1. Yang Guo Thomson Princeton Lab
Mesh or Multiple-Tree: A Comparative Study of P2P Live Streaming Approaches
Nazanin Magharei, Reza Rejaie
University of Oregon
Yang Guo
Thomson Princeton Lab
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2. Nazanin Magharei Infocom 2007
Motivation
Live P2P streaming has become increasingly popular approach for streaming live content to many receivers, IPTV
e.g. SOPCAST, PPStream and TVUPlayer.
Existing approaches can be divided into two classes:
Tree-shaped overlay + push content delivery
Mesh-shaped overlay + pull content delivery (swarming)
Previous studies on live P2P streaming have often focused on design or evaluation of a particular mechanism.
What are the differences and similarities between these two classes? How do the differences affect their performance?
But this question remains unanswered
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3. Overview: Live P2P Streaming
A live P2P streaming mechanism has two components:
Overlay construction: How overlay is maintained, mesh or tree
Content delivery: How overlay is used for content delivery, push or pull
Goal: Maximizing delivered quality to peers in a scalable fashion
Challenges: in-time delivery of packet despite churn, BW heterogeneity & asymmetry
To cope with BW heterogeneity, stream is often encoded into multiple independent sub-streams, e.g. MDC
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4. Multiple-tree approach
Overview
Multiple-tree approach
Basic idea:
Maintain multiple diverse trees
Each description of MDC content is pushed through a particular tree
Each node joins a proper number of trees based on its incoming BW
A common approach to maintain diverse stable trees (e.g. CoopNet, Splitstream) is to:
Place a node as an internal node in only one tree and leaf node in all other trees
Also try to maintain balanced and short trees
Key design issue: tree construction
SRC
Tree 2 A
Tree 1 D B C E F
Key issue is tree construction and maintenance
Mention that internal nodes can be distinguished by their colors from leaf nodes
Internal node is placed as a child for the node with the lowest depth that has an empty slot Or has a child that is a leaf
A node upon joining is placed at the first empty slot or if it is internal it can preempt an existing leaf node
Might need to omit animation for leaf node joining D E F G A B C G
2 descriptions delivered
to 7-peer overlay
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5. Mesh-based approach Randomly connected uni-directional overlay mesh
Overview
Mesh-based approach
Randomly connected uni-directional overlay mesh
Packet scheduling mech at each peer independently determines what packets to pull from individual parents
Key design issue: packet scheduling at each peer
SRC 2 1 3 5 4 6 7
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6. This paper
Illustrates the similarities & differences between tree- and mesh-based approaches to live P2P streaming, i.e. push- vs pull-based approaches.
Examines to what extent their differences affect the performance of these two approaches, and why.
Leveraging the notion of delivery tree per packet
Our focus is on live P2P streaming
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7. Similarities Overall shape of the resulting overlay is very similar
SRC
Overlay construction:
Overall shape of the resulting overlay is very similar
Aggregate view of multiple trees is a directed mesh
Content delivery:
Peers receive different pieces of content through different paths
Peer level: each peer receives content from multiple parents and sends it to multiple children
System level: the collection of all edges used for delivery of a single packet to all peers form a source-rooted tree, called delivery tree 1
Tree 1 2
Tree 2 3 4 5 6 2 5 6 7 1 3 A 4 7
SRC 2
What does it mean ? In peer level .. 1 3 5 4 6 7
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8. Differences Formation of delivery tree for individual packets2 1 3 6 7 5 4
SRC
Formation of delivery tree for individual packets
Tree-based: Static per description
Pinned down by overlay construction mechanism
A drop in BW of a connection affects all the downstream peers
Mesh-based: Dynamic per packet
Direct result of packet scheduling at all peers
How does a drop in BW of a connection affect other peers in a mesh?
How is a delivery tree dynamically formed in a mesh-based approach?
SRC 2
Don’t forget to mention that the dynamic formation is the effect of packet scheduling 1 3 5 4 6 7
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9. PRIME: Delivery tree formation
Organized view: group peers into levels based on their shortest distance from source
Delivery of packets has two phases:
Diffusion phase
Peers pull packets from parents in the higher level or source, these peers form a diffusion sub-tree
Packets flow away from source
Swarming phase
Peers pull packets from parents in the same or lower levels
SRC 1 2
Level 1
Diffusion direction 3 4 5 6
Level 2
Define diffusion subtree
Now we saw that delivery tree consists of a diffusion subtree and a combination of swarming connection hanging from that 7 8 9 10 11 12 13 14
Level 3
Swarming direction
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10. PRIME: Delivery tree formation
SRC 1 1 2 2 2 5 12 13 11 6 14 10 9 7 1 8 3 4 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11 12 12 13 13 14 14
Lets take a closer look at an example of a delivery tree
Mention many possible combination, see the paper for different types of swarming connections that can be attached to different locations
Lets get back to that basic question of drop in BW of a connection ..
Delivery tree has two components:
whole or part of a diffusion subtree
attached swarming connections
Swarming connections can attach to the diffusion subtree in certain ways (see paper for details)
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11. Evaluation Methodology
Many parameters could affect performance of P2P streaming mechanism
overlay properties, peer dynamics, peer buffer.
We adopt the following methodology to separate the effect of these parameters:
Content delivery in static overlay (ns-2)
Per-connection BW, Peer degree/number of trees, BW heterogeneity and Group size
Content delivery in a distorted overlay (ns-2)
Cohesion of the overlay structure under persistent churn (psim)
Please see the paper for all the results
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12. Extended effect of BW bottleneck
Results – Static overlay
Extended effect of BW bottleneck
% of connections (CDF)
% of connections (CDF)
Avg. BW utilization (%) , Tree-based, degree=8
Avg. BW utilization (%) , Mesh-based, degree=8
In tree, lower level connections have a lower average utilization  a BW bottleneck in upper levels has an extended effect on all downstream connections
In mesh, connections regardless of their location have a high BW utilization (>95%)
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13. Peer degree Results – Static overlay
Avg. BW utilization (%)
Degree
Increasing degree in tree reduces depth  reduces the probability of cumulative effect upstream bottleneck
Increasing degree in mesh-based approach increases diversity among parents
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14. Pattern of content delivery
Results – Static overlay
Pattern of content delivery
% of population (CDF)
% of population (CDF)
Avg. hop count – Mesh-based
Avg. hop count – Tree-based
Avg. hop count in mesh is larger due to the flexibility of packet scheduling to receive a packet from longer paths
For both approaches, increasing peer degree decreases avg. hop count but for different reasons:
In mesh-based approach due to the increase in diversity
In tree-based approach due to the decrease in depth
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15. Nazanin Magharei Infocom 2007
Results – Dynamic overlay
Distorted overlay 5 2 12 10
Avg. BW utilization (%) 4
Source 7 1 3 8 11 6 9 13
% of departed peers
Distorted overlay represents worst case connectivity with churn
In tree, due to the diverse placement of peers in trees, departure of any peer reduces the quality of all of its descendants
In mesh, flexibility in utilizing swarming connections minimize the impact of peer departure
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16. Stability of path from source
Results – Dynamic overlay
Stability of path from source
Avg. interval between ancestor change (sec)
Avg. interval between ancestor change (sec)
Group size – Mesh-based
Group size – Tree-based
Stability of path from source
Increasing group size, decreases stability of path from source in both approaches due to an increase in avg. distance of peers.
Long-lived peers observe a higher degree of stability among their ancestor in mesh
Long-lived peers gradually move to higher levels and increase stability
Paths from source are more stable in mesh
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17. Conclusions
Mesh-based approach consistently exhibits a superior performance over tree-based
Mesh is able to effectively utilize available resources as the overlay grows mainly due to the dynamic mapping of content to parents – tree can not
Peer departure has a local effect on a mesh but it always affect a sub-tree (where it is internal node) in a tree because of the diverse placement of peers
Ongoing work: examining other tree construction algorithms, especially those that dynamically change parents to improve performance, e.g. ChunkySpread.
For further information on our projects on P2P streaming visit
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