1. Implicit group messaging in peer-to-peer networks Daniel Cutting, 28th April 2006 Advanced Networks Research Group

2. Implicit group messaging April 28, 2006 Slide 2 Outline. Motivation and problem Implicit groups Implicit group messaging (IGM) P2P model Evaluation

3. Implicit group messaging April 28, 2006 Slide 3 Motivation. It’s now very easy to publish content on the Internet: blogs, podcasts, forums, iPhoto “photocasting”, … More and more publishers of niche content Social websites like Flickr, YouTube, MySpace, etc. are gateways for connecting publishers and consumers Similar capability would also be desirable in P2P Collaboration and sharing without central authority No reliance on dedicated infrastructure No upfront costs, requirements

4. Implicit group messaging April 28, 2006 Slide 4 Problem. As more new niches are created, consumers need to search/filter more to find and collate varied content How can we connect many publishers and consumers? The publisher already knows the intended audience Can often describe the audience in terms of interests Does not know the names of individual audience members So, address them as an implicit group

5. Implicit group messaging April 28, 2006 Slide 5 Implicit groups. Explicit groups Members named Pre-defined by publisher or consumers need to join Wolfgang, Julie Implicit groups Members described Publisher defines “on the fly”, consumers don’t need to join Soccer & Brazil

6. Implicit group messaging April 28, 2006 Slide 6 Implicit group messaging. CAST messages from any source to any implicit group at any time in a P2P network Each peer described by attributes (capabilities, interests, services, …), e.g. “Soccer”, “Brazil” Implicit groups are specified as logical expressions of attributes, e.g. “(Soccer OR Football) AND Brazil” System delivers messages from sources to all peers matching target expressions

7. Implicit group messaging April 28, 2006 Slide 7 P2P model. A fully distributed, structured overlay network Peers maintain a logical Cartesian surface (like CAN) Each peer owns part of the surface and knows neighbours Peers store data hashed to their part of the surface Peers geometrically ROUTE to locations by passing from neighbour to neighbour Quadtree-based surface addressing Smoothly combine two major techniques for efficient CAST delivery to groups of any size

8. Implicit group messaging April 28, 2006 Slide 8 P2P model. Attribute partitioning: “attribute  peer” index for small groups Summary hashing: for reaching BIG groups Hybrid CAST algorithm: reactive multicast algorithm combining the above

9. Implicit group messaging April 28, 2006 Slide 9 Quadtree-based addressing. Surfaces can be any dimensionality d An address is a string of digits of base 2 d Map from an address to the surface using a quadtree decomposition Quadrants called extents

10. Implicit group messaging April 28, 2006 Slide 10 Attribute partitioning. A distributed index from each attribute to all peers Indices are stored at rendezvous points (RPs) on the surface by hashing the attribute to an address

11. Implicit group messaging April 28, 2006 Slide 11 Attribute partitioning (registration). Every peer registers at each of its attributes RPs Every registration includes IP address and all attributes

12. Implicit group messaging April 28, 2006 Slide 12 Attribute partitioning (CASTing). To CAST, select one term from target Route CAST to its RP RP finds all matches and unicasts to each

13. Implicit group messaging April 28, 2006 Slide 13 Attribute partitioning. Simple, works well for small groups and rare attributes Fast: just one overlay route followed by unicasts Fair: each peer responsible for similar number of attributes BUT common attribute  lots of registrations at one RP Heavy registration load on some unlucky peers ALSO big groups  many identical unicasts required Heavy link stress around RPs SO, in these cases share the load with your peers!

14. Implicit group messaging April 28, 2006 Slide 14 Summary hashing. Spreads registration and delivery load over many peers In addition to attribute registrations, each peer stores a back-pointer and a summary of their attributes at one other location on the surface Location of summary encodes its attributes Given a target expression, any peer can calculate all possible locations of matching summaries (and thus find pointers to all group members) Summaries distributed over surface; a few at each peer

15. Implicit group messaging April 28, 2006 Slide 15 Summary hashing (registration). Wolfgang {Soccer, Brazil} Benoit {Argentina, Soccer} Kim {Brazil} Julie {Soccer, Argentina, Brazil} Each peer creates a Bloom Filter {Soccer,Brazil}01101 01100 | 01001 Treat bits as an address 01101(0)  122 (2D) Store summary at that address on the surface

16. Implicit group messaging April 28, 2006 Slide 16 Summary hashing (CASTing). Can find all summaries matching a CAST by calculating all possible extents where they must be stored Convert CAST to Bloom Filter, replace 0s with wildcards Soccer & Brazil  {Soccer, Brazil}  *11*1 01100 | 01001 Any peer with both attributes must have (at least) the 2nd, 3rd and 5th bits set in their summary address The wildcards may match 1s or 0s depending on what other attributes the peer has

17. Implicit group messaging April 28, 2006 Slide 17 Summary hashing (CASTing). Find extents with 2nd, 3rd and 5th bits are set {Soccer,Brazil}  *11*1(*) = {122, 123, 132, 133, 322, 323, 332, 333 }

18. Implicit group messaging April 28, 2006 Slide 18 Summary hashing (CASTing). Start anywhere and intersect unvisited extents with target expression Cluster remainder and forward towards each one until none remain When summaries are found, unicast to peers Called Directed Amortised Routing (DAR)

19. Implicit group messaging April 28, 2006 Slide 19 IGM on P2P summary. Peers store their summary on the surface and register at the RP for each of their attributes If an RP receives too many registrations for a common attribute, it simply drops them To CAST, a source peer picks any term from target expression and tries a Partition CAST (through an RP) If RP doesn’t know all matching members (because it’s a common attribute) or the group is too large to unicast to each one, it resorts to a DAR

20. Implicit group messaging April 28, 2006 Slide 20 Evaluation. 2,000 peer OMNeT++/INET simulation of campus-scale physical networks, 10 attributes per peer (Zipf) 8,000 random CASTs of various sizes (0 to ~900 members) Comparison to a Centralised server model Metrics Delay penalty Peer stress (traffic and storage)

21. Implicit group messaging April 28, 2006 Slide 21 Evaluation (delay penalty). Ratio of Average Delay (RAD) and Ratio Maximum Delay (RMD) compared to Centralised model 80% of CASTs have average delay less than 6 times Centralised model 95% have maximum delay less than 6 times Centralised

22. Implicit group messaging April 28, 2006 Slide 22 Evaluation (peer stress). Order of magnitude fewer maximum packets handled by any one peer over the Centralised server Higher average stress since more peers involved in delivering CASTs Even spread of registrations over peers

23. Implicit group messaging April 28, 2006 Slide 23 Conclusion Implicit groups are a useful way of addressing a group when you know what they have in common but not who they are IGM is also applicable to other applications Software updates to those who need them Distributed search engines P2P implicit group messaging is fast and efficient Does not unfairly stress any peers or network links Can deliver to arbitrary implicit groups with large size variation

24. Implicit group messaging April 28, 2006 Slide 24 Questions?