1. CHORD – peer to peer lookup protocol Shankar Karthik Vaithianathan & Aravind Sivaraman University of Central Florida

2. Outline History General definition of a DHT system Chord Applications Implementation Demo/Open questions

3. serverindex tabledata client aclient bclient cclient d query data transferring History: client-server model

4. server index table dataclient a client b client c client ddata query data transferring History: peer-to-peer model (Napster)

5. query data transferring index table dataclient a client b client c client d index table data index table data index table data History: peer-to-peer model (Gnutella)

6. query data transferring index table dataclient a client b client c client d index table data index table data index table data History: peer-to-peer model (DHT systems)

7. What is normal hashing ? Putting items into say n buckets, often solved by a static solution(e.g H(key)mod N) Problems N changes ? We must move every thing (simple solution) or could we move just (#items) / (#buckets) on an average (optimal solution) what if we do not know the number of buckets ? Distributed Hash Table

8. Consistent Hashing  Let A & B be Hashing. They send an object k to two different buckets, say y and z, respectively. Then either A cannot see bucket z or B cannot see bucket y.  Each bucket is a assigned a number in (0,1), and each item gets a value (0,1). You could then be in according to least distance or always round down(or up) and wrap at the edges.

9. Distributed Hash Table Consistent Hashing c’td If a new bucket is added, we therefore need to move the Contents of bucket immediately before it. If we let each bucket have m elements and (M = log n) then each bucket gets a fair share of load and on an average we move only the average contents of the bucket around. If we let each bucket have m elements and (M = log n) then each bucket gets a fair share of load and on an average we move only the average contents of the bucket around.

10. Distributed Hash Table a new class of peer-to-peer routing infrastructures. support a hash table-like functionality on Internet-like scale Consider the buckets to be computers on the internet and that given a key, they should find the value(IP address) The challenges with such a system are: Load balancing Scalability dynamic nature no critical point deterministic.

11. an overlay space and its partition approach a routing protocol  local routing table  next-hop decision a base hash function  variants: proximity-aware, locality-preserving, etc. Basic DHT components

12. Consistent hashing data server Overlay space Hashing

13. Overlay space - one-dimensional unidirectional key space 0 – 2 m -1.  Given two m-bit identifiers x and y, d(x, y)=(y-x+ 2 m ) % 2 m - Each node is assigned a random m-bit identifier. - Each node is responsible for all keys equal to or before its identifier until its predecessor node’s identifier in the key space. Chord [Stoica et al. Sigcomm2001]

14. Routing table (finger table) - (at most) m entries. The i th entry of node n contains the pointer to the first node that succeeds n by at least 2 (i-1) on the key space, 1  i  m. Next-hop decision: - For the given target key k, find the closest finger before (to) k and forward the request to it. - Ending condition: The request terminates when k lies between the ID range of current node and its successor node. - The routing path length is O(log n) for a n-nodes network with high probability (w.h.p.). Chord – routing protocol

15. A Chord network with 8 nodes and 8-bit key space 0 32 64 96 128 160 192 224 256 Network node 0 256 Data 120 Chord – an example (m=8)

16. Chord – routing table setup A Chord network with 8 nodes and 8-bit key space Network node 255 0 64 128 32 192 96 160224 [1,2) Range 1 [2,4) Range 2 [4,8) Range 3 [8,16) Range 4 [16,32) Range 5 [32,64) Range 6 [64,128) Range 7 [128,256) Range 8 Data Pointer

17. Chord – a lookup for key 120

18. How to look up a key quickly ? We need finger table

19. How to look up a key quickly ?(cont.) u finger table for node 1

20. How to look up a key quickly ?(cont.) Node 3: Am I predecessor(1) ? Predecessor(1)  successor(1) Node 3: Try entry 3, and find node 0 Node 3: Send lookup to node 0Node 0: Am I predecessor(1) ?Node 0: successor(1) is node 1 return to node 3 (RPC) Value of key 1 ?

21. Node joins u Two challenges  Each node’s finger table is correctly filled  Each key k is stored at node successor(k) u Three operations  Initialize the predecessor and fingers of the new node n  Update the fingers and predecessors of existing nodes  Copy to n all keys for which node n has became their successor

22. Initialize the predecessor and fingers of node n Idea: Ask an existing node for information needed Join in

23. Update the fingers and predecessors of existing nodes Observation: when node n joins the network, n will become the i th finger of a node p when the following two conditions meet:  P proceeds n by at least 2 i-1  The i th finger of node p succeeds n u Solution: Try to find predecessor(n- 2 i-1 ) for all 1<=i<=m; and check whether n is their i th finger, and whether n is their predecessor’s i th finger.

24. Update the fingers and predecessors of existing nodes (cont.) Predecessor(6-2 1-1 ) =3, update 66 Predecessor(3) =1, no update Predecessor(6-2 2-1 ) =3, update 66 Predecessor(6-2 3-1 ) =1, update 66 Predecessor(1) =0, update Predecessor(3) =1, no update 66 Predecessor(0) =3, no update Join in

25. Copy to n all keys for which node n has became their successor Idea: Node n can become the successor only for keys stored by the node immediately following n Join in

26. Extended Chord protocol Concurrent joins Failures and replication Beyond are project scope. [Ref. Stoica et, all sigcomm 2001]

27. Example applications:  Co-operative Mirroring  Time-shared storage  Distributed indexes  Large-Scale combinatorial search

28. Implementation Our current implementation is simulation of the Chord protocol in O(log n) steps in java. The framework can be shown as below

29. Methods Implemented

30. Conclusion Questions ? Reference : Chord: A Scalable Peer to peer Lookup Service for Internet Applications Ion Stoica, Robert Morris, David Karger, M. Frans Kaashoek, Hari Balakrishnan MIT Laboratory for Computer Science chord@lcs.mit.edu http://pdos.lcs.mit.edu/chord/

31. THANK YOU