Designing IP Multicast Services © 2004 Cisco Systems, Inc. All rights reserved. Examining IP Multicast Services ARCH v1.28-1

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Unicast applications send one copy of each packet to every client unicast address. Unicast Traffic

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Unicast Traffic (Cont.)

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Unicast Traffic (Cont.)

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v A multicast server sends out a single data stream to multiple clients using a special broadcast address. Multicast Traffic

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Applications Current Video conferencing IP telephony music on hold Corporate-wide communications Distance learning Software distribution Any one-to-many data push applications Emerging Broadband access Videoconferencing Digital TV Digital audio Entertainment Personal digital assistants and home appliances

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Data Delivery Principles

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Characteristics Transmits to a host group Delivers with best-effort reliability Supports dynamic membership Supports diverse numbers and locations Supports membership in more than one group Supports multiple streams host

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Multicast Advantages Enhanced efficiency: Controls network traffic and reduces server and CPU loads Optimized performance: Eliminates traffic redundancy Distributed applications: Makes multipoint applications possible

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Multicast Disadvantages: Multicast Is UDP-Based Best-effort delivery Drops are to be expected. No congestion avoidance Lack of TCP windowing and slow-start mechanisms can result in network congestion. Duplicates Some protocol mechanisms result in the occasional generation of duplicate packets. Out-of-order delivery Some protocol mechanisms result in out-of-order delivery of packets.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Multicast Forwarding Multicast routing is backwards from unicast routing. –Unicast routing is concerned about where the packet is going. –Multicast routing is concerned about where the packet came from. Multicast routing uses Reverse Path Forwarding. –A router forwards a multicast datagram only if received on the up stream interface towards the source. –The routing table used for multicasting is checked against the source IP address in the packet.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Reverse Path Forwarding: RPF Checking

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Reverse Path Forwarding: RPF Check Fails

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Reverse Path Forwarding: RPF Check Succeeds

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Group Membership You must be a member of a group to receive data. If you send to a group address, all members receive it. You do not have to be a member of a group to send to a group.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Source Distribution Trees Uses more memory Supports optimal paths from source to all receivers Minimizes delay

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Shared Distribution Trees Uses less memory May result in sub-optimal paths from source to all receivers May introduce extra delay

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v PIM-DM Flood & Prune: Initial Flooding

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v PIM-DM Flood & Prune: Pruning Unwanted Traffic

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v PIM-DM Flood & Prune: Results After Pruning

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v PIM-SM: Shared Tree Join

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v PIM-SM Sender Registration

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v PIM-SM SPT Switchover

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Selecting PIM-DM or PIM-SM PIM-DM Advantages: Easy to configure Simple flood and prune mechanism Effective for small pilot networks Potential issues: Inefficient flood and prune behavior Mixed control and data planes –Results in (S, G) state in every router in the network –Can result in non- deterministic topological behaviors PIM-SM Advantages: Effective for sparse or dense distribution of multicast receivers Traffic only sent down joined branches Switching to optimal source trees for high-traffic sources dynamically Unicast routing protocol- independent Potential issues: Requires an RP during the initial distribution tree setup RPs can become bottlenecks unless selected carefully

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Components

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v The host sends an IGMP report to join the group. IGMP: Joining a Group

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IGMP: Maintaining a Group The router sends periodic queries to One member per group per subnet reports Other members suppress reports

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v The host quietly leaves the group. The router sends three general queries (60 seconds apart). No IGMP report for the group is received. The group times out (worst-case delay is about three minutes). IGMPv1: Leaving a Group

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IGMPv2: Leaving a Group The host sends a Leave message to The router sends a group specific query to No IGMP report is received within about 3 seconds. Group times out.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IGMPv3

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v IP Multicast Control Mechanisms Cisco Group Management Protocol –Protocol that allows Catalyst switches to leverage IGMP information on Cisco routers to make Layer 2 forwarding decisions Internet Group Management Protocol snooping –IP multicast constraining mechanism that runs on a data link layer LAN switch

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Summary IP multicast is an alternative to unicast and broadcast that sends packets to a subset of network hosts simultaneously. By requiring only a single copy of each packet to be sent on each interface, multicast helps reduce network traffic. IP multicast packets are replicated only at routers where paths diverge to reach the intended recipients. Multicast routing uses RPF to flood packets out all interfaces except packets incoming from the source. An IP multicast packets destination is a virtual group address. Members join a group and then begin receiving multicast packets addressed to that group.

© 2004 Cisco Systems, Inc. All rights reserved. ARCH v Summary (Cont.) PIM uses unicast routing information to perform the multicast forwarding function. PIM uses the unicast routing table to perform the RPF check function instead of building up a completely independent multicast routing table. By default, a data link layer switch will forward all multicast traffic to every port that belongs to the destination LAN. IP multicast control mechanisms limit multicast traffic to the ports that need to receive the data.