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

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

6 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

7 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

Multipath Routing for Point-to- Point Video Delivery

11 A Point-to-Point Video Network

12 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

13 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

14 A Multipath Routing Heuristics 1. Find the max-flow sub-graph G of the network 2. Find the shortest-path in the sub-graph G 3. If the aggregated bandwidth of the path(s) found is sufficient, return 4. Subtract the bandwidth from G along the path just found 5. Repeat steps 2 to 4

15 An Example s v1v1 v3v3 v2v2 v5v5 v4v4 t (20,6) (10,5) (15,7) (8,13) (20,7) (10,8) (15,6) (10,12) (15,7) s v1v1 v3v3 v2v2 v5v5 v4v4 t (20,6) (10,5) (15,7) (8,13) (20,7) (10,8) (15,6) (10,14) (10,10) (5,13) (10,12) (15,7)

16 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

17 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

18 High Success Rate

19 High Bandwidth Achieved

20 Low Average Delay

21 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

22 An example s t Upper hierarchy Lower hierarchy

Server Scheduling Algorithm

24 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

25 A Simple Case Two paths with the same bandwidth of B/2 but different delays d 1 and d 2 (d 1 < d 2 ) Without server scheduling, the start-up delay equals the delay of the longer path, i.e., d 2

26 The Theoretical Minimum Delay Data production and consumption curves The difference is the buffer requirement In the example, the minimum start-up delay is (d 1 +d 2 )/2 minimized delay Data d1d1 d2d2 Time 0 Slope=B/2 Slope=B original delay

27 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

28 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 To path 1 To path 2 Video data

29 General Case of Scheduling Video timet 1 t K-1 t 2 t 0 t 3... p 1 p K p 2 p 1 p 1 p 1 p 3 p 2 p

30 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

31 Rescheduling Achieves a Delay Comparable to the Shortest Path

Extension to Point-to-Multipoint Video Delivery

33 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

34 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

35 An Example s Users Base layer tree 1 Base layer tree 2 Enh. layer tree 1

36 Problem Formulation: Bandwidth- Constrained Cost-Optimized Problem Given A source s A set of destinations Y (= {y 1, y 2,…, y n }) Bandwidth requirement B (= {b 1, b 2,…, b n } ) Find multiple trees T to achieve Bandwidth no less than b i for y i 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

37 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

38 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

39 Bandwidth Assignment of Layers 1. Group the trees spanning the same set of users 2. Arrange these groups according to decreasing number of users covered The previous set of users is the superset of the latter 3. The aggregate bandwidth of the first tree-group is the bandwidth of the base layer 4. The aggregate bandwidth of the 2nd group is the bandwidth of the enhancement layer 1, and so on

40 An Example on Layering s Users Base layer tree 1 Base layer tree 2 Enh. layer tree 1

41 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

42 High Success Rate

43 High Average Bandwidth

44 Slightly Higher Cost

45 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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