. Overview. Why Multicast? A user’s perspective. The ABC’s of Multicast – some important acronyms and what they mean. I. Why Multicast?.

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Matthew Wolf

College of Computing

Georgia Institute of Technology

  • Why Multicast? A user’s perspective.
  • The ABC’s of Multicast – some important acronyms and what they mean.
i why multicast
I. Why Multicast?
  • The last time Bill Gates gave a web-cast speech, how many simultaneous connections did they need to use?
    • OK, I don’t know, but it was lots.
  • Multicast supports a group communication model.
    • Everyone who’s interested in the web-cast is a group → only one outgoing video feed.
  • Huge reductions in bandwidth!
example access grid
Example: Access Grid
  • The Access Grid
    • 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.
access grid cont
Access Grid (cont)
  • Realities: Mainly video & audio right now, along with distributed PowerPoint.
  • This isn’t 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
multicast to the rescue
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.
ii the abc s of multicast
II. The ABC’s of Multicast

Flood & Prune

Messages automatically go everywhere, except where excluded. Router builds table from prune messages.

pim protocol independent multicast
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.
pim sm


Sparse Mode



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.

  • 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
problems with ism
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...
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)
discovery of resources
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.
miscellaneous future developments
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.
for more information
For More Information
  • Internet2 WG – at
  • 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.