A Framework for Architecting Peer-to- Peer Receiver-driven Overlays Reza Rejaie, Shad Stafford Mirage Research Group Department of Computer Science University of Oregon NOSSDAV 2004 Cork, Ireland

Introduction Problem: non-interactive streaming in P2P Networks Design goals: –Maximum delivered quality to each peer. –Scalability (of data and control traffic) with group size –Robust to dynamics of peer participation –Adaptable to changing network dynamics (variations in BW) –Accommodating peer BW heterogeneity and asymmetry Common approach: form a P2P overlay  Existing solutions are unable to form a bandwidth- aware overlay => can not maximize delivered quality

Basic Design Issues Structured vs unstructured P2P? – Unstructured can gracefully cope with churn Single vs multiple parents in the overlay? –Only multiple parent overlay (mesh) can accommodate BW Hetro. & Asym. Pair-wise connections must be congestion controlled Delivery: how to deliver a single stream from multiple congestion controlled parents? –Requires tight coordination among senders to efficiently “stream” maximum deliverable quality from multiple parents

Main Components 1) Peer Discovery: the mechanism to find other peers –Must scale with group size 2) Parent Selection: the mechanism to select “good” parents –Selection Criteria: 1) Minimizing overall Delay, & 2) maximizing overall Bandwidth from all parents.  Minimizing Delay and Maximizing BW might be conflicting goals Scalable Delay estimation using Global Network Positioning (GNP) Estimation of pair-wise available BW between all peers requires periodic N*N measurement => Does not scale  Selecting a parent could affect BW from others (i.e. shared bottleneck)

Existing Solutions Layered encoded content, each layer is delivered through a separate source-rooted tree –accommodate BW Heterogeneity, but does not perform congestion control Peer Discovery: Mesh-first or centralized approach – Neither scalable nor bandwidth-aware Parent Selection: only using delay as selection criteria –Delivered bandwidth over a tree structure is inherently limited by minimum outgoing bandwidth among upstream peers –Multiple trees may share congested links

Proposed approach Decouple Delivery & Overlay construction 1) Delivery (PALS): A receiver-driven mechanism to stream from multiple congestion-controlled senders [NOSSDAV 2003]  Higher bandwidth => Higher quality 2) Overlay construction (PRO):  Each receiver selfishly, independently, dynamically searches for parents to maximize its own BW, and thus its own quality

PRO: An Overview Search for good parents by each peer is conducted in two phases: 1) Gossip-based Peer Discovery (PD): –Identifies potentially good parents in the overlay through periodic gossiping. –Good parents are maintained in a local image –Reducing scope of search from the entire group to a small number of peers 2) Parent selection (PS) –PS searches for a subset of parents within the local image that maximize performance –Their actual BW is verified through passive measurement

Gossip-based Peer Discovery Peer P i (originator) periodically selects a target peer (P j ) to gossip with Gathered info about good parents are maintained in a local, incomplete image. Per peer info: –P k : IPAdd k, GNP-Coor k, inBW k, outBW k, TS [gossip target] P j can identify good parents P k from its local image for gossip originator P i since: –Well known Utility func: U(outBW k, D ki ) enables P j to assess value of any peer P k in its local image for P j, e.g. joint-ranking –D ki : Relative Distance P k - P j is estimated from their GNP cor. –outBW k is used as a “hint” for potential ave BW from P k Pk : IPAddk Pj Pi Pk originator targetLocal Image Good parents for me? Pk,Pn,Pm

Gossiping: Main Components 1) Target selection: how to select gossip target? –Random selection, biased towards peers with higher utility 2) Content selection: how should the target peer select content of a gossip message? –Select N peers from its local image with Max utility for the gossip originator. 3) Image maintenance: how to improve “quality” of local image? [there are two orthogonal dimension for img quality] –Utility: Maximizing overall utility [drop peers w lower utility] –Freshness: Maximizing freshness (ts) of image [drop older ts]  Tradeoff between freshness and utility of image  Degree of overlap among images affects reachability, search efficiency, and robustness to peer dynamics

Parent Selection Local image contains potentially good parents Each peer selfishly, progressively searches for a subset of parents from the local image in order to 1)Maximize total BW/Quality from all parents 2)Minimize total delay from all parents 3)Maximize path diversity from parents Best subset of parents is not unique, and changes with time PS strategy by individual peers collectively determines shape of the overlay.  What is a proper search & evaluation strategy to dynamically find the best subset of parent?

Parent Selection: Main Components 1) When to select a new parent? –Tradeoff between responsiveness and stability –Hysteresis and binning (of BW and distance) => stability –Select a new parent to increase BW or decrease delay 2) Which peer to select as new parent? –Non-uniform random selection, biased with peer’s utility 3) How should a parent be evaluated? [both new and existing] –Monitor available BW from parents & correlation among them  Correlation in available BW => shared bottleneck  [Degree of correlation => level of mux on a shared bottleneck]  [Maximize path diversity and/or overall BW from parents]

The Key Idea Congestion controlled BW from parents serves as an implicit signal for each peer to detect: –Any relevant change in the overlay, mainly peers that share a parent –Any measurable/major shared bottleneck among connection from parents –Major, long-term changes in BW due to congestion  CC bandwidth signals peers to properly change their parents  dynamically reshape the overlay

Conclusion & Future Work PRO is a simple, receiver-driven framework to architect unstructured P2P overlays, that –Maximizes delivered bandwidth to heterogeneous peers –Gracefully accommodate dynamics of peer participation –Incorporates scalable peer discovery, and selfish parent selection –Dynamically adapts to changes in overlay or network conditions. PRO trades simplicity with performance –Optimal performance is very hard to achieve anyway! Gossip-based peer discovery is applicable to any P2P net. Future Work –Detailed evaluations of various components are being conducted –Plan to prototype the PD and PS functions as a generic middleware See paper for further details.