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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.