Middleware issues: From P2P systems to Ad Hoc Networks Franca Delmastro franca.delmastro@iit.cnr.it 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 2128 - 1

The Pastry Ring L1 X2 L2 X1 (K1 ,V) (K2 ,V) L1 = H(K1) L2 = H(K2 ) X1 = H(N1) N2 X2 = H(N2) L2 L1 2128 - 1 X2 X1

Pastry routing tables 2128 - 1 02212102 10233001 10233102 10233232 2128 - 1 02212102 11301233 10323302

Pastry Multi-hop Routing 23301101 23301201 33301201 2128 - 1 22301203 10233102 02212102 bestMatch(RoutingTable[0], 23302121 ) = 22301203 Route(23302121) 23000101 Compare(10233102, 23302121) = 0

Joining the Ring X has to know a own physical neighbor already present in the ring (node A) A sends a message with key equal to X 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 2128 - 1 X Route(X) Z Bk B2 X Join(X) A 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: Cons: 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 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 LINK- STATE routing protocol, each node knows the entire network NeSt Applications Middleware Transport Network MAC

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

Nodes providing service S CROSS-ROAD overlay Working hypothesis: The Network level gives a graph representing the network topology to the Nest Each node has to be characterized by at least: (IPaddress, Services, 1hop-neighbors) The middleware level can access to this graph and recover relevant information for itself Middleware level A B A Network level B Nodes providing service S Ad Hoc Network nodes

Interactions with NeSt Network Data Abstraction Middleware Node A Middleware Network NeSt Data Abstraction Node B Local Provided services LSU routing pkt containing services publications and topology updates Application messages Topology update and remote services

CROSS-ROAD Routing Tables Each node defines the ring autonomously CROSS-ROAD routing table contains only nodes provide the service (Leafset and Neighborset disappear) Middleware routing protocol is limited to a peer-to-peer connection Messages forwarding is realized by the network routing protocol CROSS-ROAD Routing Table Destination ID = H(IP address) Cost Network Routing Table Destination IP address Next Hop IP add. Cost Services

Pastry vs CROSS-ROAD CROSS-ROAD: PASTRY: Join operation does not require remote connections: each node can build its ring autonomously PASTRY: Join operation requires many remote connections to recover routing tables contents HIGH COST at middleware level NO COST at middleware level

Pastry vs CROSS-ROAD PASTRY: CROSS-ROAD: The detection of disconnection events requires polling cycles towards remote nodes CROSS-ROAD: Disconnection events are detected by the netwrok routing protocol through LSU packets HIGH COST at middleware level NO COST at middleware level

Pastry vs CROSS-ROAD PASTRY: CROSS-ROAD: Routing table fixed size involves a not complete knowledge of the network CROSS-ROAD: Routing table size depends on the number of nodes taking part to the service HIGH COST for tables management due to remote connections following topology updates NO COST at middleware level: local interactions with NeSt are sufficient to update routing tables

Pastry vs CROSS-ROAD PASTRY: CROSS-ROAD: Subject-based routing involves a multi-hop middleware routing CROSS-ROAD: Subject-based routing involves a peer-to-peer connection O(log(N)) middleware lookup cost O(1) middleware lookup cost, the remaining is a routing task

CROSS-ROAD Software Architecture