2 IP Multicast Distribution Trees and Control Protocols Overview This lesson represents an entry point to IP multicast services, presents the functional model of IP multicasting and gives an overview of technologies present in IP multicasting. The student will grasp the idea of IP multicasting, its benefits and associated caveats, determine various types of multicast applications, and gain an understanding of the IP multicast conceptual model and its implementation prerequisites. Objectives Upon completion of this chapter, the student will be able to: n List the functions performed with a multicast-enabled network n Identify the types of multicast distribution trees and the way they are built n Explain various multicast routing protocols and their place in different network / application environments n Identify the protocols for group membership reporting 2-2 IP Multicast Technology Copyright  2000, Cisco Systems, Inc. Functions of Multicast Enabled Networks Objectives Upon completion of this section, the student will be able to: n Identify the functions of multicast-enabled routers n Explain how multicast data is forwarded loop free to receiver segments n Determine methods for scoping multicast streams Copyright  2000, Cisco Systems, Inc. IP Multicast Distribution Trees and Control Protocols 2-3 © 2000, Cisco Systems, Inc. www.cisco.com IP Multicast Primer Chapter2 Page5 Multicast Forwarding Multicast Forwarding •Multicast Routing works the opposite way of Unicast Routing • Unicast Routing is concerned with where the packet is going • Multicast Routing is concerned with where the packet comes from •Multicast Routing uses “Reverse Path Forwarding” to prevent forwarding loops In unicast routing, when the router receives the packet, the decision about where to forward the packet is made depending on the packet’s destination address. In multicast routing the decision about where to forward the multicast packet depends on where the packet came from. Multicast routers must know the packet’s origin, rather than its destination, which is just the opposite in unicast routing. In multicast origination, the IP address denotes the known source and the destination IP address denotes a group of unknown receivers. Multicast routing uses mechanism called Reverse Path Forwarding (RPF) to prevent forwarding loops and to ensure the shortest path from the source to the receivers. 2-4 IP Multicast Technology Copyright  2000, Cisco Systems, Inc. © 2000, Cisco Systems, Inc. www.cisco.com IP Multicast Primer Chapter2 Page6 Reverse Path Forwarding (RPF) Reverse Path Forwarding (RPF) • What is RPF? • A router forwards a multicast datagram only if received on the upstream interface to the source (I.e. it follows the distribution tree) • The RPF Check • The routing table for unicast is checked against the “source” address in the multicast datagram • If the datagram arrived on the interface specified in the routing table for the source address: – The RPF check succeeds – Otherwise, the RPF Check fails When a router receives a multicast packet, it checks its routing tables (usually unicast) to see if the interface the packet came from provides the shortest path back to the source. If the interface provides the shortest path to the source, the router will forward the packet. Otherwise, if the multicast packet is received from some other interface that does not provide the shortest path to the source, it’s silently discarded. This mechanism that multicast routing utilizes is called Reverse Path Forwarding (RPF). RPF ensures that multicast packets will follow the shortest path from the source to the receivers and that there will be no loops on that path. Copyright  2000, Cisco Systems, Inc. IP Multicast Distribution Trees and Control Protocols 2-5 © 2000, Cisco Systems, Inc. www.cisco.com IP Multicast Primer Chapter2 Page7 Reverse Path Forwarding (cont.) Reverse Path Forwarding (cont.) •Reverse Path Forwarding (RPF) Check: • If the RPF check succeeds, the datagram is forwarded • If the RPF check fails, the datagram is typically silently discarded • When a datagram is forwarded, it is sent out of each interface in the outgoing interface list • The packet is never sent back out of the RPF interface! After the router receives a multicast packet, it performs an RPF check. If the RPF check succeeds, the packet is forwarded; otherwise, it’s silently discarded. The multicast packet is forwarded out of each interface that is in the Outgoing Interface List (OIL). OIL entries point to the current router’s downstream multicast neighbors. The incoming interface (or RPF interface) on which the packet was received is never in the OIL; therefore, the packet is never forwarded back out of the RPF interface. 2-6 IP Multicast Technology Copyright  2000, Cisco Systems, Inc. Routers perform an RPF check to insure that arriving multicast packets were received via the interface that is on the most direct path to the source that sent the packets. In the above example, each router forwards received multicast packets to each of its neighbor routers. (This is indicated by the arrows that indicate the initial multicast traffic flow from source 151.10.3.21 throughout the network.) Observe that the two routers in the middle of the picture are each receiving multicast packets via the most direct path from the source indicated by the green arrows. These received packets arrived via the RPF interface (indicated by the green arrows) so both routers forward the multicast packets to all neighbors; in this case, each other. This results in the two routers receiving packets via the non-RPF interface (i.e. an interface that is not on the shortest path to the source) as shown by the red arrows. This causes the RPF check to fail and the packets to be silently discarded. Copyright  2000, Cisco Systems, Inc. IP Multicast Distribution Trees and Control Protocols 2-7 Let’s take a closer look at a failed RPF check. The router in our slide receives a multicast packet from source 151.10.3.21 on interface S0. The router performs the RPF check by looking into the unicast routing table. The unicast routing table indicates that interface S1 is the shortest path to the network 151.10.0.0/16. Because interface S0 is not the shortest path to the network from which the packet from the source 151.10.3.21 came from, the RPF check fails and the packet is discarded. 2-8 IP Multicast Technology Copyright  2000, Cisco Systems, Inc. Now let’s take a closer look at a successful RPF check. The router in our slide receives a multicast packet from source 151.10.3.21 on interface S1. The router performs the RPF check by looking into the unicast routing table. The unicast routing table indicates that interface S1 is the shortest path to the network 151.10.0.0/16. Since interface S1 is the shortest path to the network from which the packet from the source 151.10.3.21 came from, the RPF check succeeds and the packet is forwarded on every interface in OIL. In our example, OIL for current multicast packet consists of interfaces S2 and E0, so the packet is forwarded on interfaces S2 and E0. Copyright  2000, Cisco Systems, Inc. IP Multicast Distribution Trees and Control Protocols 2-9 An RPF check is always done with respect to the incoming interface – the RPF interface. The RPF check will succeed if the incoming interface is the shortest path to the source. The RPF interface is determined either by the underlying unicast routing protocol or the dedicated multicast routing protocol (e.g. DVMRP, MBGP, etc.). Note that changes in the unicast topology will not necessarily immediately reflect a change in RPF if the multicast routing protocol relies on underlying unicast routing tables. It depends on how frequently the RPF check is performed on a multicast forwarding entry - every five seconds is the current Cisco default. 2-10 IP Multicast Technology Copyright  2000, Cisco Systems, Inc. Sometimes we want to define the boundaries for certain multicast traffic. We don’t want all our multicast traffic to be heard all over our network or outside of our network boundaries, and, also, we may not want some external multicast traffic to enter our network. In order to achieve this goal we can use TTL Thresholds. TTL Thresholds may be set on a multicast router interface to limit the forwarding of multicast traffic to outgoing packets with TTLs greater than the TTL Threshold. A Zero TTL Threshold implies that no threshold has been set. As a multicast packet arrives, TTL is decremented by one. If the resulting TTL is less or equal to 0, it’s dropped. If a multicast packet is to be forwarded out of an interface with a non-zero TTL Threshold, then its TTL is checked against the TTL Threshold. If a packet’s TTL is less than the specified threshold, it is not forwarded out of the interface. [...]... the meaning of “push” and “pull” models and their association with dense and sparse mode multicast protocols IP Multicast Technology Copyright © 2000, Cisco Systems, Inc Multicast Distribution Trees define the path from the source to receivers over which the multicast traffic flows There are two types of Multicast Distribution Trees Source-rooted or Shortest Path Trees and Shared Trees In the case of... Interface and how is it identified? n How can you constrain multicast streams to be propagated outside a certain scope? Copyright © 2000, Cisco Systems, Inc IP Multicast Distribution Trees and Control Protocols 2-15 Multicast Distribution Trees and Protocol Types Objectives Upon completion of this section, the student will be able to: n n 2-16 Distinguish between source-rooted and shared distribution trees. .. IP Multicast Distribution Trees and Control Protocols 2-23 Dense Mode protocols implement the “PUSH” model for multicast data delivery This model is also called broadcast and prune or flood and prune because initial traffic is flooded to all points in the network After the initial flood, branches without receivers are pruned After a timeout, traffic is flooded throughout the network again 2-24 IP Multicast. .. multicasting Copyright © 2000, Cisco Systems, Inc IP Multicast Distribution Trees and Control Protocols 2-27 There are two types of multicast routing protocols Dense mode protocols use the PUSH model where multicast traffic is flooded throughout the network Sparse mode routing protocols are a better choice because they use the PULL method By using the PULL method, multicast traffic is sent only to where it’s... and G is the multicast group address 2-20 IP Multicast Technology Copyright © 2000, Cisco Systems, Inc In this example of a Shared Distribution Tree, we can see that sources Source1 and Source2 are sending multicast packets towards a Rendezvous Point via Source Path Trees, and, from the Rendezvous Point, the multicast packets are flowing via a Shared Distribution Tree towards receivers Receiver1 and. .. administration of multicast addresses difficult Copyright © 2000, Cisco Systems, Inc IP Multicast Distribution Trees and Control Protocols 2-13 In the example above, the company uses private multicast address range 239.128.0.0/10 and does Address Scoping on its boundaries Departments within the company use the subset of that range Eng and Mkt departments use the subset ranges 239.129.0.0/16 and 239.130.0.0/16... Copyright © 2000, Cisco Systems, Inc IP Multicast Distribution Trees and Control Protocols 2-17 The example above shows a Shortest Path Tree (SPT) between a source Source1 and receivers Receivers1 and Receiver2 We assume that the path between source and receivers over routers A, C and E is the path with the lowest cost Packets are forwarded according to Source and Group Address pair down the SPT For... 2000, Cisco Systems, Inc IP Multicast Distribution Trees and Control Protocols 2-29 Distribution trees are built in a variety of ways, depending upon the multicast routing protocol employed: n PIM utilizes the underlying unicast routing table (any unicast routing protocol) plus: – – Prune: routers prune their interfaces and/ or send PIM-PRUNE messages upstream to remove them from the distribution tree when... between source-rooted and shared distribution trees n Identify the meaning of push and pull models and their association with dense and sparse mode multicast protocols Review Questions n n Where is the traffic “pulled” from in an explicit sparse mode approach? n What are basic differences between (S, G) and (*, G) entries, and how are the entries created? n 2-26 Why are dense mode protocols associated... intensive? IP Multicast Technology Copyright © 2000, Cisco Systems, Inc Overview of Multicast Routing Protocols Objectives Upon completion of this section, the student will be able to: n Describe the issues of intradomain and interdomain multicast routing n List the representatives of dense mode and sparse mode intradomain protocols n Determine the current situation and protocols for interdomain multicasting . Inc. IP Multicast Distribution Trees and Control Protocols 2-17 Multicast Distribution Trees define the path from the source to receivers over which the multicast. the IP address of the source and G is the multicast group address. Copyright  2000, Cisco Systems, Inc. IP Multicast Distribution Trees and Control Protocols