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DHCP for Multi-hop Wireless Ad-Hoc Networks

DHCP for Multi-hop Wireless Ad-Hoc Networks. Presented by William List. Problem Statement. DHCP is designed to work on the wired internet with bridges, switches and routers DHCP messages sent from a client are broadcast, not unicast Nodes in an ad hoc network need to know what address to use!.

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DHCP for Multi-hop Wireless Ad-Hoc Networks

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  1. DHCP for Multi-hop Wireless Ad-Hoc Networks Presented by William List

  2. Problem Statement • DHCP is designed to work on the wired internet with bridges, switches and routers • DHCP messages sent from a client are broadcast, not unicast • Nodes in an ad hoc network need to know what address to use!

  3. DHCP Basics (RFC 2131) • When a client boots, it attempts to obtain a usable IP address • A DHCPDISCOVER message is broadcast to locate available DHCP servers • Server responds with a DHCPOFFER message containing the new address • DHCPREQUEST and DHCPACK confirm the assignment

  4. DHCP Relays • A DHCP relay will forward DHCP messages back and forth between a client and server • The relay knows the address of the DHCP server • DHCP packets allow for only one relay (gateway)

  5. DHCP Packet Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | op (1) | htype (1) | hlen (1) | hops (1) | +---------------+---------------+---------------+---------------+ | xid (4) | +-------------------------------+-------------------------------+ | secs (2) | flags (2) | +-------------------------------+-------------------------------+ | ciaddr (4) | +---------------------------------------------------------------+ | yiaddr (4) | +---------------------------------------------------------------+ | siaddr (4) | +---------------------------------------------------------------+ | giaddr (4) | +---------------------------------------------------------------+ | | | chaddr (16) | | | | | +---------------------------------------------------------------+

  6. DHCP Packet Fields • ciaddr : The client’s previous IP address • yiaddr : The address offered by the server • giaddr : Relay address • chaddr : Client’s hardware address

  7. DHCP Message Exchange • DHCPDISCOVER • Hello!? I’d like an IP address please! • DHCPOFFER • How about 192.168.0.1? • DHCPREQUEST • Sounds good. • DHCPACK • Glad to hear it. Have fun! Client Server Discover Offer Request ACK

  8. Using a Relay Client Relay Server 0.0.0.0 giaddr: 192.168.0.2 192.168.0.1 The relay can be another host or a router

  9. Solution • Make every node in the network run the DHCP relay once configured • Include the DHCP server IP address in AP beacons • Unicast messages towards DHCP server rather than broadcasting

  10. Assumptions • Every node in the network can reach the Access Point • Either directly or through multiple hops • DCHP server(s) assigns IP addresses • Allows for network monitoring and accounting • Should be able to run over any ad-hoc routing protocol

  11. Network Model 0.0.0.0 DHCP Server 192.168.0.130 192.168.0.1 192.168.0.129 192.168.0.128

  12. Client Node • Waits for a beacon from an AP • Once the IP address of the DHCP server is obtained: • Add an entry in routing table specifying same next-hop as for the associated AP • Generate a DHCPREQUEST message • Unicast towards DHCP server

  13. Neighboring Nodes • Next hop towards DHCP server receives the DHCP request from source • The giaddr field is filled in with the node’s IP address • Packet is forwarded to DHCP server

  14. Intermediate Nodes • Nodes between the designated relay node and the DHCP server • Are unaware of the DHCP messages and simply forwards packets back and forth

  15. DHCP Server • Access Point must know the IP address of the DHCP server (for beacons) • DHCP server uses giaddr as the destination for responses • Source address must remain 0.0.0.0 • IP masquerading and NAT complicated • Best if ad-hoc nodes are on the same subnet as the DHCP server

  16. Alternate Model • Client broadcasts DHCP messages • Do not need to wait for a beacon • Multiple neighbors serve as relays • Client might receive multiple responses from the same server • Client will select best offer • Relays unicast to the DHCP server

  17. Other Alternatives • The Access Point knows the location of the DHCP server • Client could unicast DHCP messages to AP

  18. Issues • DHCP server probes for used addresses • ICMP ping packets will be forwarded by nodes in the ad hoc network • What if the packet is dropped somewhere? • Broadcasting DHCP messages introduces added network traffic • Address assignment is only done once, unless the node is highly mobile

  19. Conclusion • DHCP fundamentally works as-is in a multi-hop environment • Future Work • Run more scenarios • AP could broadcast ping packets several times for the DHCP server, rather than just once • Integration with a Internet gateway

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