Highly Dynamic DestinationSequenced DistanceVector Routing

DSDV for Mobile Computers

Pravin Bhagwat Charles E Perkins

Computer Science Department IBM TJ Watson ResearchCenter

University of Maryland Hawthorne NY

College Park MD

Abstract users are free to move ab out at their convenience with

out b eing constrained by wires

An adhoc network is the co op erative engagementof

This is a revolutionary development in p ersonal com

a collection of Mobile Hosts without the required inter

puting Battery p owered untethered computers are

vention of anycentralized Access Point In this pap er

likely to b ecome a p ervasive part of our computing in

we present an innovative design for the op eration of such

frastructure As p eople b egin to have mobile computers

adho c networks The basic idea of the design is to op

handy for whatever purp oses sharing information b e

erate each Mobile Host as a sp ecialized router which

tween the computers will b ecome a natural requirement

p erio dically advertises its view of the interconnection

Currently such sharing is made dicult by the need for

top ology with other Mobile Hosts within the network

users to p erform administrative tasks and set up static

This amounts to a new sort of routing proto col We

bidirectional links b etween their computers However

haveinvestigated mo dications to the basic Bellman

if the wireless communications systems in the mobile

Ford routing mechanisms as sp ecied by RIP to

computers supp ort a broadcast mechanism much more

make it suitable for a dynamic and selfstarting network

exible and useful ways of sharing information can b e

mechanism as is required by users wishing to utilize ad

imagined For instance anynumb er of p eople could

ho c networks Our mo dications address some of the

conceivably enter a conference ro om and agree to sup

previous ob jections to the use of BellmanFord related

p ort communications links b etween themselves with

to the p o or lo oping prop erties of such algorithms in the

out necessarily engaging the services of any preexisting

face of broken links and the resulting time dep endent

equipment in the ro om ie without requiring anypre

nature of the interconnection top ology describing the

existing communications infrastructure Thus one of

links b etween the Mobile Hosts Finallywe describ e

our primary motivations is to allow the construction of

the ways in which the basic networklayer routing can

temp orary networks with no wires and no administra

b e mo died to provide MAClayer supp ort for adho c

tiveintervention required In this pap er suchainter

networks

connection b etween the mobile computers will b e called

an adhoc network in conformance with current usage

Intro duction

within the IEEE sub committee

Recently there has b een tremendous growth in the

Adho c networks dier signicantly from existing

sales of laptop and p ortable computers These smaller

networks First of all the top ology of interconnections

computers nevertheless can b e equipp ed with hun

may b e quite dynamic Secondly most users will not

dreds of megabytes of disk storage high resolution color

wish to p erform any administrative actions to set up

displays p ointing devices and wireless communications

suchanetwork In order to provide service in the most

adapters Moreover since many of these small in size

general situation we do not assume that every computer

only computers op erate for hours with battery p ower

is within communication range of every other computer

This lack of complete connectivitywould certainly b e a

reasonable characteristic of say a p opulation of mo

bile computers in a large ro om which relied on infrared

transceivers to eect their data communications

Currently there is no metho d available which enables

mobile computers with wireless data communications

equipment to freely roam ab out while still maintaining

metho d Eachnodemaintains a view of the network connections with each other unless sp ecial assumptions

top ology with a cost for eachlinkTokeep these views are made ab out the way the computers are situated with

consistent each no de p erio dically broadcasts the link resp ect to each other One mobile computer mayoften

costs of its outgoing links to all other no des using a b e able to exchange data with two other mobile comput

proto col such as o o ding As a no de receives this in ers which cannot themselves directly exchange data As

formation it up dates its view of the network top ology a result computer users in a conference ro om maybe

and applies a shortestpath algorithm to cho ose its next unable to predict which of their asso ciates computers

hop for each destination Some of the link costs in a could b e relied up on to maintain network connection

no des view can b e incorrect b ecause of long propaga esp ecially as the users moved from place to place within

tion delays partitioned network etc Such inconsistent the ro om

views of network top ologies might lead to formation of

Routing proto cols for existing networks havenot

routing lo ops These lo ops however are shortlived

b een designed sp ecically to provide the kind of dy

b ecause they disapp ear in the time it takes a message

namic selfstarting b ehavior needed for adho c net

to traverse the diameter of the network

works Most proto cols exhibit their least desirable b e

havior when presented with a highly dynamic inter

connection top ology Although we thought that mo

DistanceVector In distancevector algorithms

bile computers could naturally b e mo deled as routers

every no de i maintains for each destination x a set

it was also clear that existing routing proto cols would

x

g where j ranges over the neighb ors of of distances fd

ij

place to o heavy a computational burden on each mobile

iNodei treats neighbor k as a nexthop for a packet

computer Moreover the convergence characteristics of

x x

destined for x if d equals min fd g The succession

j

ij

ik

existing routing proto cols did not seem go o d enough

of next hops chosen in this manner lead to x along the

to t the needs of adho c networks Lastly the wire

shorest path In order to keep the distance estimates

less medium diers in imp ortantways from wired me

uptodate each no de monitors the cost of its outgo

dia whichwould require that wemake mo dications to

ing links and p erio dically broadcasts to each one its

whichever routing proto col we mightcho ose to exp er

neighb ors its current estimate of the shortest distance

iment with For instance mobile computers maywell

to every other no de in the network

have only a single network interface adapter whereas

The ab ove distancevector algorithm is the classical

most existing routers havenetwork interfaces to connect

Distributed BellmanFord DBF algorithm Com

two separate networks together Besides wireless media

pared to linkstate metho d it is computationally more

are of limited and variable range in distinction to exist

ecient easier to implement and requires muchless

ing wired media Since we had to makelotsofchanges

storage space However it is well known that this algo

anywaywe decided to follow our adho c network mo del

rithm can cause the formation of b oth shortlived and

as far as we could and ended up with a substantially

longlived lo ops The primary cause for formation

new approach to the classic distancevector routing

of routing lo ops is that no des cho ose their nexthops in

a completely distributed fashion based on information

Overview of Routing Metho ds

which can p ossibly b e stale and therefore incorrect

In our environment the problem of routing is essen

Almost all prop osed mo dications to DBF algorithm

tially the distributed version of the shortest path prob

eliminate the lo oping problem by forcing all

lem Eachnodeinthenetwork maintains for each

no des in the network to participate in some form of in

destination a preferred neighb or Each data packet con

terno dal co ordination proto col Suchinterno dal co ordi

tains a destination no de identier in its header When

nation mechanisms might b e eective when top ological

a no de receives a data packet it forwards the packet

changes are rare However within an adho c mobile en

to the preferred neighb or for its destination The for

vironment enforcing anysuchinterno dal co ordination

warding pro cess continues until the packet reaches its

mechanism will b e dicult due to the rapidly changing

destination The manner in which routing tables are

top ology of the underlying routing network

constructed maintained and up dated diers from one

Simplicity is one of the primary attributes which

routing metho d to another Popular routing metho ds

makes any routing proto col preferred over others for

however attempt to achieve the common ob jectiveof

implementation within op erational networks RIP

routing packets along the optimal path The nexthop

is a classical example Despite the countingtoinnity

routing metho ds can b e categorized into two primary

problem it has proven to b e very successful within small

classes linkstate and distancevector

size internetworks The usefulness of RIP within adho c

environment however is limited as it was not designed

to handle rapid top ological changes Furthermore the LinkState The linkstate approach is closer to the

techniques of splithorizon and poisonedreverse are centralized version of the shortest path computation

tween arrival of the rst and the arrival of the best route not useful within the wireless environment due to the

for each particular destination Based on this data a broadcast nature of the transmission medium Our de

decision may b e made to delayadvertising routes which sign goal therefore has b een to design a routing metho d

are ab out to change so on thus damping uctuations for adho c networks whichpreserves the simplicityof

of the route tables The advertisementofrouteswhich RIPyet at the same time avoids the lo oping problem

maynothave stabilized yet is delayed in order to reduce Our approachistotageach route table entry with a

the numb er of rebroadcasts of p ossible route entries that sequence numb er so that no des can quickly distinguish

normally arrive with the same sequence number stale routes from the new ones and thus avoid formation

of routing lo ops

The DSDV proto col requires each mobile station to

advertise to each of its current neighb ors its own rout

DestinationSequenced Distance Vec

ing table for instance by broadcasting its entries The

tor DSDV Proto col

entries in this list maychange fairly dynamically over

time so the advertisementmust b e made often enough

Our prop osed routing metho d allows a collection of

to ensure that every mobile computer can almost al

mobile computers whichmay not b e close to anybase

ways lo cate every other mobile computer of the collec

station and can exchange data along changing and arbi

tion In addition each mobile computer agrees to relay

trary paths of interconnection to aord all computers

data packets to other computers up on request This

among their numb er a p ossibly multihop path along

agreement places a premium on the ability to deter

which data can b e exchanged In addition our solution

mine the shortest numb er of hops for a route to a desti

must remain compatible with op eration in cases where

nation wewould liketoavoid unnecessarily disturbing

a base station is available By the metho ds outlined in

mobile hosts if they are in sleep mo de In this wayamo

this pap er not only will routing b e seen to solvethe

bile computer may exchange data with any other mobile

problems asso ciated with adho c networks but in ad

computer in the group even if the target of the data is

dition we will describ e ways to p erform such routing

not within range for direct communication If the noti

functions at Layer which traditionally has not b een

cation of which other mobile computers are accessible

utilized as a proto col level for routing

from any particular computer in the collection is done

Packets are transmitted b etween the stations of the

at layer then DSDVwillwork with whatever higher

network by using routing tables which are stored at

layer eg Network Layer proto col might b e in use

each station of the network Each routing table at each

All the computers interop erating to create data paths

of the stations lists all available destinations and the

between themselves broadcast the necessary data p e

numb er of hops to each Each route table entry is tagged

rio dicallysay once every few seconds In a wireless

with a sequence numb er which is originated by the des

medium it is imp ortanttokeep in mind that broad

tination station To maintain the consistency of routing

casts are limited in range bythephysical characteris

tables in a dynamically varying top ologyeach station

tics of the medium This is dierent than the situation

p erio dically transmits up dates and transmits up dates

with wired media which usually haveamuchmorewell

immediately when signicant new information is avail

dened range of reception The data broadcast byeach

able Since we do not assume that the mobile hosts

mobile computer will contain its new sequence number

are maintaining any sortoftimesynchronization we

and the following information for each new route

also make no assumption ab out the phase relationship

of the up date p erio ds b etween the mobile hosts These

The destinations address

packets indicate which stations are accessible from each

station and the numb er of hops necessary to reach these

The numb er of hops required to reach the destina

accessible stations as is often done in distancevector

tion and

routing algorithms It is not the purp ose of this pap er to

prop ose any new metrics for route selection other than

The sequence numb er of the information received

the freshness of the sequence numb ers asso ciated with

regarding that destination as originally stamp ed

the route cost or other metrics might easily replace the

by the destination

numb er of hops in other implementations The packets

may b e transmitted containing either layer MAC

The transmitted routing tables will also contain the

addresses or layer network addresses

hardware address and if appropriate the network ad

dress of the mobile computer transmitting them within Routing information is advertised by broadcasting or

the headers of the packet The routing table will also multicasting the packets which are transmitted p erio d

include a sequence numb er created by the transmitter ically and incrementally as top ological changes are de

Routes with more recent sequence numb ers are always tected for instance when stations movewithinthe

preferred as the basis for making forwarding decisions network Data is also kept ab out the length of time b e

casts of the routing information packets In order to re but not necessarily advertised Of the paths with the

duce the amount of information carried in these packets same sequence numb er those with the smallest metric

twotyp es will b e dened One will carry all the avail will b e used By the natural way in which the routing

able routing information called a full dump The tables are propagated the sequence number is senttoall

other typ e will carry only information changed since mobile computers whichmayeach decide to maintain a

the last full dump called an incremental By design routing entry for that originating mobile computer

an incremental routing up date should t in one net

Routes received in broadcasts are also advertised by

work proto col data unit NPDU The full dump will

the receiver when it subsequently broadcasts its routing

most likely require multiple NPDUs even for relatively

information the receiver adds an increment to the met

small p opulations of Mobile Hosts Full dumps can b e

ric b efore advertising the route since incoming pack

transmitted relatively infrequently when no movement

ets will require one more hop to reach the destination

of Mobile Hosts is o ccurring When movement b ecomes

namely the hop from the transmitter to the receiver

frequent and the size of an incremental approaches the

Again we do not explicitly consider here the changes

size of a NPDU then a full dump can b e scheduled so

required to use metrics which do not use the hop count

that the next incremental will b e smaller It is exp ected

to the destination

that mobile no des will implement some means for de

termining which route changes are signicant enough to One of the most imp ortant parameters to b e chosen is

be sent out with each incremental advertisement For the time b etween broadcasting the routing information

instance when a stabilized route shows a dierentmet packets However when any new or substantially mo di

ric for some destination that would likely constitute ed route information is received by a Mobile Host the

a signicantchange that needed to b e advertised after new information will b e retransmitted so on sub ject to

stabilization If a new sequence numberforarouteis constraints imp osed for damping route uctuations ef

received but the metric stays the same that would b e fecting the most rapid p ossible dissemination of routing

unlikely to b e considered as a signicantchange information among all the co op erating Mobile Hosts

This quick rebroadcast intro duces a new requirement

When a Mobile Host receives new routing informa

for our proto cols to converge as so on as p ossible It

tion usually in an incremental packet as just describ ed

would b e calamitous if the movement of a Mobile Host

that information is compared to the information al

caused a storm of broadcasts degrading the availability

ready available from previous routing information pack

of the wireless medium

ets Any route with a more recent sequence number

is used Routes with older sequence numb ers are dis Mobile Hosts cause broken links as they move from

carded A route with a sequence numb er equal to an place to place The broken link may b e detected by

existing route is chosen if it has a b etter metric and the layer proto col or it may instead b e inferred if no

the existing route discarded or stored as less prefer broadcasts have b een received for a while from a for

able The metrics for routes chosen from the newly mer neighb or A broken link is describ ed by a metric of

received broadcast information are each incremented ie anyvalue greater than the maximum allowed

by one hop Newly recorded routes are scheduled for metric When a link to a next hop has broken any

immediate advertisement to the current Mobile Hosts route through that next hop is immediately assigned an

neighb ors Routes whichshowanimproved metric are metric and assigned an up dated sequence numb er

scheduled for advertisement at a time which dep ends Since this qualies as a substantial route change such

on the average settling time for routes to the particular mo died routes are immediately disclosed in a broad

destination under consideration cast routing information packet Building information

to describ e broken links is the only situation when the

Timing skews b etween the various Mobile Hosts are

sequence numberisgeneratedbyany Mobile Host other

exp ected The broadcasts of routing information bythe

than the destination Mobile Host Sequence numb ers

Mobile Hosts are to b e regarded as somewhat asyn

dened by the originating Mobile Hosts are dened to

chronous events even though some regularityisex

be even numb ers and sequence numb ers generated to

p ected In such a p opulation of indep endently trans

indicate metrics are o dd numb ers In this wayany

mitting agents some uctuation could develop using the

real sequence numb ers will sup ersede an metric

ab ove pro cedures for up dating routes It could turn out

When a no de receives an metric and it has a later

that a particular Mobile Host would receive new routing

sequence numb er with a nite metric it triggers a route

information in a pattern which causes it to consistently

up date broadcast to disseminate the imp ortantnews

change routes from one next hop to another even when

ab out that destination

the destination Mobile Host has not moved This hap

In a very large p opulation of Mobile Hosts adjust p ens b ecause there are twoways for new routes to b e

ments will likely b e made in the time b etween broad chosen they mighthave a later sequence number or

vertise whichLayer proto cols it supp orts and each they mighthave a b etter metric A Mobile Host could

Mobile Host advertising reachability to that destina conceivably always receivetwo routes to the same des

tion would include along with the advertisement the tination with a newer sequence numb er one after an

information ab out the Layer proto cols supp orted at other via dierent neighb ors but always get the route

that destination This information would only haveto with the worse metric rst Unless care is taken this

b e transmitted when it changes which o ccurs rarely will lead to a continuing burst of new route transmittals

Changes would b e transmitted as part of each incremen up on every new sequence numb er from that destination

tal dump Since each Mobile Host could supp ort several Each new metric is propagated to every Mobile Host in

Layer proto cols and many will this list would have the neighb orho o d which propagates to their neighb ors

to b e variable in length and so on

One solution is to delaytheadvertisementofsuch

Extending Base Station Coverage

routes when a Mobile Host can determine that a route

Mobile computers will frequently b e used in conjunc

with a b etter metric is likely to showupsoon The

tion with base stations whichallowthemtoexchange

route with the later sequence number must b e available

data with other computers connected to the wired net

for use but it do es not havetobeadvertised imme

work By participating in the DSDV proto col base

diately unless it is a route to a destination whichwas

stations can extend their coverage b eyond the range

previously unreachable Thus there will b e tworout

imp osed by their wireless transmitters When a base

ing tables keptateach Mobile Host one for use with

station participates in DSDV it is shown as a default

forwarding packets and another to b e advertised via

route in the tables transmitted by a mobile station In

incremental routing information packets To determine

this way mobile stations within range of a base station

the probability of imminentarrival of routing informa

can co op erate to eectively extend the range of the base

tion showing a b etter metric the Mobile Host has to

station to serve other stations outside the range of the

keep a history of the weighted average time that routes

base station as long as those other mobile stations are

to a particular destination uctuate until the route with

close to some other mobile station that is within range

the b est metric is received We hop e that suchapro

cedure will allow us to predict howlongtowait b efore

Examples of DSDV in op eration

advertising new routes

Op erating DSDVatLayer MH MH

3 4 MH

The addresses stored in the routing tables will cor 5

resp ond to the layer at which this adho c networking

proto col is op erated That is op eration at Layer will

MH

work layer addresses for the next hop and des use net MH 8

2 MH6

tination addresses and op eration at Layer will use

Layer Media Access Control MAC addresses

MH MH

C addresses for the forwarding table do es

Using MA 1 7

intro duce a new requirement however The diculty

MH1

is that Layer network proto cols provide communica

tion based on network addresses and a waymust b e

provided to resolvetheseLayer addresses into MAC

addresses Otherwise a multiplicity of dierent address

Figure Movement in an adho c network

resolution mechanisms would b e put into place and a

corresp onding loss of bandwidth in the wireless medium

Consider MH in Figure Table shows a p ossible

would b e observed whenever the resolution mechanisms

structure of the forwarding table whichismaintained



were utilized This could b e substantial since such

at MH Supp ose the address ofeach Mobile Host is

mechanisms would require broadcasts and retransmit

represented as MH Supp ose further that all sequence

i

ted broadcasts byevery Mobile Host in the adho c net

MH where MH sp ec numb ers are denoted SNNN

i i

work Thus unless sp ecial care is taken every address

ies the computer that created the sequence number

resolution mightlooklike a glitch in the normal op er

and SNNN is a sequence number value Also supp ose

ation of the network whichmaywell b e noticeable to

that there are entries for all other Mobile Hosts with se

any active users

MH b efore MH moves away quence numb ers SNNN

i 

from MH The install time eld helps determine when



The solution prop osed here for op eration at Layer



is to include Layer proto col information along with the

If DSDV is op erated at level then MH denotes the MAC

i

address otherwise it denotes a level address Layer information Each destination host would ad

Destination NextHop Metric Sequence number Install Flags Stable data

MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


Table Structure of the MH forwarding table

tion up dates until the next full dump o ccurs When to delete stale routes With our proto col the deletion

MH moved into the vicinityofMH and MH it of stale routes should rarely o ccur since the detection

  

triggered an immediate incremental routing informa of link breakages should propagate through the adho c

tion up date whichwas then broadcast to MH MH network immediatelyNevertheless we exp ect to con

 

having determined that signicant new routing infor tinue to monitor for the existence of stale routes and

mation had b een received also triggered an immediate take appropriate action

up date which carried along the new routing informa

From table one could surmise for instance that

tion for MH MH up on receiving this information



all the computers b ecame available to MH at ab out

would then broadcast it at every interval until the next

time for most of them is the same time since its install

full routing information dump At MH the incremen

ab out the same One could also surmise that none of the

tal advertised routing up date would have the form as

links b etween the computers were broken b ecause all of

shown in table

the sequence numb er elds have times with even digits

In this advertisement the information for MH in the units place Ptr MH would all b e p ointers

i

comes rst since it is doing the advertisement The to null structures b ecause there are not any routes in

information for MH comes next not b ecause it has a Figure which are likely to b e sup erseded or comp ete



lower address but b ecause MH is the only one which with other p ossible routes to any particular destination



has any signicant route changes aecting it As a gen

Table shows the structure of the advertised route

eral rule routes with changed metrics are rst included

table of MH

in each incremental packet The remaining space is used

to include those routes whose sequence numb ers have

changed

Destination Metric Sequence number

MH S MH

 

In this example one no de has changed its routing in

MH S MH

 

formation since it is in a new lo cation All no des have

MH S MH

 

transmitted new sequence numb ers recently If there

MH S MH

were to o many up dated sequence numb ers to t in a

MH S MH

 

single packet only the ones whichtwould b e trans

MH S MH

 

mitted These would b e selected with a view to fairly

MH S MH

 

transmitting them in their turn over several incremental

MH S MH

 

up date intervals There is no such required format for

the transmission of full routing information packets As

Table Advertised route table by MH

many packets are used as are needed and all available

information is transmitted The frequency of transmit

ting full up dates would b e reduced if the volume of data

b egan to consume a signicant fraction of the available

Now supp ose that MH moves into the general vicin



capacity of the medium

ityofMH and MH andaway from the others es

 

p ecially MH The new internal forwarding tables at



Damping Fluctuations

MH might then app ear as shown in table

The following describ es how the settling time table is Only the entry for MH shows a new metric but in



used to prevent uctuations of routing table entry ad the intervening time many new sequence numb er en

vertisements The general problem arises b ecause route tries have b een received The rst entry thus must b e

up dates are selected according to the following criteria advertised in subsequent incremental routing informa

Destination NextHop Metric Sequence number Install Flags Stable data

MH MH S MH T MH M Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


MH MH S MH T MH Ptr MH

  


Table MH forwarding table up dated

p ose that MH receives the higher metric next hop rst

Destination Metric Sequence number

and so on after gets another next hop with a lower met

MH S MH

ric but the same sequence numb er This could happ en

MH S MH

 

when there are a lot of Mobile Hosts transmitting their

MH S MH

 

up dates not quite regularly Alternatively if the Mo

MH S MH

 

bile Hosts are acting indep endently and with markedly

MH S MH

 

dierent transmission intervals the situation could o c

MH S MH

 

cur with corresp ondingly fewer hosts Supp ose in any

MH S MH

 

event in Figure that there are enough Mobile Hosts to

MH S MH

 

cause the problem in two separate collections of Mobile

Hosts b oth connected to a common destination MH

Table MH advertised table up dated

but with no other Mobile Hosts in common Supp ose

further that all Mobile Hosts are transmitting up dates

approximately every seconds that Mobile Host MH



has a route to MH with hops and Mobile Host

Routes are always preferred if the sequence num

MH has a route to MH with hops Moreover sup



b ers are newer

p ose that the routing information up date from MH



Otherwise routes are preferred if the sequence

arrives at MH approximately seconds b efore the

numb ers are the same and yet the metric is b et

routing information up date from MH This will o ccur



ter lower

every time that a new sequence numb er is issued from

Mobile Host MH In fact the time dierential can b e

y Mobile Host in collection I I b egins to is

MH drastic if an

umb er up dates in multiple incremental 9 sue its sequence n

up date intervals as would happ en for instance when

there are to o many hosts with new sequence number

up dates for them all to t within a single incremental

ket up date In general the larger the number of

Mobile Host Mobile Host pac

tials b etween delivery of

Collection I Collection I I hops the more drastic dieren

the up dates can b e exp ected in Figure

The settling time data is stored in a table with the

following elds keyed by the rst eld MH MH

2 6

Destination address

MH Last settling time

4

Average settling time

Figure Receiving uctuating routes

To see the problem supp ose that two routes with The settling time is calculated bymaintaining a run

identical sequence numb ers are received by a Mobile ning weighted average over the most recent up dates of

Host but in the wrong order In other words sup the routes for each destination

more stale routing entries but would also allow for more Supp ose a new routing information up date arrives

transmission errors Transmission errors are likely to at MH The sequence numb er in the new entry is

o ccur when a CSMAtyp e broadcast medium is used the same as the sequence numb er in the currently used

as maywell b e the case for many wireless implementa entry and the newer entry has a worse ie higher

tions When the link breaks an metric route should metric Then MH must use the new entry in making

be advertised for it as well as for the routes that dep end subsequent forwarding decisions However MH do es

on it not havetoadvertise the new route immediately and

can consult its route settling time table to decide how

There are additional data elds other than those

long to wait b efore advertising it The average settling

stated ab ove which might b e transmitted as part of

time is used for this determination For instance MH

eachentry in the routing tables which are broadcast by

may decide to delayaverage settling time x b efore

each participating computer mobile or base station

advertising a route

These elds may dep end for instance on higher level

proto cols or other proto cols dep ending on the op eration This can b e quite b enecial b ecause if the p ossi

of layer For instance to enable correct ARP op era bly unstable route were advertised immediately the ef

tion each routing table entry must also contain an asso fects would ripple through the network and this bad

ciation b etween the Internet Proto col IP address and eect would probably b e rep eated every time Mobile

the MAC address of the destination This would also Host MH s sequence numb er up dates rippled through

enable an intermediate computer when serving a rout the adho c network On the other hand if a link via

ing function for its neighb ors to also issue proxy ARP Mobile Host MH truly do es break the advertisement



replies instead of routing ARP broadcasts around How of a route via MH should pro ceed immediatelyTo



ever if packet forwarding is based on MAC addresses achieve this when there is a history of uctuations at

hop efully such techniques will b e unnecessary And if Mobile Host MH the link breakage should b e detected

forwarding is done based on IP addresses no ARP is fast enough so that an intermediate host in Collection I I

strictly necessary as long as neighb oring no des keep nds out the problem and b egins a triggered incremen

track of asso ciations gleaned from route table broad tal up date showing an metric for the path along the

casts Note also that layer op eration violates the nor way to Mobile Host MH Routes with an metric are

mal subnet mo del of op eration since even if two Mobile required by this proto col to b e advertised immediately

Hosts share the same subnet address there is no guar without delay

antee they will b e directly connected in other words

In order to bias the damping mechanism in favor of

within range of each other This is compatible with the

recentevents the most recent measurement of the set

mo del of op eration oered by the mobileIP working

tling time of a particular route must b e counted with

group of the IETF

a higher weighting factor than are less recent measure

The new routing algorithm was particularly devel ments And imp ortantly a parameter must b e selected

op ed for enabling the creation of adhoc networks which which indicates how long a route has to remain stable

are most sp ecically targeted for the op eration of mo b efore it is counted as truly stable This amounts to

bile computers However the routing algorithm itself sp ecifying a maximum value for the settling time for

and the op eration of an adho c network can b e b ene the destination in the settling time table Anyroute

cially used in situations which do not include mobile more stable than this maximum value will cause a trig

computers For instance the routing algorithm could b e gered up date if it is ever replaced by another route with

applied in any situation where reduced memory require a dierent next hop or metric

ments are desired compared to linkstate routing algo

When a new routing up date is received from a neigh

rithms The op eration of an adhoc network could b e

b or during the same time that the up dates are applied

applied to wired as well as wireless mobile computers In

to the table pro cessing also o ccurs to delete stale en

general then we provide a new destinationsequenced

tries Stale entries are dened to b e those for which

routing algorithm and this algorithm is supplemented

no up date has b een applied within the last few up date

by a technique for damping uctuations

p erio ds Each neighb or is exp ected to send regular up

dates when no up dates are receivedforawhilethe

Prop erties of the DSDV Proto col

receiver may make the determination that the corre

sp onding computer is no longer a neighb or When that

At all instants the DSDV proto col guarantees lo op

o ccurs any route using that computer as a next hop

free paths to each destination Toseewhy this prop erty

should b e deleted including the route indicating that

holds consider a collection of N mobile hosts forming

computer as the actual formerly neighb oring destina

an instance of an adho c style network Further assume

tion Increasing the numberofupdateperiodsthatmay

that the system is in steadystate ie routing tables

transpire b efore entries are determined would result in

of all no des have already converged to the actual short

Comparison with other Metho ds est paths At this instant the next no de indicators to

each destination induce a tree ro oted at that destina

tion Thus routing tables of all no des in the network

The table presents a quick summary of some of the

can b e collectively visualized as forming N trees one

main features of a few chosen routing proto cols The

ro oted at each destination In the following discussion

chosen set although small is representativeofavari

well fo cus our attention on one sp ecic destination x

ety of routing techniques most commonly employed in

and follow the changes o ccurring on the directed graph

op erational data networks Except for the linkstate

x x

Gx dened by no des i and arcs i p where p de

i i

approach all routing metho ds shown in the table are

notes the nexthop for destination x at no de i Op er

avariant of the basic distancevector approach The

ation of DSDV algorithm ensures that at every instant

comparison criteria reects some of the most desirable

Gx is lo opfree or rather it is a set of disjointdi

features that a routing algorithm should p ossess for it

rected trees Each such tree is ro oted either at x or at

b e useful in a dynamic adho c style environment In

a no de whose nexthop is nil Since this prop ertyholds

wireless medium communication bandwidth is the most

with resp ect to each destination x all paths induced by

precious and scarce resource The formation of any kind

routing tables of DSDV algorithm are indeed lo op free

of routing lo ops therefore is highly undesirable In

at all instants

case of infrared LANS which employ pure CSMA pro

to col lo oping packets can not only consume the com

Potentially a lo op may form each time no de i changes

munication bandwidth but they can further degrade the

its nexthop This can happ en in two cases First when

p erformance by causing more collisions in the medium

no de i detects that the link to its nexthop is broken the

x

A common technique employed for lo opprevention is

to nil Clearly this action cannot form no de resets p

i

what we call internodalcoordination whereby strong

aloopinvolving i The second scenario o ccurs when

constraints on the ordering of the up dates among no des

no de i receives from one of its neighbors k a route

x

is imp osed The resulting interno de proto cols tend to b e

and metric m whichis to x with sequence number s

k

x

complex Furthermore their up date co ordination may

selected to replace the current route it has through p

i

x

restrict a no des ability to obtain alternate paths quickly

denote the value of the sequence numb er stored Let s

i

x

in an environment where top ology changes are relatively

denote the distance estimate from i to at no de i and d

i

frequent The last criteria used for comparison is the

x just prior to receiving route from k i will change its

x

space requirement of the routing metho d No des in

nexthop from p to k only if either of the following two

i

an adho c network may b e battery p owered laptops

happ ens

or even handheld noteb o oks which do not havethe

kind of memory that NSFNET dedicated routers are

the new route contains a newer sequence numb er

x x

exp ected to have Therefore economy of space is of

s ie s

i k

imp ortance

x x

but the new the sequence number s is same as s

i

k

The primary concern with using a Distributed Bell

route oers a shorter path to x ie md

ix

man Ford algorithm in adho c environment is its suscep

tibilitytowards forming routing lo ops and countingto

In the rst case bycho osing k as its new nexthop

innity problem RIP whichisvery similar to DBF

no de i cannot close a lo op This can b e easily deduced

algorithm also suers from the same problem Unlike

from the following observation A no de i propagates se

DBF RIP only keeps track of the b est route to each

x

to its neighb ors only after receiving quence number s

i

destination which results in some space saving at no

it from its current nexthop Therefore at all times the

extra p erformance hit RIP also employs techniques

sequence number value stored at the nexthop is always

known as splithorizon and poisonedreverse to avoid a

greater or equal to the value stored at i Starting from

pingp ong style of lo oping but these techniques are not

no de iifwe followthechain of nexthop p ointers the

powerful enough to avoid lo ops involving more than two

sequence number values stored at visited no des would

hops The primary cause of lo op formation in BF style

form a nondecreasing sequence Now supp ose no de i

algorithms is that no des make unco ordinated mo dica

forms a lo op bycho osing k as its nexthop This would

tions to their routing tables based on some information

x x

imply that s s But this contradicts our initial

i

k

which could b e incorrect This problem is alleviated

x x

assumption that s s Hence lo opformation can

i

k

by employing an interno dal co ordination mechanism as

not o ccur if no des use newer sequence numb ers to pick

prop osed by Merlin and Segall in A similar tech

routes

nique but with b etter convergence results is develop ed

by Jae and Moss in However wedonotknowof The lo opfree prop erty holds in the second scenario

any op erational routing proto cols which employthese due to the theorem proved in which states that in

complex co ordination metho ds to achieve lo opfreedom presence of static or decreasing link weights distance

which leads us to the conclusion that from a practical vector algorithms always maintain lo opfree paths

Routing Metho d Lo oping Interno dal Space

Co ordination Complexity

Bellman Ford sl O nd



Link State s O n

Lo opfree BF s O nd

RIP sl O n

Merlin Segall lo op free Required O nd

Jae Moss lo op free Required O nd

DSDV lo op free O n

s short term lo op l long term lo op

n number of nodes d maximum degree of a no de

Table Comparison of various routing metho ds

p oint of view the usefulness of such complex metho ds is Full up date p erio d

diminished

Settling time applied to triggered up dates

Linkstate algorithms are also free of the counting

toinnity problem However they need to maintain

Note that the measurement of the convergence times

the uptodate version of the entire network top ology

may dep end heavily on manyinteresting parameters

at every no de whichmay constitute excessive storage

suchastheaverage velo city of the mobile hosts up date

and communicationoverhead in a highly dynamic net

p erio ds size of the mobile host p opulation geographical

work Besides linkstate algorithms prop osed or imple

placement of mobile hosts existence of base stations

mented to date do not eliminate the creation of temp o

and average pro cessing loads at the mobile computers

rary routinglo ops

Summary

It is evident that within adho c environment design

tradeos and the constraints under which a routing

Providing convenient connectivity for mobile com

metho d has to op erate are quite dierent The pro

puters in adho c computers is a challenge that is only

p osed DSDV approach oers a very attractivecombi

now b eing met Wehavepresented an innovativeap

nation of desirable features Its memory requirementis

proach DSDV which mo dels the mobile computers as

very mo derate O n It guarantees lo opfree paths at all

routers which are co op erating to forward packets as

instants and it do es so without requiring no des to par

needed to each other We make go o d use of the prop

ticipate in any complex up date co ordination proto col

erties of the wireless broadcast medium Our approach

The worst case convergence b ehavior of the DSDVpro

can b e utilized at either the network layer layer or

to col is certainly nonoptimal but in the average case

b elow the network layer but still ab ove the MAClayer

it is exp ected that convergence will b e quite rapid We

software in layer In the latter case certain additional

are in the pro cess of conducting simulation studies to

information should b e included along with the routing

verify this claim

tables for the most convenient and ecient op eration

The information in the routing tables is similar to what

Current Status and Future Work

is found in routing tables with to days distance vec

tor BellmanFord algorithms but includes a sequence

Wehave implemented a preliminary version of this

numb er as well as settlingtime data useful for damping

proto col for use with mobile computers in our lab Cur

out uctuations in route table up dates

rently we are making necessary mo dications to the

MARS simulator for use in creating the appropria

All sequence numb ers are generated by the destina

tion simulation environment for our needs We hop e to

tion computer in each route table entry except for the

discover go o d op erational values via simulation for the

cases when a link has b een broken the latter case is de

following quantities

scrib ed byan metric and a sequence number which

cannot b e correctly generated byany destination com

puter Wehave sp ecied that metric route entries

Average convergence times

have sequence numb ers whichareoddnumbers and

sequence numb ers generated byeach destination com

Incremental up date p erio d

puter is an even numb er By the natural op eration of

the proto col the sequence numbers chosen to represent Settling time averaging metho d

broken links will b e sup erseded by real routes propa routing proto col without b ouncing eect In ACM

gated from the newly lo cated destination as so on as SIGCOMM pages Septemb er

p ossible Any newly propagated routes will necessarily

Wim Diepstraten Greg Ennis and Phil Be

use a sequence numb er greater than what was used for

langer DFWMAC Distributed Foundation Wire

the broken link since the latter sequence number is cho

less Medium Access Control IEEE Do cument

sen to b e one more than the last valid routes sequence

P Nov

numb er This allows real route data to quickly sup er

sede temp orary link outages when a mobile computer

C Hedrick Routing Information Proto col RFC

moves from one place to another

June

Wehaveborrowed the existing mechanism of trig

J M Jae and FH Moss A resp onsive dis

gered up dates to make sure that p ertinent route table

tributed routing algorithm for computer networks

changes can b e propagated throughout the p opulation

IEEE Transactions on Communications COM

of mobile hosts as quickly as p ossible whenever any

July

top ology changes are noticed This includes movement

from place to place as well as the disapp earance of a

J J Garcia LunaAceves A unied approach

mobile host from the interconnect top ology p erhaps as

to lo opfree routing using distance vectors or link

a result of turning o its p ower

states In ACM SIGCOMM pages

In order to combat problems arising with large p op

J M McQuillan I Richer and E C Rosen

ulations of mobile hosts which can cause route up dates

The new routing algorithm for the ARPANET

to b e received in an order delaying the b est metrics until

IEEE Transactions on Communications COM

after p o orer metric routes are received wehave sepa

May

rated the route tables into two distinct structures The

actual routing is done according to information kept P M Merlin and A Segall A failsafe distributed

in the internal route table but this information is not routing proto col IEEE Transactions on Commu

always advertised immediately up on receipt Wehave nications COM September

dened a mechanism whereby routes are not advertised

Charles Perkins Mobile IP as seen by the IETF

until it is likely based up on past history that they are

Connexions pages Mar

stable This measurement of the settling time for each

route is b e biased towards the most recent measure

M Schwartz and TE Stern Routing tech

ments for the purp oses of computing an average

niques used in computer communication networks

IEEE Transactions on Communications COM

Wehave found that mobile computers mo deled

April

as routers can eectively co op erate to build adho c

networks We hop e to explore further the necessary

Bill Simpson draftietfmobileipproto coltxt

applicationlevel supp ort needed to automatically en

Draft RFC work in progress May

able use of the networklayer route capabilities to pro

vide simple access to conferencing and workplace to ols

for collab oration and information sharing

Acknowledgment

We thank the reviewers for their manyvaluable com

ments also thanks to Arvind Krishna and Josh Knight

for their suggestions and improvements

References

C Alaettinoglu K DussaZieger I Matta A U

Shankar and O Gudmundsson Intro ducing mars

a routing testb ed ACM Computer Communication

Review

D Bertsekas and R Gallager Data Networks

pages PrenticeHall Inc

C Cheng R RileySP R Kumar and J J

GarciaLunaAceves A lo opfree BellmanFord