INTERNET-DRAFT T. Maufer C. Semeria Category: Informational 3Com Corporation March 1997 Introduction to IP Multicast Routing Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the internet-drafts Shadow Directories on: ftp.is.co.za (Africa) nic.nordu.net (Europe) ds.internic.net (US East Coast) ftp.isi.edu (US West Coast) munnari.oz.au (Pacific Rim) FOREWORD This document is introductory in nature. We have not attempted to describe every detail of each protocol, rather to give a concise overview in all cases, with enough specifics to allow a reader to grasp the essential details and operation of protocols related to multicast IP. Every effort has been made to ensure the accurate representation of any cited works, especially any works-in-progress. For the complete details, we refer you to the relevant specification(s). If internet-drafts are cited in this document, it is only because they are the only sources of certain technical information at the time of this writing. We expect that many of the internet-drafts which we have cited will eventually become RFCs. See the shadow directories above for the status of any of these drafts, their follow-on drafts, or possibly the resulting RFCs. Maufer & Semeria [Page 1] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ABSTRACT The first part of this paper describes the benefits of multicasting, the MBone, Class D addressing, and the operation of the Internet Group Management Protocol (IGMP). The second section explores a number of different techniques that may potentially be employed by multicast routing protocols: o Flooding o Spanning Trees o Reverse Path Broadcasting (RPB) o Truncated Reverse Path Broadcasting (TRPB) o Reverse Path Multicasting (RPM) o "Shared-Tree" Techniques The third part contains the main body of the paper. It describes how the previous techniques are implemented in multicast routing protocols available today (or under development). o Distance Vector Multicast Routing Protocol (DVMRP) o Multicast Extensions to OSPF (MOSPF) o Protocol-Independent Multicast - Dense Mode (PIM-DM) o Protocol-Independent Multicast - Sparse Mode (PIM-SM) o Core-Based Trees (CBT) Table of Contents Section 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . INTRODUCTION 1.1 . . . . . . . . . . . . . . . . . . . . . . . . . Multicast Groups 1.2 . . . . . . . . . . . . . . . . . . . . . Group Membership Protocol 1.3 . . . . . . . . . . . . . . . . . . . . Multicast Routing Protocols 1.3.1 . . . . . . . . . . . Multicast Routing vs. Multicast Forwarding 2 . . . . . . . . MULTICAST SUPPORT FOR EMERGING INTERNET APPLICATIONS 2.1 . . . . . . . . . . . . . . . . . . . . . . . Reducing Network Load 2.2 . . . . . . . . . . . . . . . . . . . . . . . . Resource Discovery 2.3 . . . . . . . . . . . . . . . Support for Datacasting Applications 3 . . . . . . . . . . . . . . THE INTERNET'S MULTICAST BACKBONE (MBone) 4 . . . . . . . . . . . . . . . . . . . . . . . . MULTICAST ADDRESSING 4.1 . . . . . . . . . . . . . . . . . . . . . . . . Class D Addresses 4.2 . . . . . . . Mapping a Class D Address to an IEEE-802 MAC Address 4.3 . . . . . . . . . Transmission and Delivery of Multicast Datagrams 5 . . . . . . . . . . . . . . INTERNET GROUP MANAGEMENT PROTOCOL (IGMP) 5.1 . . . . . . . . . . . . . . . . . . . . . . . . . . IGMP Version 1 5.2 . . . . . . . . . . . . . . . . . . . . . . . . . . IGMP Version 2 5.3 . . . . . . . . . . . . . . . . . . . . . . . . . . IGMP Version 3 6 . . . . . . . . . . . . . . . . . . . MULTICAST FORWARDING TECHNIQUES 6.1 . . . . . . . . . . . . . . . . . . . . . "Simpleminded" Techniques 6.1.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flooding 6.1.2 . . . . . . . . . . . . . . . . . . . . . . . . . . Spanning Tree 6.2 . . . . . . . . . . . . . . . . . . . Source-Based Tree Techniques Maufer & Semeria [Page 2] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 6.2.1 . . . . . . . . . . . . . . . . . Reverse Path Broadcasting (RPB) 6.2.1.1 . . . . . . . . . . . . . Reverse Path Broadcasting: Operation 6.2.1.2 . . . . . . . . . . . . . . . . . RPB: Benefits and Limitations 6.2.2 . . . . . . . . . . . Truncated Reverse Path Broadcasting (TRPB) 6.2.3 . . . . . . . . . . . . . . . . . Reverse Path Multicasting (RPM) 6.2.3.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . Operation 6.2.3.2 . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations 6.3 . . . . . . . . . . . . . . . . . . . . . . Shared Tree Techniques 6.3.1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Operation 6.3.2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Benefits 6.3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations 7 . . . . . . . . . . . . . . . . . . . "DENSE MODE" ROUTING PROTOCOLS 7.1 . . . . . . . . Distance Vector Multicast Routing Protocol (DVMRP) 7.1.1 . . . . . . . . . . . . . . . . . Physical and Tunnel Interfaces 7.1.2 . . . . . . . . . . . . . . . . . . . . . . . . . Basic Operation 7.1.3 . . . . . . . . . . . . . . . . . . . . . DVMRP Router Functions 7.1.4 . . . . . . . . . . . . . . . . . . . . . . . DVMRP Routing Table 7.1.5 . . . . . . . . . . . . . . . . . . . . . DVMRP Forwarding Table 7.2 . . . . . . . . . . . . . . . Multicast Extensions to OSPF (MOSPF) 7.2.1 . . . . . . . . . . . . . . . . . . Intra-Area Routing with MOSPF 7.2.1.1 . . . . . . . . . . . . . . . . . . . . . Local Group Database 7.2.1.2 . . . . . . . . . . . . . . . . . Datagram's Shortest Path Tree 7.2.1.3 . . . . . . . . . . . . . . . . . . . . . . . Forwarding Cache 7.2.2 . . . . . . . . . . . . . . . . . . Mixing MOSPF and OSPF Routers 7.2.3 . . . . . . . . . . . . . . . . . . Inter-Area Routing with MOSPF 7.2.3.1 . . . . . . . . . . . . . . . . Inter-Area Multicast Forwarders 7.2.3.2 . . . . . . . . . . . Inter-Area Datagram's Shortest Path Tree 7.2.4 . . . . . . . . . Inter-Autonomous System Multicasting with MOSPF 7.3 . . . . . . . . . . . . . . . Protocol-Independent Multicast (PIM) 7.3.1 . . . . . . . . . . . . . . . . . . . . PIM - Dense Mode (PIM-DM) 8 . . . . . . . . . . . . . . . . . . . "SPARSE MODE" ROUTING PROTOCOLS 8.1 . . . . . . . Protocol-Independent Multicast - Sparse Mode (PIM-SM) 8.1.1 . . . . . . . . . . . . . . Directly Attached Host Joins a Group 8.1.2 . . . . . . . . . . . . Directly Attached Source Sends to a Group 8.1.3 . . . . . . . Shared Tree (RP-Tree) or Shortest Path Tree (SPT)? 8.1.4 . . . . . . . . . . . . . . . . . . . . . . . Unresolved Issues 8.2 . . . . . . . . . . . . . . . . . . . . . . Core Based Trees (CBT) 8.2.1 . . . . . . . . . . . . . . . . . . Joining a Group's Shared Tree 8.2.2 . . . . . . . . . . . . . . . . . . . . . Data Packet Forwarding 8.2.3 . . . . . . . . . . . . . . . . . . . . . . . Non-Member Sending 8.2.4 . . . . . . . . . . . . . . . . . CBT Multicast Interoperability 9 . . . . . . INTEROPERABILITY FRAMEWORK FOR MULTICAST BORDER ROUTERS 9.1 . . . . . . . . . . . . . Requirements for Multicast Border Routers 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFERENCES 10.1 . . . . . . . . . . . . . . . . . . . Requests for Comments (RFCs) 10.2 . . . . . . . . . . . . . . . . . . . . . . . . . Internet-Drafts 10.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . Textbooks 10.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other 11 . . . . . . . . . . . . . . . . . . . . . . SECURITY CONSIDERATIONS 12 . . . . . . . . . . . . . . . . . . . . . . . . . . ACKNOWLEDGEMENTS 13 . . . . . . . . . . . . . . . . . . . . . . . . . AUTHORS' ADDRESSES Maufer & Semeria [Page 3] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 1. INTRODUCTION There are three fundamental types of IPv4 addresses: unicast, broadcast, and multicast. A unicast address is used to transmit a packet to a single destination. A broadcast address is used to send a datagram to an entire subnetwork. A multicast address is designed to enable the delivery of datagrams to a set of hosts that have been configured as members of a multicast group across various subnetworks. Multicasting is not connection-oriented. A multicast datagram is delivered to destination group members with the same "best-effort" reliability as a standard unicast IP datagram. This means that multicast datagrams are not guaranteed to reach all members of a group, nor to arrive in the same order in which they were transmitted. The only difference between a multicast IP packet and a unicast IP packet is the presence of a 'group address' in the Destination Address field of the IP header. Instead of a Class A, B, or C IP destination address, multicasting employs a Class D address format, which ranges from 224.0.0.0 to 239.255.255.255. 1.1 Multicast Groups Individual hosts are free to join or leave a multicast group at any time. There are no restrictions on the physical location or the number of members in a multicast group. A host may be a member of more than one multicast group at any given time and does not have to belong to a group to send packets to members of a group. 1.2 Group Membership Protocol A group membership protocol is employed by routers to learn about the presence of group members on their directly attached subnetworks. When a host joins a multicast group, it transmits a group membership protocol message for the group(s) that it wishes to receive, and sets its IP process and network interface card to receive frames addressed to the multicast group. This receiver-initiated join process has excellent scaling properties since, as the multicast group increases in size, it becomes ever more likely that a new group member will be able to locate a nearby branch of the multicast delivery tree. [This space was intentionally left blank.] Maufer & Semeria [Page 4] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== _ _ _ _ |_| |_| |_| |_| '-' '-' '-' '-' | | | | <- - - - - - - - - -> | | v Router ^ / \ _ ^ + + ^ _ |_|-| / \ |-|_| '_' | + + | '_' _ | v v | _ |_|-|- - >|Router| <- + - + - + -> |Router|<- -|-|_| '_' | | '_' _ | | _ |_|-| |-|_| '_' | | '_' v v LEGEND <- - - -> Group Membership Protocol <-+-+-+-> Multicast Routing Protocol Figure 1: Multicast IP Delivery Service ======================================================================= 1.3 Multicast Routing Protocols Multicast routers execute a multicast routing protocol to define delivery paths that enable the forwarding of multicast datagrams across an internetwork. 1.3.1 Multicast Routing vs. Multicast Forwarding Multicast routing protocols establish or help establish the distribution tree for a given group, which enables multicast forwarding of packets addressed to the group. In the case of unicast, routing protocols are also used to build a forwarding table (commonly called a routing table). Unicast destinations are entered in the routing table, and associated with a metric and a next-hop router toward the destination. The key difference between unicast forwarding and multicast forwarding is that multicast packets must be forwarded away from their source. If a packet is ever forwarded back toward its source, a forwarding loop could have formed, possibly leading to a multicast "storm." Maufer & Semeria [Page 5] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 Each routing protocol constructs a forwarding table in its own way; the forwarding table tells each router that for a certain source, or for a given source sending to a certain group (called a (source, group) pair), packets are expected to arrive on a certain "inbound" or "upstream" interface and must be copied to certain (set of) "outbound" or "downstream" interface(s) in order to reach all known subnetworks with group members. 2. MULTICAST SUPPORT FOR EMERGING INTERNET APPLICATIONS Today, the majority of Internet applications rely on point-to-point transmission. The utilization of point-to-multipoint transmission has traditionally been limited to local area network applications. Over the past few years the Internet has seen a rise in the number of new applications that rely on multicast transmission. Multicast IP conserves bandwidth by forcing the network to do packet replication only when necessary, and offers an attractive alternative to unicast transmission for the delivery of network ticker tapes, live stock quotes, multiparty videoconferencing, and shared whiteboard applications (among others). It is important to note that the applications for IP Multicast are not solely limited to the Internet. Multicast IP can also play an important role in large commercial internetworks. 2.1 Reducing Network Load Assume that a stock ticker application is required to transmit packets to 100 stations within an organization's network. Unicast transmission to this set of stations will require the periodic transmission of 100 packets where many packets may in fact be traversing the same link(s). Multicast transmission is the ideal solution for this type of application since it requires only a single packet stream to be transmitted by the source which is replicated at forks in the multicast delivery tree. Broadcast transmission is not an effective solution for this type of application since it affects the CPU performance of each and every station that sees the packet. Besides, it wastes bandwidth. 2.2 Resource Discovery Some applications utilize multicast instead of broadcast transmission to transmit packets to group members residing on the same subnetwork. However, there is no reason to limit the extent of a multicast transmission to a single LAN. The time-to-live (TTL) field in the IP header can be used to limit the range (or "scope") of a multicast transmission. Maufer & Semeria [Page 6] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 2.3 Support for Datacasting Applications Since 1992, the IETF has conducted a series of "audiocast" experiments in which live audio and video were multicast from the IETF meeting site to destinations around the world. In this case, "datacasting" takes compressed audio and video signals from the source station and transmits them as a sequence of UDP packets to a group address. Multicast delivery today is not limited to audio and video. Stock quote systems are one example of a (connectionless) data-oriented multicast application. Someday reliable multicast transport protocols may facilitate efficient inter-computer communication. Reliable multicast transport protocols are currently an active area of research and development. 3. THE INTERNET'S MULTICAST BACKBONE (MBone) The Internet Multicast Backbone (MBone) is an interconnected set of subnetworks and routers that support the delivery of IP multicast traffic. The goal of the MBone is to construct a semipermanent IP multicast testbed to enable the deployment of multicast applications without waiting for the ubiquitous deployment of multicast-capable routers in the Internet. The MBone has grown from 40 subnets in four different countries in 1992, to more than 3400 subnets in over 25 countries by March 1997. With new multicast applications and multicast-based services appearing, it seems likely that the use of multicast technology in the Internet will keep growing at an ever-increasing rate. The MBone is a virtual network that is layered on top of sections of the physical Internet. It is composed of islands of multicast routing capability connected to other islands by virtual point-to-point links called "tunnels." The tunnels allow multicast traffic to pass through the non-multicast-capable parts of the Internet. Tunneled IP multicast packets are encapsulated as IP-over-IP (i.e., the protocol number is set to 4) so they look like normal unicast packets to intervening routers. The encapsulation is added on entry to a tunnel and stripped off on exit from a tunnel. This set of multicast routers, their directly-connected subnetworks, and the interconnecting tunnels comprise the MBone. Since the MBone and the Internet have different topologies, multicast routers execute a separate routing protocol to decide how to forward multicast packets. The majority of the MBone routers currently use the Distance Vector Multicast Routing Protocol (DVMRP), although some portions of the MBone execute either Multicast OSPF (MOSPF) or the Protocol-Independent Multicast (PIM) routing protocols. The operation of each of these protocols is discussed later in this paper. Maufer & Semeria [Page 7] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== +++++++ / |Island | \ /T/ | A | \T\ /U/ +++++++ \U\ /N/ | \N\ /N/ | \N\ /E/ | \E\ /L/ | \L\ ++++++++ +++++++ ++++++++ | Island | | Island| ---------| Island | | B | | C | Tunnel | D | ++++++++ +++++++ --------- ++++++++ \ \ | \T\ | \U\ | \N\ | \N\ +++++++ \E\ |Island | \L\| E | v v | _ |_|-|- - >|Router| <- + - + - + -> |Router|<- -|-|_| '_' | | '_' _ | | _ |_|-| |-|_| '_' | | '_' v v LEGEND <- - - -> Group Membership Protocol <-+-+-+-> Multicast Routing Protocol Figure 1: Multicast IP Delivery Service ======================================================================= 1.3 Multicast Routing Protocols Multicast routers execute a multicast routing protocol to define delivery paths that enable the forwarding of multicast datagrams across an internetwork. 1.3.1 Multicast Routing vs. Multicast Forwarding Multicast routing protocols establish or help establish the distribution tree for a given group, which enables multicast forwarding of packets addressed to the group. In the case of unicast, routing protocols are also used to build a forwarding table (commonly called a routing table). Unicast destinations are entered in the routing table, and associated with a metric and a next-hop router toward the destination. The key difference between unicast forwarding and multicast forwarding is that multicast packets must be forwarded away from their source. If a packet is ever forwarded back toward its source, a forwarding loop could have formed, possibly leading to a multicast "storm." Maufer & Semeria [Page 5] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 Each routing protocol constructs a forwarding table in its own way; the forwarding table tells each router that for a certain source, or for a given source sending to a certain group (called a (source, group) pair), packets are expected to arrive on a certain "inbound" or "upstream" interface and must be copied to certain (set of) "outbound" or "downstream" interface(s) in order to reach all known subnetworks with group members. 2. MULTICAST SUPPORT FOR EMERGING INTERNET APPLICATIONS Today, the majority of Internet applications rely on point-to-point transmission. The utilization of point-to-multipoint transmission has traditionally been limited to local area network applications. Over the past few years the Internet has seen a rise in the number of new applications that rely on multicast transmission. Multicast IP conserves bandwidth by forcing the network to do packet replication only when necessary, and offers an attractive alternative to unicast transmission for the delivery of network ticker tapes, live stock quotes, multiparty videoconferencing, and shared whiteboard applications (among others). It is important to note that the applications for IP Multicast are not solely limited to the Internet. Multicast IP can also play an important role in large commercial internetworks. 2.1 Reducing Network Load Assume that a stock ticker application is required to transmit packets to 100 stations within an organization's network. Unicast transmission to this set of stations will require the periodic transmission of 100 packets where many packets may in fact be traversing the same link(s). Multicast transmission is the ideal solution for this type of application since it requires only a single packet stream to be transmitted by the source which is replicated at forks in the multicast delivery tree. Broadcast transmission is not an effective solution for this type of application since it affects the CPU performance of each and every station that sees the packet. Besides, it wastes bandwidth. 2.2 Resource Discovery Some applications utilize multicast instead of broadcast transmission to transmit packets to group members residing on the same subnetwork. However, there is no reason to limit the extent of a multicast transmission to a single LAN. The time-to-live (TTL) field in the IP (hex); to be clear, the range from 01-00-5E-00-00-00 to 01-00-5E-FF-FF-FF is reserved for IP multicast groups. A simple procedure was developed to map Class D addresses to this reserved MAC-layer multicast address block. This allows IP multicasting to easily take advantage of the hardware-level multicasting supported by network interface cards. The mapping between a Class D IP address and an IEEE-802 (e.g., FDDI, Ethernet) MAC-layer multicast address is obtained by placing the low- order 23 bits of the Class D address into the low-order 23 bits of IANA's reserved MAC-layer multicast address block. This simple procedure removes the need for an explicit protocol for multicast address resolution on LANs akin to ARP for unicast. All LAN stations know this simple transformation, and can easily send any IP multicast over any IEEE-802-based LAN. Figure 4 illustrates how the multicast group address 234.138.8.5 (or EA-8A-08-05 expressed in hex) is mapped into an IEEE-802 multicast address. Note that the high-order nine bits of the IP address are not mapped into the MAC-layer multicast address. The mapping in Figure 4 places the low-order 23 bits of the IP multicast group ID into the low order 23 bits of the IEEE-802 multicast address. Note that the mapping may place up to multiple IP groups into the same IEEE-802 address because the upper five bits of the IP class D address are not used. Thus, there is a 32-to-1 ratio of IP class D addresses to valid MAC-layer multicast addresses. In practice, there is a small chance of collisions, should multiple groups happen to pick class D addresses that map to the same MAC-layer multicast address. However, chances are that higher-layer protocols will let hosts interpret which packets are for them (i.e., the chances of two different groups picking the same class D address and the same set of UDP ports is extremely unlikely). For example, the class D addresses 224.10.8.5 (E0-0A-08-05) and 225.138.8.5 (E1-8A-08-05) map to the same IEEE-802 MAC-layer multicast address (01-00-5E-0A-08-05) used in this example. Maufer & Semeria Informational [Page 10] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== Class D Address: 234.138.8.5 (EA-8A-08-05) | E A | 8 Class-D IP |_______ _______|__ _ _ _ Address |-+-+-+-+-+-+-+-|-+ - - - |1 1 1 0 1 0 1 0|1 |-+-+-+-+-+-+-+-|-+ - - - ................... IEEE-802 ....not......... MAC-Layer .............. Multicast ....mapped.. Address ........... |-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+ - - - |0 0 0 0 0 0 0 1|0 0 0 0 0 0 0 0|0 1 0 1 1 1 1 0|0 |-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+ - - - |_______ _______|_______ _______|_______ _______|_______ | 0 1 | 0 0 | 5 E | 0 [Address mapping below continued from half above] | 8 A | 0 8 | 0 5 | |_______ _______|_______ _______|_______ _______| Class-D IP - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-| Address | 0 0 0 1 0 1 0|0 0 0 0 1 0 0 0|0 0 0 0 0 1 0 1| - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-| \____________ ____________________/ \___ ___/ \ / | 23 low-order bits mapped | v - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-| IEEE-802 | 0 0 0 1 0 1 0|0 0 0 0 1 0 0 0|0 0 0 0 0 1 0 1| MAC-Layer - - - -|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-|-+-+-+-+-+-+-+-| Multicast |_______ _______|_______ _______|_______ _______| Address | 0 A | 0 8 | 0 5 | Figure 4: Mapping between Class D and IEEE-802 Multicast Addresses ======================================================================== Maufer & Semeria Informational [Page 11] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 4.3 Transmission and Delivery of Multicast Datagrams When the sender and receivers are members of the same (LAN) subnetwork, the transmission and reception of multicast frames is a straightforward process. The source station simply addresses the IP packet to the multicast group, the network interface card maps the Class D address to the corresponding IEEE-802 multicast address, and the frame is sent. Receivers that wish to capture the frame notify their MAC and IP layers that they want to receive datagrams addressed to the group. Things become somewhat more complex when the sender is attached to one subnetwork and receivers reside on different subnetworks. In this case, the routers must implement a multicast routing protocol that permits the construction of multicast delivery trees and supports multicast packet forwarding. In addition, each router needs to implement a group membership protocol that allows it to learn about the existence of group members on its directly attached subnetworks. 5. INTERNET GROUP MANAGEMENT PROTOCOL (IGMP) The Internet Group Management Protocol (IGMP) runs between hosts and their immediately-neighboring multicast routers. The mechanisms of the protocol allow a host to inform its local router that it wishes to receive transmissions addressed to a specific multicast group. Also, routers periodically query the LAN to determine if any group members are still active. If there is more than one IP multicast router on the LAN, one of the routers is elected "querier" and assumes the responsibility of querying the LAN for the presence of any group members. Based on the group membership information learned from the IGMP, a router is able to determine which (if any) multicast traffic needs to be forwarded to each of its "leaf" subnetworks. Multicast routers use this information, in conjunction with a multicast routing protocol, to support IP multicasting across the Internet. 5.1 IGMP Version 1 IGMP Version 1 was specified in RFC-1112. According to the specification, multicast routers periodically transmit Host Membership Query messages to determine which host groups have members on their directly-attached networks. IGMP Query messages are addressed to the all-hosts group (224.0.0.1) and have an IP TTL = 1. This means that Query messages sourced from a router are transmitted onto the directly-attached subnetwork but are not forwarded by any other multicast routers. When a host receives an IGMP Query message, it responds with a Host Membership Report for each group to which it belongs, sent to each group to which it belongs. (This is an important point: While IGMP Queries Maufer & Semeria Informational [Page 12] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== Group 1 _____________________ ____ ____ | multicast | | | | | | router | |_H2_| |_H4_| |_____________________| ---- ---- +-----+ | | | <-----|Query| | | | +-----+ | | | | |---+----+-------+-------+--------+-----------------------+----| | | | | | | ____ ____ ____ | | | | | | |_H1_| |_H3_| |_H5_| ---- ---- ---- Group 2 Group 1 Group 1 Group 2 Figure 5: Internet Group Management Protocol-Query Message ======================================================================== are sent to the "all hosts on this subnet" class D address (224.0.0.1), IGMP Reports are sent to the group(s) to which the host(s) belong. IGMP Reports, like Queries, are sent with the IP TTL = 1, and thus are not forwarded beyond the local subnetwork.) In order to avoid a flurry of Reports, each host starts a randomly- chosen Report delay timer for each of its group memberships. If, during the delay period, another Report is heard for the same group, every other host in that group must reset its timer to a new random value. This procedure spreads Reports out over a period of time and thus minimizes Report traffic for each group that has at least one member on a given subnetwork. It should be noted that multicast routers do not need to be directly addressed since their interfaces are required to promiscuously receive all multicast IP traffic. Also, a router does not need to maintain a detailed list of which hosts belong to each multicast group; the router only needs to know that at least one group member is present on a given network interface. Multicast routers periodically transmit IGMP Queries to update their knowledge of the group members present on each network interface. If the router does not receive a Report from any members of a particular group after a number of Queries, the router assumes that group members Maufer & Semeria Informational [Page 13] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 are no longer present on an interface. Assuming this is a leaf subnet (i.e., a subnet with group members but no multicast routers connecting to additional group members further downstream), this interface is removed from the delivery tree(s) for this group. Multicasts will continue to be sent on this interface only if the router can tell (via multicast routing protocols) that there are additional group members further downstream reachable via this interface. When a host first joins a group, it immediately transmits an IGMP Report for the group rather than waiting for a router's IGMP Query. This reduces the "join latency" for the first host to join a given group on a particular subnetwork. "Join latency" is measured from the time when a host's first IGMP Report is sent, until the transmission of the first packet for that group onto that host's subnetwork. Of course, if the group is already active, the join latency is precisely zero. 5.2 IGMP Version 2 IGMP version 2 was distributed as part of the Distance Vector Multicast Routing Protocol (DVMRP) implementation ("mrouted") source code, from version 3.3 through 3.8. Initially, there was no detailed specification for IGMP version 2 other than this source code. However, the complete specification has recently been published in which will update the specification contained in the first appendix of RFC-1112. IGMP version 2 extends IGMP version 1 while maintaining backward compatibility with version 1 hosts. IGMP version 2 defines a procedure for the election of the multicast querier for each LAN. In IGMP version 2, the multicast router with the lowest IP address on the LAN is elected the multicast querier. In IGMP version 1, the querier election was determined by the multicast routing protocol. IGMP version 2 defines a new type of Query message: the Group-Specific Query. Group-Specific Query messages allow a router to transmit a Query to a specific multicast group rather than all groups residing on a directly attached subnetwork. Finally, IGMP version 2 defines a Leave Group message to lower IGMP's "leave latency." When the last host to respond to a Query with a Report wishes to leave that specific group, the host transmits a Leave Group message to the all-routers group (224.0.0.2) with the group field set to the group being left. In response to a Leave Group message, the router begins the transmission of Group-Specific Query messages on the interface that received the Leave Group message. If there are no Reports in response to the Group-Specific Query messages, then (if this is a leaf subnet) this interface is removed from the delivery tree(s) for this group (as was the case of IGMP version 1). Again, multicasts will continue to be sent on this interface if the router can tell (via multicast routing protocols) that there are additional group members further downstream reachable via this interface. Maufer & Semeria Informational [Page 14] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 "Leave latency" is measured from a router's perspective. In version 1 of IGMP, leave latency was the time from a router's hearing the last Report for a given group, until the router aged out that interface from the delivery tree for that group (assuming this is a leaf subnet, of course). Note that the only way for the router to tell that this was the LAST group member is that no reports are heard in some multiple of the Query Interval (this is on the order of minutes). IGMP version 2, with the addition of the Leave Group message, allows a group member to more quickly inform the router that it is done receiving traffic for a group. The router then must determine if this host was the last member of this group on this subnetwork. To do this, the router quickly queries the subnetwork for other group members via the Group-Specific Query message. If no members send reports after several of these Group- Specific Queries, the router can infer that the last member of that group has, indeed, left the subnetwork. The benefit of lowering the leave latency is that prune messages can be sent as soon as possible after the last member host drops out of the group, instead of having to wait for several minutes worth of Query intervals to pass. If a group was experiencing high traffic levels, it can be very beneficial to stop transmitting data for this group as soon as possible. 5.3 IGMP Version 3 IGMP version 3 is a preliminary draft specification published in . IGMP version 3 introduces support for Group- Source Report messages so that a host can elect to receive traffic from specific sources of a multicast group. An Inclusion Group-Source Report message allows a host to specify the IP addresses of the specific sources it wants to receive. An Exclusion Group-Source Report message allows a host to explicitly identify the sources that it does not want to receive. With IGMP version 1 and version 2, if a host wants to receive any traffic for a group, the traffic from all sources for the group must be forwarded onto the host's subnetwork. IGMP version 3 will help conserve bandwidth by allowing a host to select the specific sources from which it wants to receive traffic. Also, multicast routing protocols will be able to make use this information to conserve bandwidth when constructing the branches of their multicast delivery trees. Finally, support for Leave Group messages first introduced in IGMP version 2 has been enhanced to support Group-Source Leave messages. This feature allows a host to leave an entire group or to specify the specific IP address(es) of the (source, group) pair(s) that it wishes to leave. Note that at this time, not all existing multicast routing protocols have mechanisms to support such requests from group members. This is one issue that will be addressed during the development of IGMP version 3. Maufer & Semeria Informational [Page 15] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 6. MULTICAST FORWARDING TECHNIQUES IGMP provides the final step in a multicast packet delivery service since it is only concerned with the forwarding of multicast traffic from a router to group members on its directly-attached subnetworks. IGMP is not concerned with the delivery of multicast packets between neighboring routers or across an internetwork. To provide an internetwork delivery service, it is necessary to define multicast routing protocols. A multicast routing protocol is responsible for the construction of multicast delivery trees and enabling multicast packet forwarding. This section explores a number of different techniques that may potentially be employed by multicast routing protocols: o "Simpleminded" Techniques - Flooding - Spanning Trees o Source-Based Tree (SBT) Techniques - Reverse Path Broadcasting (RPB) - Truncated Reverse Path Broadcasting (TRPB) - Reverse Path Multicasting (RPM) o "Shared-Tree" Techniques Later sections will describe how these algorithms are implemented in the most prevalent multicast routing protocols in the Internet today (e.g., Distance Vector Multicast Routing Protocol (DVMRP), Multicast extensions to OSPF (MOSPF), Protocol-Independent Multicast (PIM), and Core-Based Trees (CBT). 6.1 "Simpleminded" Techniques Flooding and Spanning Trees are two algorithms that can be used to build primitive multicast routing protocols. The techniques are primitive due to the fact that they tend to waste bandwidth or require a large amount of computational resources within the multicast routers involved. Also, protocols built on these techniques may work for small networks with few senders, groups, and routers, but do not scale well to larger numbers of senders, groups, or routers. Also, the ability to handle arbitrary topologies may not be present or may only be present in limited ways. 6.1.1 Flooding The simplest technique for delivering multicast datagrams to all routers in an internetwork is to implement a flooding algorithm. The flooding procedure begins when a router receives a packet that is addressed to a multicast group. The router employs a protocol mechanism to determine whether or not it has seen this particular packet before. If it is the first reception of the packet, the packet is forwarded on all interfaces Maufer & Semeria Informational [Page 16] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 (except the one on which it arrived) guaranteeing that the multicast packet reaches all routers in the internetwork. If the router has seen the packet before, then the packet is discarded. A flooding algorithm is very simple to implement since a router does not have to maintain a routing table and only needs to keep track of the most recently seen packets. However, flooding does not scale for Internet-wide applications since it generates a large number of duplicate packets and uses all available paths across the internetwork instead of just a limited number. Also, the flooding algorithm makes inefficient use of router memory resources since each router is required to maintain a distinct table entry for each recently seen packet. 6.1.2 Spanning Tree A more effective solution than flooding would be to select a subset of the internetwork topology which forms a spanning tree. The spanning tree defines a structure in which only one active path connects any two routers of the internetwork. Figure 6 shows an internetwork and a spanning tree rooted at router RR. Once the spanning tree has been built, a multicast router simply forwards each multicast packet to all interfaces that are part of the spanning tree except the one on which the packet originally arrived. Forwarding along the branches of a spanning tree guarantees that the multicast packet will not loop and that it will eventually reach all routers in the internetwork. A spanning tree solution is powerful and would be relatively easy to implement since there is a great deal of experience with spanning tree protocols in the Internet community. However, a spanning tree solution can centralize traffic on a small number of links, and may not provide the most efficient path between the source subnetwork and group members. Also, it is computationally difficult to compute a spanning tree in large, complex topologies. 6.2 Source-Based Tree Techniques The following techniques all generate a source-based tree by various means. The techniques differ in the efficiency of the tree building process, and the bandwidth and router resources (i.e., state tables) used to build a source-based tree. 6.2.1 Reverse Path Broadcasting (RPB) A more efficient solution than building a single spanning tree for the entire internetwork would be to build a spanning tree for each potential source [subnetwork]. These spanning trees would result in source-based delivery trees emanating from the subnetworks directly connected to the Maufer & Semeria Informational [Page 17] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== A Sample Internetwork #----------------# / |\ / \ | | \ / \ | | \ / \ | | \ / \ | | \ / \ | | #------# \ | | / | \ \ | | / | \ \ | \ / | \-------# | \ / | -----/| | #-----------#----/ | | /|\--- --/| \ | | / | \ / \ \ | | / \ /\ | \ / | / \ / \ | \ / #---------#-- \ | ----# \ \ | / \--- #-/ A Spanning Tree for this Sample Internetwork # # \ / \ / \ / \ / \ / #------RR | \ | \ | \-------# | #-----------#---- /| | \ / | \ \ / \ | \ / \ | \ # # | # | # LEGEND # Router RR Root Router Figure 6: Spanning Tree ======================================================================== Maufer & Semeria Informational [Page 18] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 source stations. Since there are many potential sources for a group, a different delivery tree is constructed rooted at each active source. 6.2.1.1 Reverse Path Broadcasting: Operation The fundamental algorithm to construct these source-based trees is referred to as Reverse Path Broadcasting (RPB). The RPB algorithm is actually quite simple. For each source, if a packet arrives on a link that the local router believes to be on the shortest path back toward the packet's source, then the router forwards the packet on all interfaces except the incoming interface. If the packet does not arrive on the interface that is on the shortest path back toward the source, then the packet is discarded. The interface over which the router expects to receive multicast packets from a particular source is referred to as the "parent" link. The outbound links over which the router forwards the multicast packet are called "child" links for this source. This basic algorithm can be enhanced to reduce unnecessary packet duplication. If the local router making the forwarding decision can determine whether a neighboring router on a child link is "downstream," then the packet is multicast toward the neighbor. (A "downstream" neighbor is a neighboring router which considers the local router to be on the shortest path back toward a given source.) Otherwise, the packet is not forwarded on the potential child link since the local router knows that the neighboring router will just discard the packet (since it will arrive on a non-parent link for the source, relative to that downstream router). ======================================================================== Source | ^ | : shortest path back to the | : source for THIS router | : "parent link" _ ______|!2|_____ | | --"child -|!1| |!3| - "child -- link" | ROUTER | link" |_______________| Figure 7: Reverse Path Broadcasting - Forwarding Algorithm ======================================================================== Maufer & Semeria Informational [Page 19] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 The information to make this "downstream" decision is relatively easy to derive from a link-state routing protocol since each router maintains a topological database for the entire routing domain. If a distance- vector routing protocol is employed, a neighbor can either advertise its previous hop for the source as part of its routing update messages or "poison reverse" the route toward a source if it is not on the distribution tree for that source. Either of these techniques allows an upstream router to determine if a downstream neighboring router is on an active branch of the delivery tree for a certain source. Please refer to Figure 8 for a discussion describing the basic operation of the enhanced RPB algorithm. ======================================================================== Source Station------>O A # +|+ + | + + O + + + 1 2 + + + + + + B + + C O-#- - - - -3- - - - -#-O +|+ -|+ + | + - | + + O + - O + + + - + + + - + 4 5 6 7 + + - + + + E - + + + - + D #- - - - -8- - - - -#- - - - -9- - - - -# F | | | O O O LEGEND O Leaf + + Shortest-path - - Branch # Router Figure 8: Reverse Path Broadcasting - Example ======================================================================== Maufer & Semeria Informational [Page 20] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 Note that the source station (S) is attached to a leaf subnetwork directly connected to Router A. For this example, we will look at the RPB algorithm from Router B's perspective. Router B receives the multicast packet from Router A on link 1. Since Router B considers link 1 to be the parent link for the (source, group) pair, it forwards the packet on link 4, link 5, and the local leaf subnetworks if they contain group members. Router B does not forward the packet on link 3 because it knows from routing protocol exchanges that Router C considers link 2 as its parent link for the source. Router B knows that if it were to forward the packet on link 3, it would be discarded by Router C since the packet would not be arriving on Router C's parent link for this source. 6.2.1.2 RPB: Benefits and Limitations The key benefit to reverse path broadcasting is that it is reasonably efficient and easy to implement. It does not require that the router know about the entire spanning tree, nor does it require a special mechanism to stop the forwarding process (as flooding does). In addition, it guarantees efficient delivery since multicast packets always follow the "shortest" path from the source station to the destination group. Finally, the packets are distributed over multiple links, resulting in better network utilization since a different tree is computed for each source. One of the major limitations of the RPB algorithm is that it does not take into account multicast group membership when building the delivery tree for a source. As a result, datagrams may be unnecessarily forwarded onto subnetworks that have no members in a destination group. 6.2.2 Truncated Reverse Path Broadcasting (TRPB) Truncated Reverse Path Broadcasting (TRPB) was developed to overcome the limitations of Reverse Path Broadcasting. With information provided by IGMP, multicast routers determine the group memberships on each leaf subnetwork and avoid forwarding datagrams onto a leaf subnetwork if it does not contain at least one member of a given destination group. Thus, the delivery tree is "truncated" by the router if a leaf subnetwork has no group members. Figure 9 illustrates the operation of TRPB algorithm. In this example the router receives a multicast packet on its parent link for the Source. The router forwards the datagram on interface 1 since that interface has at least one member of G1. The router does not forward the datagram to interface 3 since this interface has no members in the destination group. The datagram is forwarded on interface 4 if and only if a downstream router considers this subnetwork to be part of its "parent link" for the Source. Maufer & Semeria Informational [Page 21] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ====================================================================== Source | : : | : (Source, G1) v | "parent link" | "child link" ___ G1 _______|2|_____ \ | | G3\\ _____ ___ ROUTER ___ ______ / G2 \| hub |--|1| |3|-----|switch|/ /|_____| ^-- ___ --^ |______|\ / ^ |______|4|_____| ^ \ G1 ^ .^--- ^ G3 ^ .^ | ^ ^ .^ "child link" ^ Forward | Truncate Figure 9: Truncated Reverse Path Broadcasting - (TRPB) ====================================================================== TRPB removes some limitations of RPB but it solves only part of the problem. It eliminates unnecessary traffic on leaf subnetworks but it does not consider group memberships when building the branches of the delivery tree. 6.2.3 Reverse Path Multicasting (RPM) Reverse Path Multicasting (RPM) is an enhancement to Reverse Path Broadcasting and Truncated Reverse Path Broadcasting. RPM creates a delivery tree that spans only: o Subnetworks with group members, and o Routers and subnetworks along the shortest path to subnetworks with group members. RPM allows the source-based "shortest-path" tree to be pruned so that datagrams are only forwarded along branches that lead to active members of the destination group. 6.2.3.1 Operation When a multicast router receives a packet for a (source, group) pair, the first packet is forwarded following the TRPB algorithm across all Maufer & Semeria Informational [Page 22] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 routers in the internetwork. Routers on the edge of the network (which have only leaf subnetworks) are called leaf routers. The TRPB algorithm guarantees that each leaf router will receive at least the first multicast packet. If there is a group member on one of its leaf subnetworks, a leaf router forwards the packet based on this group membership information. ======================================================================== Source | : | : (Source, G) | v | | o-#-G |********** ^ | * , | * ^ | * o , | * / o-#-o #*********** ^ |\ ^ |\ * ^ | o ^ | G * , | , | * ^ | ^ | * , | , | * # # # /|\ /|\ /|\ o o o o o o G o G LEGEND # Router o Leaf without group member G Leaf with group member *** Active Branch --- Pruned Branch ,>, Prune Message (direction of flow --> Figure 10: Reverse Path Multicasting (RPM) ======================================================================== If none of the subnetworks connected to the leaf router contain group members, the leaf router may transmit a "prune" message on its parent link, informing the upstream router that it should not forward packets for this particular (source, group) pair on the child interface on which it received the prune message. Prune messages are sent just one hop back toward the source. Maufer & Semeria Informational [Page 23] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 An upstream router receiving a prune message is required to store the prune information in memory. If the upstream router has no recipients on local leaf subnetworks and has received prune messages from each downstream neighbor on each of the child interfaces for this (source, group) pair, then the upstream router does not need to receive additional packets for the (source, group) pair. This implies that the upstream router can also generate a prune message of its own, one hop further back toward the source. This cascade of prune messages results in an active multicast delivery tree, consisting exclusively of "live" branches (i.e., branches that lead to active receivers). Since both the group membership and internetwork topology can change dynamically, the pruned state of the multicast delivery tree must be refreshed periodically. At regular intervals, the prune information expires from the memory of all routers and the next packet for the (source, group) pair is forwarded toward all downstream routers. This allows "stale state" (prune state for groups that are no longer active) to be reclaimed by the multicast routers. 6.2.3.2 Limitations Despite the improvements offered by the RPM algorithm, there are still several scaling issues that need to be addressed when attempting to develop an Internet-wide delivery service. The first limitation is that multicast packets must be periodically flooded across every router in the internetwork, onto every leaf subnetwork. This flooding is wasteful of bandwidth (until the updated prune state is constructed). This "flood and prune" paradigm is very powerful, but it wastes bandwidth and does not scale well, especially if there are receivers at the edge of the delivery tree which are connected via low-speed technologies (e.g., ISDN or modem). Also, note that every router participating in the RPM algorithm must either have a forwarding table entry for a (source, group) pair, or have prune state information for that (source, group) pair. It is clearly wasteful (especially as the number of active sources and groups increase) to place such a burden on routers that are not on every (or perhaps any) active delivery tree. Shared tree techniques are an attempt to address these scaling issues, which become quite acute when most groups' senders and receivers are sparsely distributed across the internetwork. 6.3 Shared Tree Techniques The most recent additions to the set of multicast forwarding techniques are based on a shared delivery tree. Unlike shortest-path tree algorithms which build a source-based tree for each source, or each (source, group) pair, shared tree algorithms construct a single delivery tree that is shared by all members of a group. The shared tree approach is quite similar to the spanning tree algorithm except it allows the Maufer & Semeria Informational [Page 24] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 definition of a different shared tree for each group. Stations wishing to receive traffic for a multicast group must explicitly join the shared delivery tree. Multicast traffic for each group is sent and received over the same delivery tree, regardless of the source. 6.3.1 Operation A shared tree may involve a single router, or set of routers, which comprise(s) the "core" of a multicast delivery tree. Figure 11 illustrates how a single multicast delivery tree is shared by all sources and receivers for a multicast group. ======================================================================== Source Source Source | | | | | | v v v [#] * * * * * [#] * * * * * [#] * ^ * ^ | * | join | * | join | [#] | [x] [x] : : member member host host LEGEND [#] Shared Tree "Core" Routers * * Shared Tree Backbone [x] Member-hosts' directly-attached routers Figure 11: Shared Multicast Delivery Tree ======================================================================== Similar to other multicast forwarding algorithms, shared tree algorithms do not require that the source of a multicast packet be a member of a destination group in order to send to a group. 6.3.2 Benefits In terms of scalability, shared tree techniques have several advantages Maufer & Semeria Informational [Page 25] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 over source-based trees. Shared tree algorithms make efficient use of router resources since they only require a router to maintain state information for each group, not for each source, or for each (source, group) pair. (Remember that source-based tree techniques required all routers in an internetwork to either a) be on the delivery tree for a given source or (source, group) pair, or b) to have prune state for that source or (source, group) pair: So the entire internetwork must participate in the source-based tree protocol.) This improves the scalability of applications with many active senders since the number of source stations is no longer a scaling issue. Also, shared tree algorithms conserve network bandwidth since they do not require that multicast packets be periodically flooded across all multicast routers in the internetwork onto every leaf subnetwork. This can offer significant bandwidth savings, especially across low-bandwidth WAN links, and when receivers sparsely populate the domain of operation. Finally, since receivers are required to explicitly join the shared delivery tree, data only ever flows over those links that lead to active receivers. 6.3.3 Limitations Despite these benefits, there are still several limitations to protocols that are based on a shared tree algorithm. Shared trees may result in traffic concentration and bottlenecks near core routers since traffic from all sources traverses the same set of links as it approaches the core. In addition, a single shared delivery tree may create suboptimal routes (a shortest path between the source and the shared tree, a suboptimal path across the shared tree, a shortest path between the egress core router and the receiver's directly attached router) resulting in increased delay which may be a critical issue for some multimedia applications. (Simulations indicate that latency over a shared tree may be approximately 10% larger than source-based trees in many cases, but by the same token, this may be negligible for many applications.) Finally, expanding-ring searches are not supported inside shared-tree domains. 7. "DENSE MODE" ROUTING PROTOCOLS Certain multicast routing protocols are designed to work well in environments that have plentiful bandwidth and where it is reasonable to assume that receivers are rather densely distributed. In such scenarios, it is very reasonable to use periodic flooding, or other bandwidth-intensive techniques that would not necessarily be very scalable over a wide-area network. In section 8, we will examine different protocols that are specifically geared toward efficient WAN operation, especially for groups that have widely dispersed (i.e., sparse) membership. Maufer & Semeria Informational [Page 26] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 These routing protocols include: o Distance Vector Multicast Routing Protocol (DVMRP), o Multicast Extensions to Open Shortest Path First (MOSPF), o Protocol Independent Multicast - Dense Mode (PIM-DM). These protocols' underlying designs assume that the amount of protocol overhead (in terms of the amount of state that must be maintained by each router, the number of router CPU cycles required, and the amount of bandwidth consumed by protocol operation) is appropriate since receivers densely populate the area of operation. 7.1. Distance Vector Multicast Routing Protocol (DVMRP) The Distance Vector Multicast Routing Protocol (DVMRP) is a distance- vector routing protocol designed to support the forwarding of multicast datagrams through an internetwork. DVMRP constructs source-based multicast delivery trees using the Reverse Path Multicasting (RPM) algorithm. Originally, the entire MBone ran only DVMRP. Today, over half of the MBone routers still run some version of DVMRP. DVMRP was first defined in RFC-1075. The original specification was derived from the Routing Information Protocol (RIP) and employed the Truncated Reverse Path Broadcasting (TRPB) technique. The major difference between RIP and DVMRP is that RIP calculates the next-hop toward a destination, while DVMRP computes the previous-hop back toward a source. Since mrouted 3.0, DVMRP has employed the Reverse Path Multicasting (RPM) algorithm. Thus, the latest implementations of DVMRP are quite different from the original RFC specification in many regards. There is an active effort within the IETF Inter-Domain Multicast Routing (IDMR) working group to specify DVMRP version 3 in a standard form. The current DVMRP v3 Internet-Draft is: , or 7.1.1 Physical and Tunnel Interfaces The ports of a DVMRP router may be either a physical interface to a directly-attached subnetwork or a tunnel interface to another multicast- capable island. All interfaces are configured with a metric specifying cost for the given port, and a TTL threshold that limits the scope of a multicast transmission. In addition, each tunnel interface must be explicitly configured with two additional parameters: The IP address of the local router's tunnel interface and the IP address of the remote router's interface. Maufer & Semeria Informational [Page 27] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== TTL Scope Threshold ________________________________________________________________________ 0 Restricted to the same host 1 Restricted to the same subnetwork 15 Restricted to the same site 63 Restricted to the same region 127 Worldwide 191 Worldwide; limited bandwidth 255 Unrestricted in scope Table 1: TTL Scope Control Values ======================================================================== A multicast router will only forward a multicast datagram across an interface if the TTL field in the IP header is greater than the TTL threshold assigned to the interface. Table 1 lists the conventional TTL values that are used to restrict the scope of an IP multicast. For example, a multicast datagram with a TTL of less than 16 is restricted to the same site and should not be forwarded across an interface to other sites in the same region. TTL-based scoping is not always sufficient for all applications. Conflicts arise when trying to simultaneously enforce limits on topology, geography, and bandwidth. In particular, TTL-based scoping cannot handle overlapping regions, which is a necessary characteristic of administrative regions. In light of these issues, "administrative" scoping was created in 1994, to provide a way to do scoping based on multicast address. Certain addresses would be usable within a given administrative scope (e.g., a corporate internetwork) but would not be forwarded onto the global MBone. This allows for privacy, and address reuse within the class D address space. The range from 239.0.0.0 to 239.255.255.255 has been reserved for administrative scoping. While administrative scoping has been in limited use since 1994 or so, it has yet to be widely deployed. The IETF MBoneD working group is working on the deployment of administrative scoping. For additional information, please see or its successor, entitled "Administratively Scoped IP Multicast." 7.1.2 Basic Operation DVMRP implements the Reverse Path Multicasting (RPM) algorithm. According to RPM, the first datagram for any (source, group) pair is forwarded across the entire internetwork (providing the packet's TTL and router interface thresholds permit this). Upon receiving this traffic, leaf routers may transmit prune messages back toward the source if there are no group members on their directly-attached leaf subnetworks. The Maufer & Semeria Informational [Page 28] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 prune messages remove all branches that do not lead to group members from the tree, leaving a source-based shortest path tree. After a period of time, the prune state for each (source, group) pair expires to reclaim stale prune state (from groups that are no longer in use). If those groups are actually still in use, a subsequent datagram for the (source, group) pair will be flooded across all downstream routers. This flooding will result in a new set of prune messages, serving to regenerate the source-based shortest-path tree for this (source, group) pair. In current implementations of RPM (notably DVMRP), prune messages are not reliably transmitted, so the prune lifetime must be kept short to compensate for lost prune messages. DVMRP also implements a mechanism to quickly "graft" back a previously pruned branch of a group's delivery tree. If a router that had sent a prune message for a (source, group) pair discovers new group members on a leaf network, it sends a graft message to the previous-hop router for this source. When an upstream router receives a graft message, it cancels out the previously-received prune message. Graft messages cascade (reliably) hop-by-hop back toward the source until they reach the nearest "live" branch point on the delivery tree. In this way, previously-pruned branches are quickly restored to a given delivery tree. 7.1.3 DVMRP Router Functions In Figure 13, Router C is downstream and may potentially receive datagrams from the source subnetwork from Router A or Router B. If Router A's metric to the source subnetwork is less than Router B's metric, then Router A is dominant over Router B for this source. This means that Router A will forward any traffic from the source subnetwork and Router B will discard traffic received from that source. However, if Router A's metric is equal to Router B's metric, then the router with the lower IP address on its downstream interface (child link) becomes the Dominant Router for this source. Note that on a subnetwork with multiple routers forwarding to groups with multiple sources, different routers may be dominant for each source. 7.1.4 DVMRP Routing Table The DVMRP process periodically exchanges routing table updates with its DVMRP neighbors. These updates are logically independent of those generated by any unicast Interior Gateway Protocol. Since the DVMRP was developed to route multicast and not unicast traffic, a router will probably run multiple routing processes in practice: One to support the forwarding of unicast traffic and another to support the forwarding of multicast traffic. (This can be convenient: A router can be configured to only route multicast IP, with no unicast Maufer & Semeria Informational [Page 29] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== To .-<-<-<-<-<-<-Source Subnetwork->->->->->->->->--. v v | | parent link parent link | | _____________ _____________ | Router A | | Router B | | | | | ------------- ------------- | | child link child link | | --------------------------------------------------------------------- | parent link | _____________ | Router C | | | ------------- | child link | Figure 12. DVMRP Dominant Router in a Redundant Topology ======================================================================== IP routing. This may be a useful capability in firewalled environments.) Again, consider Figure 12: There are two types of routers in this figure: dominant and subordinate; assume in this example that Router B is dominant, Router A is subordinate, and Router C is part of the downstream distribution tree. In general, which routers are dominant or subordinate may be different for each source! A subordinate router is one that is NOT on the shortest path tree back toward a source. The dominant router can tell this because the subordinate router will 'poison-reverse' the route for this source in its routing updates which are sent on the common LAN (i.e., Router A sets the metric for this source to 'infinity'). The dominant router keeps track of subordinate routers on a per-source basis...it never needs or expects to receive a prune message from a subordinate router. Only routers that are truly on the downstream distribution tree will ever need to send prunes to the dominant router. If a dominant router on a LAN has received either a poison-reversed route for a source, or prunes for all groups emanating from that source subnetwork, then it may itself send a prune upstream Maufer & Semeria Informational [Page 30] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 toward the source (assuming also that IGMP has told it that there are no local receivers for any group from this source). A sample routing table for a DVMRP router is shown in Figure 13. Unlike ======================================================================== Source Subnet From Metric Status TTL Prefix Mask Gateway 128.1.0.0 255.255.0.0 128.7.5.2 3 Up 200 128.2.0.0 255.255.0.0 128.7.5.2 5 Up 150 128.3.0.0 255.255.0.0 128.6.3.1 2 Up 150 128.3.0.0 255.255.0.0 128.6.3.1 4 Up 200 Figure 13: DVMRP Routing Table ======================================================================== the table that would be created by a unicast routing protocol such as the RIP, OSPF, or the BGP, the DVMRP routing table contains Source Prefixes and From-Gateways instead of Destination Prefixes and Next-Hop Gateways. The routing table represents the shortest path (source-based) spanning tree to every possible source prefix in the internetwork--the Reverse Path Broadcasting (RPB) tree. The DVMRP routing table does not represent group membership or received prune messages. The key elements in DVMRP routing table include the following items: Source Prefix A subnetwork which is a potential or actual source of multicast datagrams. Subnet Mask The subnet mask associated with the Source Prefix. Note that the DVMRP provides the subnet mask for each source subnetwork (in other words, the DVMRP is classless). From-Gateway The previous-hop router leading back toward a particular Source Prefix. TTL The time-to-live is used for table management and indicates the number of seconds before an entry is removed from the routing table. This TTL has nothing at all to do with the TTL used in TTL-based scoping. Maufer & Semeria Informational [Page 31] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 7.1.5 DVMRP Forwarding Table Since the DVMRP routing table is not aware of group membership, the DVMRP process builds a forwarding table based on a combination of the information contained in the multicast routing table, known groups, and received prune messages. The forwarding table represents the local router's understanding of the shortest path source-based delivery tree for each (source, group) pair--the Reverse Path Multicasting (RPM) tree. ======================================================================== Source Multicast TTL InIntf OutIntf(s) Prefix Group 128.1.0.0 224.1.1.1 200 1 Pr 2p3p 224.2.2.2 100 1 2p3 224.3.3.3 250 1 2 128.2.0.0 224.1.1.1 150 2 2p3 Figure 14: DVMRP Forwarding Table ======================================================================== The forwarding table for a sample DVMRP router is shown in Figure 14. The elements in this display include the following items: Source Prefix The subnetwork sending multicast datagrams to the specified groups (one group per row). Multicast Group The Class D IP address to which multicast datagrams are addressed. Note that a given Source Prefix may contain sources for several Multicast Groups. InIntf The parent interface for the (source, group) pair. A 'Pr' in this column indicates that a prune message has been sent to the upstream router (the From-Gateway for this Source Prefix in the DVMRP routing table). OutIntf(s) The child interfaces over which multicast datagrams for this (source, group) pair are forwarded. A 'p' in this column indicates that the router has received a prune message(s) from a (all) downstream router(s) on this port. Maufer & Semeria Informational [Page 32] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 7.2. Multicast Extensions to OSPF (MOSPF) Version 2 of the Open Shortest Path First (OSPF) routing protocol is defined in RFC-1583. OSPF is an Interior Gateway Protocol (IGP) that distributes unicast topology information among routers belonging to a single OSPF "Autonomous System." OSPF is based on link-state algorithms which permit rapid route calculation with a minimum of routing protocol traffic. In addition to efficient route calculation, OSPF is an open standard that supports hierarchical routing, load balancing, and the import of external routing information. The Multicast Extensions to OSPF (MOSPF) are defined in RFC-1584. MOSPF routers maintain a current image of the network topology through the unicast OSPF link-state routing protocol. The multicast extensions to OSPF are built on top of OSPF Version 2 so that a multicast routing capability can be incrementally introduced into an OSPF Version 2 routing domain. Routers running MOSPF will interoperate with non-MOSPF routers when forwarding unicast IP data traffic. MOSPF does not support tunnels. 7.2.1 Intra-Area Routing with MOSPF Intra-Area Routing describes the basic routing algorithm employed by MOSPF. This elementary algorithm runs inside a single OSPF area and supports multicast forwarding when a source and all destination group members reside in the same OSPF area, or when the entire OSPF Autonomous System is a single area (and the source is inside that area...). The following discussion assumes that the reader is familiar with OSPF. 7.2.1.1 Local Group Database Similar to all other multicast routing protocols, MOSPF routers use the Internet Group Management Protocol (IGMP) to monitor multicast group membership on directly-attached subnetworks. MOSPF routers maintain a "local group database" which lists directly-attached groups and determines the local router's responsibility for delivering multicast datagrams to these groups. On any given subnetwork, the transmission of IGMP Host Membership Queries is performed solely by the Designated Router (DR). However, the responsibility of listening to IGMP Host Membership Reports is performed by not only the Designated Router (DR) but also the Backup Designated Router (BDR). Therefore, in a mixed LAN containing both MOSPF and OSPF routers, an MOSPF router must be elected the DR for the subnetwork. This can be achieved by setting the OSPF RouterPriority to zero in each non-MOSPF router to prevent them from becoming the (B)DR. The DR is responsible for communicating group membership information to all other routers in the OSPF area by flooding Group-Membership LSAs. Similar to Router-LSAs and Network-LSAs, Group-Membership LSAs are only flooded within a single area. Maufer & Semeria Informational [Page 33] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 7.2.1.2 Datagram's Shortest Path Tree The datagram's shortest path tree describes the path taken by a multicast datagram as it travels through the area from the source subnetwork to each of the group members' subnetworks. The shortest path tree for each (source, group) pair is built "on demand" when a router receives the first multicast datagram for a particular (source, group) pair. When the initial datagram arrives, the source subnetwork is located in the MOSPF link state database. The MOSPF link state database is simply the standard OSPF link state database with the addition of Group- Membership LSAs. Based on the Router- and Network-LSAs in the OSPF link state database, a source-based shortest-path tree is constructed using Dijkstra's algorithm. After the tree is built, Group-Membership LSAs are used to prune the tree such that the only remaining branches lead to subnetworks containing members of this group. The output of these algorithms is a pruned source-based tree rooted at the datagram's source. ======================================================================== S | | A # / \ / \ 1 2 / \ B # # C / \ \ / \ \ 3 4 5 / \ \ D # # E # F / \ \ / \ \ 6 7 8 / \ \ G # # H # I LEGEND # Router Figure 15. Shortest Path Tree for a (S, G) pair ======================================================================== Maufer & Semeria Informational [Page 34] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 To forward multicast datagrams to downstream members of a group, each router must determine its position in the datagram's shortest path tree. Assume that Figure 15 illustrates the shortest path tree for a given (source, group) pair. Router E's upstream node is Router B and there are two downstream interfaces: one connecting to Subnetwork 6 and another connecting to Subnetwork 7. Note the following properties of the basic MOSPF routing algorithm: o For a given multicast datagram, all routers within an OSPF area calculate the same source-based shortest path delivery tree. Tie-breakers have been defined to guarantee that if several equal-cost paths exist, all routers agree on a single path through the area. Unlike unicast OSPF, MOSPF does not support the concept of equal-cost multipath routing. o Synchronized link state databases containing Group-Membership LSAs allow an MOSPF router to build a source-based shortest- path tree in memory, working forward from the source to the group member(s). Unlike the DVMRP, this means that the first datagram of a new transmission does not have to be flooded to all routers in an area. o The "on demand" construction of the source-based delivery tree has the benefit of spreading calculations over time, resulting in a lesser impact for participating routers. Of course, this may strain the CPU(s) in a router if many new (source, group) pairs appear at about the same time, or if there are a lot of events which force the MOSPF process to flush and rebuild its forwarding cache. In a stable topology with long-lived multicast sessions, these effects should be minimal. 7.2.1.3 Forwarding Cache Each MOSPF router makes its forwarding decision based on the contents of its forwarding cache. Contrary to DVMRP, MOSPF forwarding is not RPF- based. The forwarding cache is built from the source-based shortest- path tree for each (source, group) pair, and the router's local group database. After the router discovers its position in the shortest path tree, a forwarding cache entry is created containing the (source, group) pair, its expected upstream interface, and the necessary downstream interface(s). The forwarding cache entry is now used to quickly forward all subsequent datagrams from this source to this group. If a new source begins sending to a new group, MOSPF must first calculate the distribution tree so that it may create a cache entry that can be used to forward the packet. Maufer & Semeria Informational [Page 35] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 Figure 16 displays the forwarding cache for an example MOSPF router. The elements in the display include the following items: Dest. Group A known destination group address to which datagrams are currently being forwarded, or to which traffic was sent "recently" (i.e., since the last topology or group membership or other event which (re-)initialized MOSPF's forwarding cache. Source The datagram's source host address. Each (Dest. Group, Source) pair uniquely identifies a separate forwarding cache entry. ======================================================================== Dest. Group Source Upstream Downstream TTL 224.1.1.1 128.1.0.2 11 12 13 5 224.1.1.1 128.4.1.2 11 12 13 2 224.1.1.1 128.5.2.2 11 12 13 3 224.2.2.2 128.2.0.3 12 11 7 Figure 16: MOSPF Forwarding Cache ======================================================================== Upstream Datagrams matching this row's Dest. Group and Source must be received on this interface. Downstream If a datagram matching this row's Dest. Group and Source is received on the correct Upstream interface, then it is forwarded across the listed Downstream interfaces. TTL The minimum number of hops a datagram must cross to reach any of the Dest. Group's members. An MOSPF router may discard a stem is a single area (and the source is inside that area...). The following discussion assumes that the reader is familiar with OSPF. 7.2.1.1 Local Group Database Similar to all other multicast routing protocols, MOSPF routers use the Internet Group Management Protocol (IGMP) to monitor multicast group membership on directly-attached subnetworks. MOSPF routers maintain a "local group database" which lists directly-attached groups and determines the local router's responsibility for delivering multicast datagrams to these groups. On any given subnetwork, the transmission of IGMP Host Membership Queries is performed solely by the Designated Router (DR). However, the responsibility of listening to IGMP Host Membership Reports is performed by not only the Designated Router (DR) but also the Backup Designated Router (BDR). Therefore, in a mixed LAN containing both MOSPF and OSPF routers, an MOSPF router must be elected the DR for the subnetwork. This can be achieved by setting the OSPF RouterPriority to zero in each non-MOSPF router to prevent them from becoming the (B)DR. The DR is responsible for communicating group membership information to all other routers in the OSPF area by flooding Group-Membership LSAs. Similar to Router-LSAs and Network-LSAs, Group-Membership LSAs are only flooded within a single area. Maufer & Semeria Informational [Page 33] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 7.2.1.2 Datagram's Shortest Path Tree The datagram's shortest path tree describes the path taken by a multicast datagram as it travels through the area from the source subnetwork to each of the group members' subnetworks. The shortest path tree for each (source, group) pair is built "on demand" when a router receives the first multicast datagram for a particular (source, group) pair. When the initial datagram arrives, the source subnetwork is located in the MOSPF link state database. The MOSPF link state database is simply the standard OSPF link state database with the addition of Group- Membership LSAs. Based on the Router- and Network-LSAs in the OSPF link state database, a source-based shortest-path tree is constructed using Dijkstra's algorithm. After the tree is built, Group-Membership LSAs are used to prune the tree such that the only remaining branches lead to subnetworks containing members of this group. The output of these algorithms is a pruned source-based tree rooted at the datagram's source. ======================================================================== S | | A # / \ / \ 1 2 / \ B # # C / \ \ / \ \ 3 4 5 / \ \ D # # E # F / \ \ / \ \ 6 7 8 / \ \ G # # H # I LEGEND # Router Figure 15. Shortest Path Tree for a (S, G) pair ======================================================================== Maufer & Semeria Informational [Page 34] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 To forward multicast datagrams to downstream members of a group, each router must determine its position in the datagram's shortest path tree. Assume that Figure 15 illustrates the shortest path tree for a given (source, group) pair. Router E's upstream node is Router B and there are two downstream interfaces: one connecting to Subnetwork 6 and another connecting to Subnetwork 7. Note the following properties of the basic MOSPF routing algorithm: o For a given multicast datagram, all routers within an OSPF is flooded into the backbone, but group membership from the backbone (or from any other non-backbone areas) is not flooded into any non-backbone area(s). To permit the forwarding of multicast traffic between areas, MOSPF introduces the concept of a "wild-card multicast receiver." A wild-card multicast receiver is a router that receives all multicast traffic generated in an area. In non-backbone areas, all inter-area multicast forwarders operate as wild-card multicast receivers. This guarantees that all multicast traffic originating in any non-backbone area is delivered to its inter-area multicast forwarder, and then if necessary into the backbone area. Since the backbone knows group membership for all areas, the datagram can be forwarded to the appropriate location(s) in the OSPF autonomous system, if only it is forwarded into the backbone by the source area's multicast ABR. 7.2.3.2 Inter-Area Datagram's Shortest-Path Tree In the case of inter-area multicast routing, it is usually impossible to build a complete shortest-path delivery tree. Incomplete trees are a fact of life because each OSPF area's complete topological and group membership information is not distributed between OSPF areas. Topological estimates are made through the use of wild-card receivers and OSPF Summary-Links LSAs. If the source of a multicast datagram resides in the same area as the router performing the calculation, the pruning process must be careful to ensure that branches leading to other areas are not removed from the tree. Only those branches having no group members nor wild-card multicast receivers are pruned. Branches containing wild-card multicast receivers must be retained since the local routers do not know whether there are any group members residing in other areas. If the source of a multicast datagram resides in a different area than the router performing the calculation, the details describing the local topology surrounding the source station are not known. However, this information can be estimated using information provided by Summary-Links LSAs for the source subnetwork. In this case, the base of the tree Maufer & Semeria Informational [Page 38] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 begins with branches directly connecting the source subnetwork to each of the local area's inter-area multicast forwarders. Datagrams sourced from outside the local area will enter the area via one of its inter- area multicast forwarders, so they all must be part of the candidate distribution tree. Since each inter-area multicast forwarder is also an ABR, it must maintain a separate link state database for each attached area. Thus each inter-area multicast forwarder is required to calculate a separate forwarding tree for each of its attached areas. 7.2.4 Inter-Autonomous System Multicasting with MOSPF Inter-Autonomous System multicasting involves the situation where a datagram's source or some of its destination group members are in different OSPF Autonomous Systems. In OSPF terminology, "inter-AS" communication also refers to connectivity between an OSPF domain and another routing domain which could be within the same Autonomous System from the perspective of an Exterior Gateway Protocol. To facilitate inter-AS multicast routing, selected Autonomous System Boundary Routers (ASBRs) are configured as "inter-AS multicast forwarders." MOSPF makes the assumption that each inter-AS multicast forwarder executes an inter-AS multicast routing protocol which forwards multicast datagrams in a reverse path forwarding (RPF) manner. Since the publication of the MOSPF RFC, a term has been defined for such a router: Multicast Border Router. See section 9 for an overview of the MBR concepts. Each inter-AS multicast forwarder is a wildcard multicast receiver in each of its attached areas. This guarantees that each inter-AS multicast forwarder remains on all pruned shortest-path trees and receives all multicast datagrams. The details of inter-AS forwarding are very similar to inter-area forwarding. On the "inside" of the OSPF domain, the multicast ASBR must conform to all the requirements of intra-area and inter-area forwarding. Within the OSPF domain, group members are reached by the usual forward path computations, and paths to external sources are approximated by a reverse-path source-based tree, with the multicast ASBR standing in for the actual source. When the source is within the OSPF AS, and there are external group members, it falls to the inter- AS multicast forwarders, in their role as wildcard receivers, to make sure that the data gets out of the OSPF domain and sent off in the correct direction. 7.3 Protocol-Independent Multicast (PIM) The Protocol Independent Multicast (PIM) routing protocols have been developed by the Inter-Domain Multicast Routing (IDMR) working group of the IETF. The objective of the IDMR working group is to develop one--or possibly more than one--standards-track multicast routing protocol(s) Maufer & Semeria Informational [Page 39] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 that can provide scaleable multicast routing across the Internet. PIM is actually two protocols: PIM - Dense Mode (PIM-DM) and PIM - Sparse Mode (PIM-SM). In the remainder of this introduction, any references to "PIM" apply equally well to either of the two protocols... there is no intention to imply that there is only one PIM protocol. While PIM-DM and PIM-SM share part of their names, and they do have related control messages, they are actually two completely independent protocols. PIM receives its name because it is not dependent on the mechanisms provided by any particular unicast routing protocol. However, any implementation supporting PIM requires the presence of a unicast routing protocol to provide routing table information and to adapt to topology changes. PIM makes a clear distinction between a multicast routing protocol that is designed for dense environments and one that is designed for sparse environments. Dense-mode refers to a protocol that is designed to operate in an environment where group members are relatively densely packed and bandwidth is plentiful. Sparse-mode refers to a protocol that is optimized for environments where group members are distributed across many regions of the Internet and bandwidth is not necessarily widely available. It is important to note that sparse-mode does not imply that the group has a few members, just that they are widely dispersed across the Internet. The designers of PIM-SM argue that DVMRP and MOSPF were developed for environments where group members are densely distributed, and bandwidth is relatively plentiful. They emphasize that when group members and senders are sparsely distributed across a wide area, DVMRP and MOSPF do not provide the most efficient multicast delivery service. The DVMRP periodically sends multicast packets over many links that do not lead to group members, while MOSPF can send group membership information over links that do not lead to senders or receivers. 7.3.1 PIM - Dense Mode (PIM-DM) While the PIM architecture was driven by the need to provide scaleable sparse-mode delivery trees, PIM also defines a new dense-mode protocol instead of relying on existing dense-mode protocols such as DVMRP and MOSPF. It is envisioned that PIM-DM would be deployed in resource rich environments, such as a campus LAN where group membership is relatively dense and bandwidth is likely to be readily available. PIM-DM's control messages are similar to PIM-SM's by design. [This space was intentionally left blank.] Maufer & Semeria Informational [Page 40] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 PIM - Dense Mode (PIM-DM) is similar to DVMRP in that it employs the Reverse Path Multicasting (RPM) algorithm. However, there are several important differences between PIM-DM and DVMRP: o To find routes back to sources, PIM-DM relies on the presence of an existing unicast routing table. PIM-DM is independent of the mechanisms of any specific unicast routing protocol. In contrast, DVMRP contains an integrated routing protocol that makes use of its own RIP-like exchanges to build its own unicast routing table (so a router may orient itself with respect to active source(s)). MOSPF augments the information in the OSPF link state database, thus MOSPF must run in conjunction with OSPF. o Unlike the DVMRP which calculates a set of child interfaces for each (source, group) pair, PIM-DM simply forwards multicast traffic on all downstream interfaces until explicit prune messages are received. PIM-DM is willing to accept packet duplication to eliminate routing protocol dependencies and to avoid the overhead inherent in determining the parent/child relationships. For those cases where group members suddenly appear on a pruned branch of the delivery tree, PIM-DM, like DVMRP, employs graft messages to re-attach the previously pruned branch to the delivery tree. 8. "SPARSE MODE" ROUTING PROTOCOLS The most recent additions to the set of multicast routing protocols are called "sparse mode" protocols. They are designed from a different perspective than the "dense mode" protocols that we have already examined. Often, they are not data-driven, in the sense that forwarding state is set up in advance, and they trade off using bandwidth liberally (which is a valid thing to do in a campus LAN environment) for other techniques that are much more suited to scaling over large WANs, where bandwidth is scarce and expensive. These emerging routing protocols include: o Protocol Independent Multicast - Sparse Mode (PIM-SM), and o Core-Based Trees (CBT). While these routing protocols are designed to operate efficiently over a wide area network where bandwidth is scarce and group members may be quite sparsely distributed, this is not to imply that they are only suitable for small groups. Sparse doesn't mean small, rather it is meant to convey that the groups are widely dispersed, and thus it is wasteful to (for instance) flood their data periodically across the entire internetwork. Maufer & Semeria Informational [Page 41] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 8.1 Protocol-Independent Multicast - Sparse Mode (PIM-SM) As described previously, PIM also defines a "dense-mode" or source-based tree variant. Again, the two protocols are quite unique, and other than control messages, they have very little in common. Note that while PIM integrates control message processing and data packet forwarding among PIM-Sparse and -Dense Modes, PIM-SM and PIM-DM must run in separate regions. All groups in a region are either sparse-mode or dense-mode. PIM-Sparse Mode (PIM-SM) has been developed to provide a multicast routing protocol that provides efficient communication between members of sparsely distributed groups--the type of groups that are likely to be common in wide-area internetworks. PIM's designers observed that several hosts wishing to participate in a multicast conference do not justify flooding the entire internetwork periodically with the group's multicast traffic. Noting today's existing MBone scaling problems, and extrapolating to a future of ubiquitous multicast (overlaid with perhaps thousands of small, widely dispersed groups), it is not hard to imagine that existing multicast routing protocols will experience scaling problems. To eliminate these potential scaling issues, PIM-SM is designed to limit multicast traffic so that only those routers interested in receiving traffic for a particular group "see" it. PIM-SM differs from existing dense-mode protocols in two key ways: o Routers with adjacent or downstream members are required to explicitly join a sparse mode delivery tree by transmitting join messages. If a router does not join the pre-defined delivery tree, it will not receive multicast traffic addressed to the group. In contrast, dense-mode protocols assume downstream group membership and forward multicast traffic on downstream links until explicit prune messages are received. Thus, the default forwarding action of dense-mode routing protocols is to forward all traffic, while the default action of a sparse-mode protocol is to block traffic unless it has been explicitly requested. o PIM-SM evolved from the Core-Based Trees (CBT) approach in that it employs the concept of a "core" (or rendezvous point (RP) in PIM-SM terminology) where receivers "meet" sources. [This space was intentionally left blank.] Maufer & Semeria Informational [Page 42] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== S1 S2 ___|___ ___|___ | | | | # # \ / \ / \_____________RP______________/ ./|\. ________________// | \\_______________ / _______/ | \______ \ # # # # # ___|___ ___|___ ___|___ ___|___ ___|___ | | | | | | R R R R R R LEGEND # PIM Router R Multicast Receiver Figure 17: Rendezvous Point ======================================================================== When joining a group, each receiver uses IGMP to notify its directly- attached router, which in turn joins the multicast delivery tree by sending an explicit PIM-Join message hop-by-hop toward the group's RP. A source uses the RP to announce its presence, and act as a conduit to members that have joined the group. This model requires sparse-mode routers to maintain a bit of state (the RP-set for the sparse-mode region) prior to the arrival of data. In contrast, because dense-mode protocols are data-driven, they do not store any state for a group until the arrival of its first data packet. There is only one RP-set per sparse-mode domain, not per group. Moreover, the creator of a group is not involved in RP selection. Also, there is no such concept as a "primary" RP. Each group has precisely one RP at any given time. In the event of the failure of an RP, a new RP-set is distributed which does not include the failed RP. 8.1.1 Directly Attached Host Joins a Group When there is more than one PIM router connected to a multi-access LAN, the router with the highest IP address is selected to function as the Designated Router (DR) for the LAN. The DR may or may not be responsible for the transmission of IGMP Host Membership Query messages, but does send Join/Prune messages toward the RP, and maintains the status of the active RP for local senders to multicast groups. Maufer & Semeria Informational [Page 43] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 When the DR receives an IGMP Report message for a new group, the DR determines if the group is RP-based or not by examining the group address. If the address indicates a SM group (by virtue of the group- specific state that even inactive groups have stored in all PIM routers), the DR performs a deterministic hash function over the sparse-mode region's RP-set to uniquely determine the RP for the group. ======================================================================== Source (S) _|____ | | # / \ / \ / \ # # / \ Designated / \ Host | Router / \ Rendezvous Point -----|- # - - - - - -#- - - - - - - -RP for group G (receiver) | ----Join--> ----Join--> | LEGEND # PIM Router RP Rendezvous Point Figure 18: Host Joins a Multicast Group ======================================================================== After performing the lookup, the DR creates a multicast forwarding entry for the (*, group) pair and transmits a unicast PIM-Join message toward the primary RP for this specific group. The (*, group) notation indicates an (any source, group) pair. The intermediate routers forward the unicast PIM-Join message, creating a forwarding entry for the (*, group) pair only if such a forwarding entry does not yet exist. Intermediate routers must create a forwarding entry so that they will be able to forward future traffic downstream toward the DR which originated the PIM-Join message. 8.1.2 Directly Attached Source Sends to a Group When a source first transmits a multicast packet to a group, its DR forwards the datagram to the primary RP for subsequent distribution along the group's delivery tree. The DR encapsulates the initial Maufer & Semeria Informational [Page 44] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 multicast packets in a PIM-SM-Register packet and unicasts them toward the primary RP for the group. The PIM-SM-Register packet informs the RP of a new source which causes the active RP to transmit PIM-Join messages back toward the source's DR. The routers between the RP and the source's DR use the received PIM-Join messages (from the RP) to create forwarding state for the new (source, group) pair. Now all routers from the active RP for this sparse-mode group to the source's DR will be able to forward future unencapsulated multicast packets from this source subnetwork to the RP. Until the (source, group) state has been created in all the routers between the RP and source's DR, the DR must continue to send the source's multicast IP packets to the RP as unicast packets encapsulated within unicast PIM-Register packets. The DR may stop forwarding multicast packets encapsulated in this manner once it has received a PIM-Register-Stop message from the active RP for this group. The RP may send PIM-Register-Stop messages if there are no downstream receivers for a group, or if the RP has successfully joined the (source, group) tree (which originates at the source's DR). ======================================================================== Source (S) _|____ | | # v /.\ , / ^\ v / .\ , # ^# v / .\ , Designated / ^\ v Host | Router / .\ , | Host -----|-#- - - - - - -#- - - - - - - -RP- - - # - - -|----- (receiver) | <~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~> | (receiver) LEGEND # PIM Router RP Rendezvous Point > , > PIM-Register < . < PIM-Join ~ ~ ~ Resend to group members Figure 19: Source sends to a Multicast Group ======================================================================== Maufer & Semeria Informational [Page 45] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 8.1.3 Shared Tree (RP-Tree) or Shortest Path Tree (SPT)? The RP-tree provides connectivity for group members but does not optimize the delivery path through the internetwork. PIM-SM allows routers to either a) continue to receive multicast traffic over the shared RP-tree, or b) subsequently create a source-based shortest-path tree on behalf of their attached receiver(s). Besides reducing the delay between this router and the source (beneficial to its attached receivers), the shared tree also reduces traffic concentration effects on the RP-tree. A PIM-SM router with local receivers has the option of switching to the source's shortest-path tree (i.e., source-based tree) once it starts receiving data packets from the source. The change-over may be triggered if the data rate from the source exceeds a predefined threshold. The local receiver's last-hop router does this by sending a Join message toward the active source. After the source-based SPT is active, protocol mechanisms allow a Prune message for the same source to be transmitted to the active RP, thus removing this router from the shared RP-tree. Alternatively, the DR may be configured to continue using the shared RP-tree and never switch over to the source-based SPT, or a router could perhaps use a different administrative metric to decide if and when to switch to a source-based tree. ======================================================================== Source (S) _|____ | %| % # % / \* % / \* % / \* Designated % # #* Router % / \* % / \* Host | <-% % % % % % / \v -----|-#- - - - - - -#- - - - - - - -RP (receiver) | <* * * * * * * * * * * * * * * | LEGEND # PIM Router RP Rendezvous Point * * RP-Tree (Shared) % % Shortest-Path Tree (Source-based) Figure 20: Shared RP-Tree and Shortest Path Tree (SPT) ======================================================================== Maufer & Semeria Informational [Page 46] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 Besides a last-hop router being able to switch to a source-based tree, there is also the capability of the RP for a group to transition to a source's shortest-path tree. Similar controls (bandwidth threshhold, administrative weights, etc.) can be used at an RP to influence these decisions. 8.2 Core Based Trees (CBT) Core Based Trees is another multicast architecture that is based on a shared delivery tree. It is specifically intended to address the important issue of scalability when supporting multicast applications across the public Internet. Similar to PIM-SM, CBT is protocol-independent. CBT employs the information contained in the unicast routing table to build its shared delivery tree. It does not care how the unicast routing table is derived, only that a unicast routing table is present. This feature allows CBT to be deployed without requiring the presence of any specific unicast routing protocol. Another similarity to PIM-SM is that CBT has adopted the core discovery mechanism ("bootstrap" ) defined in the PIM-SM specification. For inter-domain discovery, efforts are underway to standardize (or at least separately specify) a common RP/Core discovery mechanism. The intent is that any shared tree protocol could implement this common discovery mechanism using its own protocol message types. In a significant departure from PIM-SM, CBT has decided to maintain it's scaling characteristics by not offering the option of shifting from a Shared Tree (e.g., PIM-SM's RP-Tree) to a Shortest Path Tree (SPT) to optimize delay. The designers of CBT believe that this is a critical decision since when multicasting becomes widely deployed, the need for routers to maintain large amounts of state information will become the overpowering scaling factor. Finally, unlike PIM-SM's shared tree state, CBT state is bi-directional. Data may therefore flow in either direction along a branch. Thus, data from a source which is directly attached to an existing tree branch need not be encapsulated. 8.2.1 Joining a Group's Shared Tree A host that wants to join a multicast group issues an IGMP host membership report. This message informs its local CBT-aware router(s) that it wishes to receive traffic addressed to the multicast group. Upon receipt of an IGMP host membership report for a new group, the local CBT router issues a JOIN_REQUEST hop-by-hop toward the group's core router. Maufer & Semeria Informational [Page 47] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 If the JOIN_REQUEST encounters a router that is already on the group's shared tree before it reaches the core router, then that router issues a JOIN_ACK hop-by-hop back toward the sending router. If the JOIN_REQUEST does not encounter an on-tree CBT router along its path towards the core, then the core router is responsible for responding with a JOIN_ACK. In either case, each intermediate router that forwards the JOIN_REQUEST towards the core is required to create a transient "join state." This transient "join state" includes the multicast group, and the JOIN_REQUEST's incoming and outgoing interfaces. This information allows an intermediate router to forward returning JOIN_ACKs along the exact reverse path to the CBT router which initiated the JOIN_REQUEST. As the JOIN_ACK travels towards the CBT router that issued the JOIN_REQUEST, each intermediate router creates new "active state" for this group. New branches are established by having the intermediate routers remember which interface is upstream, and which interface(s) is(are) downstream. Once a new branch is created, each child router monitors the status of its parent router with a keepalive mechanism, the CBT "Echo" protocol. A child router periodically unicasts a CBT_ECHO_REQUEST to its parent router, which is then required to respond with a unicast CBT_ECHO_REPLY message. ======================================================================== #- - - -#- - - - -# | \ | # | # - - - - # member | | host --| | | --Join--> --Join--> --Join--> | |- [DR] - - - [:] - - - -[:] - - - - [@] | <--ACK-- <--ACK-- <--ACK-- | LEGEND [DR] CBT Designated Router [:] CBT Router [@] Target Core Router # CBT Router that is already on the shared tree Figure 21: CBT Tree Joining Process ======================================================================== Maufer & Semeria Informational [Page 48] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 If, for any reason, the link between an on-tree router and its parent should fail, or if the parent router is otherwise unreachable, the on-tree router transmits a FLUSH_TREE message on its child interface(s) which initiates the tearing down of all downstream branches for the multicast group. Each downstream router is then responsible for re-attaching itself (provided it has a directly attached group member) to the group's shared delivery tree. The Designated Router (DR) is elected by CBT's "Hello" protocol and functions as THE single upstream router for all groups using that link. The DR is not necessarily the best next-hop router to every core for every multicast group. The implication is that it is possible for a JOIN_REQUEST to be redirected by the DR across a link to the best next-hop router providing access a given group's core. Note that data traffic is never duplicated across a link, only JOIN_REQUESTs, and the volume of this JOIN_REQUEST traffic should be negligible. 8.2.2 Data Packet Forwarding When a JOIN_ACK is received by an intermediate router, it either adds the interface over which the JOIN_ACK was received to an existing forwarding cache entry, or creates a new entry if one does not already exist for the multicast group. When a CBT router receives a data packet addressed to the multicast group, it simply forwards the packet over all outgoing interfaces as specified by the forwarding cache entry for the group. 8.2.3 Non-Member Sending Similar to other multicast routing protocols, CBT does not require that the source of a multicast packet be a member of the multicast group. However, for a multicast data packet to reach the active core for the group, at least one CBT-capable router must be present on the non-member source station's subnetwork. The local CBT-capable router employs IP-in-IP encapsulation and unicasts the data packet to the active core for delivery to the rest of the multicast group. 8.2.4 CBT Multicast Interoperability Multicast interoperability is currently being defined. Work is underway in the IDMR working group to describe the attachment of stub-CBT and stub-PIM domains to a DVMRP backbone. Future work will focus on developing methods of connecting non-DVMRP transit domains to a DVMRP backbone. CBT interoperability will be achieved through the deployment of domain border routers (BRs) which enable the forwarding of multicast traffic between the CBT and DVMRP domains. The BR implements DVMRP and CBT on different interfaces and is responsible for forwarding data across the domain boundary. Maufer & Semeria Informational [Page 49] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 ======================================================================== /---------------\ /---------------\ | | | | | | | | | DVMRP |--[BR]--| CBT Domain | | Backbone | | | | | | | \---------------/ \---------------/ Figure 22: Domain Border Routers (BRs) ======================================================================== The BR is also responsible for exporting selected routes out of the CBT domain into the DVMRP domain. While the CBT stub domain never needs to import routes, the DVMRP backbone needs to import routes to any sources of traffic which are inside the CBT domain. The routes must be imported so that DVMRP can perform its RPF check. 9. INTEROPERABILITY FRAMEWORK FOR MULTICAST BORDER ROUTERS In late 1996, the IETF IDMR working group began discussing a formal structure that would describe the way different multicast routing protocols should interact inside a multicast border router (MBR). The work can be found in the following internet draft: --Join--> --Join--> | |- [DR] - - - [:] - - - -[:] - - - - [@] | <--ACK-- <--ACK-- <--ACK-- | LEGEND [DR] CBT Designated Router [:] CBT Router [@] Target Core Router # CBT Router that is already on the shared tree Figure 21: CBT Tree Joining Process ======================================================================== Maufer & Semeria Informational [Page 48] INTERNET-DRAFT Introduction to Ilticast: Protocol Specification," , A. J. Ballardie, March 1997. "Core Based Tree (CBT) Multicast Border Router Specification for Connecting a CBT Stub Region to a DVMRP Backbone," , A. J. Ballardie, March 1997. "Distance Vector Multicast Routing Protocol," , T. Pusateri, February 19, 1997. "Internet Group Management Protocol, Version 2," , William Fenner, January 22, 1997. "Internet Group Management Protocol, Version 3," , Brad Cain, Ajit Thyagarajan, and Steve Deering, Expired. "Protocol Independent Multicast-Dense Mode (PIM-DM): Protocol Specification," , D. Estrin, D. Farinacci, A. Helmy, V. Jacobson, and L. Wei, September 12, 1996. Maufer & Semeria Informational [Page 52] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 "Protocol Independent Multicast-Sparse Mode (PIM-SM): Motivation and Architecture," , S. Deering, D. Estrin, D. Farinacci, V. Jacobson, C. Liu, and L. Wei, November 19, 1996. "Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification," , D. Estrin, D. Farinacci, A. Helmy, D. Thaler; S. Deering, M. Handley, V. Jacobson, C. Liu, P. Sharma, and L. Wei, October 9, 1996. (Note: Results of IESG review were announced on December 23, 1996: This internet-draft is to be published as an Experimental RFC.) "PIM Multicast Border Router (PMBR) specification for connecting PIM-SM domains to a DVMRP Backbone," , D. Estrin, A. Helmy, D. Thaler, Febraury 3, 1997. "Administratively Scoped IP Multicast," , D. Meyer, December 23, 1996. "Interoperability Rules for Multicast Routing Protocols," , D. Thaler, November 7, 1996. See the IDMR home pages for an archive of specifications: 10.3 Textbooks Comer, Douglas E. Internetworking with TCP/IP Volume 1 Principles, Protocols, and Architecture Second Edition, Prentice Hall, Inc. Englewood Cliffs, New Jersey, 1991 Huitema, Christian. Routing in the Internet, Prentice Hall, Inc. Englewood Cliffs, New Jersey, 1995 Stevens, W. Richard. TCP/IP Illustrated: Volume 1 The Protocols, Addison Wesley Publishing Company, Reading MA, 1994 Wright, Gary and W. Richard Stevens. TCP/IP Illustrated: Volume 2 The Implementation, Addison Wesley Publishing Company, Reading MA, 1995 10.4 Other Deering, Steven E. "Multicast Routing in a Datagram Internetwork," Ph.D. Thesis, Stanford University, December 1991. Maufer & Semeria Informational [Page 53] INTERNET-DRAFT Introduction to IP Multicast Routing March 1997 Ballardie, Anthony J. "A New Approach to Multicast Communication in a Datagram Internetwork," Ph.D. Thesis, University of London, May 1995. "Hierarchical Distance Vector Multicast Routing for the MBone," Ajit Thyagarajan and Steve Deering, July 1995. 11. SECURITY CONSIDERATIONS Security issues are not discussed in this memo. 12. ACKNOWLEDGEMENTS This RFC would not have been possible without the encouragement of Mike O'Dell and the support of Joel Halpern and David Meyer. Also invaluable was the feedback and comments of the IETF MBoneD and IDMR working groups. Certain people spent considerable time commenting on and discussing this paper with the authors, and deserve to be mentioned by name: Tony Ballardie, Steve Casner, Jon Crowcroft, Steve Deering, Bill Fenner, Hugh Holbrook, Cyndi Jung, Shuching Shieh, Dave Thaler, and Nair Venugopal. Our apologies to anyone we unintentionally neglected to list here. 13. AUTHORS' ADDRESSES Tom Maufer 3Com Corporation 5400 Bayfront Plaza P.O. Box 58145 Santa Clara, CA 95052-8145 Phone: +1 408 764-8814 Email: Chuck Semeria 3Com Corporation 5400 Bayfront Plaza P.O. Box 58145 Santa Clara, CA 95052-8145 Phone: +1 408 764-7201 Email: Maufer & Semeria Informational [Page 54]