Vikash Agarwal, Reza Rejaie

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Adaptive Multi-Source Streaming in Heterogeneous
Peer-to-Peer Networks

• Vikash Agarwal, Reza Rejaie
• Computer and Information Science Department
• University of Oregon
• http//mirage.cs.uoregon.edu
• January 19, 2005

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Introduction

• P2P streaming becomes increasingly popular
• Participating peers form an overlay to
  cooperatively stream content among themselves
• Overlay-based approach is the only way to
  efficiently support multi-party streaming apps
  without multicast
• Two components
• Overlay construction
• Content delivery
• Each peer desires to receive max. quality that
  can be streamed through its access link
• Peers have asymmetric heterogeneous BW
  connectivity
• Each peer should receive content from multiple
  parent peers gt Multi-source streaming.
• Multi-parent overlay structure rather than tree

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Benefits of Multi-source Streaming

• Higher bandwidth to each peer
• higher delivered quality
• Better load balancing among peers
• Less congestion across the network
• More robust to dynamics of peer participation
• Multi-source streaming introduces new challenges
  

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Multi-source streaming Challenges

• Congestion controlled connections from different
  parent peers exhibit
• independent variations in BW
• different RTT, BW, loss rate
• Aggregate bandwidth changes over time
• Streaming mechanism should be quality adaptive
• Static one-layer-per-sender approach is
  inefficient
• There must be a coordination mechanism among
  senders in order to
• Efficiently utilize aggregate bandwidth
• Gracefully adapt delivered quality with BW
  variations
• This paper presents a receiver-driven
  coordination mechanism for multi-source streaming
  called PALS

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Previous Studies

• Congestion control was often ignored
• Server/content placement for streaming MD content
  Apostolopoulos et al.
• Resource management for P2P streaming Cue et
  al.
• Multi-sender streaming Nguyen et al, but they
  assumed
• Aggregate BW is more than stream BW
• RLM is receiver-driven but ..
• RLM tightly couples coarse quality adaptation
  with CC
• PALS only determines how aggregate BW is used
• P2P content dist. mechanism can not accomodate
  streaming apps
• e.g. BitTorrent, Bullet

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Overall Architecture

• Overall architecture for P2P streaming
• PRO Bandwidth-aware overlay construction
• Identifying good parents in the overlay
• PALS Multi-source adaptive streaming
• Streaming content from selected parents
• Distributed multimedia caching
• Decoupling overlay construction from delivery
  provides great deal of flexibility
• PALS is a generic multi-source streaming protocol
  for non-interactive applications

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Assumptions Goals

• Goals
• To fully utilize aggregate bandwidth to
  dynamically maximize delivered quality
• Deliver max no of layers
• Minimize variations in quality

• Assumptions
• All peers/flows are cong. controlled
• Content is layered encoded
• All layers are CBR with the same cons. rate
• All senders have all layers (relax this later)
• Limited window of future packets are available at
  each sender
• Live but non-interactive
• Not requirements

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P2P Adaptive Layered Streaming (PALS)

• Receiver periodically requests an ordered list
  of packets/segments from each sender.
• Sender simply delivers requested packets with
  the given order at the CC rate
• Benefits of ordering the requested list
• Provide flexibility for the receiver to closely
  control delivered packets
• Graceful degradation in quality when bandwidth
  suddenly drops
• Periodic requests gt stability less overhead

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Basic Framework

• Receiver passively monitors EWMA BW from each
  sender
• EWMA aggregate BW
• Estimate total no of pkts to be delivered during
  next window (K)
• Allocate K pkts among active layers (Quality
  Adaptation)
• Controlling bw0(t), bw1(t), ,
• Controlling evolution of buf. state.
• Assign a subset of pkts to each sender (Packet
  assignment)
• Allocating each senders bw among active layers

Demux
bw (t)
bw (t)
bw (t)
2
1
3
bw (t)
0
buf
buf
buf
buf
1
2
3
0
C
C
C
C
Decoder
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Key Components of PALS

• Sliding Window (SW) to keep all senders busy
  loosely synchronized with receiver playout time
• Quality adaptation (QA) to determine quality of
  delivered stream, i.e. required packets for all
  layers during one window
• Packet Assignment (PA) to properly distribute
  required packets among senders

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Sliding Window

• Buffering window range of timestamps for packets
  that must be requested in one window.
• Window is slided forward in a step-like fashion
• Requested packets per window can be from
• Playing window (loss recovery)
• Buffering window (main group)
• Future windows (buffering)

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Sliding Window (contd)

• Window size determines the tradeoff between
  smoothness or signaling overhead responsiveness
• Should be a function of RTT since it specifies
  timescale of variations in BW
• Multiple of max smoothed RTT among senders
• Receiver might receive duplicates
• Re-requesting the packet that is in flight!
• Ratio of duplicates are very low and can be
  reduced by increasing window

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Coping with BW variations

• Sliding window is insufficient
• Coping with sudden drop in BW by
• Overwriting request at senders
• Ordering requested packets
• Coping with sudden increase in BW by
• Requesting extra packets

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Quality Adaptation

• Determining required packets from future windows
• Coarse-grained adaptation
• Add/drop layer
• Fine-grained adaptation
• Controlling bw0(t), bw1(t), ,
• Loosely controlling evolution of receiver buffer
  state/dist.
• What is a proper buffer dist?
• Buffer distribution determines what degree of BW
  variations can be smoothed.

Demux
bw (t)
bw (t)
bw (t)
2
1
3
bw (t)
0
buf
buf
buf
buf
1
2
3
0
C
C
C
C
Decoder
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Buffer Distribution

• Impact on delivered quality
• Conservative buf. distribution achieves long-term
  smoothing
• Aggressive buf. distribution achieves short-term
  improvement
• PALS leverages this tradeoff in a balanced
  fashion
• Window size affects buffering
• Amount of future buffering
• Slope of buffer distribution
• Multiple opportunities to request a packet (see
  paper)
• Implicit loss recovery

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Packet Assignment

• How to assign an ordered list of selected pkts
  from diff. layers to individual senders?
• Number of assigned pkts to each sender must be
  proportional to its BW contribution
• More important pkts should be delivered
• Weighted round robin pkt assignment strategy
• Extended this strategy to support partially
  available content at each peer
• Please see paper for further details

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Performance Evaluation

• Using ns simulation to control BW dynamics
• Focused on three key dynamics in P2P systems BW
  variations, Peer participation, Content
  availability
• Senders with heterogeneous RTT BW
• Decouple underlying CC mechanism from PALS
• Performance Metrics BW Utilization, Delivered
  Quality
• Two strawman mechanisms with static layer
  assignment to each sender
• Single Layer per Sender (SLS) Sender i delivers
  layer i
• Multiple Layer per Sender (MLS) Sender i
  delivers layer jlti

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Necessity of Coordination

• SLS MLS exhibit high variations in quality
• No explicit loss recovery
• No coordination
• Inter-layer dependency magnifies the problem
• PALS effectively utilizes aggregate BW delivers
  stable quality in all cases

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Delay-Window Tradeoff

• Avg. delivered quality only depends on agg. BW
  Heterogeneous senders
• Higher Delay gt smoother quality
• Duplicates exponentially decrease with window
  size
• Avg. per-layer buffering linearly increases with
  Delay
• Increasing window leads to even buffer dist.
• See paper for more results.

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Conclusion Future Work

• PALS is a receiver-driven coordination mechanism
  for streaming from multiple cong. controlled
  senders.
• Simulation results are very promising
• Future work
• Further simulation to examine further details
• Prototype implementation for real experiments
• Integration with other components of our
  architecture for P2P streaming

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Partially available content

• Effect of segment size and redundancy

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Packet Dynamics