adhocslides.pdf

November 2003 Peer-to-Peer Networking 1

An Introduction to Ad Hoc Networking

Lars Michael KristensenDepartment of Computer ScienceUniversity of [email protected]


Background


Ad Hoc Networking

The network operates autonomously to provide connectivity: No centralised control/components. No fixed infrastructure (e.g, base stations and wired links).

.. wireless communication between mobile nodes (MNs).


Ad Hoc Networking

B

A

C

D

.. a group of laptops operating in wireless ad hoc mode:

Important characteristics: The mobile nodes acts as routers (multi-hop). Highly dynamic topology due to mobility. Varying and constrained bandwidth on links. Power constrained (laptops, mobile phones, PDAs).


Example: On-demand Ad Hoc Routing

B

A

C

D

RREQ(D)

RREQ(D)

RREQ(D)

RREQ(D)

RREP(D)RREP(D)


Origins of Ad Hoc NetworkingDefence Advanced Research Project Agency (DARPA) Packet Radio NETwork (PRNET) project (1972):

SURAN - Survivable Radio Networks (1983).

DARPA Global Mobile (GloMo) Information Systems Program (1994).

Based on broadcast property of radios to send/receive data packets.


Ad Hoc Networking TodayMobile wireless computing devices (Laptops, PDAs, mobile phones) are now relatively inexpensive and widely used.

Wireless networking products (Wireless LAN,) with still increasing bandwidth are becoming available.Much of the communication is still based on fixed infrastructure(e.g., WLAN) or point-to-point communication (e.g., infrared).


Envisioned Application Areas

Sensor networks for environmental monitoring.Rescue operations in remote areas.Remote construction sites.Mobile conferencing.Home networking.Wireless Personal Area Networks.

Military ad hoc networking:


Wireless Ad Hoc Networking

Sometimes there may be no network infrastructure available: Remote areas. Unplanned meetings. Emergency relief personnel quickly deployed into an area. Military where infrastructure has been destroyed or is untrusted.

Sometimes users wont want to use the available infrastructure: Time to access and register on the service. Cost of using the service. Performance of the existing service and infrastructure. Capacity of the existing service and infrastructure.

Can dynamically extend coverage range of any infrastructure: Allow users to be further away from infrastructure serving them.


Protocols for Ad Hoc Networking

Network Layer: Routing between end-points.

Data Link Layer: Point-to-point Communication


Link and Medium Access Control Layers


Wireless Links

Communication is based on radio waves.

Signal strength diminishes with distance and may vary significantly.

Radio waves may be blocked or absorbed by objects such as buildings, mountains, and rain.

Radio waves may be reflected off objects: allows sender to reachreceiver although direct path is blocked, but creates multipaths.

Wireless links may be unidirectional.


Wireless LinksMedium Access Control (MAC) needs to be exercised to deal with interference between transmissions on the wireless medium (ether):

A number of MAC protocols have been defined to ensure coordinated use of the wireless medium.

Typically based on sending Request To Send (RTS) and Clear To Send (CTS) messages on a control channel to manage reservations.


Hidden Terminal Problem

RTS(A,B)

CTS(B,A)

Nodes that senses RTS or CTS must not transmit.

The Network Allocation Vector specifies time media is reserved

Two nodes (A and C) not within transmission range of each other transmits simultaneously to a third node (B).

C is hidden from A and classical carrier sense is not appropriate.


Exposed Node Problem

Reserved by A to B communication

C is not allowed to transmit because it receives RTS from A.

C could transmit to D without interfering with transmission from A to B.

This may significantly reduce performance of wireless networks.

Data transmission from neighboring nodes can inhibit an exposed node from transmitting data.


Link Layer Technologies.. a number of link layer and MAC technologies have been proposed for ad hoc networking.IEEE 802.11 Wireless LAN Standard:

Distributed Coordination Function for ad hoc networks.Distributed Foundation Wireless MAC (CSMA/CA).

HiperLan/2 (European Telecomunication Standards Institute):Home Enviroment Profile for one-hop ad hoc networks. Time Division Multiple Access using central controller electedamong mobile terminals.


Link Layer Technologies

Bluetooth (Bluetooth Special Interest Group):Piconets with one master and several slaves (star topology).Scatternet can be formed by multiple overlapping piconets.

Home Radio Frequency (Home RF):Shared Wireless Access Protocol (SWAP).

MAC layer similar to IEEE 802.11.


Link Layer Technologies

A number of other link layer technologies are emerging.


Routing in Ad Hoc Networks


The Need for Multi-hop Routing

Obtaining full connectivity not feasible in general (e.g., battery power).Ad hoc networking puts additional requirements on (mobile) nodes:

must act as routers forwarding packets to other peer nodes. need to find new routes as nodes move or link conditions change.

.. the distinction between hosts and routers disappears.

A B C

.. some nodes may be out of transmission range of others:


Routing Protocol Zoo for Ad Hoc NetworksIETF Mobile Ad Hoc Networks (MANET) working group:

The WG will operate under a reduced scope by targeting the promotion of a number of core routing protocol specifications to EXPERIMENTAL RFC status.

First goal is to standardise intra-domain unicast routing protocols:

A suite of routing protocols is most likely needed: Many possible mobility scenarios/patterns. Large variety of link layer/transmission technologies.

Reactive/on demand

DSDV

WRP

OLSR

ABR

ZRP

TORA

LAR

SSR

Proactive/periodic

DSR AODV

TBRPF


IETF Standardisation Process

We reject kings, presidents, and voting.

We believe in rough consensus and running code.

- Dr. David Clark, MIT (1992)

.. emphasis on operational experience.

RFC

Moving from Proposed to Draft Standard requires at least two independent and interoperable implementations.

Current status: little consensus and some running code.


Routing Protocol ChallengesConventional Routing Protocols:

Impossible to exchange information immediately and routing information will in general be slightly out of date.

Routing information is not inexpensive to distribute and acquire. Link conditions (e.g., congestion) may change frequently. Scalability is important (e.g., size of routing tables).

Ad hoc Routing Protocols: The nodes are often battery powered which makes the

consumption of energy a constraint. Highly dynamic topology due to frequent movement of nodes

and nodes being powered off or entering sleep mode. Wireless links are bandwidth constrained compared to wired

links, and might be unidirectional. No natural hierarchy on the network addresses in the ad hoc

network.


Graph Model of the NetworkCan represent network as a graph G = (V, E):

V = nodes in the network. E = between two nodes if they can communicate directly (1 hop).

Directed graph in case of unidirectional links.

Distance will be measured in the number of hops.


Routing Protocols: OverviewConventional next-hop unicast routing protocols:

Link-state: each node maintains a complete view of the topology and periodically broadcasts information about its outgoing links to all nodes.

Example: Open Shortest Path First (OSPF).

Distance-vector: each node maintains information about distance to each destination via neighbors and periodically broadcasts to its neighbors estimate of shortest distance for each destination.

Example: Routing Information Protocol (RIP).

Ad hoc routing protocols:

Periodic / proactive / table-driven: nodes periodically exchange routing information and attempt to keep up to date routing information.

On-demand / reactive: nodes only try to find a route to a destination when actually needed for communication.


Example Routing Protocols

Destination-Sequenced Distance Vector (DSDV). Ad-hoc On-Demand Distance Vector Routing Protocol (AODV). Dynamic Source Routing Protocol (DSR).


DSDV: Destination-Sequenced Distance Vector Routing

A modification of standard distance-vector routing (e.g., Routing Information Protocol (RIP)) based on the Bellman-Ford algorithm.

Problems with standard distance-vector routing for ad hoc networks: Performs at its worst with many network changes. Easily forms routing loops. Ad hoc networking is thus a very difficult case for distance-vector.

DSDV enhancements to distance-vector routing: Guarantees no loops by adding sequence numbers to updates. Bandwidth efficiency through incremental updates. Delay route advertisements to damp fluctuations.


Bellman-Ford AlgorithmNode i periodically broadcast routing tables entries to neighbours:

Dest. Metric First HopB 1 = D(i,B) AC 2 = D(i,C) F

Nexthop neighbour for destination x is the neighbour j that minimizes the estimated distance:

D(i,x) = 1 + D(j,x)

Routing table entry at node i for destination x is updated if:

D(i,x) < D(i,x) (a smaller distance becomes available) or

D(i,x) > D(i,x) (increase for current next hop neighbour j)

estimate from neighbour j


Distance-Vector Example

Routing table at node A:Dest. Metric First Hop

B 1 BC 2 BD 4 BE 3 B

Routing table at node D:Dest. Metric First Hop

A 4 EB 3 EC 2 EE 1 E

Current topologyof the network:


Distance-Vector Example

New routing table at A:Dest. Metric First Hop

B 1 BC 2 BD 1 DE 2 D

Suppose node Dmoves closerto node A

Routing update from D:Dest. Metric First Hop

A 4 EB 3 EC 2 EE 1 E


Routing Loops

Bellman-Ford based protocols are prune to routing loops:

A B C D x

Routing tables entries with respect to destination x:

(4,B) (3,C) (2,D) (1,x)

(4,B) (3,C) (4,B) (3,C)

(4,B) (5,C) (4,B) (5,C)

(6,B) (5,C) (6,B) (5,C)

Counting to infinity since routing information is summarised.

More complex scenarios are also possible.

C relearns distance from B


DSDV Sequence Numbers

Each node maintains a sequence number for itself. Increment by 2 each time the local neighbor list changes, i.e.,

links are created or destroyed around the node. Uses only even sequence numbers.

Each entry in routing table has a sequence number: Routing update contains sequence number from each table entry. Receiving a routing update: new entry always overrides old entry if

sequence number is greater.When a link failure to a nexthop neighbor is noticed:

Pretend to have received new update from that neighborwith metric infinity and next odd sequence number.

Routing loops cannot arise with the use of sequence numbers.


AODV: Ad Hoc On-demand Distance Vector Routing Protocol

Based on: Next-hop distance-vector concept for routing table entries. Sequence numbers in routing table entries to prevent creation

of routing loops. Create route to destination when communication requires it.

Overall protocol operation: Route (path) discovery for creating a route to destination. Route (path) maintenance for dealing with topology changes.


Route Discovery.. flooding the network with route requests (RREQ) initiated by the source:

Broadcast sequence number and source address is used to detect duplicate RREQ that have been received before.

Broadcast sequence number incremented whenever the node initiates a route discovery.

RREQ

Creating route from node A to node I


Route DiscoveryRoute Reply (RREP) is unicast back along reverse route:

RREP

Reverse route information in nodes not on reverse route will eventually time out.

Intermediate nodes having a route to destination may send RREP.

Nodes on the discovered route automatically learns route to destination via RREP.


Path Maintenance

Link layer or periodic hello messages used to detect link failures.

Link layer failure:

Upstream neighbor sends RREP with next destination sequence and infinity metric to active neighbors.

Ensures that routing tables entries for this route is overwritten when RREP is propagated backwards.

Reinitiating of route discovery:

By source node or another (earlier) upstream neighbor.

Uses a larger destination sequence number than received in the RREP.

Ensures that a new route is created or a never route is reused.


AODV at 58th IETF

Implementation of AODV protocol demonstrated at 58th IETF Meeting in Minneapolis, November 9-14, 2003.

Based on IEEE 802.11 ad-hoc mode.

Implemented as a routing daemon for Linux and Microsoft Windows operating systems.

First ever large-scale, publicly-usable ad hoc network using the AODV routing protocol.


Dynamic Source Routing (DSR)

On-demand routing protocol based on source routing.

Divides routing problem into two parts: Route Discovery: finds a route to a given destination

when the node needs a route and do not have one. Route Maintenance: responds to changes in topology that

affect a route currently in use.Characteristics of DSR:

Source routing avoids keeping information in intermediate nodes and guarantees loop-free operation.

On-demand operation implies no overhead when nodes are stationary and routes have already been created.

Supports unidirectional links and asymmetric routes.


Route DiscoveryInitiator initiates flooding of the network by broadcasting a Route

Request with a unique request identification in it.When receiving a Route Request:

If the target node is yourself, return the recorded route to theinitiator in a Route Reply; initiator caches the route.

Else, if recently seen a request with this id, drop the Route Request.

Otherwise, append own address to a route record in the packet and rebroadcast the Route Request.


Route DiscoveryThe Route Reply can be returned in various ways:

Target node uses a previously cached route to the source node (initiator).

Destination performs a route discovery for the source with the route reply piggybacked to avoid an infinite loop.

The reverse sequence of nodes can be use if all links are bidirectional.

Rate of route requests limited using exponential back-off in case the destination is not reachable.


Route MaintenanceAfter transmitting a packet to the next hop:

Listen for link-level per-hop acknowledgement (present in many wireless LANs), or

Listen for the next-hop node sending packet to its next hop (passive acknowledgement), or

Set a bit in packet to request explicit next-hop acknowledgement.

When a problem with forwarding is detected: Send a Route Error message to original sender, identifying the broken link. Sender may use another cached routes or do new Route Discovery if needed.


Route Discovery OptimizationsLearn routes from forwarded and overheard packets:

Forwarded data packets, Route Requests, Route Replies, and Route Errors (subject to unidirectional links).

Packets overheard while in promiscuous mode.Generation of Reply to a Route Request by intermediate nodes:

Intermediate nodes check route cache for a route to the target. If route is in cache, send a route Reply with concatenation of

record from the Route Request plus route from cache.Reduce Route Requests flooding using expanding ring search:

Use small hop limit for first attempt at Route Discovery. If no reply, increase hop limit for each subsequent re-attempt. Useful if target node is close to initiator, but adds latency to the

Route Discovery procedure.


Automatic Route ShorteningShorten route if a downstream node overhear a packet early:

Suppose node A is transmitting to next-hop node B Suppose C overhears this directly:

Node C can detect this since B is still listed as next hop Node C can send a gratuitous Route Reply to A indicating the

shorter route A C D


DSR Implementation and TestbedTested and demonstrated regularly from Dec 1998Mar 1999:

5 cars driving 2025 MPH, looping between A and B 2 stationary nodes (E1 and E2) about 3 radio hops apart using

off-the-shelf WaveLAN wireless LAN radios All routing between ad hoc

network nodes done with DSR Integrated into Internet and

Mobile IP, allowing nodes tojoin the ad hoc network

Traffic included ftp, telnet,UDP CBR audio, real-timekinematic (RTK) GPScorrection packets, real-timestatistics and position logging


Performance of Ad Hoc Routing Protocols

The relative performance of a number of ad-hoc routing protocols has been studied in Broach et. al, 1998 using the ns-2 simulator.

Routing protocols considered: DSDV, AODV, TORA, DSR.

Simulation model: 50 wireless mobile node moving in a 1500mx300 square. Each node can buffer up to 50 packets waiting for transmission. Routing protocols evaluated on the same 210 scenarios. Nodes move between random points with a speed chosen

uniformly within [0,maxspeed]. The node is then stationary for a certain pause time. A number of traffic sources (10,20,30) generates packets at a

constant bit rate.


Application Data Packet Delivery

DSDV slow in responding to link breaks (propagate from destination).

AODV and DSR allows local repair and multiple routes.


Routing Overhead

Routing overhead large in AODV due to flooding.

DSR allows routes to be learned by overheard packets.


Path Optimality


ConclusionsPacket Delivery Ratio:

Small pause time and high speed in movement: AODV and DSR clear winners.

DSDV performs reasonable for medium pause time and low speed movement.

Routing Overhead: DSR overhead lowest in number of packets. AODV lowest in the number of bytes.

Path optimality: DSDV and DSR performs best.

No comparison done on packet delay.Load balancing is not considered.


Conclusions

Mobile Ad Hoc Networking: Peer-to-peer wireless forwarding of packets without infrastructure. Very dynamic, potentially rapidly changing network topology. All communication is wireless over very limited shared resource.

Very active research in this area, increasing every year.

Main research issues currently: Scalability: current flat routing protocol supports 200-300 nodes. Client- server model? Quality of service: bandwidth and constrained applications. Security. Internet connectvity: changing point of attachments. Power control.


References

C. Perkins. Ad Hoc Networking An Introduction. Chapter 1 in C. Perkins: Ad Hoc Networking, Addison-Wesley, 2002.

E. M. Royer et al. A Review of Current Routing Protocols for Ad Hoc Mobile Wireless Networks. IEEE Personal Communication, April 1999.

J. Broch et al. A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Routing Protocols. Proc. of MobiCom 1998.

S. Uskela. Link Technology Aspects of Multi Ad Hoc Networks. Seminar on Ad Hoc Networking, Espoo, April 12-13, 2002. Networking Laboratory, HelsinkiUniversity of Technology.

D. B. Johnson. Routing in Mobile and Wireless Data Networks. Tutorial at FORTE2002.

adhocslides.pdf

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home networking

wireless communication

available infrastructure

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