wireless routing in the beginning - tu berlin · 2016-08-31 · outline review of last time wired...

Wireless Internet Routing

Wireless Routing in the Beginning

1

Wireless Internet Routing

 Today: routing in MANET/ad hoc networks and mesh networks o  Deployed as access networks (and sensor network

deployments) or wireless backhaul o  Internet? = Gateway(s) wireless/wired

2

Outline

 Review of last time  Wired routing algorithms: why are they not usable in

wireless scenarios  In the beginning: (unicast) routing for MANET

o  Proactive vs on-demand o  DSR, AODV, DSDV

3

Review: Mesh Networks with Internet Access

AP

AP AP

AP

AP

Internet

4

Internet

 Wireless infrastructure (backbone) o  Static backbone

 Gateways to the Internet o  Eg: Freifunk, Seattle Wireless

 Clients connects to AP o  Transparent wireless network o  Mobility, roaming

Review: Multi-Hop Wireless Networks

5

 No infrastructure: ad-hoc network o  MANET: Mobile Ad Hoc Networks o  Sensor networks o  VANET

 Origins: packet radio networks (1978, “Advances in packet radio technology”)

 Nodes: transmit, receive, forward  Nodes can be mobile  Wireless routing becomes necessary

Review: Wireless Physical Layer and its effect on the upper layers

Unlike a wired connection!  Characteristics

o  Time-varying channel: propagation, connectivity and available capacity

o  Shared medium: interference, collisions

 Consequences for the upper layers: links exhibit o  Time-varying behavior: connectivity, rate, delay o  Low reliability: packets are lost (typical 10^-2 PER) o  Smaller bandwidth than wired counterpart (10 to 100 Mbit/s) o  Asymmetric / bidirectional and unidirectional links

6

Review: Time-Varying Environment

7

Connectivity is not a unit-disc

Received power is time-varying

Variable packet loss

Review: What is Routing

Control Plane:  Topology discovery

o  Neighborhood discovery  Route discovery and maintenance

o  Fault recovery

Data Plane:  Forwarding

8

Challenges of Wireless Routing

 Routing in a wireless network = routing in a network with: o  Links: unreliable, asymmetric, time-varying o  Dynamic topology o  Potentially high time variability

9

Routing in Wireless Networks: Disclaimer!

 There is not a unique solution! o  No one size fits all

 Lots of different algorithms and proposals o  Different design goals and objectives

o Mesh: maximizes throughput o Sensor network: maximizes battery life or minimize

delay (fire detection)

10

Wired Routing Recipes

 Distance vector algorithms (e.g. RIP) o  Decentralized information o  Router knows physically-connected neighbors, link costs to

neighbors o  Iterative process of computation, exchange of info with

neighbors

 Link-state algorithms (e.g. OSPF) o  Global information o  All routers have complete topology, link cost info

11

A Distance Vector Routing Algorithm Decentralized algorithm:  Router knows its neighbors and link costs to

neighbors  Iterative computation, exchange of info with

neighbors Bellman-Ford Equation (dynamic programming) Define dx(y) := cost of least-cost path from x to y Then

dx(y) = min {c(x,v) + dv(y)}

where min is taken over all neighbors v of x  Calculate direction and distance to any link in a

network 12

v

Distance Vector Algorithm Iterative, asynchronous:   Each local iteration caused

by: o  Local link cost change o  DV update message from

neighbor Distributed:   Each node notifies

neighbors only when its Distance Vector changes o  Neighbors then notify

their neighbors if necessary

o  Region not concerned by topology change are not affected

13

wait for (change in local link cost of msg from neighbor)

recompute estimates

if Distance Vector to any dest has changed, notify neighbors

Each node:

A Link-State Routing Algorithm   Net topology, link costs

known to all nodes o  Accomplished via “link

state broadcast” o  All nodes have same info

  Computes least cost paths from one node (‘source”) to all other nodes o  Gives routing table for

that node   Example:

Dijkstra’s algorithm o  Iterative: after k iterations,

know least cost path to k dest.’s

Notation: Dijkstra’s algorithm

 c(i,j): link cost from node i to j. cost infinite if not direct neighbors

 D(v): current value of cost of path from source to dest. v

 p(v): predecessor node along path from source to v

 N’: set of nodes whose least cost path definitively known

14

Link State Routing

 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

15

Example

 Java applet:

http://www.mathiaz.com/index.php?n=Routage.Routage

16

Distance Vector and Link State in Wireless Networks  Link state

o  Fully network topology must be distributed throughout the network

o  Can lead to short terms loop o OSPF elected router to solve distribution issues

Not practical in wireless: mobility induces lots of topology changes, need to flood information throughout the network

 Distance vector o  Monitor neighbors and outgoing links o  Periodically broadcast shortest distance table to every

neighbors Not practical in wireless: periodic update wastes bandwidth/power,

links/connectivity time-varying, solutions to prevent loops are not directly applicable

17

Distance Vector Example

 Symmetric links, link cost = 1  A, B… E are addresses  Local knowledge

18

A B

D E

C

Example borrowed from “Routing in the Internet”, Huitema, 1995

1 2

3 4 5

6

Cold Start

 Tx: DV(A): A=0 [1,3]

19

A B

D E

C 1 2

3 4 5

6

From A to Link Cost A Local 0

From B to Link Cost B Local 0

Update at B and D

 Tx: DV(B): B=0, A=1 [1,2,4]  Tx: DV(D): D=0, A=1 [3,6]

20

A B

D E

C 1 2

3 4 5

6

From A to Link Cost A Local 0

From B to Link Cost B Local 0 A 1 1

From D to Link Cost D Local 0 A 3 1

Message from B Received First

 Rx: DV(B): B=0, A=1  Rx: DV(D): D=0, A=1

21

A B

D E

C 1 2

3 4 5

6 From A to Link Cost A Local 0 B 1 1 D 3 1

From C to Link Cost C Local 0 B 2 1 A 2 1

From E to Link Cost E Local 0 B 4 1 A 4 2

E Receives Message From D

22

A B

D E

C 1 2

3 4 5

6

 Rx: DV(D): D=0, A=1

 Tx: DV(A): A=0,B=1,D=1 [1,3]  Tx: DV(C): C=0, B=1, A=2 [2,5]  Tx: DV(E): E=0,B=1,A=2,D=1 [4,5,6]

From E to Link Cost E Local 0 B 4 1 A 4 2 D 6 1

No update of the entry for A

Routing Tables at B,D and E are Updated

23

A B

D E

C 1 2

3 4 5

6

From D to Link Cost D Local 0 A 3 1 B 3 2 E 6 1

From B to Link Cost B Local 0 A 1 1 D 1 2 C 2 1 E 4 1

From E to Link Cost E Local 0 B 4 1 A 4 2 D 6 1 C 5 1

And New DVs are Sent

24

A B

D E

C 1 2

3 4 5

6

 DV(B): B=0,A=1,D=2, C=2, E=1 [1,3,4]  DV(D): D=0, A=1, B=2, E=1 [3,6]  DV(E): E=0,B=1,A=2,D=1,C=1 [4,5,6]

Routing Tables at A,C and D are Updated

25

A B

D E

C 1 2

3 4 5

6

From C to Link Cost C Local 0 B 2 1 A 2 2 E 5 1 D 5 2 From A to Link Cost

A Local 0 B 1 1 D 3 1 C 1 2 E 1 2

From D to Link Cost D Local 0 A 3 1 B 3 2 E 6 1 C 6 2  No more update necessary

From B to Link Cost B Local 0 A 1 Inf D 1 Inf C 2 1 E 4 1

If Link 1 Breaks?

26

A B

D E

C 1 2

3 4 5

6

 Update at A and B: inf for all routes through link 1

 Tx: DV(A) … [3]  Tx: DV(B) … [4,2]  …  Low convergence time

From A to Link Cost A Local 0 B 1 Inf D 3 1 C 1 Inf E 1 Inf

Distance Vector Issues: Temporary Loop

 Cost(5)=10  Link 2 breaks

27

D E

C 1 2

3 4 5 (Cost 10)

6

From X to C Link Cost A 1 2 B 2 1 C Local 0 D 3 3 E 4 2

A B

Distance Vector Issues: Temporary Loop

28

D E

C 1 2

3 4 5 (Cost 10)

6

A B From X to C Link Cost A 1 2 B 2 Inf C Local 0 D 3 3 E 4 2

  B notices failure  A sends DV refresh

before B: DV(A) A=0,B=1,C=2,D=1,E=2

  B sends DV update DV(B) A=1,B=0,C=3,D=2,E=1

 Temp. loop A-B to C

From X to C Link Cost A 1 4 B 1 3 C Local 0 D 3 3 E 4 4

From X to C Link Cost

A 1 2

B 2 1

C Local 0

D 3 3

E 4 2

Distance Vector Issue: Count to Infinity

29

A B

D E

C 1 2

3 4 5

6

From D to Link Cost D Local 0 A 3 1 B 3 2 E 6 Inf C 6 Inf

  Link 6 breaks after link 2  D notices failure  A sends DV refresh

before D: DV(A) A=0,B=3,C=3,D=1,E=3

  Temp. loop A-B

From D to Link Cost D Local 0 A 3 1 B 3 4 E 3 4 C 3 4

Split Horizon and Poisonous Reverse

30

A B

D E

C 1 2

3 4 5

6

 Split horizon: if A to X through B, then B should not try to reach X through A o  A does not announce route to X to B

 With poisonous reverse: A announces route but set cost to Inf

Split Horizon and Poisonous Reverse

31

 Split Horizon: DV(A) empty [3]  With poisonous reverse:

DV(A) B=Inf, D=Inf, C=Inf, E=Inf [3]

A B

D E

C 1 2

3 4 5

6

From A to Link Cost A Local 0 B 3 3 D 3 1 C 3 3 E 3 2

Another example: link 1 broke , all traffic from A goes through D

 E-D failure

  DV(E) … D=Inf [4,5] , tx error on 5   DV(C) E=Inf,D=Inf,B=1 [5] or DV(C)

B=1[5], DV(C) E=1,D=2 [2]   DV(B) D=3… [4]   Loop B-C-E to D

Split Horizon Does Not Prevent all Loops

32

B

E

C 1 2

4 5

6

From X to D Link Cost B 4 2 C 5 2 E 6 Inf

From X to D Link Cost B 4 Inf C 5 2 E 6 Inf

From X to D Link Cost B 2 3 C 5 2 E 4 4

Summary: Problems of conventional routing algorithms in wireless networks

(RIP, OSPF) are not designed for dynamic topologies

 Links are asymmetric, many links available  Topology is dynamic  Periodic information (route,topology) update or dump

broadcast o  Bandwidth o  Power

33

Early Design of Wireless Routing Protocols: Some Assumptions  Fully symmetric environment: all nodes/stations have

identical capabilities and responsibilities  Nodes don’t sleep  Energy consumption is not an issue  ID or IP address assignment has been performed  Unicast

34

Route Discovery and Maintenance (control plane): Proactive vs Reactive  Proactive protocols

o  Determine routes independent of traffic pattern o  Traditional link-state and distance-vector routing protocols

are proactive

 Reactive protocols (on demand) o  Maintain routes only if needed

 Hybrid protocols

35

Reactive vs Proactive: Trade-Off

  Latency of route discovery o  Proactive protocols may have lower latency since routes are

maintained at all times o  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 o  Reactive protocols may have lower overhead since routes are

determined only if needed o  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

36

Taxonomy of Routing Protocols for Wireless Networks

Response time, bandwidth Energy

(Mobile) ad hoc networks mesh networks Sensor networks

Proactive protocols

Reactive protocols

Destination-Sequenced Distance-Vector (DSDV)

Optimized Link- State Routing (OLSR)

Ad Hoc On-Demand Distance-Vector (AODV)

Dynamic Source Routing (DSR)

Geography- based routing

Cluster-based (or hierarchical) routing

37

Flooding

 The wireless channel is a broadcast medium after all

38

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: 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

39


40

B

A

S E F

H

J

D

C

G

I K

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

Z

Y

Represents a node that has received packet P

M

N

L


41

B

A

S E F

H

J

D

C

G

I K

Represents transmission of packet P

Represents a node that receives packet P for the first time

Z

Y Broadcast transmission

M

N

L


42

B

A

S E F

H

J

D

C

G

I K

  Node H receives packet P from two neighbors: potential for collision

Z

Y

M

N

L


43

B

A

S E F

H

J

D

C

G

I K

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

Z

Y

M

N

L


44

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

  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

N

L


45

B

A

S E F

H

J

D

C

G

I K

Z

Y

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

M

N

L


46

B

A

S E F

H

J

D

C

G

I K

  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)

Z

Y

M

N

L


47

B

A

S E F

H

J

D

C

G

I K

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

Z

Y

M

N

L

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 o  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 o  Because packets may be delivered to the destination on multiple

paths o  (we will talk about it later)

48

Flooding for Data Delivery: Disadvantages

 Potentially, very high overhead o  Data packets may be delivered to too many nodes who do not

need to receive them

 Potentially lower reliability of data delivery o  Flooding uses broadcasting -- hard to implement reliable

broadcast delivery without significantly increasing overhead o Broadcasting in IEEE 802.11 MAC is unreliable

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

o In this case, destination would not receive the packet at all

49

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

50

Summary: Flooding

 Flooding is used in many protocols, such as Dynamic Source Routing (DSR)

 Problems associated with flooding o  Collisions o  Redundancy

 Collisions may be reduced by “jittering” (waiting for a random interval before propagating the flood)

 Redundancy may be reduced by selectively re-broadcasting packets from only a subset of the nodes

51

DSR: Dynamic Source Routing

 RFC 4728: http://tools.ietf.org/html/rfc4728

 David B. Johnson. Routing in Ad Hoc Networks of Mobile Hosts. Proceedings of the Workshop on Mobile Computing Systems and Applications, pp. 158-163, IEEE Computer Society, Santa Cruz, CA, December 1994

 Reactive protocol o  Source routing for data delivery o  Route discovery / route maintenance

52

Dynamic Source Routing

 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

53

Route Discovery in DSR

54

B

A

S E F

H

J

D

C

G

I K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L


55

B

A

S E F

H

J

D

C

G

I K

Represents transmission of RREQ

Z


M

N

L

[S]

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


56

B

A

S E F

H

J

D

C

G

I K

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

Z

Y

M

N

L

[S,E]

[S,C]


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

57

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

[S,C,G]

[S,E,F]


58

B

A

S E F

H

J

D

C

G

I K

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]


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

59

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

[S,E,F,J,M]


 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

60

Route Reply in DSR

61

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

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

Represents RREP control message

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 o  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 o  Unless node D already knows a route to node S o  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) o  Hidden hypothesis here?

62

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 o  Hence the name source routing

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

63

Data Delivery in DSR

64

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

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

Packet header size grows with route length

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 o  Is there any optimization possible?

65

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

66

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 o  Can speed up route discovery o  Can reduce propagation of route requests

67

Use of Route Caching

68

B

A

S E F

H

J

D

C

G

I K

[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

Use of Route Caching: Can Speed up Route Discovery

69

B

A

S E F

H

J

D

C

G

I K

Z

M

N

L


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

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

[J,F,E,S]

RREQ

When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route

[K,G,C,S] RREP

Use of Route Caching: Can Reduce Propagation of Route Requests

70

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L


[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

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

71

Route Maintenance

 If node is moving: can reinitiate route discovery  If a link breaks: Route Error packets

72

Route Error (RERR)

73

B

A

S E F

H

J

D

C

G

I K

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

Dynamic Source Routing: Advantages

 Routes maintained only between nodes who need to communicate o  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

74

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 o  Insertion of random delays before forwarding RREQ

  Increased contention if too many route replies come back due to nodes replying using their local cache o  Route Reply Storm problem o  Reply storm may be eased by preventing a node from sending

RREP if it hears another RREP with a shorter route

75

Dynamic Source Routing: Disadvantages  Delay for route establishment

 Route maintenance is not a local process

 An intermediate node may send Route Reply using a stale cached route, thus polluting other caches o  This problem can be eased if some mechanism to purge

(potentially) invalid cached routes is incorporated. o  For some proposals for cache invalidation, see

[Hu00Mobicom]: static timeouts / adaptive timeouts based on link stability

76

Ad Hoc On-Demand Distance Vector Routing (AODV)  RFC 3561: http://tools.ietf.org/html/rfc3561

 Charles E. Perkins and Elizabeth M. Royer. "Ad hoc On-Demand Distance Vector Routing." Proceedings of the 2nd IEEE Workshop on Mobile Computing Systems and Applications, 1999

 Reactive protocol o  Distance vector o  Destination sequence number

77

Why AODV?

 DSR includes source routes in packet headers

  Resulting large headers can sometimes degrade performance o  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

78

AODV Overview

 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 o  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

79

AODV overview

 Path discovery  Reverse path setup  Forward path setup

80

Route Requests in AODV

81

B

A

S E F

H

J

D

C

G

I K

Z

Y

Represents a node that has received RREQ for D from S

M

N

L


82

B

A

S E F

H

J

D

C

G

I K

Represents transmission of RREQ

Z


M

N

L


83

B

A

S E F

H

J

D

C

G

I K

Represents links on Reverse Path

Z

Y

M

N

L

Reverse Path Setup in AODV

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

84

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L


85

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L


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

86

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

Route Reply in AODV

87

B

A

S E F

H

J

D

C

G

I K

Z

Y

Represents links on path taken by RREP

M

N

L

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 o  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

88

Forward Path Setup in AODV

89

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

Forward links are setup when RREP travels along the reverse path

Represents a link on the forward path

Data Delivery in AODV

90

B

A

S E F

H

J

D

C

G

I K

Z

Y

M

N

L

Routing table entries used to forward data packet.

Route is not included in packet header.

DATA

Timeouts

 A routing table entry maintaining a reverse path is purged after a timeout interval o  Timeout should be long enough to allow RREP to come back o  Used to remove unused reverse path

 A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval o  If no data is 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)

91

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

92

Route Error  When node X is unable to forward packet P (from node S

to node D) on link (X,Y), it generates a RERR message

 Node X increments the destination sequence number for D cached at node X

 The incremented sequence number N is included in the RERR

 When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N

 When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N

93

Why Sequence Numbers in AODV

 To avoid using old/broken routes o  To determine which route is newer

 To prevent formation of loops

o  Assume that A does not know about failure of link C-D because RERR sent by C is lost

o  Now C performs a route discovery for D. Node A receives the RREQ (say, via path C-E-A)

o  Node A will reply since A knows a route to D via node B o  Results in a loop (for instance, C-E-A-B-C )

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A B C D

E

AODV Sequence Number Rule

 If RREQ sequence number > sequence number in table, intermediate node does not answer RREQ

 If RREP sequence number < sequence number at source node, the RREP is ignored

 Destination always update its sequence number to the largest received

95

AODV Uses “Route Request” Sequence Number

 E unable to forward packet P (from E to D) on link 6, generates a RERR message

 E increments the destination sequence number for D cached at E

 Incremented sequence number N included in RERR  E initiates RREQ for D, uses destination sequence

number › N  D receives RREQ with destination sequence number

N, D set its sequence number to N, unless it is already larger than N 96

B C 1 2

4 5 E

With Previous Example

 E sends to D, link 6 breaks  E sends RREP(D,Inf), lost on link 4  E sends RREQ(D,N+2)  B does not send RREP since N+2 > N

97

B

E

C 1 2

4 5

6

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

98

Optimization: Expanding Ring Search

 Route Requests are initially sent with small Time-to-Live (TTL) field, to limit their propagation o  DSR also includes a similar optimization

 If no Route Reply is received, then larger TTL tried

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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 o  Multi-path extensions can be designed o  DSR may maintain several routes for a single destination

 Unused routes expire even if topology does not change

100

Fixing Distance Vector in Wireless Networks: DSDV  DSDV: destination sequence distance vector

 Adapting distance vector algorithms for wireless network

 Solves routing loop problem

 Perkins, Charles E. and Bhagwat, Pravin . Highly Dynamic Destination-Sequenced Distance-Vector Routing (DSDV) for Mobile Computers, Sigcomm 1994

101

DSDV Overview

 Routing information distribution o  Full dumps infrequently o  Incremental updates (smaller) more frequently

102

DSDV Overview

 Each node maintains a routing table which stores o  Next hop towards each destination o  A cost metric for the path to each destination o  A destination sequence number that is created by the

destination itself o Sequence numbers used to avoid formation of loops

 Each node periodically forwards the routing table to its neighbors o  Each node increments and appends its sequence number when

sending its local routing table o  This sequence number will be attached to route entries

created for this node 103

DSDV: Sequence Number

 Assume that node X receives routing information from Y about a route to node Z

 Let S(X) and S(Y) denote the destination sequence number for node Z as stored at node X, and as sent by node Y with its routing table to node X, respectively

104

X Y Z

DSDV: Sequence Number and Routing Update  Node X takes the following steps:

1.  If S(X) > S(Y), then X ignores the routing information received from Y

2.  If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z

3.  If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)

105

X Y Z

DSDV Use Destination Sequence Numbers to Avoid Loops  (destination) sequence number for each node

o  Each node increments and appends its sequence number when sending its local routing table

o  This sequence number attached to route entries created for this node

106

B

E

C 1 2

4 5

6

DSDV Use Destination Sequence Numbers to Avoid Loops

107

B

E

C 1 2

4 5

6

From B to Link Cost Seq A 4 3 50 B Local 0 60 C 2 1 70 D 4 2 80 E 4 1 90

From C to Link Cost Seq A 5 3 50 B 2 1 60 C Local 0 70 D 5 2 80 E 5 1 90

From E to Link Cost Seq A 6 2 50 B 4 1 60 C 5 1 70 D 6 1 80 E Local 0 90

With Previous Example

  DV(E) … D=Inf(80) [4,5] , tx error on 5   DV(C) E=Inf(90),… [5], DV(C) E=1(90),D=2(80) [2]

o  Identical sequence number, but longer route: rule 2, update rejected

108

B

E

C 1 2

4 5

6

From B to Link Cost Seq A 4 3 50 B Local 0 60 C 2 1 70 D 4 2 80 E 4 1 90

Summary

 All protocols discussed so far perform some form of flooding o  DSR, AODV o  DSDV

 Use hop count metric  Connection to the Internet is not addressed

o  Gateway selection problem

109

DSR

 On-demand algorithm  Source routing  RREQ, RREP, RERR  Caching  Many optimizations

110

AODV

 On-demand algorithm  Distance vector: routing table at nodes  RREQ, RREP

o  Path discovery, reverse path setup, forward path setup  Sequence number

111

DSDV

 DSDV: Destination Sequence Distance Vector  Distance vector adapted to wireless  Sequence number against loops  Full dump infrequently, incremental updates more

frequently

112

Flooding/Broadcast Storm

 See S.-Y. Ni, Y.-C. Tseng, Y.-S. Chen, and J.-P. Sheu, "The broadcast storm problem in a mobile ad hoc network," in MobiCom 1999

113

Broadcast Storm Problem

 When node A broadcasts a route query, nodes B and C both receive it

 B and C both forward to their neighbors  B and C transmit at about the same time since they

are reacting to receipt of the same message from A  This results in a high probability of collisions

114

B

D

C

A

Broadcast Storm Problem

 Redundancy: A given node may receive the same route request from too many nodes, when one copy would have sufficed

 Node D may receive from nodes B and C both

115

B

D

C

A

Solutions for Broadcast Storm

 Probabilistic scheme: On receiving a route request for the first time, a node will re-broadcast (forward) the request with probability p

 Also, re-broadcasts by different nodes should be staggered by using a collision avoidance technique (wait a random delay when channel is idle) o  This would reduce the probability that nodes B and C would

forward a packet simultaneously in the previous example

116

Solutions for Broadcast Storms

 Counter-Based Scheme: If node E hears more than k neighbors broadcasting a given route request, before it can itself forward it, then node E will not forward the request

 Intuition: k neighbors together have probably already forwarded the request to all of E’s neighbors

117

B

D

C

A

F

E


 Distance-Based Scheme: If node E hears RREQ broadcasted by some node Z within physical distance d, then E will not re-broadcast the request

 Intuition: Z and E are too close, so transmission areas covered by Z and E are not very different o  if E re-broadcasts the request, not many nodes who have not

already heard the request from Z will hear the request

118

E Z

<d


 Let the physical layer characteristics handle it o  High packet loss rate

119

wireless routing in the beginning - tu berlin · 2016-08-31 · outline review of last time wired...

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