Kademlia: A peer-to peer information system based on XOR. based on XOR Metric,by P. Maymounkov and D. Mazieres

Transcription

1 : A peer-to peer information system based on XOR Metric,by P. Maymounkov and D. Mazieres March 10, 2009 : A peer-to peer information system based on XOR

2 Features From past p2p experiences, it has been felt a need to: decentralize the index server (as opposed to Napster, which was prone to single point of failure). less time desired information retrieval (as opposed to Gnutella which is using a flooding query model) guarantee that data would be found (as opposed to Freenet) Solution: move to a new DHT implementation based on specific features.. : A peer-to peer information system based on XOR

3 Features features: minimizes the number of configuration messages nodes must send to learn about each other. nodes have enough knowledge and flexibility to route queries through low-latency paths (via parallel asynchronous queries) resistance against denial of service attacks (for registering new nodes) uses XOR metric for distance between points (helps in learning useful routing information from queries they receive) uses a single routing algorithm from start to finish (other systems use 2 algorithms) : A peer-to peer information system based on XOR

4 protocol Keys, and IDs are 160-bit identifiers publish and find < key, value > pairs with help of XOR distance between identifiers (i.e. for x and y identifiers, d(x,y)=x y) d(x,x)=0 d(x, y) > 0 if x y symmetry: x, y : d(x, y) = d(y, x) triangle property: d(x, y) + d(y, z) d(x, z) : A peer-to peer information system based on XOR

5 protocol Each node keeps the following info: i, 0 i < 160, a sorted k-buckets list of <IP address, UDP port, Node ID> triples for nodes of distance between 2 i &2 i+1 from itself the k-buckets are stored in FIFO manner (tail has the most recently seen, and head the oldest seen) for small values of i, the k-bucket will generally be empty; for big values of i, the k-bucket will have up to k (system-wider replication parameter, ex. 20) : A peer-to peer information system based on XOR

6 protocol At every received message, updates the appropriate k-bucket for the sender s node ID, base on 3 cases: sender s ID already exists in the recipient s k-bucket node not yet in the k-bucket list, and the list is not full node not yet in the k-bucket list, and the list is full By this method, higher priorities are given to live nodes, and system is less prone to DoS attacks. : A peer-to peer information system based on XOR

7 protocol RPCs: PING - probes a node to see if it s still online STORE - instructs a node to store a <key,value> pair for later retrieval FIND NODE - takes 160-bit ID as argument, and returns <IP address,udp port, Node ID>for the k-nodes it knows about closest to the target ID (nodes are retrieved recursively if needed) FIND VALUE - same as FIND NODE, but if recipient had a STORE RPC for the key, it just returns the stores value last 2 RPCs are used during the node lookup : A peer-to peer information system based on XOR

8 node lookup: protocol pick α(generally 3) nodes from its closest non-empty k-bucket send parallel, asynchronous FIND NODE RPCs to the α nodes it has chosen. node storage: participant locates k closest nodes to the key, and sends them STORE RPCs to all of them each node republishes the < key, value > pairs that it has, every hour. every 24 hours the original publisher of < key, value > must republish it value lookup - uses FIND VALUE and halts immediately when any node returns a value. Once a value is found, the requesting node stores the < key, value > pair at the closest node it observed to the key that did not return the value : A peer-to peer information system based on XOR

9 protocol problem: high popularity for a certain key, the system might cache it at many nodes, dealing with over-caching solution: make the expiration time of a < key, value > pair in any node s database exponentially inversely proportional to the number of nodes between the current node and the closest node to the key ID : A peer-to peer information system based on XOR

10 protocol In the end the paper proves correctness of the system, and the fact that operations take O(logn) + c, where c is a small constant. Systems that use some similar algorithm to the XOR-topology based routing: Pastry, Tapestry, and Plaxtron All 3 need special approach on how to deal with nodes that share the same prefix, but differ in the next b-bit digit. The solution, however increase bootstrapping and maintenance costs : A peer-to peer information system based on XOR

11 protocol is the first p2p system that combines provable consistency and performance, latency minimization routing, and symmetric, unidirectional topology. introduces a concurrency parameter α, which lets people trade a constant factor in bandwidth for asynchonous lowest-latency hop selection and delay-free fault recovery exploits the fact that node failure are inversely related to uptime : A peer-to peer information system based on XOR

12 Thank you for your attention! Questions? :) : A peer-to peer information system based on XOR