1. Research: Group communication in distributed interactive applications Student: Knut-Helge Vik Institute: University of Oslo, Simula Research Labs

2. RELAY - ND seminar Outline  MiSMoSS  Motivation Group Communication Application Layer Multicast Tree algorithms  Research  Conclusions

3. RELAY - ND seminar MiSMoSS Project  Investigate Large-scale interactive applications  Main issue: Latency  Three sub-projects: Latency hiding  prediction Group communication management  Overlay multicast Transmission protocol optimization  Thin streams Real-World Proximity Virtual World Proximity

4. RELAY - ND seminar Motivation - Group communication management  Large-scale interactive applications Users interact in groups Communication demands vary within an application  low latency demands  high bandwidth demands  frequent group membership changes  consistency Build overlay routing  with a small diameter  with degree limitations  using algorithms with low execution times  with stable reconfigurations

5. RELAY - ND seminar Motivation  Example application: Massively Multiplayer Online Games Large scale (thousands of simultaneous users) Central server-based (experience high latency) Clients far apart in physical world, but near in virtual world Issues: Event distribution Goal: Reduce latency, decrease server load, increase MMOG size  Group Communication – Application Layer Multicast Overlay Multicast – Must handle group dynamics Current overlay multicast protocols lack efficient dynamic handling  Goal: Create a dynamic overlay multicast protocol Real-World Proximity Virtual World Proximity

6. RELAY - ND seminar Research - Summary  Group membership - join/leave Insert or remove group members to an existing topology  Overlay multicast – fully meshed graph: Optimization techniques – edge pruning, core selection Multicast trees  Investigating tree problems: Shortest path tree Minimum spanning tree Steiner minimum tree – SPH, DNH, ADH Minimum diameter degree limited spanning tree Dynamic tree algorithms – insert and remove  Tree algorithm constraints: unconstrained degree and/or delay constrained  Metrics: Stress - degree Diameter – maximum pairwise latency Total tree cost – sum of edge weights Reconfiguration time – time it takes to complete reconfiguration Edge change – number of link changes in a reconfiguration

7. RELAY - ND seminar Research - Optimization  Application layer graphs are fully meshed Ex: |V|=1000, |E| = 499500 edges, |E_T|= |V| - 1 (using 0.02 % of the edges) Tree algorithms build trees using graphs  Graph optimization techniques Edge pruning algorithms: k-Best links Limit nodes to group members – steiner minimum trees?  Core selection heuristics: Include stronger nodes in the input graph – higher stress capacity Especially suitable for SMT heuristics Group center, topological center, MDDL center  Goal: Reduce reconfigure time while preserving tree quality

8. RELAY - ND seminar Research - Group Dynamics  Dynamic membership – nodes join and leave the multicast tree dynamically Must insert and remove nodes online  Needs algorithm to reconfigure the tree  Contradictory goals: Low reconfigure time efficient tree tree stability

9. RELAY - ND seminar Research – Reconfiguration Set  Reconfiguration set – nodes involved in reconfiguration  Entire group: Pros: Tree efficiency Cons: High reconfiguration time, tree stability  Reduced size of reconfiguration set Pros: Low reconfiguration time, increased stability Cons: Reduced tree efficiency

10. RELAY - ND seminar Research – Reconfiguration Set  Reconfiguration set – nodes involved in reconfiguration  Entire group: Pros: Tree efficiency Cons: High reconfiguration time, tree stability  Reduced size of reconfiguration set Pros: Low reconfiguration time, increased stability Cons: Reduced tree efficiency

11. RELAY - ND seminar Tree Algorithms  Tree algorithms – reconfigures entire tree Problems in P: Minimum spanning tree (MST), Shortest path tree (SPT) Problems in NP: Steiner minimum tree (SMT), Minimum diameter degree limited tree, Degree constrained MST, SPT, SMT  Main issues: reconfiguration time is high and tree stability suffers Heuristics are especially slow  Addressing issues: Reduce number of edges in input graph, include strong cores Pros: Reduced reconfiguration time, increased stability Cons: Tree efficiency is also reduced pruned Reconfiguration time Total tree cost

12. RELAY - ND seminar Dynamic Algorithms  Dynamic Algorithms – insert/remove (reconfigure smaller parts of a tree) basic edge optimization goals: Minimum cost edge, Minimum diameter edge, Minimum cost to source Prune non member nodes  Main issues: tree efficiency suffers Always local optimizations Crowded with non member nodes  Addressing issues: Vary reconfiguration set size, prune non-members, switch non members to stronger cores Cons: Increased reconfiguration time, reduced stability Pros: Tree efficiency SPH MDDBST Edge changes – remove algorithms (100 nodes) Edge changes MDDBST SPH Worst case insert

13. RELAY - ND seminar Insert Algorithms  Basic insertion choices – Insert as leaf – no edge change Insert and reconfigure – increased tree efficiency but reconfiguration time!  Implemented a number of insert algorithms – ex: I-MC : insert minimum cost edge I-MDDL : insert minimum diameter degree limited edge Node is joining Connect to tree as leaf Insert strong coreUse as intersection Three configuration examples

14. RELAY - ND seminar Remove Algorithm  Basic remove choices – Remove leaf – no edge changes (easy) Remove non-leaf – MUST reconfigure  reconfigure and add/remove non-MN  Implemented a number of algorithms – ex: RTR-MC – neighbors RTR-P – pruning non members Keep as non-memberUse stronger coreReconnect neighborsNode is leaving Three configuration examples

15. RELAY - ND seminar Dynamic Algorithms – Insert/Remove MDDL I-MDDL 25 20 15 10 5 020406080100 diameter group size / number of nodes Remove strategy: RTR-MC MDDL I-MDDL 25 20 15 10 5 020406080100 diameter group size / number of nodes Remove strategy: RTR-P RTR-MCRTR-P leaving reconfigure set

16. RELAY - ND seminar Conclusions and Future Work  Current algorithms are centralized  Implement distributed algorithms  PlanetLab implementation Implement overlay multicast protocol  Investigate mesh vs. trees  Questions?