Page 1/18 Hongbeom Ahn A Survey of Application-Layer Multicast Protocols MOJTABA HOSSEINI, DEWAN TANVIR AHMED, SHERVIN SHIRMOHAMMADI, AND NICOLAS D. GEORGANAS, UNIVERSITY OF OTTAWA IEEE COMMUNICATIONS SURVEYS, 2007

Page 2/18 Contents AbstractMulticastingIP Multicast vs. Application Layer MulticastALM Protocol Design Application domain Deployment level Group management Routing mechanism Popular ALM Protocols ZIGZAG NICE OMNI Open Issues

Page 3/18 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

Page 4/18 Multicast Background  IP Multicast –Router actively participate in multicast Making copies of packet Forwarding toward multicast receivers –Multicast is more efficient than multiple unicast connections  Unicast –Source sends N unicast datagrams, one addressed to each of N receivers –Not scalable Network is going to collapse

Page 5/18 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

Page 6/18 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

Page 7/18 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

Page 8/18 IP Multicast vs. Application Layer Multicast (ALM) : Conceptual comparison

Page 9/18 IP Mutlicast 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 MulticastALM Total cost = 37 Total cost = 39

Page 10/18 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 90 ʼ s) –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

Page 11/18 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

Page 12/18 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

Page 13/18 ALM Protocol Design : Deployment Level  Proxy-based (infrastructure-level) ALM –Requires dedicated server/proxies in the Internet –Creates overlay only among proxies –Provides a transparent multicast service 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 specific application domains

Page 14/18 ALM Protocol Design : Group Management  Key decisions regarding group / node management –How to find out about / join / leave groups? –Sending allowed when not joined? –Centralized or 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

Page 15/18 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)

Page 16/18 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

Page 17/18 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

Page 18/18 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 cost spanning all members  Cluster structure –Build hierarchical clusters  Peer-to-Peer structure –Typically use reverse-path forwarding

Page 19/18 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

Page 20/18 Survey and Classification of ALM Protocols : Zigzag  Operation The Highest Layer Only have links to its foreign subordinates (Only except for the server) Non-head member Cannot get the content from their head

Page 21/18 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 L 0 –Cluster leaders in layer L i join layer L i +1 –Each host maintains state about all clusters it belongs to and about its super-cluster (leader`s cluster)

Page 22/18 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

Page 23/18 Survey and Classification of ALM Protocols : NICE  Control and data paths –Source-specific tree

Page 24/18 Survey and Classification of ALM Protocols : NICE  Join process –Find the closet to itself –Using Rendezvous Point!

Page 25/18 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 delivery backbone such that the overlay latency to the client set is minimized –Minimum average-latency degree- degree bounded spanning tree

Page 26/18 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 L r,d on 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 MSN i –Aggregate subtree latency Summation of overlay latency of each MSN in the subtree from MSN i M : is the set of all MSN C i : the number of clients served by the MSN i Children(i) : the set of children of i in the overlay tree

Page 27/18 Survey and Classification of ALM Protocols : OMNI  Operations –Initial join –Local transformation Child-promote Parent-Child Swap Iso-level-2 transfer Aniso+level-1-2 swap

Page 28/18 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