Background of Ad hoc Wireless Networks

Student Presentations

Wireless Communication Technology and Research

Ad hoc Routing and Mobile IP and Mobility

Wireless Sensor and Mesh Networks

Mobile and Ad hoc Networks

Adhoc Network RoutingAdhoc Network Routinghttp://web.uettaxila.edu.pk/CMS/SP2012/teAWNms/

Routing Overview

Network with nodes, edges Goal: Devise scheme for

transferring message from one node to another Which path? Who decides – source or

intermediate nodes?

msg

Which path?

Routing generally tries to optimize something: Shortest path (fewest hops) Shortest time (lowest latency) Shortest weighted path (utilize available

bandwidth) Etc…

Who determines route?

Two general approaches: Source (“path”) routing

Source specifies entire route: places complete path to destination in message header: A – D – F – G

Intermediate nodes just forward to specified next hop: D would look at path in header, forward to F

Like airline travel – get complete set of tickets to final destination before departing…

Destination (“hop-by-hop”) routing Source specifies only destination in message

header: G Intermediate nodes look at destination in

header, consult internal tables to determine appropriate next hop

Like postal service – specify only the final destination on an envelope, and intermediate post offices select where to forward next…

Comparison

Source routing Moderate source storage

(entire route for each desired dest.)

No intermediate node storage

Higher routing overhead (entire path in message header, route discovery messages)

Destination routing No source storage High intermediate node

storage (table with routing instructions for all possible dests.)

Lower routing overhead (just dest in header, only routers need to deal with route discovery)

Ad Hoc Routing

Every node participates in routing: no distinction between “routers” and “end nodes”

No external network setup: “self-configuring” Especially useful when network topology is dynamic

(frequent network changes – links break, nodes come and go)

Common Application

Mobile wireless hosts Only subset within range at given time Want to communicate with any other node

Ad Hoc Routing Protocols

Standardization effort led by IETF Mobile Ad-hoc Networks (MANET) task group http://www.ietf.org/html.charters/manet-charter

.html 9 routing protocols in draft stage, 4 drafts

dealing with broadcast / multicast / flow issues Other protocols being researched

utilize geographic / GPS info, Ant-based techniques, etc.

10

Why is Routing in MANET 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

11

Unicast Routing Protocols

Many protocols have been proposed

Some have been invented specifically for MANET

Others are adapted from previously proposed protocols for wired networks

No single protocol works well in all environments some attempts made to develop adaptive protocols

12

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

Leading MANET Contenders

DSR: Dynamic Source Routing Source routing protocol

AODV: Ad-hoc On-demand Distance Vector Routing “Hop-by-hop” protocol

Both are “on demand” protocols: route information discovered only as needed

Dynamic Source Routing

Draft RFC at http://www.ietf.org/internet-drafts/draft-ietf-manet-dsr-07.txt

Source routing: entire path to destination supplied by source in packet header Utilizes extension header following standard IP

header to carry protocol information (route to destination, etc.)

DSR Protocol Activities

Route discovery Undertaken when source needs a route to a

destination Route maintenance

Used when link breaks, rendering specified path unusable

Routing (easy!)

Route Discovery Route Request:

Source broadcasts Route Request message for specified destination

Intermediate node: Adds itself to path in message Forwards (broadcasts) message toward destination

Route Reply Destination unicasts Route Reply message to source

will contain complete path built by intermediate nodes

Details, details…

Intermediate nodes cache overheard routes “Eavesdrop” on routes contained in headers Reduces need for route discovery

Intermediate node may return Route Reply to source if it already has a path stored

Route Maintenance Used when link breakage occurs

Link breakage may be detected using link-layer ACKs, DSR ACK request

Route Error message sent to source of message being forwarded when break detected

Intermediate nodes “eavesdrop”, adjust cached routes Source deletes route; tries another if one cached, or issues

new Route Request Piggybacks Route Error on new Route Request to clear

intermediate nodes’ route caches, prevent return of invalid route

19

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

20

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

21

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

[S]

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

22

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

• Node H receives packet RREQ from two neighbors: potential for collision

Z

Y

M

N

L

[S,E]

[S,C]

23

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

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

Z

Y

M

N

L

[S,C,G]

[S,E,F]

24

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

• Nodes J and K both broadcast RREQ to node D• Since nodes J and K are hidden from each other, their transmissions may collide

N

L

[S,C,G,K]

[S,E,F,J]

25

Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

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

M

N

L

[S,E,F,J,M]

26

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

27

Route Reply in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

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

Represents RREP control message

28

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. If IEEE 802.11 MAC is used to send data, then links have to be

bi-directional (since Ack is used)

29

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

30

Data Delivery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

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

Packet header size grows with route length

31

When to Perform a Route Discovery

When node S wants to send data to node D, but does not know a valid route node D

32

DSR Optimization: Route Caching Each node caches a new route it learns by any means When node S finds route [S,E,F,J,D] to node D, node

S also learns route [S,E,F] to node F When node K receives Route Request [S,C,G]

destined for node, node K learns route [K,G,C,S] to node S

When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node D

When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D

A node may also learn a route when it overhears Data packets

33

Use of Route Caching

When node S learns that a route to node D is broken, it uses another route from its local cache, if such a route to D exists in its cache. Otherwise, node S initiates route discovery by sending a route request

Node X on receiving a Route Request for some node D can send a Route Reply if node X knows a route to node D

Use of route cache can speed up route discovery can reduce propagation of route requests

34

Use of Route Caching

B

A

S E

F

H

J

D

C

G

IK

[P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format)

M

N

L

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

[C,S]

[G,C,S]

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

[J,F,E,S]

Z

35

Use of Route Caching:Can Speed up Route Discovery

B

A

S E

F

H

J

D

C

G

IK

Z

M

N

L

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

[C,S][G,C,S]

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

[J,F,E,S]

RREQ

When node Z sends a route requestfor node C, node K sends back a routereply [Z,K,G,C] to node Z using a locallycached route

[K,G,C,S] RREP

36

Use of Route Caching:Can Reduce Propagation of Route Requests

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

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

[C,S][G,C,S]

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

[J,F,E,S]

RREQ

Assume that there is no link between D and Z.Route Reply (RREP) from node K limits flooding of RREQ.In general, the reduction may be less dramatic.

[K,G,C,S]RREP

37

Route Error (RERR)

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

RERR [J-D]

J sends a route error to S along route J-F-E-S when its attempt to forward the data packet S (with route SEFJD) on J-D fails

Nodes hearing RERR update their route cache to remove link J-D

38

Route Caching: Beware!

Stale caches can adversely affect performance With passage of time and host mobility, cached

routes may become invalid A sender host may try several stale routes (obtained

from local cache, or replied from cache by other nodes), before finding a good route

An illustration of the adverse impact on TCP is discussed in tutorial by [Holland99]

39

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

40

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

41

Dynamic Source Routing: Disadvantages

An intermediate node may send Route Reply using a stale cached route, thus polluting other caches

This problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated.

For some proposals for cache invalidation, see [Hu00Mobicom] Static timeouts Adaptive timeouts based on link stability

Issues Scalability

Discovery messages broadcast throughout network Broadcast / Multicast

Use Route Request packets with data included Duplicate rejection mechanisms prevent “storms”

Multicast treated as broadcast; no multicast-tree operation defined Scalability issues

http://www.ietf.org/internet-drafts/draft-ietf-manet-simple-mbcast-01.txt

Ad-hoc On-demand Distance Vector Routing Draft RFC at

http://www.ietf.org/internet-drafts/draft-ietf-manet-aodv-10.txt

“Hop-by-hop” protocol: intermediate nodes use lookup table to determine next hop based on destination

Utilizes only standard IP header

AODV Protocol Activities

Route discovery Undertaken whenever a node needs a “next hop” to

forward a packet to a destination Route maintenance

Used when link breaks, rendering next hop unusable

Routing (easy!)

Route Discovery Route Request:

Source broadcasts Route Request (RREQ) message for specified destination

Intermediate node: Forwards (broadcasts) message toward

destination Creates next-hop entry for reverse path to

source, to use when sending reply (assumes bidirectional link…)

Route Reply Destination unicasts Route Reply (RREP) message to

source RREP contains sequence number, hop-count field

(initialized to 0) Will be sent along “reverse” path hops created by

intermediate nodes which forwarded RREQ Intermediate node:

Create next-hop entry for destination as RREP is received, forward along “reverse path” hop

Increment hop-count field in RREP and forward Source:

If multiple replies, uses one with lowest hop count

Details again… Each node maintains non-decreasing sequence number

Sent in RREQ, RREP messages; incremented with each new message

Used to “timestamp” routing table entries for “freshness” comparison

Intermediate node may return RREP if it has routing table entry for destination which is “fresher” than source’s (or equal with lower hop count)

Routing table entries assigned “lifetime”, deleted on expiration Unique ID included in RREQ for duplicate rejection

Route Maintenance Used when link breakage occurs

Link breakage detected by link-layer ACK, AODV “Hello” messages

Detecting node may attempt “local repair” Send RREQ for destination from intermediate node

Route Error (RERR) message generated Contains list of unreachable destinations Sent to “precursors”: neighbors who recently sent

packet which was forwarded over broken link Propagated recursively

49

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

50

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

51

Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

52

Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

53

Route Requests in AODV

B

A

S E

F

H

J

D

C

G

IK

Represents links on Reverse Path

Z

Y

M

N

L

54

Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

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

Z

Y

M

N

L

55

Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

56

Reverse Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

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

M

N

L

57

Route Reply in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

Represents links on path taken by RREP

M

N

L

58

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

59

Forward Path Setup in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

Forward links are setup when RREP travels alongthe reverse path

Represents a link on the forward path

60

Data Delivery in AODV

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

Routing table entries used to forward data packet.

Route is not included in packet header.

DATA

61

Timeouts

A routing table entry maintaining a reverse path is purged after a timeout interval timeout should be long enough to allow RREP to

come back A routing table entry maintaining a forward path is

purged if not used for a active_route_timeout interval if no is data being sent using a particular routing

table entry, that entry will be deleted from the routing table (even if the route may actually still be valid)

62

Link Failure Reporting

A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry

When the next hop link in a routing table entry breaks, all active neighbors are informed

Link failures are propagated by means of Route Error messages, which also update destination sequence numbers

63

Link Failure Detection

Hello messages: Neighboring nodes periodically exchange hello message

Absence of hello message is used as an indication of link failure

Alternatively, failure to receive several MAC-level acknowledgement may be used as an indication of link failure

Issues Scalability

No inherent “subnetting” provision in routing tables – one entry per destination

Directionality Assumes there is at least one bidirectional path

between any two nodes

Issues (cont.) Multicast

True multicast-tree generation and maintenance Detailed in supplementary (expired…) draft:

http://www.watersprings.org/pub/id/draft-ietf-manet-maodv-00.txt

Broadcast Suggested use of IP Ident field for duplicate detection http://www.ietf.org/internet-drafts/draft-ietf-manet-bcas

t-00.txt

Protocol Performance Tests

“A Performance Comparison of Multi-Hop Wireless Ad Hoc Network Rotuing Protocols”, D. Johnson et al., MobiCom ’98 Proceedings. By the creators of DSR

“Performance Comparison of Two On-Demand Routing Protocols for Ad Hoc Networks”, C. Perkins et al., IEEE Personal Communications, February 2001. By the creators of AODV

Both used ns-2 simulator, simulated 802.11 link layer

Johnson et al Compared DSR, AODV, DSDV, TORA

Varied number of sources, node mobility, traffic load 50 nodes total, 64-byte data packets

Looked at packet delivery ratio, routing overhead Conclusions:

DSR, AODV similar on packet delivery ratio DSR much lower routing traffic overhead (excluding

DSR’s routing header extension in each data packet) TORA, DSDV performed very poorly in certain

situations (low packet delivery ratio)

Perkins et al Compared DSR and AODV

Varied number of sources, node mobility, traffic load 50 and 100 nodes, 512-byte data packets

Looked at packet delivery ratio, packet delay, routing overhead, total network throughput

Conclusions: DSR outperforms with fewer nodes, lower traffic load, less

node mobility AODV outperforms when have more nodes, higher traffic

load, greater node mobility DSR always lower routing overhead (excluding routing header) DSR poor delivery ratio when many nodes, many sources, high

mobility

Linux Implementations

DSR Sourceforge “PicoNet” project, Alex Song:http://sourceforge.net/projects/piconet/

AODV NIST “Kernel AODV” implementation, Luke

Klein-Berndt:http://w3.antd.nist.gov/wctg/aodv_kernel/

Optimized Link State Routing (OLSR)Overview An optimization of the link-state protocol Introduces serial numbers to handle stale routes The Goal is to

Reduce the flooding of topology control messages Reduce size of topology control messages

OLSR Overview (2)

Uses MultiPoint Relays (MPRs), that relays all traffic from a node

Each node advertises the nodes that have selected it as an MPR

From this information other nodes can build a "last hop" table and calculate the routing table from this

OLSR Overview (3)

v

Retransmitting nodeor multipoint relay

• A subset of 1-hop neighbors are selected as MPRs

• The set of MPRs must cover all 2-hop neighbors

Topology Broadcast based on Reverse Path Forwarding (TBRPF) Overview Two modes of operation

Full Topology, (FT) Suited for small networks with few nodes

Partial Topology, (PT) Suited for dense networks Node only reports changes to its source tree Each node can compute min-hop distance to every other

node Neighbor discovery

By sending out differentiated HELLO’s New and recently lost neighbors

TBRPF Overview (2) Routing function

By means of a reportable sub-tree Links to all neighbors Branch of the source tree rooted at node j if node i determines that i

is the next hop of some neighbor k to reach j.

1

6

2

10

7

3

11

8

4

12

9

5

13

2’s reportable subtree6’s reportable subtree10’ reportable subtree

ji

k

75

TBRPF Overview (3)

Link failure and rerouting Reorganisation of 2’s and 6’s reportable subtree

1

6

2

10

7

3

11

8

4

12

9

5

13

2’s reportable subtree6’s reportable subtree10’ reportable subtree

Comparison

Reactive Suitable for networks with small subset of nodes

communicating High latency when establishing new routes Low storage and message overhead Difficult to implement QoS

Pro-active Suitable for networks with large subset of nodes

communicating Low latency when establishing connections Overhead to maintain routes to all destinations Relatively easy to implement QoS

Reactive Protocols

DSR Hop-by-hop route to destination

stored in cache Multiple routes possible Route cache is only updated in

terms of RREPs and RERRs. Generally low routing load Better performance in small

netoworks

AODV Next-hop routing towards

destination One route to each destination Timers and sequence numbers to

avoid stale routes High routing load for small

numbers of nodes Better performance in larger

network

Pro-Active Routing

OLSR Suitable for dense networks Use of MPRs reduce the set of

active nodes Small packet size Dependent on special HELLO

messages

TBRPF More topology information is

available to nodes No need for special HELLO

messages if network supports link sensing

Very good results in simulations

Conclusion

As more and more devices becomes network aware, good ad-hoc routing algorithms will become increasingly important

The choice of what ad-hoc routing algorithm to use is highly dependent on network layout and traffic characteristics

Ad-hoc routing is still a hot research topic

Conclusion (2)

The standardization process AODV is generating the most interest in the research

community and is furthest along in the standardization process

Not much current research on DSR TBRPF seems to be a relatively closed project, very few

independent articles on TBRPF are published OLSR is not generating many articles either More independent performance comparisons are needed,

especially for TBRPF and OLSR

Assignment #6

Find and Compare Papers related to TBRPF and OLSR.

Q&A

?