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 Slice 1 Slice 2 Slice 3 Source Slice Source 2 Slice Source 1 Slice Source 3 Multicasts the slice Source sends one slice to each node - acts as slice source to a tree Source selects random neighbors using Swaplinks

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 ML = Maximum Load TL+δ TL-δ Children Improve Latency 0 TL = Target Load Adds children Sheds children Never enter this load region Higher Load (more children) Lower Load (less children)

Parent/Child Switch Child Parent for slice k (Load>Satisfactory threshold) Potential Parents (Load<Satisfactory Threshold) 1)Child sends info about A and B 2) Gets info from all children 4) Child requests A A 5) A says yes if still underloaded B 3) Chooses A and asks child to switch

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) 0 ML=(1.5).(TL) TL+δ TL-δ TL = Target Load

Control over load Flash crowd scenario 2.5K nodes join a 7.5K node network at 100 joins/sec Nodes within 20% of TL even with latency reduction (δ=12.5%) Peak of 40 control messages per node per second with latency reduction Median of 10 messages per node per second during period of join [(Load-TL)/TL]% Snap shot of system after nodes finished fine-tuning trees Trees optimized ~95s after all nodes join with latency reduction With latency

Latency Improvement No Latency With Latency 90 th percentile network stretch of 9 ~ small buffer Maximum latency ~ Buffer capacity without node failures Flash crowd scenario

Burst Failure Recovery Time Recovery time within a few seconds Buffering: Dominant over effects of latency Neighbor failure timeout set at 4 seconds FEC codes improve recovery time 0 Redundant slices 1 Redundant slice 3 Redundancy Failure Burst: 1K nodes fail in a 10K-node network at the same time instant Shown with various Redundancy levels CDF of Disconnect duration with latency reduction

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