Multicast Matthew Wolf College of Computing Georgia Institute of Technology

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

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 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) 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

A Day in the Lobby:

A Typical Day in the Lobby

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

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 RP IGMP

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

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