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NAT-Based Internet Connectivity for Multi-Homed On-Demand Ad Hoc Networks Paal Engelstad and Geir Egeland University of Oslo (UniK) / Telenor R&D, 1331 Fornebu, Norway Presented by: Paal Engelstad http://www.unik.no/~paalee/PhD.htm 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 2 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 3 Background (1): MIP-FA External Host Overview Internet – A gateway with FA-support (MIPFA) which understands AODV – A MANET node with MIPv4 support – The MANET registers the MIP-FA Gateway with its Home Agent Home Agent Foreign Agent Drawbacks: – High complexity – MIP and AODV makes unsynchronized modifications to routing table – MIP requires global IPv4 addresses Source Node MANET 4 Background (2): NAT External Host Overview Internet Drawbacks – The well-known drawbacks with the use of NATs – Dynamic change of gateways must be solved by MIPv4 3 2 1 Advantages – Less complex, easy to implement and deploy – Does not rely on MIPv4 deployment and fixed IPv4 address Network Address Translator 4 Source Node MANET 5 Route Discovery with Proxy RREP F F External Host Source Node (SN) broadcasts a RREQ to establish route to External Host (XH) Gateway impersonates XH, by sending a RREP on behalf of XH. Internet – Uses XHs IP address as Source IP Address in RREP – This is a “Proxy RREP” F F Gateway 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”? 6 Source Node MANET RREQ: Route Request RREP: Route Reply XH: External Host NAT: Network Address Translation 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...) 7 Proxy RREPs and Multi Homing F External Host The Source Node (SN) broadcasts a RREQ to establish route to the external Host (XH) F Both gateways send a Proxy RREP on behalf of the XH F The Source Node forwards packets to XH using the route established by one of the Proxy RREPs. F The “winning” gateway forwards the packet to the XH 8 Internet NAT NAT Source Node MANET RREQ: Route Request RREP: Route Reply XH: External Host NAT: Network Address Translation Race Conditions - a route needs to be re-discovered ? F F F F F F External Host 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 9 Internet GW1 (NAT) GW2 (NAT) Source Node MANET RREQ: Route Request RREP: Route Reply XH: External Host GW: Gateway 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... 10 Test bed experiment (1) AODV-implementation by Uppsala University External Host – IEEE 802.11 – Linux – MAC-layer filtering Internet 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% 11 GW1 (NAT) GW2 (NAT) Intermediate Node MANET Source Node Share of RREPs received Test bed experiment (2) 11 Tmax [ms] 12 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 13 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) 50 % Session breakages/Data Packet Range 10 25 % 0% 500 Range 50 1000 1500 2000 2500 3000 3500 4000 Packet Transmission Interval (ms) 14 4500 5000 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) 50 % Share of Network Configurations 45 % 40 % 35 % 30 % Range 10m 25 % Range 50m 20 % 15 % 10 % 5% 0% 0% 20 % 40 % 60 % 80 % Session Breaks/Packet for different Network Configurations 15 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) 60 % Session breakages/Data Packet 50 % 40 % Range 10 30 % Range 50 20 % 10 % 0% 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 (50) (100) (150) (200) (250) (300) (350) (400) Size of Sides of Terrain Square (m) 16 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) 50 % 45 % Session breakages/Packet 40 % 35 % 30 % 25 % Range 10 20 % Range 50 15 % 10 % 5% 0% 0 (0) 1 (5) 2 (10) 3 (15) 4 (20) 5 (25) 6 (30) Max Random Speed (m/sec) 17 7 (35) 8 (40) 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) 18 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 19 Proposed working solution External Host 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 src=SN IP-payload dst=XH Inner IPheader src=SN src=SN IP-payload dst=GW1 dst=XH GW tunnels return traffic from XH to SN – This is necessary due to specifics in the AODV specification Intermediate Node MANET Source Node RREQ: Route Request RREP: Route Reply XH: External Host SN: Source Node 20 GW2 (NAT) GW1 (NAT) Outer IP- Inner IPheader header – GW decapsulates and forwards to XH Internet Questions?