Ad Hoc Networks

Ad Hoc Networks

Dirk GrunwaldUniversity of Colorado, Boulder

Outline

Review of routing in wired networked Routing domains & levels RIP routing

• Distance Vector Routing• Examples of RIP information & management

OSPF routing• Link state routing

Routing in ad-hoc wireless networks

Gateway Hierarchy

InternetCore

AutonomousSystem

(AS)

AutonomousSystem

(AS)

Two levels of Routing Protocols

RoutingDomain

RoutingDomain

RoutingDomain

EGP EGP

EGP

IGP

IGP

IGP

Intra-domainrouting protocol

Exteriorrouting protocol

Routing Protocols

Intra-domain Gateway Protocols RIP RIP V2 OSPF - open shortest path first IS-IS (similar to OSPF)

Exterior Gateway Protocols EGP BGP

RIP

Distance vector routing algorithm based on hops that communicates between routers using UDP

On initialization, router determines all available interfaces and sends a ROUTE REQUEST packet out each interface. Special request for “send everything”

On receipt of request, Either return everything Or, for each requested destination, return distance to that

destination + 1 On response

Update routing tables

RIP V1 Protocol

Command Version MBZ

32-bit IP address

Address Family MBZ

MBZ

MBZ

Metric (value of 1..16)

Up to 24 more routes in same format...

Metrics

R1

R2

N1

N2

N3

N2 is 1 hop

N3 is 1 hop

N1 is 1 hop

N2 is 1 hop

Route to N3via R2 with

hop count of 2

Problems

Hop count limited to 15 Can only be used within an AS where maximum network

diameter of 15

It’s based on HOPS, not e.g., latency or bandwidth

No notion of subnet addressing in RIP V1

RIP V2 Protocol

Command Version Routing domain

32-bit IP address

Address Family Route tag

32-bit subnet mask

32-bit next-hop IP address

Metric (value of 1..16)

Up to 24 more routes in same format...

RIP V2

Routing domain is an identifier of the routing daemon Process ID in UNIX …So you can run multiple instances of RIP

Route tag carries an autonomous system number for EGP and BGP

Next op address is where packets corresponding to that (sub)network should be sent. A value of zero means send to the system sending RIP info.

Simple authentication scheme with clear-text password

Distance Vector Routing

Also called Bellman-Ford or Ford-Fulkerson algorithms Used by RIP

Each router is responsible for keeping track of and informing its neighbors of its distance to each destination

The router computes its distance to a destination based on its neighbors distance to the destination

Router must know it’s own ID and the cost of its links to each neighbor

Distance Vector Routing For Address “D”

R

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Link cost

Link number


R

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11829

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Cost from neighbor to

destination D


R

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123 70

Cost for Rto get to Dvia this link

Minimumcost route


R

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70

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Cost fromR to D

Problems With Distance Vector

Slow convergence to the lowest cost route

Slow recovery time if there are link failures

Slow recovery leads to routing problems during recovery Router loops Count to infinity

Count To Infinity (worse case loop)

A

B

C

A

B1

2

1

2 A

B2

3 A

B3

4

OSPF - Open Shortest Path First

OSPF uses IP directly (I.e., like ICMP) Routes calculated based on Type of Service (TOS) Each interface is assigned a dimensionless cost,

for each TOS If several equal-cost routes are available,

traffic is load-balanced Subnets are associated with each advertised route Supports authentication Uses multicast to distribute information

Link State Routing

Used by OSPF and IS-IS

Construct a Link State Packet (LSP) that lists neighbors and costs to get to those neighbors

LSP = (destination, path cost, forwarding direction)

Use Dijkstra’s algorithm to compute global routes as a tree from the current router

Dijkstra’s Algorithm

Two sets of paths: PATH and TENT(ative) LSP = (destination, path cost, forwarding direction)

(1) Place (self, 0,0) in path

(2) For any (D,C,F) placed in PATH, examine D’s LSP. For each Neighbor of D listed in D’s LSP with a link cost to N of c, see if (N, c, …) is already in TENT or PATH. If not listed, or if c is less than existing cost, add (N, C+c,…) to TENT

(3) Terminate if TENT is empty, otherwise find (D,C,F) with minimum C and add that to PATH. Goto (2)

Example of Dijkstra’s Algorithm

C(0)

B(2)G(5)

F(2)

Cost

Destination


C(0)

B(2)G(5)

F(2)

Since B and F have samecost, select one at random.

Place “F” in PATH.

G(3)E(6)

Cost relative to node C


C(0)

B(2)G(5)

F(2)

Better pathto G exists G(3)

E(6)


C(0)

B(2)F(2)

G(3)E(6)

Add B toPath

A(8)E(3)


C(0)

B(2)F(2)

G(3)E(6)

A(8)E(3)

Better pathto E exists


C(0)

B(2)F(2)

G(3)

A(8)E(3)

D(5)

Add E to path

Original Distribution of LSP’s

Neighbor-to-neighbor protocol for LSP distribution Each LSP consists of

Identity of router generating the LSP A sequence number The time left until the LSP should be discarded A list of neighbors, and the cost of links to each

LSP is broadcast to all neighbors LSP’s have a 64 second lifetime - each router must

resend each minute Wrap-around comparison on sequence numbers used

Problem in 1991

A router announced LSP’s with random TTL’s a < b < c <a

Each router that processed one of these LSP’s over-wrote the one in memory and generated more copies of the bad LSP, since it thought that this LSP was newer & needed to be propagated to neighbors

The TTL field never changed because of the rapid rate of updates

Revised method using 32-bit sequences

LSP’s compared only by their sequence numbers - if you get LSP with sequence S & T, and S < T, then T is more recent (no matter what TTL says)

LSP’s are purged when old, based on TTL. Every router decrements TTL

When TTL reaches 0, router refloods the LSP. Given (sequence, TTL) (S,x) & (T,y) then S < T if x = 0, but not otherwise

LSP’s generated much less frequently (~1hr) Router starts with the lowest sequence number. If the

network retains old LSP, it will be flooded back to source, which then knows “current” sequence number to use

Now, Ad-Hoc Routing

A Simple Ad-Hoc Network

A can not communicate directly with C

D. Johnson and D. Maltz

Applications of Ad Hoc Networks

Useful for forming instant networks, without administration and reduced set-up cost.

Conferences / meetings / lectures – shared notes, whiteboard, file sharing, etc

Emergency Services / Search & Rescue – location, situation information, status & messaging

People may move between an infrastructure network and an adhoc network

May be important for vehicles – cars could form an “adhoc” network while on the move

Challenges & Future Research Areas

Multicast QoS support

“plain” and “adaptive” QoS

Power-aware Routing Location-Aided Routing

Media Access Control Security Service Discovery

Taxonomy of Adhoc Routing Algorithms

C G S R

C lus te rhe ad G ate w ay

S w it c h R o u t ing

DS DV

De s t inat io n S eque nc e

D is tanc e V e c tor

W R P

W ire le s s R o u t ing

P ro to c o l

T a b le

Dr iven

A O DC

A d -H o c O n -De m a nd

Dis tanc e V e c tor

DS R

Dyna m ic S o u rc e

R o u t ing

T O R A

T e m po ra lly-O rde red

R o u t ing A lgo r ithm

L M R

L igh tw e ight

M o b ile R o u t ing

S S R

S igna l S ta b il it y

R o u t ing

A B R

A s s o c ia t iv ity

B a s e d R o u t ing

S o u rc e -I n it ia ted

O n -De m and Dr iven

A d -H o c R o u t ing P ro to c o ls

Table Driven Methods

Attempt to maintain consistent, up-to-date routing information from each node to every other node in the network

Perfect for “connectionless” traffic where you may send traffic to any node at any time

Respond to changes by propagating updates throughout the network to maintain a consistent network view.

Source Initiated Methods

Routes are only created when desired by the source node When a node requires a route, it initiates a “route discovery

process”. The route discovery finishes either when a route is discovered

or all possible communication paths have been examined.

Once a route is established, is it maintained by some sort of route maintenance protocol

Taxonomy of Table DrivenRouting Algorithms

C G S R

C lu s te rh e ad G ate w ay

S w it c h R o u t ing

DS DV

De s t inat io n S equ e nc e

D is tan c e V e c tor

W R P

W ire le s s R o u t ing

P ro to c o l

T a b le

Dr iven

DSDV – Destination Sequenced Distance Vector Routing

Based on Bellman-Ford routing mechanism Each node maintains routing table that records all

possible destinations and the number of hops to those destinations

Each entry is marked with a sequence number assigned by the destination node. These sequence numbers are used to distinguish “old” vs “new” routes.

Routing tables are periodically transmitted to maintain table consistency

A “full dump” record reports on the entire routing table. Lots of traffic, since each table is probably larger than the network protocol data unit.

Incremental updates provide information since last “full dump”

DSDV

A new route broadcasts contain The address of the destination The number of hops to reach the destination The sequence number of the information Sequence number unique to this broadcast

Route labeled with most recent sequence number is always used

If two event have the same sequence number, the route with the smaller metric (== shorter route) is used

Hosts also keep track of the the settling time of routes, or the weighted average time that routes to a destination will fluctuate before the route with the best metric is received

Clusterhead Gateway Switch Routing (CGSR)

Clustered multi-hop routing protocol Node routes to “cluster head” Clusterhead routes to other “cluster-head” by gateways Eventually, clusterhead routes to specific node

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Gateway

ClusterHead

CGSR

Clusters allow channel & bandwidth allocation Nodes only speak to cluster heads Gateways speak to one or more cluster heads

Cluster election algorithm picks cluster head A “least cluster change” algorithm is used to reduce the

election overhead Re-election only occurs when two cluster heads move within

communication range or a node is “lost” by moving out of communication range.

DSDV used as the underlying scheme

CGSR Example

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CGSR

Each node must keep a “cluster member table” where it stores destination cluster head for each mobile node

Can that really be true? Shouldn’t the cluster head be the only one that needs to maintain this?

Cluster tables are broadcast using DSDV algorithm

Each node also maintains a routing table, which is used to specify the next hop towards the destination

To route, the node looks in the cluster table to determine the nearest cluster head and then uses routing table to determine how to reach that cluster head.

Makes no sense.

Wireless Routing Protocol

Each node maintains Distance Table Routing Table Link-Cost Table Message Retransmission List

Updates are exchanged with neighbor nodes Destination, distance to destination, predecessor of the

destination (next hop) Messages are sent when a link status changes or after updates

from their neighbors

Nodes learn of the existence of their neighbors from the receipt of ACKs

Taxonomy of Source InitiatedRouting Algorithms

A O DC

A d -H o c O n -De m a nd

Dis tanc e V e c tor

DS R

Dyna m ic S o u rc e

R o u t ing

T O R A

T e m po ra lly-O rde red

R o u t ing A lgo r ithm

L M R

L igh tw e ight

M o b ile R o u t ing

S S R

S ign a l S ta b il it y

R o u t ing

A B R

A s s o c ia t iv ity

B a s e d R o u t ing

S o u rc e -I n it ia ted

O n -De m and Dr iven

Ad-Hoc On Demand Distance Vector (AODV)

Based on DSDV, but builds routes on demand. If node S has a message for D, but no route, it starts a

path discovery process. Broadcasts / floods a Route Request (RREQ) Eventually, either the destination (D) or an intermediate node

with a “fresh enough” route cache is reached.

Each node maintains a sequence number as well as a broadcast ID

Broadcast ID is incremented for each RREQ Combination of node address & broadcast ID uniquely

identifies a RREQ

AODV

The source RREQ also includes the most recent sequence number for the destination

Intermediate nodes can only respond if they have “fresher” information -- prevents replying with stale information

Once the message reaches the destination (or intermediary), a Route Reply (RREP) is sent back along the path by which each node first received a route request

Nodes in that route establish forward routes for the destination

Only supports symmetric links!

AODV

If a source node moves, it can re-initiate the route discovery protocol to find a new route

If a node along the route moves, upstream neighbors notice and propogate a “link failure notification” message to each upstream neighbor.

This is propagated “until the source node is reached” How is this known? Must be imprecise statement

“Hello” messages are used to maintain local connectivity

Dynamic Source Routing

The source specifies entire route Assumes the network is fairly small But, no periodic routing advertisements

Uses a source route cache Route discovery protocol uses a route request message

containing Destination address Source address Unique ID

Each node receiving a RREQ checks to see if it knows of a route to the destination. If note, it adds its own address to the Route Record and then forwards the packet on out-going links.


The request is forwarded iff the request has not been seen (hence ID) and if the node address is not in the list

Once the message gets to the destination or reaches a node with an unexpired cached entry, a Route Reply is sent.

If it is the destination, the final route record is placed into the route reply

If it is an intermediate node, it appends the cached route to the route record

The replying node must have a route back to the source This may require another route discovery The route reply is piggy-backed on the new route request

Route Discovery Protocol

D. Johnson and D. Maltz


Route maintenance accomplished through route error packets Route error packets are generated at a node when the data link layer

encounters a fatal transmission Causes routes to be removed from route cache

Route maintenance: active: utilize MAC level ACK active: utilize transport or application ACKs passive: free hop-by-hop surveillance using promiscuous receive

(A hears B sending to C) when a problem is detected, alternate route should be discovered

Optimizations: use promiscuous mode to listen to arbitrary routes in use. Replace with shorter routes

But, being in promiscuous mode takes energy

Temporally-Ordered Routing Algorithm (TORA)

Uses global synchronize clocks (e.g. GPS) Proposed for a highly-dynamic environment Provides multiple routes for S->D

Nodes maintain routing information about adjacent nodes and the routing algorithm attempts to contain how far any route change actually flows

TORA Route Creation

A DAG is created from the nodes

Each node is assigned a “height” (distance from root) using some distributed algorithm not described

Links are assigned a “direction” based on height

The DAG is broken when nodes move

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Associativity Based Routing (ABR)

The goal of ABR is to derive long-lived routes Based on the degree of association stability

Nodes generate beacons When beacons are received by neighbors, the beacon count is

incremented Routing decisions favor routes with a larger beacon count

ABR Route Discovery

Route discovery uses a broadcast query and reply All nodes receiving the query append their addresses and their

associatively ticks to the neighbors, along with QoS info Successive nodes remove the information about neighbors of

the previous node and replace it with their own neighbor information

When the destination is reached, the RREQ contains the associativity information of the discovered route

The destination may receive multiple RREQ’s and selects the path with the largest associativity

A REPLY is issued that follows that associativity chain. Nodes along the REPLY path mark the route as valid

ABR Route Reconstruction

Route Reconstruction may consist of Partial route discovery Invalid route erasure Valid route updates New route discovery

Route reconstruction uses localized messages, although a full “Route Delete” message causes a broadcast route deletion

Signal Stability Routing (SSR)

Selects routes based on signal strength Two protocols: dynamic routing protocol and static

routing protocol DRP maintains signal stability table (SST) and routing table (RT) All transmissions are received by DRP, which updates tables

and then hands packet to SRP

SRP processes packets by accepting it if it is the receiver or looking up the destination in the RT for the destination.

If there’s no known route, a route search is started. During route search, route requests are only passed over a

“strong channel” If no route is found that way, weak channels are searched

Multicast

Not all protocols directly support multicast

Documents

Ad Hoc Networks