Ahmed Helmy - UF1 IP-Multicast (outline) -Motivation and Background -Multicast vs. unicast -Multicast Applications -Delivery of Multicast -Local delivery and multicast addressing -WAN delivery and its model -Group Membership Protocol (IGMP) -Multicast Algorithms and Concepts -Flooding, Spanning Tree, Reverse Path Broadcasting (RPB), Truncated RPB, Reverse Path Multicasting, Center-Based Trees

Ahmed Helmy - UF2 -Multicast Routing Protocols -Dense vs. Sparse Multicast -DVMRP -MOSPF -PIM (PIM-DM, PIM-SM) -Multicast and the Internet -The MBONE -Recent deployment Outline (Contd.)

Ahmed Helmy - UF3 Unicast vs. Multicast Multicast provides multipoint-to-multipoint communication Today majority of Internet applications rely on point-to-point transmission (e.g., TCP). IP-Multicast conserves bandwidth by replicating packets in the network only when necessary

Ahmed Helmy - UF4 Unicast vs. Multicast S R1 R2 R3 R4 S R1 R2 R3 R4 Multiple unicasts Multicast

Ahmed Helmy - UF5 Example Multicast Applications One-to-Many –Scheduled audio/video distribution: lectures, presentations –Push media: news headlines, weather updates –Caching: web site content & other file-based updates sent to distributed replication/caching sites –Announcements: network time, configuration updates –Monitoring: stock prices, sensor equipment

Ahmed Helmy - UF6 IP Multicast Applications (contd.) Many-to-One –Resource discovery –Data collection and sensing –Auctions –Polling

Ahmed Helmy - UF7 –Many-to-Many Multimedia Teleconferencing (audio, video, shared whiteboard, text editor) Collaboration Multi-Player Games Concurrent Processing Chat Groups Distributed Interactive Simulation IP Multicast Applications (contd.)

Ahmed Helmy - UF8

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Ahmed Helmy - UF11 Resource Discovery – Multicast may be used (instead of broadcast) to transmit to group members on the same LAN. –Multicast may be used for resource discovery within a specific scope using the TTL field in the IP header. More Applications...

Ahmed Helmy - UF12 Multicast Scope Control: TTL Expanding-Ring Search to reach or find a nearby subset of a group s 1 2 3

Ahmed Helmy - UF13 Multicast Scope Control: Administrative TTL Boundaries to keep multicast traffic within an administrative domain, e.g., for privacy reasons an administrative domain TTL threshold set on interfaces to these links, greater than the diameter of the admin. domain the rest of the Internet

Ahmed Helmy - UF14 Multicast Scope Control: Administratively-Scoped Addresses an administrative domain address boundary set on interfaces to these links the rest of the Internet –RFC 1112 –uses address range —

Ahmed Helmy - UF15 Over the same (LAN): –The source addresses the IP packet to the multicast group –The network interface card maps the Class D address to the corresponding IEEE-802 multicast address –Receivers notify their IP layer to receive datagrams addressed to the group. –Key issue is ‘addressing’ & filtration Transmission and Delivery of Multicast Datagrams

Ahmed Helmy - UF16 Over different subnets: –Routers implement a multicast routing protocol that constructs the multicast delivery trees and supports multicast data packet forwarding. –Routers implement a group membership protocol to learn about the existence of group members on directly attached subnets. –Hosts implement the group membership protocol that provides the ‘IP-multicast host model’

Ahmed Helmy - UF17

Ahmed Helmy - UF18 Multicast Addressing An IP multicast group is identified by a Class D address. Multicast group addresses range from ( ) to ( ).

Ahmed Helmy - UF19 The Internet Assigned Numbers Authority (IANA) registers IP multicast groups. The block of multicast addresses ranging from ( ) to ( ) is reserved for local LAN multicast: –used by routing protocols and other low-level topology discovery or maintenance protocols –E.g., "all-hosts" group ( ), "all-routers” group ( ), "all DVMRP routers", etc. The range ( ) to ( ) are used for site-local "administratively scoped" applications.

Ahmed Helmy - UF20 Hosts can join or leave a group at any time A host may be a member of multiple groups Senders need not be members of the group Participants do not know about each other The two components of IP-multicast: –the group membership protocol –the multicast routing protocol The Multicast Host Model

Ahmed Helmy - UF21 Routers need to learn about the presence of group members on directly attached subnets When a host joins a group: –it transmits a group membership message for the group(s) that it wishes to receive –sets its IP process and network interface card to receive packets sent to those groups. Group Membership Protocol

Ahmed Helmy - UF22 Multicast Routing Protocols –Run on routers and establish the multicast distribution tree to forward packets from sender(s) to group members. Based on unicast routing concepts: –DVMRP is a distance-vector routing protocol, –MOSPF is an extension to the OSPF link-state unicast routing protocol. Center-based trees (e.g., CBT & PIM-SM) introduce the notion of the tree ‘core’.

Ahmed Helmy - UF23 Multicast Forwarding Algorithms A multicast routing protocol is responsible for the establishment of the multicast distribution tree and for performing packet forwarding.

Ahmed Helmy - UF24 Several algorithms may be employed by multicast routing protocols:  Flooding  Spanning Trees  Reverse Path Broadcasting (RPB)  Truncated Reverse Path Broadcasting (TRPB)  Reverse Path Multicasting (RPM)  Core-Based Trees

Ahmed Helmy - UF25 Flooding The simplest technique for multicast delivery. When a router receives a multicast packet it determines whether or not this is the first time it has seen this packet. –On first reception, a packet is forwarded on all interfaces except the one on which it arrived. –If the router has seen the packet before, it is discarded.

Ahmed Helmy - UF26 A router does not maintain a routing table, but needs to keep track of recently seen packets. Flooding does not scale for Internet-wide application: -Generates a large number of duplicate packets and uses all available paths across the internetwork. -Routers maintain a distinct table entry for each recently seen packet (consumes memory).

Ahmed Helmy - UF27 Spanning Tree More effective than flooding Defines a tree structure where one active path connects any two routers on the Internet. Spanning Tree rooted at R

Ahmed Helmy - UF28 A router forwards each multicast packet to interfaces that are part of the spanning tree except the receiving interface. A spanning tree avoids looping of multicast packets and reaches all routers in the network.

Ahmed Helmy - UF29 A spanning tree algorithm is easy to implement However, a spanning tree solution: –may centralize traffic on small number of links –may not provide the most efficient path between the source and the group members.

Ahmed Helmy - UF30 Reverse Path Broadcasting (RPB) More efficient than building a single spanning tree for the entire Internet. Establishes source-rooted distribution trees for every source subnet. A different spanning tree is constructed for each active (source, group) pair.

Ahmed Helmy - UF31 RPB Algorithm For each (source, group) pair –if a packet arrives on a link that the router considers to be the shortest path back to the source of the packet then the router forwards the packet on all interfaces except the incoming interface. –Otherwise, the packet is discarded.

Ahmed Helmy - UF32 The interface over which a router accepts multicast packets from a particular source is called the "parent" link. The outbound links over which a router forwards the multicast packets are called the "child" links.

Ahmed Helmy - UF33 Reverse Path Broadcasting (RPB) Forwarding

Ahmed Helmy - UF34 Enhancement to reduce packet duplication: –A router determines if a neighboring router considers it to be on the shortest path back to the source. –If Yes, the packet is forwarded to the neighbor. –Otherwise, the packet is not forwarded on that potential child link.

Ahmed Helmy - UF35 To derive the parent-child information: –link-state routing protocol already has it (since each router maintains a topological database for the entire routing domain). –distance-vector routing protocol uses ‘poison reverse’: a neighbor can either advertise its previous hop for the source subnet as part of its routing update messages or "poison reverse" the route.

Ahmed Helmy - UF36 Example of Reverse Path Broadcasting

Ahmed Helmy - UF37 Benefits Reasonably efficient and easy to implement. Does not require keeping track of previous packets, as flooding does. Multicast packets follow the "shortest" path from the source to the group members. Avoids concentration over single spanning tree

Ahmed Helmy - UF38 Reverse Path Multicasting (RPM) RPM enhances TRPB. In RPM, non-member branches are pruned Packets are forwarded only along branches leading to group members.

Ahmed Helmy - UF39 RPM Operation The first multicast packet is forwarded (using TRPB) to all routers in the network. Routers at edges of the network with no downstream routers are called ‘leaf’routers. A leaf router with no downstream members sends a "prune" message on its parent link to stop packet flow down that branch.

Ahmed Helmy - UF40 Prune messages are sent hop-by-hop back toward the source. A router receiving a prune message stores the prune state in memory. A router with no local members that receives prunes on all child interfaces sends a prune one hop back toward the source. This succession of prune messages creates a multicast forwarding tree that contains only branches that lead to group members.

Ahmed Helmy - UF41 Reverse Path Multicasting (RPM)

Ahmed Helmy - UF42 Limitations Despite improvements over RPM, there are scaling issues and limitations: –Multicast packets are periodically forwarded to every router in the network. –Routers maintain prune state off-tree for all (source,group) pairs. These limitations are amplified with increase in number of sources and groups.

Ahmed Helmy - UF43 Center/Core-Based Trees (CBT) Earlier algorithms build source-based trees CBT builds a single delivery tree (rooted at the core) that is shared by all group members. Multicast traffic for each group is sent and received over the shared tree, regardless of the source.

Ahmed Helmy - UF44 A core-based tree involves one or more cores in the CBT domain. Each leaf-router of a group sends a hop-by- hop "join" message toward the "core tree" of that group. Routers need to know the group core to send the join request. Senders unicast packets toward the core. CBT Operation

Ahmed Helmy - UF45 Benefits Advantages over RPM, in terms of scalability: –A router maintains state information for each group, not for each (source, group) pair. –Multicast packets only flow down branches leading to members (not periodically broadcast). –Only join state is kept on-tree

Ahmed Helmy - UF46 Limitations CBT may result in traffic concentration near the core since traffic from all sources traverses the same set of links as it approaches the core. A single shared delivery tree may create sub- optimal routes resulting in increased delay. Core management issues –dynamic core selection –core placement strategies

Ahmed Helmy - UF47 Multicast Routing Protocols In general, there are two classes of multicast routing protocols: –Dense-mode protocols (broadcast-and-prune) DVMRP, PIM-DM, (MOSPF!) –Sparse-mode protocols (explicit-join) PIM-SM, CBT, BGMP

Ahmed Helmy - UF48 Dense vs. Sparse Mode Multicast R1 R2 R3 R4 S Dense-Mode Multicast

Ahmed Helmy - UF49 Dense vs. Sparse Mode Multicast S R1 R2 R3 R4 Root R1 R2 R3 R4 S Dense-Mode Multicast Sparse-Mode Multicast

Ahmed Helmy - UF50 Distance Vector Multicast Routing Protocol (DVMRP) DVMRP constructs source-rooted trees using variants of RPM (broadcast-and- prune). The first packet for any (source, group) pair is broadcast to the entire network. Leaf routers with no local members send prune messages back toward the source.

Ahmed Helmy - UF51 Example DVMRP Scenario gg s g

Ahmed Helmy - UF52 Initial Broadcast using Truncated Broadcast gg s g

Ahmed Helmy - UF53 Prune non-member branches gg s prune (s,g) g

Ahmed Helmy - UF54 graft (s,g) Graft new members gg s g g report (g)

Ahmed Helmy - UF55 DVMRP Distribution Tree gg s g g

Ahmed Helmy - UF56 DVMRP Forwarding Table

Ahmed Helmy - UF57 Multicast Extensions to OSPF (MOSPF) MOSPF routers maintain current image of the network topology via OSPF. Basic MOSPF runs in OSPF domain MOSPF uses IGMP to discover members on directly attached subnets. The subnet router floods Group- Membership Link State Advertisements (LSAs) throughout the OSPF domain.

Ahmed Helmy - UF58 The shortest path tree for (S, G) pair is built "on demand" when a router receives the first packet for (S,G). When the initial packet arrives, the source subnet is located in MOSPF link state database. –MOSPF LS-DB = OSPF LS-DB + Group-Membership LSAs Building the Shortest Path Tree

Ahmed Helmy - UF59 Forwarding cache entry contains the (source, group) pair, the upstream node, and the downstream interfaces. MOSPF Forwarding Cache Forwarding Cache

Ahmed Helmy - UF60 Limitations  Limited to OSPF domains  Flooding membership information does not scale well for Internet-wide multicsat

Ahmed Helmy - UF61 Protocol-Independent Multicast (PIM) Design Rationale: –Broadcast and prune keeps state off-tree and is suitable when members are densely distributed –Explicit join/center-based approach keeps state on-tree and is suitable when members are sparsely distributed –PIM attempts to combine the best of both worlds

Ahmed Helmy - UF62 Design Choices Shared trees or shortest path trees? –Both: use shared trees to ‘Rendezvous’ then switch to shortest path to deliver DV or LS for routing? –Use routing tables regardless of which protocol created them (hence the name ‘Protocol Independent’)

Ahmed Helmy - UF63 PIM Operation Modes PIM provides both dense-mode (DM) and sparse-mode (SM) protocols PIM-DM: similar to DVMRP but does not build its own routing table PIM-SM: similar to CBT but provides switching to SPT and bootstrap mechanism for electing the tree center dynamically

Ahmed Helmy - UF64 How PIM-SM works A Rendezvous Point (RP) is chosen as tree center per group to enable members and senders to “meet” Members send their explicit joins toward the RP Senders send their packets to the RP Packets flow only where there is join state (*,G) [any-source,group] state is kept in routers between receivers and the RP

Ahmed Helmy - UF65 How PIM-SM works When should we use shared-trees versus source- trees? –Source-trees tradeoff low-delay from source with more router state –Shared-trees tradeoff higher-delay from source with less router state Switch to the source-tree if the data rate is above a certain threshold

Ahmed Helmy - UF66 How PIM-SM works Source B E AD C RP Receiver 2Receiver 1 Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF67 How PIM-SM works BAD RP Source Receiver 2Receiver 1 (*, G) Join EC Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF68 How PIM-SM works BAD RP Source Receiver 2Receiver 1 EC Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF69 How PIM-SM works BAD RP Receiver 2Receiver 1 Source Register EC Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF70 How PIM-SM works BAD RP Receiver 2Receiver 1 Source (S, G) Join (S, G) Join EC Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF71 How PIM-SM works BAD RP Receiver 2Receiver 1 Source Register-Stop EC Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF72 How PIM-SM works BAD RP Receiver 2Receiver 1 Source (S, G) Join EC Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF73 How PIM-SM works BAD RP Receiver 2Receiver 1 Source (S, G) RP Bit Prune EC (S, G) Prune Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF74 How PIM-SM works BAD RP Receiver 2Receiver 1 Source EC (*, G) Join Link (*,G) Data (S,G) Data Control

Ahmed Helmy - UF75 How PIM-SM works BAD RP Receiver 2Receiver 1 Source EC Link (*,G) Data (S,G) Data Control