Post on 20-Jan-2016
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Hongbeom Ahn
A Survey of Application-LayerMulticast Protocols
MOJTABA HOSSEINI, DEWAN TANVIR AHMED, SHERVIN SHIRMOHAMMADI, ANDNICOLAS D. GEORGANAS, UNIVERSITY OF OTTAWA
IEEE COMMUNICATIONS SURVEYS, 2007
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Contents
Abstract
Multicasting
IP Multicast vs. Application Layer Multicast
ALM Protocol Design• Application domain• Deployment level• Group management• Routing mechanism
Popular ALM Protocols• ZIGZAG• NICE• OMNI
Open Issues
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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
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Multicast Background
IP Multicast– Router actively participate in mul-
ticast• Making copies of packet• Forwarding toward multicast re-
ceivers
– Multicast is more efficient than multiple unicast connections
Unicast– Source sends N unicast data-
grams, one addressed to each of N receivers
– Not scalable• Network is going to collapse
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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
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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
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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
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IP Multicast vs. Application Layer Multicast (ALM): Conceptual comparison
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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 Multicast ALM
Total cost = 37 Total cost = 39
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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
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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
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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
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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 specificapplication domains
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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
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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)
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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 destina-tion
• 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
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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
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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
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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
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Survey and Classification of ALM Protocols: Zigzag
Operation
The Highest LayerOnly have links to itsforeign subordinates(Only except for the
server)
Non-head memberCannot get the content
from their head
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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)
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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
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Survey and Classification of ALM Protocols: NICE
Control and data paths– Source-specific tree
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Survey and Classification of ALM Protocols: NICE
Join process– Find the closet to itself– Using Rendezvous Point!
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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 overlaylatency to the client set is minimized
– Minimum average-latency degree-degree bounded spanning tree
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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,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 MSNi
– Aggregate subtree latency• Summation of overlay latency
of each MSN in the subtree from MSNi
M : is the set of all MSNCi : the number of clients served by the MSN iChildren(i) : the set of chil-dren of i in the overlay tree
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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
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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 indi-
vidual destinations– Minimizing the total number of hops to forward the packet to all the
destinations