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Middleware issues: From P2P systems to Ad Hoc Networks Franca Delmastro [email protected] Pervasive Computing & Networking Lab (PerLab) IIT-CNR Pisa MobileMAN Project Helsinki, June 7th 2004 Middleware for Ad Hoc networks Ad Hoc networks and P2P systems: The distribution of services and data can fit well with the decentralized nature of Ad Hoc networks In both cases nodes interactions are temporary Resources are distributed among heterogeneous nodes Systems must be reliable in case of node failures and disconnections events The fundamentals of the structured overlay network CAN, Chord, Pastry et al. represents a family of middleware solutions for large-scale P2P systems They use a Distributed Hash Function (DHT) to map physical addresses to a logical address space Information and workload are uniformely distributed on the network System scalability with the number of nodes is the main objective for ad hoc networks The Pastry model The overlay network is a circular address space where nodes and data are logically mapped. There is no correspondence between logical and physical distances Subject-based routing reduce the lookup cost to a logarithmic cost There are a lot of applications implemented for this solution, and many others are adaptable to this routing policy Logical & Physical distances Pastry Ring 0 2128 - 1 The Pastry Ring (K2 ,V) (K1 ,V) L2 = H(K2 ) L1 = H(K1) 0 2128 - 1 L1 X2 N1 L2 N2 X1 X1 = H(N1) X2 = H(N2) Pastry routing tables 02212102 0 2128 - 1 10233001 10233102 10233232 10323302 11301233 Pastry Multi-hop Routing 02212102 0 2128 - 1 33301201 23301201 23301101 Route(23302121) 10233102 23000101 22301203 Compare(10233102, 23302121) = 0 bestMatch(RoutingTable[0], 23302121 ) = 22301203 Joining the Ring X has to know a own physical neighbor already present in the ring (node A) 0 2128 - 1 A sends a message with key equal to X X Z Pastry Routing table of node X is initialized using routing tables of contacted nodes: LS(X) = LS(Z) NS(X) = NS(A) RT(X) is a join of the routing tables of other nodes, according to the prefix shared metric Bk Route(X) B2 A X Join(X) B1 Disconnection from the ring Each node executes a polling procedure to discover “remote” nodes status (referred only to routing table knowledge). A “remote” node is considered disconnected from the Pastry network if it doesn’t answer to a polling message before a timeout expiration After a disconnection event, the sender of the polling message has to update its routing tables contacting other “remote” nodes to fill in entries related to that node. Pastry Pros & Cons Pros: DHT allows an uniform distribution of IDs and workload on nodes providing the service The subject-based routing defines a logarithmic lookup cost on the network dimension (O(log N)) A lot of application can adapt their contents to this routing strategy Cons: Routing tables management based on remote connections can be a big overhead for ad hoc networks Forcing the network routing with the subject-based policy can reduce network performances Using Cross-Layer to CROSS-ROAD CROSS-layer Ring Overlay for AD hoc networks Applications Middleware NeSt Transport Network MAC In order to build an overlay network, the middleware can directly use the Network Routing table. Node ID = H(IPaddress) Since each ring is associated to a service, the routing protocol has to provide Service Location Using a proactive LINKSTATE routing protocol, each node knows the entire network Hazy Sighted Link-State Optimization of Proactive routing protocols Each node sends periodical LSU packets on the network with frequencies inversary proportional to the routing hops number. “Hazy” knowledge of distant nodes. Their status is not frequently updated as the 1-hop neighbors. (*) C. Santivanez, I. Stavrakakis et al., “Making Link-State Routing Scale for Ad Hoc Networks”, MobiHoc 2001 (*) C. Santivanez, I. Stavrakakis et al., “On the Scalability of Ad Hoc Routing Protocols”, INFOCOM 2002 CROSS-ROAD overlay Working hypothesis: A Middleware level B A Network level B Nodes providing service S Each node has to be characterized by at least: (IPaddress, Services, 1hop-neighbors) Ad Hoc Network nodes The Network level gives a graph representing the network topology to the Nest The middleware level can access to this graph and recover relevant information for itself Interactions with NeSt Middleware Data Abstraction Local Provided services Node A Node B Middleware Middleware NeSt NeSt Network Data Abstraction Middleware Data Abstraction Network Network Application messages Topology update and remote services LSU routing pkt containing services publications and topology updates Network Data Abstraction CROSS-ROAD Routing Tables CROSS-ROAD Routing Table Destination ID = H(IP address) Cost Each node defines the ring autonomously CROSS-ROAD routing table contains only nodes provide the service (Leafset and Neighborset disappear) Network Routing Table Destination IP address Next Hop IP add. Cost Services Middleware routing protocol is limited to a peer-to-peer connection Messages forwarding is realized by the network routing protocol Pastry vs CROSS-ROAD PASTRY: Join operation requires many remote connections to recover routing tables contents HIGH COST at middleware level CROSS-ROAD: Join operation does not require remote connections: each node can build its ring autonomously NO COST at middleware level Pastry vs CROSS-ROAD PASTRY: The detection of disconnection events requires polling cycles towards remote nodes HIGH COST at middleware level CROSS-ROAD: Disconnection events are detected by the netwrok routing protocol through LSU packets NO COST at middleware level Pastry vs CROSS-ROAD PASTRY: Routing table fixed size involves a not complete knowledge of the network HIGH COST for tables management due to remote connections following topology updates CROSS-ROAD: Routing table size depends on the number of nodes taking part to the service NO COST at middleware level: local interactions with NeSt are sufficient to update routing tables Pastry vs CROSS-ROAD PASTRY: Subject-based routing involves a multi-hop middleware routing O(log(N)) middleware lookup cost CROSS-ROAD: Subject-based routing involves a peer-to-peer connection O(1) middleware lookup cost, the remaining is a routing task CROSS-ROAD Software Architecture CROSS-ROAD NodeHandle CROSS-ROAD Node NeSt Service Notification Endpoint Topology Abstraction (GRAPH) Hash ID Factory CROSS-ROAD Messages CROSS-ROAD Routing Table Link-State Routing Topology table Routing Table LSU Management P2P CommonAPI Client/Server Manager Routing Messages Socket Manager