1. Middleware issues: From P2P systems to Ad Hoc Networks
Franca Delmastro
Pervasive Computing & Networking Lab (PerLab)
IIT-CNR Pisa
MobileMAN Project
Helsinki, June 7th 2004

2. 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

3. 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

4. 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

5. Logical & Physical distances
Pastry Ring

6. 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 X2 X1

7. Pastry routing tables 2128 - 1 02212102 10233001 10233102 10233232

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

9. 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 X
Route(X) Z Bk B2 X
Join(X) A B1

10. 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.

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. CROSS-ROAD Software Architecture