Hongbeom Ahn

1. A Survey of Application-LayerMulticast Protocols MOJTABA HOSSEINI, DEWAN TANVIR AHMED, SHERVIN SHIRMOHAMMADI, AND NICOLAS D. GEORGANAS, UNIVERSITY OF OTTAWA IEEE COMMUNICATIONS SURVEYS, 2007 HongbeomAhn

2. Contents

3. Abstract • Internet • Born for one-to-one applications • Nowadays : requires one-to-many, many-to-many applications • IP multicast for a solution • Trade-off • Group management • Mesh-first vs. tree-first approach • Routing • Minimum spanning tree vs. cluster structure • Application domain • Multi-source vs. single-source

4. Multicast Background • Unicast • Source sends N unicast datagrams, one addressed to each of N receivers • Not scalable • Network is going to collapse • IP Multicast • Router actively participate in multicast • Making copies of packet • Forwarding toward multicast receivers • Multicast is more efficient than multiple unicast connections

5. IP Multicast is a Solution? • Practical problems • Needs to be installed at all levels of the network • From backbone to edge routers • Does ISP want to do this? -> Cost • Requires routers to maintain per-group state • Violates the stateless principle of the router construction • Vulnerable to flooding attacks without complex network management • Hard to provide reliability, congestion control • Reality • Slow to be widely adopted • A case for application-layer (or end-system) multicast

6. Application-layer Multicast • Application Layer Multicast • An alternative ways of multicasting at the application layer • End systems communicate through an overlay structure • Assuming only unicast paths provided by underlying network • Advantages • No need to change routers • Allow features to be easily incorporated • Main problems • How do end systems with limited topological information cooperate to construct good overlay structures! • Performance implications of using an overlay structure

7. IP Multicast vs. Application Layer Multicast (ALM) • IP Multicast optimal regarding tree structures • Routers in tree • ALM has overhead due to tree built among application node • Constructing non-optimal trees

8. IP Multicast vs. Application Layer Multicast (ALM): Conceptual comparison

9. IP Multicast vs. ALM: Efficiency of IP Multicast vs. ALM • IP Multicast is efficient but needs deployment of routers • ALM hosts have little information about underlying network • ALM tree building can be optimized (link/tree stretch) to incur only low penalties compared to IP Multicast Topology IP Multicast ALM Total cost = 37 Total cost = 39

10. Deployment Issues with Multicasting • No widespread deployment of IP Multicast in the Internet • Technical, administrative and business related issues • IP Multicast capable routers all levels of network required • Tendency to install simple, unintelligent (= very fast)routers • Managing and security issues (flooding attacks) • Billing and charging • MBONE [5] project (mid 90ʼs) • Unicast connections between (IP Multicast) subnetworks • IP tunneling between these “IP Multicast islands” • Problems: receiver authentication, group management, flooding • Static setup of unicast tunnels = growth problem • Not available for home Internet users through their ISPs

11. Application Layer Multicast Protocol Design • Design? • ALM protocols have a wide variety of approaches and characteristics • Customization for improving overall performance of ALM protocols by • Its requirements • Its constraints • Its assumed resources • Features • Application Domain • Deployment Level • Group Management • Routing Mechanism

12. ALM Protocol Design : Application Domain • ALM protocol design depends on the application domain • Audio/Video streaming • Single source • Large number of receivers • Audio/video conferencing • Small to medium group size • Interactive multipart conferencing session • Multiple sources • Generic multicast service • Based on specific metric (delay, BW, fan-out,…) • Reliable data broadcast and file transfer • Large data sets (distributed DB, file sharing) • Bandwidth as only metric • Typically focus on optimizing for a single application domain

13. ALM Protocol Design : Deployment Level • Proxy-based (infrastructure-level) ALM • Requires dedicated server/proxiesin the Internet • Creates overlay only among proxies • Provides a transparent multicastservice to end-users (IP multicast) • Typically generic multicast service • Expect a service charge • End-system ALM • Assumes only unicast infrastructure • Expects users to take part in the forwarding • Free as of peer-to-peer nature • Independent and cost-free • Enjoys more flexibility, optimized for specificapplication domains

14. ALM Protocol Design : Group Management • Key decisions regarding group / node management • How to find out about / join / leave groups? • Sending allowed when not joined? • Centralizedor decentralized management? • Support existing IP Multicast Islands? • Support refinement during group life-time? • Use mesh-first or tree-first approach? • Typically ALM uses • Rendez-vous points for discovery • Source-specific trees for video streaming (1:n] • Mesh-first constructed shared trees for conferencing

15. ALM Protocol Design : Group Management • Mesh-first vs. Tree-first • Configure the data distribution pathways • Mesh-first • Topology with many redundant interconnections • Source is chosen as a root and a routing algorithm • Builds P2P ‘mesh’ without the multicast • Limits multicast tree quality (depends on quality of the mesh) • More robust and better for multi-source applications • Tree-first • Builds the multicast tree directly without any mesh • Members select their parent from the known members • Require running an algorithm to detect and avoid loops • Gives direct control over the tree • Changes cause change for all descendants in tree • Lower communication overhead (simpler)

16. ALM Protocol Design : Group Management • Source-specific tress vs. shared trees • Conflicting design goals • Minimize individual path length (hops/end-to-end delay) to specific destination • Minimize sum of hops (cumulative end-to-end delay) to all destinations • Source-specific trees • Optimizes the tree for a single source • Limited efficiency for multiple sources on the same tree • Shared trees • Supports efficiently multiparty-communications • Better maintenance costs than source-specific trees

17. ALM Protocol Design : Group Management • Distributed vs. Centralized (balance simplicity vs. robustness) • Distribute workload for tree maintenance among root nodes(Robust, synchronization issues, large-scale applications) • Synchronization -> real-time media hard to ensure • Central group management for small-scale applications(Single-point of failure, simple & easy deployment) • Refinement • Optimize tree performance because of new joins and leaves • Causes interruptions & stability issues

18. ALM Protocol Design : Routing Mechanism • Shortest path trees • Uses RTT measurements to build the shortest path tree from source to end hosts • Constructs a minimum cost path from a source node to all its receivers • Commonly used by ALM protocols • Minimum spanning trees • Just tries to construct a low cost tree • Minimum total costspanning all members • Cluster structure • Build hierarchical clusters • Peer-to-Peer structure • Typically use reverse-path forwarding

19. Survey and Classification of ALM Protocols: Zigzag • Overview • A single source • Degree-bounded ALM for media streaming • Key in ZigZag • Use of a foreign head to forward the content to the other members of cluster • Purpose • Short end-to-end delay • Low control overhead • Efficient join and failure recovery • Low maintenance overhead

20. Survey and Classification of ALM Protocols: Zigzag • Operation The Highest Layer Only have links to itsforeign subordinates (Only except for the server) Non-head member Cannot get the content from their head

21. Survey and Classification of ALM Protocols: NICE • Overview • Scalable application layer multicast– hierarchical clustering approach • Support a larger number of receivers • Low bandwidth soft real-time data stream (stock and internet radio) • Features • Cluster size = k to 3k-1 (e.g. 3~8) • Cluster leader • Is the center of the cluster • Minimum maximum distance to all other hosts in cluster • All hosts are part of the layer L0 • Cluster leaders in layer Li join layer Li+1 • Each host maintains stateabout all clusters it belongsto and about its super-cluster(leader`s cluster)

22. Survey and Classification of ALM Protocols: NICE • Analysis • Control overhead • Exchange message with clusters • A host belongs only to L0 = O(k) • A host belongs to Li(highest) = O(k*i) • Worst case = O(k*logN) • Operations • Join • Maintenance • Refinement • Leave/Failure recovery • Assumption • RP(Rendezvous Point) • New host contacts the RP to initiate join process

23. Survey and Classification of ALM Protocols: NICE • Control and data paths • Source-specific tree

24. Survey and Classification of ALM Protocols: NICE • Join process • Find the closet to itself • Using Rendezvous Point!

25. Survey and Classification of ALM Protocols: OMNI • Overlay Multicast Network Infrastructure • Overlay architecture to efficiently implement media streaming • Multicast Service Nodes(MSNs) deployed by service providers • Act as a forwarding entities for a set of clients • Distributed protocol to form a delivery backbone • Goal • Construct a multicast data deliverybackbone such that the overlaylatency to the client set is minimized • Minimum average-latency degree-degree bounded spanning tree

26. Survey and Classification of ALM Protocols: OMNI • A latency of client (consists of) • The latency from the media source to the root MSN r • The latency Lr,don the path from root MSN r to destination MSN d • The latency from the MSN d to the client I • Solve! • The minimum average-latency degree-bounded by • Evaluation • Aggregate subtree clients • The entire set of clients server by all MSNs at MSNi • Aggregate subtree latency • Summation of overlay latencyof each MSN in the subtree from MSNi M : is the set of all MSN Ci : the number of clients served by the MSN i Children(i) : the set of children of i in the overlay tree

27. Survey and Classification of ALM Protocols: OMNI • Operations • Initial join • <LatencyToRoot, DegreeBound> • Local transformation • Child-promote • Parent-Child Swap • Iso-level-2 transfer • Aniso+level-1-2 swap

28. Open Issues • The Current Trend regarding ALM • Trust in overlay network • Heterogeneity of users • Providing resilience • High-bandwidth file transfer and downloading • Topologically-aware data path method • Reduce unnecessary high latency and redundant network resource usage • Confidentiality • Tree refinement • Reorganization or shuffling of the nodes in the tree • Enhance the system performance (zero-degree -> join? How to control) • Handle these points • Minimizing the length of the paths (usually in terms hops) to the individual destinations • Minimizing the total number of hops to forward the packet to all the destinations

29. Q&A