Analysis of NAT-Based Internet Connectivity for Multi-Homed
On-Demand Ad Hoc Networks Engelstad, P.E. and Egeland, G.
University of Oslo (UniK) / Telenor R&D, 1331 Fornebu, Norway
Presented by: Geir Egeland http://www.unik.no/~paalee/research.htm
CNDS 2004 (WMC 2004) San Diego, 22.01.2004 Slide 2 2 Motivation l
Already seeing users communicating with mobile terminals in an ad
hoc manner using Bluetooth (Bluejacking) l Mobile ad-hoc networks
(MANET) may need to connect to nodes in the fixed Internet Some
nodes connected to external IP-networks may operate as gateways for
other MANET nodes l Previously proposed solutions (proxy RREP):
MIP-FA based gateways making modifications to Mobile IPv4 and using
Ad- hoc On demand Distance Vector (AODV) Internet draft by
Belding-Royer et al. MSc. Thesis on MIPMANET by Alriksson F. And
Jnsson U., August 1999 NAT based gateways implementing an Network
Address Translator at the gateway Uppsala Universitys implementaton
of AODV Slide 3 3 Internet External Host Background (1): Ad-hoc on
demand Distance Vector (AODV) l Reactive ad-hoc routing protocol
Generates routes only when needed l Uses Route Request (RREQ) and
Route Reply (RREP) to form forward and return route l Maintains
routing tables at the nodes, so that data packets not have to
contain routes l A node in a MANET may want to connect to a host on
the Internet MANET Gateway Slide 4 4 Background (2): MIP-FA l
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 l Drawbacks High complexity MIP and
AODV makes unsynchronized modifications to routing table MIP
requires global IPv4 addresses l Advantages MANET nodes can use its
Home Address and be globally routable Internet Home Agent External
Host Foreign AgentGateway MANET Source Node Slide 5 5 Background
(3): NAT l Overview A gateway uses NAT to hide non-routable
addresses in MANET l Drawbacks The well-known drawbacks with the
use of NATs Mobility (i.e. Sessions through the gateway break when
the node moves to a new MANET) l Advantages Less complex, easy to
implement and deploy Does not rely on MIPv4 deployment and fixed
IPv4 address Internet External Host Network Address Translator
Gateway MANET Source Node 1 2 3 4 Slide 6 6 Route Discovery with
Proxy RREP l How gateways discover that the XH is present on the
Internet MIP-FA Gateway (Belding-Royer et.al.): Source Node sets
F-bit in RREQ AODV-UU NAT-solution: Require different IP address
spaces l Source Node (SN) broadcasts a RREQ to establish route to
External Host (XH) l Gateway impersonates XH, by sending a RREP on
behalf of XH. This is a Proxy RREP l SN forwards packets to XH
using the route established by the Proxy RREP. l The gateway
forwards the packet to XH Internet External Host Gateway (NAT)
Gateway MANET Source Node F F F F RREQ: Route Request RREP: Route
Reply XH: External Host NAT: Network Address Translation Slide 7 7
Proxy RREPs and Multi Homing l The Source Node (SN) broadcasts a
RREQ to establish route to the external Host (XH) l Both gateways
send a Proxy RREP on behalf of the XH l The Source Node forwards
packets to XH using the route established by one of the Proxy
RREPs. l The winning gateway forwards the packet to the XH Internet
External Host NAT MANET Source Node F RREQ: Route Request RREP:
Route Reply XH: External Host NAT: Network Address Translation NAT
F F F Slide 8 8 Race Conditions a route needs to be re-discovered l
The Source Node (SN) broadcasts a RREQ to establish route to the
external Host (XH) l Both gateways send a Proxy RREP on behalf of
the XH, GW1 wins l SN sends packets for XH via GW1. l After link
break or route timeout, SN broadcasts a new RREQ to re-establish
the route to XH l Both gateways send a Proxy RREP on behalf of XH,
but this time GW2 wins l SN sends subsequent packets for XH via
GW2, connection fails Internet External Host GW2 (NAT) MANET Source
Node F RREQ: Route Request RREP: Route Reply XH: External Host GW:
Gateway GW1 (NAT) F F F F F ? Slide 9 9 Test bed experiment (1) l
AODV-implementation by Uppsala University IEEE 802.11b Linux
(2.2.20 kernel) MAC-layer filtering Gateways with equal
configuration l Best performance: 14% of sessions break due to race
condition l Introduced a random delay from a uniform distribution
[0,T max ] ms in the GWs Share of sessions that breaks approx. 50%
Internet External Host GW2 (NAT) MANET Source Node GW1 (NAT)
Intermediate Node Slide 10 10 Test bed experiment (2) Share of
RREPs received 14 T max [ms] Slide 11 11 Simulation setup l
Glomosim, with AODV module l IEEE 802.11, Two-Ray channel model l
Traffic pattern: Constant Bit Rate (CBR), 1024 byte packets l 50
nodes Radio Range 50m, 200mx200m square Radio Range 10m, 40mx40m
square Slide 12 12 Simulation #1 l 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.). Slide 13 13 Simulation #2 l 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 with bad performance
Percentage of networks Slide 14 14 Simulation #3 l Testing effects
of terrain size (i.e. of node density or of strength of
connectivity): Fully connected network: Probability that session
breaks = 0.5 Problem decreases as terrain size increases, because
probability that gateways are an equal number of hops away,
decreases. Slide 15 15 Simulation #4 l 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 Slide 16 16 Summary of results l
Test bed experiment showed that race conditions occurs due to Proxy
RREPs l Simulations showed that race conditions reduce performance
in small on-demand ad hoc networks. l Race Conditions due to route
timeout represents a non-negligible problem, especially for
interactive applications where the packet transmission interval
easily exceeds the Active Route Timeout of AODV l Race Conditions
due to link breaks (e.g. caused by mobility, radio fading, etc.) is
a serious problem for all sessions, independent of packet
transmission intervals. Slide 17 17 Proposed working solution l SN
discovers that XH is not present locally after unsuccessful route
establishment on MANET l SN sets a Gateway bit in RREQ for XH l
Gateways responds with a RREP establishing route to the GW (i.e. no
race conditions will occur) l RREP contains extensions with XHs
destination IP-address The functionality/capabilities of the
gateway l SN tunnels traffic to selected GW GW decapsulates and
forwards to XH l GW tunnels return traffic from XH to SN Internet
External Host GW2 (NAT) MANET Source Node GW1 (NAT) Intermediate
Node src=SN dst=XH Inner IP- header Outer IP- header
IP-payloadsrc=SN dst=GW1 src=SN dst=XH Inner IP- header IP-payload
RREQ: Route Request RREP: Route Reply XH: External Host SN: Source
Node F F F F F F Slide 18 Slide 19 19 Route discovery in AODV A B H
S C E F I G K M L N J D Represents a node that has received RREQ
for D from S Slide 20 20 Route discovery in AODV A B H S C E F I G
K M L N J D Represents transmission of RREQ Broadcast transmission
Slide 21 21 Route discovery in AODV A B H S C E F I G K M L N J D
Represents links on Reverse Path Slide 22 22 Route discovery in
AODV A B H S C E F I G K M L N J D Node C receives RREQ from G and
H, but does not forward it again, because node C has already
forwarded RREQ once Slide 23 23 Route discovery in AODV A B H S C E
F I G K M L N J D Slide 24 24 Route discovery in AODV A B H S C E F
I G K M L N J D Slide 25 25 Route discovery in AODV A B H S C E F I
G K M L N J D Routing table entries used to forward data packet
Route is not included in packet
