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Multipath Routing for Video Delivery over Bandwidth-Limited Networks

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## Multipath Routing for Video Delivery over Bandwidth-Limited Networks

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### Multipath Routing for Video Delivery over Bandwidth-Limited Networks

### Questions and Answers

S.-H. Gary Chan Jiancong Chen

Department of Computer Science

Hong Kong University of Science and Technology

Clear Water Bay, Kowloon

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

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

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

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

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

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

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

Multipath Problem Formulation: Bandwidth-Constrained 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

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

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

(20,7)

v1

v1

v4

v4

(10,12)

(10,12)

(8,13)

(8,13)

(15,6)

(15,6)

(5,13)

s

s

v3

v3

t

t

(10,5)

(10,5)

(10,10)

(15,7)

(15,7)

(10,8)

(10,8)

(15,7)

(15,7)

(20,6)

(20,6)

v2

v2

v5

v5

(10,14)

An ExampleSimulation Model

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

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

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

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

A Simple Case

- Two paths with the same bandwidth of B/2 but different delays d1and d2 (d1< d2)
- Without server scheduling, the start-up delay equals the delay of the longer path, i.e., d2

Slope=B

Slope=B/2

Time

d2

d1

0

original delay

minimized delay

The Theoretical Minimum Delay- Data production and consumption curves
- The difference is the buffer requirement
- In the example, the minimum start-up delay is (d1+d2)/2

The Idea

- Don’t 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

To path 1

To path 2

To path 1

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

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

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

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

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 bifor 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

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(m|V|3)
- Bandwidth-first approach

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(mB|V|2)
- Cost-first approach

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

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

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

Thank you!

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