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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