Chunkyspread: Multi-tree
Unstructured Peer to Peer
Multicast
Vidhyashankar Venkataraman (Vidhya)
Paul Francis
(Cornell University)
John Calandrino
(University of North Carolina)
Introduction
Increased interest in P2P live streaming in
recent past
Existing multicast approaches
Swarming style (tree-less)
Tree-based
Swarming-based protocols
 Data-driven tree-less multicast (swarming)
getting popular
 Neighbors send notifications about data arrivals
 Nodes pull data from neighbors
 Eg. Coolstreaming, Chainsaw
 Simple, unstructured
 Latency-overhead tradeoff
 Not yet known if these protocols can have good
control over heterogeneity (upload volume)
Tree-based solutions
 Low latency and low overhead
 Tree construction/repair considered complex
 Eg. Splitstream (DHT, Pushdown, Anycast) [SRao]
 Tree repair takes time
 Requires buffering, resulting in delays
 Contribution: A multi-tree protocol that
 Is simple, unstructured
 Gives fine-grained control over load
 Has low latencies, low overhead, robust to failures
[SRao] Sanjay Rao et. Al. The Impact of Heterogeneous Bandwidth Constraints on DHT-Based
Multicast Protocols, IPTPS February 2005.
Chunkyspread – Basic Idea
 Build heterogeneity-aware unstructured
neighbor graph
 Tree building:
 Sliced data stream: one tree per slice (Splitstream)
 Simple and fast loop avoidance and detection
 Parent/child relationships locally negotiated to
optimize criteria of interest
 Load, latency, tit-for-tat, node-disjointness, etc.
Heterogeneity-aware neighbor
graph
 Neighbor graph built with simple random walks
 Using “Swaplinks”, developed at Cornell [SWAP]
 Degree of node in graph proportional to its
desired transmit load
 Notion of heterogeneity-awareness
 So that higher-capacity nodes have more children in
multicast trees
[SWAP] V. Vishnumurthy and P. Francis. On Heterogeneous Overlay Construction and Random
Node Selection in Unstructured P2P Networks. To appear in INFOCOMM, Barcelona 2006.
Sliced Data Stream
Source selects random neighbors using
Swaplinks
Slice Source 2
Multicasts
the slice
Slice
Source 1
Slice 2
Slice 1
Source
Slice 3
Slice
Source 3
Source sends one slice to each node
- acts as slice source to a tree
Building trees
Initialized by flooding control message
Pick parents subject to capacity (load)
constraints
Produces loop-free but bad trees
Subsequently fine-tune trees according to
criteria of interest
Simple and fast loop avoidance/
detection
 Proposed by Whitaker and Wetherall [ICARUS]
 All data packets carry a bloom filter
 Each node adds its mask to the filter
 Small probability of false positives
 Avoidance: advertise per-slice bloom filters to neighbors
 Detection: by first packet that traverses the loop
[ICARUS] A. Whitaker and D. Wetherall. Forwarding without loops in Icarus.
In OPENARCH, 2002.
Parent/child selection based on
load & latency
Never enter
this load region
ML = Maximum Load
Higher Load
(more children)
Lower Load
(less children)
Sheds children
TL+δ
Children
Improve Latency
TL = Target Load
TL-δ
Adds children
0
Parent/Child Switch
Potential Parents
(Load<Satisfactory Threshold)
A
B
Parent for slice k
(Load>Satisfactory threshold)
2) Gets info from all children
5) A says yes if
still underloaded
3) Chooses A and asks child to
switch
1) Child sends info
about A and B
4) Child requests A
Child
Chunkyspread Evaluation
 Discrete event-based simulator implemented in C++
 Run over transit-stub topologies having 5K routers
 Heterogeneity : Stream split into 16 slices
 TL uniformly distributed between 4 & 28 slices
 ML=(1.5)TL: Enough capacity in network
 Two cases
 No latency improvement (δ=0)
 With latency improvement:
 δ=(2/16).TL (or 12.5% of TL)
ML=(1.5).(TL)
TL+δ
TL = Target Load
TL-δ
0
Control over load
Flash crowd scenario
Nodes within 20% of
2.5K nodes join a
TL even with
7.5K node network
latency reduction
at 100 joins/sec
(δ=12.5%)
With latency
Peak of 40 control
messages
node
Snap
shot of per
system
per second with latency
after nodes finished
reduction
fine-tuning trees
Median of 10 messages
per node per second during
Trees optimized ~95s
period of join
[(Load-TL)/TL]%
after all nodes join with
latency reduction
Latency Improvement
With
Latency
Maximum latency ~
Buffer capacity without
node failures
Flash crowd scenario
No Latency
90th percentile
network stretch of 9
~ small buffer
Burst Failure Recovery Time
CDF of Disconnect duration with latency reduction
3 Redundancy
0 Redundant slices
1 Redundant slice
Failure
Burst:improve
1K nodes
FEC codes
failrecovery
in a 10K-node
time
network at the same
time instant
Neighbor failure
timeout set at 4 seconds
Recovery time within
a few seconds
Buffering: Dominant over
effectswith
of latency
Shown
various
Redundancy levels
Conclusion
 Chunkyspread is a simple multi-tree multicast protocol
 A design alternative to swarming style protocols
 Achieves fine-grained control over load with good
latencies
 Suited to non-interactive live streaming applications
 Need to do apples-to-apples comparisons with swarming
protocols and Splitstream