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!

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

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)

DHCP Packet Format | op (1) | htype (1) | hlen (1) | hops (1) | | xid (4) | | secs (2) | flags (2) | | ciaddr (4) | | yiaddr (4) | | siaddr (4) | | giaddr (4) | | | chaddr (16) | |

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

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

Using a Relay Client Server giaddr : Relay The relay can be another host or a router

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

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

Network Model DHCP Server

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

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

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

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 IP masquerading and NAT complicated Best if ad-hoc nodes are on the same subnet as the DHCP server

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

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

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

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