NAT-Based Internet Connectivity for Multi-Homed On-Demand Ad Hoc Networks

NAT-Based Internet Connectivity for Multi-Homed On-Demand Ad Hoc Networks

Paal Engelstad and Geir EgelandUniversity of Oslo (UniK) / Telenor R&D, 1331 Fornebu, Norway

Presented by: Paal Engelstad

http://www.unik.no/~paalee/PhD.htm







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Motivation

Ad hoc networks need to access the fixed Internet– Some nodes connected to external IP-networks may operate as gateways for

other MANET nodes

Previously proposed solutions:– A gateway implementing Mobile IPv4 Foreign Agent (MIP-FA)

• Internet draft by Belding-Royer et al.

• MSc. Thesis on ”MIPMANET” by Alriksson and Jönsson, KTH, August 1999

– A gateway implementing a Network Address Translator (NAT)• Uppsala University’s implementaton of AODV

NAT-based solutions have yet been poorly documented in published material

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Assume you know AODV...

Short re-cap: A Source Node discovers route to destination on demand

– It floods an RREQ to find a route to a destination– The RREQ forms a return route on each node

The Destination node responds:– It unicasts an RREP along the reverse route– The RREP forms a forward route

Every node maintains its own destination sequence number– Incremented before the flooding– Ensures loop freedom

An intermediate node may reply to RREP on behalf of Destination node if it has a valid route to the destination

With multiple RREPs, the routing protocol prefers– RREPs with higher destination sequence numbers– Fewest hops between source and destination

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Background (1): MIP-FA

Overview– A gateway with FA-support (MIP-

FA) which understands AODV – A MANET node with MIPv4

support– The MANET registers the

MIP-FA Gateway with its Home Agent

Drawbacks:– High complexity– MIP and AODV makes

unsynchronized modifications to routing table

– MIP requires global IPv4 addresses

Internet

Home AgentExternal Host

Foreign Agent

MANETSource Node

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Background (2): NAT

Overview

Drawbacks– The well-known drawbacks with the

use of NATs

– Dynamic change of gateways must be solved by MIPv4

Advantages– Less complex, easy to implement and

deploy

– Does not rely on MIPv4 deployment and fixed IPv4 address

Internet

External Host

Network Address Translator

MANETSource Node

1

2 3

4

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Route Discovery with Proxy RREP

Source Node (SN) broadcasts a RREQ to establish route to External Host (XH)

Gateway impersonates XH, by sending a RREP on behalf of XH.

– Uses XHs IP address as Source IP Address in RREP

– This is a “Proxy RREP” SN forwards packets to XH using the

route established by the Proxy RREP. The gateway forwards the packet to

XH

How about the destination sequence number in a ”Proxy RREP”?

Internet

External Host

Gateway

MANETSource Node

RREQ: Route RequestRREP: Route ReplyXH: External HostNAT: Network Address Translation


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Destination Seqence numbers in Proxy RREP MIP-FA Gateway (Belding-Royer et.al.):

– Source Node normally sets RREQ with• Unknown Seqence Number bit = 1• Destination Sequence Number = 0

– Gateway copies this into the ”Proxy RREP” (i.e. a zero destination sequence number)

AODV-UU NAT-solution: – Use Gateway’s own destination sequence number (a hack)

– Require different IP address spaces• To distinguish internal from external nodes• Not acceptible or at least very limiting

We proposed a better NAT-solution with ”Proxy RREP”: – Implementing the MIP-FA policy (above)

– Ensure that an Internal node never uses a zero destination sequence number

– Hence, a real RREP from an internal MANET node always have preference over a Proxy RREP (i.e. no problem if gateway always send Proxy RREP...)

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Proxy RREPs and Multi Homing

The Source Node (SN) broadcasts a RREQ to establish route to the external Host (XH)

Both gateways send a Proxy RREP on behalf of the XH

The Source Node forwards packets to XH using the route established by one of the Proxy RREPs.

The “winning” gateway forwards the packet to the XH

Internet

External Host

NAT

MANETSource Node



NAT

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Race Conditions - a route needs to be re-discovered The Source Node (SN) broadcasts a RREQ

to establish route to the external Host (XH)

Both gateways send a Proxy RREP on behalf of the XH, GW1 wins

SN sends packets for XH via GW1.

After link break or route timeout, SN broadcasts a new RREQ to re-establish the route to XH

Both gateways send a Proxy RREP on behalf of XH, but this time GW2 “wins”

SN sends subsequent packets for XH via GW2, connection fails

Internet

External Host

GW2(NAT)

MANETSource Node

RREQ: Route RequestRREP: Route ReplyXH: External HostGW: Gateway

RREQ: Route RequestRREP: Route ReplyXH: External HostGW: Gateway

GW1(NAT)

?

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Demonstrating Race Conditions due to route re-discovery

Testbed experiment (i.e. lab implementation)– Fewer nodes, more static– Active Route Timeout (3 sec of AODV) triggers route re-discovery

Simulations– Many nodes, more mobility, etc...– Network dynamics (such as mobility) triggers route re-discovery

I will only go through the simulations if time permits...

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Test bed experiment (1) AODV-implementation by Uppsala

University– IEEE 802.11– Linux– MAC-layer filtering

Packet Transmission Interval– 1 sec:

• OK

– 4 sec: (e.g. interactive traffic, Telnet, etc...)• Race conditions

Best performance: 11% probability of (Telnet-) session breakage due to race condition

Increased random max ”processing time” (Tmax):

=> prob. -> 50%

Internet

External Host

GW2(NAT)

MANET

Source Node

GW1(NAT)

Intermediate Node

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Test bed experiment (2)

0 %

50 %

100 %

0 1 2 3 4 5

Tmax (ms)

From GW1

From GW2

Sha

re o

f RR

EP

s re

ceiv

ed

11

Tmax [ms]

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Simulation setup

Glomosim, with AODV module

IEEE 802.11, Two-Ray channel model

Traffic pattern: Constant Bit Rate (CBR), 1024 byte packets

50 nodes– Radio Range 50m, 200mx200m square

– Radio Range 10m, 40mx40m square

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Simulation #1 Testing Race Conditions due to Route Timeout:

– Static scenario, and varying Packet Transmission Interval (PTI):– Race Conditons have a dramatic impact on performance when PTI

exceeds Active Route Timeout of AODV (of 3 sec.).

Variable Packet Transmission Interval (with fixed route timeout, fixed terrain size and no mobility)

0 %

25 %

50 %

500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Packet Transmission Interval (ms)

Ses

sio

n b

reak

ages

/Dat

a P

acke

t

Range 10

Range 50

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Simulation #2 Network configurations/ topologies that leads to

bad performance?– When gateways are an equal number of hops away from SN– (i.e. on right hand side of figure...)

Distribution of different network configurations (with fixed terrain size and no mobility)

0 %

5 %

10 %

15 %

20 %

25 %

30 %

35 %

40 %

45 %

50 %

0 % 20 % 40 % 60 % 80 %

Session Breaks/Packet for different Network Configurations

Sh

are

of

Ne

two

rk C

on

fig

ura

tio

ns

Range 10m

Range 50m

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Simulation #3 Testing effects of terrain size (i.e. of node density or of

”strength” of connectivity):– Fully connected network: Probability of 50%

• Attributed to the ”ideal” model of Glomosim

– Problem decreases as terrain size increases, because probability that gateways are an equal number of hops away, decreases.

Variable Terrain Size(with fixed route timeout, 2Kbps CBR and no mobility)

0 %

10 %

20 %

30 %

40 %

50 %

60 %

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Size of Sides of Terrain Square (m)

Se

ss

ion

bre

ak

ag

es

/Da

ta P

ac

ke

t

Range 10

Range 50

(50) (400)(100) (350)(150) (200) (250) (300)

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Simulation #4 Testing Race Conditions due to link breaks, by adding mobility:

– Random Way Point (with zero rest-time and variable max velocity)– PTI = 1 sec, i.e. safely below the Active Route Timeout of AODV– See that problem increases rapidly to unacceptably high levels, even for relatively low

levels of mobility Other non-deterministic effects (radio-fading, packet collisions, etc.)

occuring in a MANET, and is not easily caught by a simulation model– This effecs will also accellerate the problems of Race Conditions

Variable Mobility(with fixed route timeout, CBR 8 Kbps - i.e.1pkt/sec - and fixed terrain size)

0 %

5 %

10 %

15 %

20 %

25 %

30 %

35 %

40 %

45 %

50 %

0 1 2 3 4 5 6 7 8

Max Random Speed (m/sec)

Ses

sio

n b

reak

ages

/Pac

ket

Range 10

Range 50

(5) (40)(35)(30)(10)(0) (20)(15) (25)

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Summary of results - I

Our work shows that race conditions due to Proxy RREPs can be damaging in on-demand ad hoc networks

– For smaller networks (testbed)– And for larger networks (simulations)

Race Conditions represents a non-negligible problem, especially for

– Interactive applications where the packet transmission interval easily exceeds the Active Route Timeout of AODV (testbed and simulations)

– Networks with certain level of dynamics and/or mobility (simulation)

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Summary of results - II

In the paper we propose mechanisms to remove the race conditions with “Proxy RREPs”:

– By making SNs aware of gateways– Breakdown: When 2 SNs communicate with same XH over different gateways

Although results are targeted at NAT-based gateways, they also have relevance to MIP-FA based solution

– We proposed a way to avoid race conditions with Proxy RREPs– However, the problem remains due to ingress filtering

Conclusion: Using proxy RREPs is NOT the way to go!– At least not for NAT-based gateways

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Proposed working solution

SN discovers that XH is not present locally after unsuccessful route establishment on MANET

SN sets a “Gateway bit” in RREQ for XH Gateways responds with a RREP

establishing route to the GW (i.e. no race conditions will occur)

RREP contains extensions with – XH’s destination IP address

– The functionality/capabilities of the gateway

SN tunnels traffic to selected GW– GW decapsulates and forwards to XH

GW tunnels return traffic from XH to SN

– This is necessary due to specifics in the AODV specification

Internet

External Host

GW2(NAT)

MANET

Source Node

GW1(NAT)

Intermediate Nodesrc=SNdst=XH

Inner IP-header

Outer IP-header

IP-payloadsrc=SNdst=GW1

src=SNdst=XH

Inner IP-header

IP-payload

RREQ: Route RequestRREP: Route ReplyXH: External HostSN: Source Node

RREQ: Route RequestRREP: Route ReplyXH: External HostSN: Source Node

Questions?

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NAT-Based Internet Connectivity for Multi-Homed On-Demand Ad Hoc Networks