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Multicast

This article provides an overview of multicast communication and its benefits from a user's perspective. It covers important acronyms related to multicast and discusses various applications, such as the Access Grid. The article also explains the Flood & Prune concept, PIM protocols, routing, SSM, and future developments. It concludes by providing resources for further information and upcoming events related to multicast.

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Multicast

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  1. Multicast Matthew Wolf College of Computing Georgia Institute of Technology www.cercs.gatech.edu

  2. Overview • Why Multicast? A user’s perspective. • The ABC’s of Multicast – some important acronyms and what they mean.

  3. 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!

  4. 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.

  5. 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

  6. A Day in the Lobby:

  7. A Typical Day in the Lobby

  8. 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.

  9. II. The ABC’s of Multicast Flood & Prune Messages automatically go everywhere, except where excluded. Router builds table from prune messages.

  10. 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.

  11. 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.

  12. 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

  13. 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...

  14. 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)

  15. 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.

  16. 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.

  17. For More Information • Internet2 WG – at http://www.internet2.edu/multicast • 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.

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