CROSS-ROAD: CROSS-layer Ring Overlay for AD Hoc Networks Franca Delmastro IIT-CNR Pisa Cambridge, March 23 rd 2004

Ring Overlay & Pastry Model The ring overlay is a circular address space where nodes and data are logically mapped. Pastry uses a DHT to convert nodes and data identifiers on logical addresses that are used to route messages through the network There is no correspondence between logical and physical distances It provides a subject-based data routing that often requires a multi-hop network routing to reach the destination.

Logical & Physical distances Pastry Ring

The Pastry Ring N1N1 X 1 = H(N 1 ) N2N2 X 2 = H(N 2 ) L1L1 X1X1 X2X2 L2L2 (K 2,V) L 2 = H(K 2 ) (K 1,V) L 1 = H(K 1 )

Pastry routing tables

Pastry Multi-hop Routing Route( ) bestMatch(RoutingTable[0], ) = Compare( , ) = 0

Joining the Ring X has to know a own physical neighbor already present in the ring (node A) A route 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 A B1 B2 Bk Z X Join(X) Route(X) X

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 taking part to 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 on ad hoc networks  Forcing the network routing with the subject-based policy can reduce network performances

NeSt Applications Middleware Transport Network MAC Using Cross-Layer to CROSS-ROAD 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 Middleware routing can send messages directly to their FINAL destination.

Efficient proactive Link-State Proactive routing protocols are usually considered inefficient for ad hoc networks An optimization has been studied: “Hazy Sighted Link State”* (HSLS) 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. Simulations showed that HSLS scales with the network size. (*) 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

Middleware level A B CROSS-ROAD overlay Nodes providing service S Ad Hoc Network nodes Network level 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 A B

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

CROSS-ROAD Routing Tables Each node defines the ring autonomously CROSS-ROAD routing table contains only nodes taking part to 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 Network Routing Table Destination IP address Next Hop IP add. CostServices CROSS-ROAD Routing Table Destination ID = H(IP address) Cost

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

Pastry vs CROSS-ROAD PASTRY:  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:  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:  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

Further optimizations: Cost Metrics Possible metrics to determine the best route from a source to a destination:  Number of routing hops  Path reliability based on nodes mobility Mobility index: number of times that a node changed its position in a specified time interval. It can be determined by the Network routing protocol consequently to LSU packets reception. It can be used by CROSS-ROAD to define the best nodes for replicas storage.