© 2004 jörg ott · dirk kutscher why seamless? towards exploiting wlan-based intermittent...

© 2004 Jörg Ott · Dirk Kutscher

Why Seamless?Towards Exploiting WLAN-basedIntermittent Connectivity on the Road

Jörg Ott <[email protected]>

Dirk Kutscher <[email protected]>

TNC 2004 · Rhodes · 9 June 2004


Approaches to Seamless Connectivity Link Layer Mobility

Cellular Networks (GSM, GPRS): Intended for ubiquitous coverage Fundamental Service: low-bandwidth bit-pipe with transparent mobility Cell range rather large Disruptions are a every-day phenomenon

IP Layer Mobility Maintain communication relationships despite network layer mobility Handover delays

Dependent on many factors (RTT to HA etc) Vertical handovers likely to cause disruptions

Hybrid Networks Always-Best-Connected approach Utilise multiple network interfaces concurrently, select most appropriate dynamically Still no guarantee for non-disrupted service


Seamless Connectivity

100% coverage not achievableService disruptions, battery life, occasional obstacles, network saturationEconomics of service provisioning (mountains, tunnels, valleys, deserts, …)

Where available, not necessarily suitable (particularly cellular)Limited data rateCost structure

Does not address all use casesNomadic users: Switching locations with computer turned offLegal enforcement: e.g. on aircrafts

Finally: not needed for many applications!


Seamless Connectivity

100% coverage not achievableService disruptions, battery life, occasional obstacles, network saturationEconomics of service provisioning (mountains, tunnels, valleys, deserts, …)

Where available, not necessarily suitable (particularly cellular)Limited data rateCost structure

Does not address all use casesNomadic users: Switching locations with computer turned offLegal enforcement: e.g. on aircrafts

Finally: not needed for many applications!

Little point in striving for ubiquitous connectivity –service disruptions may occur anyway...

Disconnection Tolerant Networking


Disconnection Tolerant Networking

Successive periods with and without connectivityWell known: moving between office, home, hotels, Internet cafés, etc.Controlled long periods of connectivity, manually managed by user

Rather cumbersome to handle

Extreme case Drive-thru Internet: network access from vehicles Initially: rare and short connectivity periods, long periods without network

Ultimately: may potentially transition to approximate permanent connectivity

Limited influence by the mobile user, manual control not possible

Goal: conceal intermittent nature of connectivity from usersAutomate connectivity establishment, support applications In vehicles, on trains, … but also for commuting between home and office


Extreme Networking Scenario: Autobahn

1 km

6 km

4 km

2 min 3 min

0,5 min


System Architecture

ISP BISP A

Internet

Connectivity

Islands

Server


0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

Distance (m)

kbit

/s

180 km/h

120 km/h

80 km/h

Data Rate (TCP, 802.11b, plain AP)Entryphase

Productionphase

Exitphase


802.11b vs. 802.11g (High Gain Antenna)

0

2000

4000

6000

8000

10000

12000

Distance (m)

Da

ta r

ate

(k

bit

/s)

802.11b

802.11g

TCP transmission, 120 km/h, fixed mobile


802.11g Data Volume (120km/h)

0

5000

10000

15000

20000

25000

30000

35000

40000

0 80 160

240

320

400

480

560

640

720

800

880

960

1040

1120

1200

1280

1360

1440

1520

1600

1680

1760

Distance (m)

Vo

lum

e (

KB

)

25 – 30 MB

over ~1000m

in about 30 seconds


Architectural Implications

Concentrate data exchange on the production phaseMinimize activities during entry and exit phases:Lower 802.11 transmission rates and packet loss hurt overall performance

Perform autoconfiguration + authentication during the entry phaseWorkable with minimal effortAllows to make most of the production phase

Make short-lived connectivity useful for applicationsE.g. persistent transport across connectivity islandsTo exchange larger data volumes independent of the application

Application-specific support where necessary


Types of Applications Asynchronous Applications

Examples: Mail (SMTP, POP3), NetNewsBidirectional push and pull, usually initiated by mobile nodeSession-oriented communications, identifiable transactionsIndividual transactions may complete within one cloudTransactions can only be recovered as a whole

Access to information resources (WWW) Audio/Video Communication Distributed Object Synchronisation New Applications


System Architecture

ISP BISP A

Internet

ServerDrive-thru-Proxies

Drive-thru Client


Different Levels of Support

No SupportSessions must complete within connectivity windows

No SupportSessions must complete within connectivity windows

Persistent ConnectionsConcealing intermittent nature of connectivity

Persistent ConnectionsConcealing intermittent nature of connectivity

ALG servicesDrive-thru clients proxy and manage application transactions

ALG servicesDrive-thru clients proxy and manage application transactions

Advanced ALG servicesALG performs message prefetching and provides local message store

Advanced ALG servicesALG performs message prefetching and provides local message store


Application Example: E-Mail

E-mail well suited to Drive-thru environment

Sending e-mail straightforwardDrive-thru client as local SMTP relay server

Retrieving e-mail requires more thoughtOffline mode preferable

Allows for short session and automated message retrievalOnline mode more difficult

Requires persistent connections

Using POP3 as protocolCommonly supported by servers


Sample POP3 Operation

Client Server

+OK

USER foo

PASS bar

+OK

+OK

UIDL

+OK …

RETR 1

+OK 400 octets

<msg>

QUIT


Application Example

ISP BISP A

Internet

Drive-thru Proxy Server


Drive-thru POP3 Operation with ALG Support

Client ServerDrive-thru-Client Drive-thru-Proxy

Origin POP3serverMessage storeMessage store

(optional)


Application Example

ISP BISP A

Internet


Network AttachementIEEE 802.11 associationIP auto-configurationAuthentication

Must be automatic Dependent on hot-spot architecture

Network AttachementIEEE 802.11 associationIP auto-configurationAuthentication

Must be automatic Dependent on hot-spot architecture


Application Example

ISP BISP A

Internet


Application interactionInitiate sessions through local Drive-

thru client Access to e-mail with POP3

Drive-thru client provides application layer support for POP3 Careful transaction completion Monitors current connectivity status Forwards requests to actual POP3 server

via Drive-thru proxy

Application interactionInitiate sessions through local Drive-

thru client Access to e-mail with POP3

Drive-thru client provides application layer support for POP3 Careful transaction completion Monitors current connectivity status Forwards requests to actual POP3 server

via Drive-thru proxy


POP3 Operation: Client Initiates Retrieval


<connection setup> <PCMP setup><connection setup>

+OK+OK+OK

<auth><auth>

<auth>

+OK+OK

+OK


Application Example

ISP BISP A

Internet



Drive-thru POP3 Operation


+OK

<msg>

UIDL

+OK …

RETR 1

UIDL

+OK …

<msg>

RETR 2


Application Example

ISP BISP A

Internet



Application Example

ISP BISP A

Internet


Resuming interrupted sessionsRepeating outstanding and interrupted

POP3 requestsAnswering local POP3 requests

Resuming interrupted sessionsRepeating outstanding and interrupted

POP3 requestsAnswering local POP3 requests


Drive-thru POP3 Operation


RETR 1

+OK …

<msg>

UIDL+OK …

+OK …

+OK …<msg>

RETR 1

+OK …<msg>

RETR 2RETR 2

+OK …

<msg>


Application Example

ISP BISP A

Internet



Application Example

ISP BISP A

Internet


Terminating application sessions Removing ALG state Terminate Drive-thru session

Terminating application sessions Removing ALG state Terminate Drive-thru session


Application Example

ISP BISP A

Internet



Conclusion Questioning seamless connectivity

Rarely available, not always required

Disconnection Tolerant Networking Take intermittent connectivity into accountViable for many, but not for all application classesApproach: Provide different levels of infrastructure support

Drive-thru Internet approachTestbed for Disconnection Tolerant Networking with extreme characteristicsDrive-thru entities maintain persistent connections during disruptionsApplication-specific functions as optional enhancements


www.drive-thru-internet.org

© 2004 jörg ott · dirk kutscher why seamless? towards exploiting wlan-based intermittent...

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