Multipath Routing for Video Delivery over BandwidthLimited Networks

1
Multipath Routing for Video Delivery over
Bandwidth-Limited Networks

• S.-H. Gary Chan Jiancong Chen
• Department of Computer Science
• Hong Kong University of Science and Technology
• Clear Water Bay, Kowloon

2
Outline

• Introduction
• Multipath routing heuristic for point-to-point
  video delivery
• Scheduling algorithm at the server to achieve the
  theoretical minimum start-up delay
• Extension to point-to-multipoint layered video
  delivery
• Conclusion

3
Introduction
4
Research Motivation

• Deliver quality video services over
  bandwidth-limited networks (e.g., the Internet)
• Video application requirements
• High bandwidth
• Low start-up delay or network transmission cost
• Traditional routing based on single path approach
  (e.g., the shortest path routing) is no longer
  sufficient to meet the bandwidth requirement
• QoS routing

5
Negotiating and Guaranteeing QoS in the Internet

• Integrated services/Resource Reservation Protocol
  (RSVP)
• Multi-protocol label switching (MPLS)
• Differentiated services model (DiffServ)
• Traffic engineering
• Constraint-based routing

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Constraint-Based Routing

• Compute routes subject to multiple constraints
• Distribution of link state information
• Route computation
• Goals
• Select routes that can meet certain QoS
  requirements
• Increase utilization of the network

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Meeting Bandwidth Requirement with Low Delay
Multipath Routing

• The video data is transmitted over multiple paths
  in the network
• Increasing the overall aggregate delivery
  bandwidth
• Routing to meet the bandwidth requirement
• The end host needs to do reassembly
• Increasing the start up delay
• Server scheduling to reduce the delay

8
Previous Work on Multipath Routing

• Search multiple paths and select the best one
• E.g., selective probing
• Find multiple paths for a connection (e.g.,
  disjoint paths routing)
• Mainly designed for reliability rather than high
  aggregate bandwidth

9
Our Work

• A multipath heuristics for point-to-point video
  delivery
• Low delay and buffer requirement
• Efficient
• Given a set of path lengths
• The theoretical minimum delay achievable
• A scheduling algorithm to achieve that
• For point-to-multipoint communication with
  heterogeneous bandwidth requirement
• How the multicast trees should be constructed to
  minimize the cost of the tree-aggregate
• The corresponding number and bandwidth of the
  video layers

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Multipath Routing for Point-to-Point Video
Delivery
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A Point-to-Point Video Network
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Multipath Problem Formulation Bandwidth-Constrain
ed Delay-Optimized Problem

• Given
• A source s
• A destination t
• Bandwidth requirement B
• B less than the max-flow of the network
• Find routing and scheduling algorithms to achieve
• Bandwidth no less than B
• Minimum delay

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Desirable Properties of Routing Algorithms

• Efficient
• Similar complexity as the shortest path routing
• Fast route convergence
• Achieving high end-to-end bandwidth
• Preferably the max-flow of the network
• Amendable to the current Internet routing

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A Multipath Routing Heuristics

• Find the max-flow sub-graph G of the network
• Find the shortest-path in the sub-graph G
• If the aggregated bandwidth of the path(s) found
  is sufficient, return
• Subtract the bandwidth from G along the path
  just found
• Repeat steps 2 to 4

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An Example
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Simulation Model

• Hierarchical network
• 3-hierarchy nodes backbone routers, border
  routers and intra-domain routers
• Random links
• System parameters
• Network size
• Network density
• Connectivity, etc

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Comparison with the Traditional Approaches

• Shortest path
• Shortest-feasible path
• Remove the links with insufficient bandwidth
• Run the shortest path algorithm over the residual
  network
• Performance measures
• Success rate in meeting the bandwidth requirement
• Bandwidth achieved
• End-to-end delay, given by the longest path

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High Success Rate
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High Bandwidth Achieved
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Low Average Delay
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Hierarchical routing

• Logical hierarchical topology as in the Internet
• State information
• Only full local information is maintained
• Remote state information is partially maintained
• Compute multiple routes in the regions in
  parallel
• Reduce computation complexity, processing time,
  and storage

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An example
Upper hierarchy
Lower hierarchy
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Server Scheduling Algorithm
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Problem Formulation

• Given a set of path lengths
• What is the theoretical minimum start-up delay
  achievable if video data can be scheduled?
• Guarantee continuity
• Find a data scheduling algorithm at the server to
  achieve such minimum delay
• No other algorithms can achieve lower delay while
  maintaining stream continuity

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A Simple Case

• Two paths with the same bandwidth of B/2 but
  different delays d1 and d2 (d1 lt d2)
• Without server scheduling, the start-up delay
  equals the delay of the longer path, i.e., d2

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The Theoretical Minimum Delay

• Data production and consumption curves
• The difference is the buffer requirement
• In the example, the minimum start-up delay is
  (d1d2)/2

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The Idea

• Dont indiscriminately multiplex video packets
  along all the paths
• The server sends the video prefixes along the
  shorter paths
• The client plays back the prefixes with stream
  continuity
• Before the data from the longest path arrives

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The Scheduling Algorithm

• The video sequence is partitioned into segments
• All the segments are transmitted in parallel over
  the multiple paths
• The earlier segments are transmitted over the
  shorter paths

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General Case of Scheduling

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An Exact Solution Solving the Multipath Problem

• A network with unit link bandwidth
• Multipath is disjoint paths
• With scheduling, the problem is to find the
  shortest-disjoint paths (SDP)
• Bandwidth requirement B units
• Find the B-shortest-disjoint paths
• The sum of their delays is minimum
• The shortest-disjoint paths algorithm is well
  known

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Rescheduling Achieves a Delay Comparable to the
Shortest Path
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Extension to Point-to-Multipoint Video Delivery
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A Video Multicast System

• A server and multiple clients
• The clients have different bandwidth requirements
• A link is characterized by its bandwidth and cost
• Find multiple multicast trees spanning the
  multicast group
• Meeting the heterogeneous bandwidth requirements
  of the members
• With minimum cost of the tree-aggregate
• Assignment of video layers
• A base layer and several enhancement layers
• The number of video layers, and
• Their respective bandwidths

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A Simple Case

• All the users have the same requirement B
• Multiple trees are used to span all the users
• With minimum cost of the tree-aggregate
• If all the bandwidth requirements are met
• A single video layer with bandwidth B
• Otherwise, layered video can be used
• The higher layers serve users with increasing
  end-to-end bandwidth

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An Example
s
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Problem Formulation Bandwidth-Constrained
Cost-Optimized Problem

• Given
• A source s
• A set of destinations Y ( y1, y2,, yn)
• Bandwidth requirement B ( b1, b2,, bn )
• Find multiple trees T to achieve
• Bandwidth no less than bi for yi
• Minimum cost of the aggregated mesh
• The corresponding number and bandwidth of the
  layers, and along which trees a layer transmits
• Multiple trees
• To find a min-cost tree (Steiner tree) is NP-hard
• To construct such multiple trees is even harder

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Two Heuristics Multipath Extension

• Based on point-to-point multipath heuristic
• First meet the bandwidth requirement of each user
  with the multipath heuristics
• Aggregate the paths
• Construct trees out of the paths-aggregate
• Each tree has a certain bandwidth equal to the
  bandwidth of the bottleneck link
• There is at least one tree spanning all the users
• Complexity O(mV3)
• Bandwidth-first approach

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Min-Cost Tree Extension

• First find a min-cost multicast tree spanning all
  the users
• Add branches to the tree until all the bandwidth
  requirements are met
• Closest receivers
• Forming new trees
• Complexity O(mBV2)
• Cost-first approach

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Bandwidth Assignment of Layers

• Group the trees spanning the same set of users
• Arrange these groups according to decreasing
  number of users covered
• The previous set of users is the superset of the
  latter
• The aggregate bandwidth of the first tree-group
  is the bandwidth of the base layer
• The aggregate bandwidth of the 2nd group is the
  bandwidth of the enhancement layer 1, and so on

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An Example on Layering
s
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Simulation Results

• Hierarchical network
• Comparing with a single-tree approach (shortest
  path tree)
• Performance measures
• Success rate of meeting the bandwidth
  requirements of the users
• Average bandwidth achieved
• Cost

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High Success Rate
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High Average Bandwidth
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Slightly Higher Cost
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Conclusion

• Video routing over a bandwidth-limited network
• Multi-path heuristic
• Achieve high end-to-end bandwidth with low delay
• Video scheduling algorithm at the server
• Reduce the start-up delay to the theoretical
  minimum
• Extension to multicast environment
• Meeting heterogeneous bandwidth requirements
• Minimum cost of the tree-aggregate

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Questions and Answers

• Thank you!