Ad Hoc Networks (WLAN, Bluetooth)

(1)

Peer-to-Peer Networks over MANETs

1 Prof. Dr. Thomas Schmidt http:/www.informatik.haw-hamburg.de/~schmidt

Aspects of P2P over MANETs

Introduction to MANETs

Properties of MANETs

Routing in MANETs

Properties of MANETs

Unstructured P2P over MANETs

DHT over MANETs

Ekta

MADPastry

Backup Chord

Hybrid Chord

Graphics on MANET routing taken from: Nitin H. Vaidya

(2)

P2P Systems in Mobile Environments

Scenario 1: Mobile Overlay Members

Walking user at roaming devices …

Issues: Transfer personal context, location based context

Networking solution: application transparency of Mobile IP(v6)

Scenario 2: Spontaneous Overlays in Mobile Ad-Hoc Networks

Collaborative application in local, mobile environments

Issues: Adapt to efficiency & proximity needed in Manets, cope with unreliable, mobile underlay networks

P2P Systems and Manets both void infrastructure

(3)

Ad Hoc Networks (WLAN, Bluetooth)

Characteristics:

Self configuring

Infrastructure free

Wireless

Unpredictable terminal mobility

Limited radio transmission range

Goal: provide communication between nodes

(4)

The Global View:

Overlay Network Layers

regional

metropolitan area

campus-based

in-house vertical

handover

horizontal handover integration of heterogeneous fixed and

mobile networks with varying transmission characteristics

(5)

Aspects in P2P over MANETs

Manets consist of moving, unstable components

Î unsuitable for client-server, but P2P applications

P2P applications built for failure tolerance

Î potential for compensating Manet drop-outs

P2P and Manets cope with member mobility Î provide capabilities of self-restructuring

But: P2P routing (mainly) regardless of underlay capacities Î Manet limitations require optimising adaptation

P2P and Manet changes may amplify Î Issues of cross-layer synchronisation

(6)

Application Examples

Active Collaboration & Passive Information Dissemination

Single & Multiple Dedications of Nodes

Common Examples:

Military

Rescue Services

Collaborative Inter-Vehicular Communication

Sensor Networks

Personal Area Networking / Local Device Networks

Gaming, Edu-/Info-/Sociotainment

(7)

Mobile Ad Hoc Networks

Formed by wireless hosts which may be mobile

Without (necessarily) using a pre-existing infrastructure

Routes between nodes may potentially contain multiple hops

Motivations:

Ease of deployment, low costs

Speed of deployment

Decreased dependence on infrastructure

(8)

Hidden and exposed terminals

Hidden terminals

A sends to B, C cannot receive A

C wants to send to B, C senses a “free” medium (CS fails)

collision at B, A cannot receive the collision (CD fails)

A is “hidden” for C

Exposed terminals

B sends to A, C wants to send to another terminal (not A or B)

C has to wait, CS signals a medium in use

but A is outside the radio range of C, therefore waiting is not necessary

C is “exposed” to B

B

A C

(9)

Near and far terminals

Terminals A and B send, C receives

signal strength decreases proportional to the square of the distance

the signal of terminal B therefore drowns out A’s signal

C cannot receive A

If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer

Also severe problem for CDMA-networks - precise power control needed!

A B C

(10)

Mobile Ad Hoc Networks

May need to traverse multiple links to reach a destination

A

B

(11)

Mobile Ad Hoc Networks (MANET)

Mobility causes route changes

A

B

(12)

Many Variations

Fully Symmetric Environment

all nodes have identical capabilities and responsibilities

Asymmetric Capabilities

transmission ranges and radios may differ

battery life at different nodes may differ

processing capacity may be different at different nodes

speed of movement

Asymmetric Responsibilities

only some nodes may route packets

some nodes may act as leaders of nearby nodes (e.g., cluster head)

Varying Traffic Characteristics

(13)

Unicast Routing in MANETs - Why is it different ?

Host mobility

link failure/repair due to mobility may have different characteristics than those due to other causes

Rate of link failure/repair may be high when nodes move fast

New performance criteria may be used

route stability despite mobility

energy consumption

Many routing protocols proposed – no universal solution

(14)

Routing Protocols

Proactive protocols

Determine routes independent of traffic pattern

Traditional link-state and distance-vector routing protocols are proactive

Reactive protocols

Maintain routes only if needed

Hybrid protocols

(15)

Trade-Off

Latency of route discovery

Proactive protocols may have lower latency since routes are maintained at all times

Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y

Overhead of route discovery/maintenance

Reactive protocols may have lower overhead since routes are determined only if needed

Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating

Which approach achieves a better trade-off depends on the traffic and mobility patterns

(16)

Flooding for Data Delivery

Sender S broadcasts data packet P to all its neighbors

Each node receiving P forwards P to its neighbors

Sequence numbers used to avoid the possibility of forwarding the same packet more than once

Packet P reaches destination D provided that D is reachable from sender S

Node D does not forward the packet

(17)

Flooding for Data Delivery

B A

S E

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D C

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Z Y

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Represents a node that has received packet P

Represents that connected nodes are within each other’s transmission range

(18)

Flooding for Data Delivery

B A

S E

F

H

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D C

G

I

K

Z Broadcast transmission Y

M

N

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Represents a node that receives packet P for the first time

Represents transmission of packet P

(19)

Flooding for Data Delivery

B A

S E

F

H

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D C

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Z Y

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• Node H receives packet P from two neighbors:

potential for collision

(20)

Flooding for Data Delivery

B A

S E

F

H

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D C

G

I

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Z Y

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N

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• Node C receives packet P from G and H, but does not forward it again, because node C has already forwarded packet P once

(21)

Flooding for Data Delivery

B A

S E

F

H

J

D C

G

I

K

Z Y

M

N

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• Nodes J and K both broadcast packet P to node D

• Since nodes J and K are hidden from each other, their transmissions may collide

=> Packet P may not be delivered to node D at all,

despite the use of flooding

(22)

Flooding for Data Delivery

B A

S E

F

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J

D C

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Z Y

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N

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• Node D does not forward packet P, because node D is the intended destination of packet P

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Flooding for Data Delivery

B A

S E

F

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D C

G

I

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Z Y

M

N

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• Flooding completed

• Nodes unreachable from S do not receive packet P (e.g., node Z)

• Nodes for which all paths from S go through the destination D also do not receive packet P (example: node N)

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Flooding for Data Delivery

B A

S E

F

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D C

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Z Y

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N

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• Flooding may deliver packets to too many nodes (in the worst case, all nodes reachable from sender may receive the packet)

(25)

Flooding for Data Delivery:

Advantages

Simplicity

May be more efficient than other protocols when rate of information transmission is low enough that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher

this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many

topology changes occur between consecutive packet transmissions

Potentially higher reliability of data delivery

Because packets may be delivered to the destination on multiple paths

(26)

Flooding for Data Delivery:

Disadvantages

Potentially, very high overhead

Data packets may be delivered to too many nodes who do not need to receive them

Potentially lower reliability of data delivery

Flooding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead

Broadcasting in IEEE 802.11 MAC is unreliable

In our example, nodes J and K may transmit to node D simultaneously, resulting in loss of the packet

in this case, destination would not receive the packet at all

(27)

Flooding of Control Packets

Many protocols perform (potentially limited) flooding of control packets, instead of data packets

The control packets are used to discover routes

Discovered routes are subsequently used to send data packet(s)

Overhead of control packet flooding is amortized over data packets transmitted between consecutive control packet floods

(28)

Dynamic Source Routing (DSR) [Johnson96]

When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery

Source node S floods Route Request (RREQ)

Each node appends own identifier when forwarding RREQ

(29)

Route Discovery in DSR

B A

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D C

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Represents a node that has received RREQ for D from S

(30)

Route Discovery in DSR

B A

S E

F

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D C

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L [S]

Represents transmission of RREQ

[X,Y] Represents list of identifiers appended to RREQ

(31)

Route Discovery in DSR

B A

S E

F

H

J

D C

G

I

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Z Y

M

N

L [S,E]

[S,C]

• Node H receives packet RREQ from two neighbors:

potential for collision

(32)

Route Discovery in DSR

B A

S E

F

H

J

D C

G

I

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Z Y

M

N

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[S,C,G]

[S,E,F]

• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

(33)

Route Discovery in DSR

B A

S E

F

H

J

D C

G

I

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Z Y

M

N

L

[S,C,G,K]

[S,E,F,J]

• Nodes J and K both broadcast RREQ to node D

• Since nodes J and K are hidden from each other, their transmissions may collide

(34)

Route Discovery in DSR

B A

S E

F

H

J

D C

G

I

K

Z Y

M

N

L [S,E,F,J,M]

• Node D does not forward RREQ, because node D is the intended target of the route discovery

(35)

Route Discovery in DSR

Destination D on receiving the first RREQ, sends a Route Reply (RREP)

RREP is sent on a route obtained by reversing the route appended to received RREQ

RREP includes the route from S to D on which RREQ was received by node D

(36)

Route Reply in DSR

B A

S E

F

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D C

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L RREP [S,E,F,J,D]

Represents RREP control message

(37)

Route Reply in DSR

Route Reply can be sent by reversing the route in

Route Request (RREQ) only if links are guaranteed to be bi-directional

To ensure this, RREQ should be forwarded only if it received on a link that is known to be bi-directional

If unidirectional (asymmetric) links are allowed, then RREP may need a route discovery for S from node D

Unless node D already knows a route to node S

If a route discovery is initiated by D for a route to S, then the Route Reply is piggybacked on the Route Request from D.

(38)

Dynamic Source Routing (DSR)

Node S on receiving RREP, caches the route included in the RREP

When node S sends a data packet to D, the entire route is included in the packet header

hence the name source routing

Intermediate nodes use the source route included in a packet to determine to whom a packet should be

forwarded

(39)

Data Delivery in DSR

B A

S E

F

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D C

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Z Y

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N

L DATA [S,E,F,J,D]

Packet header size grows with route length

(40)

Dynamic Source Routing: Advantages

Routes maintained only between nodes who need to communicate

reduces overhead of route maintenance

Route caching can further reduce route discovery overhead

A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches

(41)

Dynamic Source Routing: Disadvantages

Packet header size grows with route length due to source routing

Flood of route requests may potentially reach all nodes in the network

Care must be taken to avoid collisions between route requests propagated by neighboring nodes

insertion of random delays before forwarding RREQ

Increased contention if too many route replies come back due to nodes replying using their local cache

Route Reply Storm problem

Reply storm may be eased by preventing a node from

sending RREP if it hears another RREP with a shorter route

(42)

Ad Hoc On-Demand Distance Vector Routing (AODV) [Perkins99Wmcsa]

DSR includes source routes in packet headers

Resulting large headers can sometimes degrade performance

particularly when data contents of a packet are small

AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data packets do not have to contain routes

AODV retains the desirable feature of DSR that routes are maintained only between nodes which need to

communicate

(43)

AODV

Route Requests (RREQ) are forwarded in a manner similar to DSR

When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source

AODV assumes symmetric (bi-directional) links

When the intended destination receives a Route Request, it replies by sending a Route Reply

Route Reply travels along the reverse path set-up when Route Request is forwarded

(44)

Route Requests in AODV

B A

S E

F

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D C

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Z Y

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Represents a node that has received RREQ for D from S

(45)

Route Requests in AODV

B A

S E

F

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D C

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N

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Represents transmission of RREQ

(46)

Route Requests in AODV

B A

S E

F

H

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D C

G

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Z Y

M

N

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Represents links on Reverse Path

(47)

Reverse Path Setup in AODV

B A

S E

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D C

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Z Y

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• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

(48)

Reverse Path Setup in AODV

B A

S E

F

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D C

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Z Y

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(49)

Reverse Path Setup in AODV

B A

S E

F

H

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D C

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Z Y

M

N

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• Node D does not forward RREQ, because node D is the intended target of the RREQ

(50)

Route Reply in AODV

B A

S E

F

H

J

D C

G

I

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Z Y

M

N

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Represents links on path taken by RREP

(51)

Route Reply in AODV

An intermediate node (not the destination) may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender S

To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used

The likelihood that an intermediate node will send a Route Reply when using AODV not as high as DSR

A new Route Request by node S for a destination is assigned a higher destination sequence number. An intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply

(52)

Forward Path Setup in AODV

B A

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D C

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Forward links are setup when RREP travels along the reverse path

Represents a link on the forward path

(53)

Data Delivery in AODV

B A

S E

F

H

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D C

G

I

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Z Y

M

N

L DATA

Routing table entries used to forward data packet.

Route is not included in packet header.

(54)

Summary: AODV

Routes need not be included in packet headers

Nodes maintain routing tables containing entries only for routes that are in active use

At most one next-hop per destination maintained at each node

Multi-path extensions can be designed

DSR may maintain several routes for a single destination

Unused routes expire even if topology does not change

(55)

Link State Routing [Huitema95]

Each node periodically floods status of its links

Each node re-broadcasts link state information received from its neighbor

Each node keeps track of link state information received from other nodes

Each node uses above information to determine next hop to each destination

(56)

Optimized Link State Routing (OLSR)

The overhead of flooding link state information is reduced by requiring fewer nodes to forward the information

A broadcast from node X is only forwarded by its multipoint relays

Multipoint relays of node X are its neighbors such that each two-hop neighbor of X is a one-hop neighbor of at least one multipoint relay of X

Each node transmits its neighbor list in periodic beacons, so that all nodes can know their 2-hop neighbors, in order to choose the multipoint relays

(57)

Optimized Link State Routing (OLSR)

Nodes C and E are multipoint relays of node A

A

B F

C

D

E H

G

K J

Node that has broadcast state information from A

(58)

Optimized Link State Routing (OLSR)

Nodes C and E forward information received from A

A

B F

C

D

E H

G

K J

(59)

Optimized Link State Routing (OLSR)

Nodes E and K are multipoint relays for node H

Node K forwards information received from H

E has already forwarded the same information once

A

B F

C

D

E H

G

K J

(60)

Summary: OLSR

OLSR floods information through the multipoint relays

The flooded information itself is for links connecting nodes to respective multipoint relays

Routes used by OLSR only include multipoint relays as intermediate nodes

(61)

Further Routing Approaches

Improvements & Optimisations of Previous Protocols

Location Aided Routing

Clustering after Landmarking

Hierarchic / Anchored Routing

Power-Aware Routing

…

(62)

Performance Properties of MANETs

One-Hop Capacity:

Consider MANET of n equal nodes, each acting as

router, with constant node density. Then the One-Hop Capacity grows linearly Î O(n)

Total Capacity surprisingly low:

Consider MANET of n equal nodes, each acting as

router in an optimal set-up, then the Node Capacity to reach an arbitrary destination reads Î O(1/◊n)

Node Capacity further decreases under wireless transmission Î O(1/◊(n ln(n))

(63)

Mobile P2P Networks:

Conceptual Aspects

Different concepts of (limited) lifetimes

Overlay: Application-/User-driven

Underlay: Mobility-/Technology-driven

In case of independence, exp. distributed MANET lifetime:

Topological embedding

How to adapt to changing underlay topologies?

Violate layers?

Resource constraints in MANETs: Data transmission costly

(1 Transmission ≈ 1.000 arithmetic operations)

(64)

Unstructured Mobile P2P Network:

The Mobile P2P Protocol (MPP)

MPP is based on Dynamic Source Routing (DSR).

Employs cross layer communication to match the virtual network on the physical DSR-network.

Æ avoids zigzag routes completely

Offers an open interface to extend MPP to support any kind of services, ranging form content delivery to service discovery in a mobile environment

MPP employs HTTP for content transmission.

possibility for downloads form multiple sources

possibility to continue an interrupted download from other sources

(65)

The Mobile P2P Protocol Stack

Physical Link Transport (TCP)

Session Presentation

Network (IP)

Application (MPP)

EDSR

MPCP

MPP: Mobile P2P Protocol MPCP: Mobile Peer Control

Protocol

EDSR: Extended Dynamic

Source Routing Protocol

(66)

Structured Mobile P2P Networks:

Ekta (Pucha et al, 2004)

Adaptation of Pastry to MANETs

Integrates Multi-hop Routing Protocol into the DHT layer

Prerequisite:

DSR protocol in MANET

All MANET nodes participate in Ekta Overlay

Node-Hash applied on Underlay Address

Achieves a late binding w.r.t. network layer routes

(67)

Ekta: Integrating the Network Layer

Modifications of Pastry’s Routing Table and Leaf Set:

Stores vector of Source Routes (instead of addresses)

Route selection according to PNS (as in Pastry)

Route employment: “Freshest among the shortest”

Route replacement: “Least Recently Discovered”

Source routes pushed to Ekta from network layer

(here we need coincidence of MANET : Ekta nodes)

Network layer routes continuously monitored (from overhearing or forwarded messages)

Prefix-based route discovery only if no route available

(68)

Ekta: Lightweight Self-Organisation

JOIN routed to node with closest node-ID

Closest node on reception of a JOIN:

responds with a copy of its leaf set

Broadcast flooding of the newly arrived node (including traversed path recording)

Leaf set members reply with recorded path

Unlike Pastry: Ekta abandons proximity probing

On Departure a node floods its LEAVE

(69)

Ekta Architecture

(70)

Ekta Performance: Packet Delivery Ratio

(71)

Ekta Performance

(72)

MADPastry (Zahn et. al., 2005)

Integrates key-based P2P routing into the network layer

Adaptation of Pastry, combined with AODV

Prerequisite: All MANET members run MADPastry

Proximity exploited via random landmarking + clustering

Idea: Forward piecewise along short, up-to-date routes

Overlay hop count not bound by O(log(n))

(73)

Random Landmarking

Select landmark keys (no fixed nodes)

Nodes owning keys become temporary landmark nodes

Clusters formed from periodic beacons (incl. hop count)

Nodes associate to closest landmark by adopting the clusters overlay id prefix

Î Nodes change their keys!

(74)

Address Resolution

As Nodes change keys, the overlay addressing semantic breaks

Healing Attempt: Node, who owns initial node key operates as “address server” (Home Agent)

Î Each node has to inquire on current destination address prior to data transmission

Î On key change, nodes need to update address server

Î On address server mobility:

Address keys need to be transferred

(75)

Routing

Degenerate Pastry Routing Table

Contains only routes into landmark clusters

Overlay Routing:

According to MADPastry’s routing table (analogue Pastry)

Underlay Routing:

Intercept packets to discover overlay proximity, AODV-routing otherwise

When physical route for an overlay hop is unknown:

Cluster-broadcast packet, if in destination cluster

AODV route discovery otherwise

(76)

MADPastry Routing

(77)

MADPastry: Performance

(78)

Hybrid Chord (Zöls et. al., 2005)

Chord-based approach for hybrid underlays

Presupposes subgroup of quasi-permanent nodes

Keys only deposited on “static” nodes

Independent of the underlay routing protocol

Defines “Context Spaces”

Grouping of shared objects in interest groups

“Info Profiles” carry keywords, hashed as keys

Suitable to optimise Boolean “AND” queries within one keyword search

(79)

HC Protocol Operations + Properties

Nodes need additional list of static successors

Chord key-based routing extended to next static successor

Info profiles distributed to (multiple) static nodes according to their keyword hashes

/ Analysis & evaluation erroneous: Authors claim improvements identical to all nodes being static

(80)

Additionally: GHTs

Geographic Hash Tables

Use out-of-band channel for geographic location

Routes developed according to geographically efficient paths

Example: GCLP.

(81)

Résumé on Proposals

Ekta: Achieves late binding through layer violation

Price to pay - DHT must be present at network layer Proximity improved by extended route set

MADPastry: Late binding through packet interception in regular routing – DHT obligatory part of network layer Proximity transferred into key space

Price to pay – destroys key semantic under mobility Hybrid Chord: Key issues discarded Î static nodes

Core performance unclear

(82)

Research Issues

Efficient late binding in overlay routes – can we leave a DHT efficient under mobility on application layer?

How to achieve effective proximity without intercepting the logic of key-based routing?

Is there (in some DHT) a degree of freedom exploitable for effective mobility management?

(83)

References

• C. Murthy and B. Manoj: Ad Hoc Wireless Networks, Pearson Prentice Hall, 2004.

• Nitin H. Vaidya: Mobile Ad Hoc Networks, Tutorial at InfoCom 2006, http://www.crhc.uiuc.edu/wireless/talks/2006.Infocom.ppt.

• P. Gupta and P. R. Kumar, “The capacity of wireless networks,” IEEE Transactions on Information Theory, vol. 46, no. 2, pp. 388–404, 2000.

• H.-J. Jeong, D. Kim, J. Song, B. Kim, and J.-S. Park, “Back-Up Chord: Chord Ring Recovery Protocol for P2P File Sharing over MANETs,” in International

Conference on Computational Science (2), ser. LNCS, V. S. Sunderam, G. D. van Albada, P. M. A. Sloot, and J. Dongarra, Eds., vol. 3515. Berlin Heidelberg:

Springer-Verlag, 2005, pp. 477–484.

• H. Pucha, S. M. Das, and Y. C. Hu, “Ekta: An Efficient DHT Substrate for

Distributed Applications in Mobile Ad Hoc Networks,” in Proceedings of the 6th IEEE Workshop on Mobile Computing Systems and Applications (WMCSA 2004).

Washington, DC, USA:IEEE Computer Society, December 2004, pp. 163–173.

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References (Cont.)

• T. Zahn and J. Schiller, “MADPastry: A DHT Substrate for Practicably Sized MANETs,” in Proc. of 5th Workshop on Applications and Services in Wireless Networks (ASWN 2005), Paris, France, June 2005.

• S. Zöls, R. Schollmeier, W. Kellerer, and A. Tarlano, “The Hybrid Chord Protocol:

A Peer-to-Peer Lookup Service for Context-Aware Mobile Applications,” in Networking – ICN 2005. 4th International Conference on Networking,

Proceedings, Part II, ser. Lecture Notes in Computer Science, P. Lorenz and P.

Dini, Eds., vol. 3421. Berlin Heidelberg: Springer-Verlag, April 2005, pp. 781–

792.

• M. Wählisch: Key-based Overlay Routing in Mobilen Ad-Hoc Netzen, Seminararbeit FU Berlin, 2006.