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# CROSS-ROAD: CROSS -layer R ing O verlay for AD Hoc Networks - PowerPoint PPT Presentation

CROSS-ROAD: CROSS -layer R ing O verlay for AD Hoc Networks. Franca Delmastro franca.delmastro@iit.cnr.it 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.

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Franca Delmastro

franca.delmastro@iit.cnr.it

IIT-CNR Pisa

Cambridge, March 23rd 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

0

2128 - 1

Pastry Ring

The Pastry Ring

(K1 ,V)

(K2 ,V)

0

2128 - 1

L1 = H(K1)

L2 = H(K2 )

L1

X2

X1 = H(N1)

L2

X1

N1

N2

X2 = H(N2)

Pastry routing tables

0

2128 - 1

02212102

10233001

10233102

10233232

10323302

11301233

Pastry Multi-hop Routing

0

2128 - 1

33301201

23301201

23301101

Route(23302121)

10233102

22301203

bestMatch(RoutingTable[0], 23302121 ) =

22301203

02212102

23000101

Compare(10233102, 23302121) = 0

Joining the Ring

X

Route(X)

X

Join(X)

• 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

0

2128 - 1

Z

Bk

B2

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:
• 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

Middleware routing can send messages directly to their FINAL destination.

• In order to build an overlay network, the middleware can directly use the Network Routing table.
• 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
• 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

A

Middleware level

B

Nodes providing service S

• Working hypothesis:
• The Network level gives a graph representing the network topology to the Nest
• Each node has to be characterized by at least:

• The middleware level can access to this graph and recover relevant information for itself

A

Network level

B

Interactions with NeSt

Node A

Node B

NeSt

Local Provided services

NeSt

Middleware

Data

Abstraction

Middleware

Data

Abstraction

Middleware

Middleware

Network

Network

Network

Data

Abstraction

Network

Data

Abstraction

Topology update and remote services

Application messages

LSU routing pkt containing services publications and topology updates

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

• 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

• Disconnection events are detected by the netwrok routing protocol through LSU packets
• PASTRY:
• The detection of disconnection events requires polling cycles towards remote nodes

NO COST at middleware level

HIGH COST at middleware level

• Routing table size depends on the number of nodes taking part to the service
• 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

NO COST at middleware level: local interactions with NeSt are sufficient to update routing tables

• Subject-based routing involves a peer-to-peer connection
• PASTRY:
• Subject-based routing involves a multi-hop middleware routing

O(1) middleware lookup cost, the remaining is a routing task

O(log(N))middleware lookup cost

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.