DHCP for Multi-hop Wireless Ad-Hoc Networks

Presented by William List

Problem StatementDHCP is designed to work on the wired internet with bridges, switches and routersDHCP messages sent from a client are broadcast, not unicastNodes 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 serversServer responds with a DHCPOFFER message containing the new addressDHCPREQUEST and DHCPACK confirm the assignment

DHCP RelaysA DHCP relay will forward DHCP messages back and forth between a client and serverThe relay knows the address of the DHCP serverDHCP packets allow for only one relay (gateway)

DHCP Packet Format0 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) | | | | |+---------------------------------------------------------------+

DHCP Packet Fieldsciaddr : The client’s previous IP addressyiaddr : The address offered by the servergiaddr : Relay addresschaddr : Client’s hardware address

DHCP Message ExchangeDHCPDISCOVER

Hello!? I’d like an IP address please!

DHCPOFFERHow about 192.168.0.1?

DHCPREQUESTSounds good.

DHCPACKGlad to hear it. Have fun!

Client Server

Discover

Offer

Request

ACK

Using a RelayClient

Server

giaddr: 192.168.0.2

0.0.0.0

Relay

192.168.0.1

The relay can be another host or a router

SolutionMake every node in the network run the DHCP relay once configuredInclude the DHCP server IP address in AP beaconsUnicast messages towards DHCP server rather than broadcasting

AssumptionsEvery node in the network can reach the Access Point

Either directly or through multiple hops

DCHP server(s) assigns IP addressesAllows for network monitoring and accounting

Should be able to run over any ad-hoc routing protocol

Network Model

DHCP Server

192.168.0.128

192.168.0.1

192.168.0.129

192.168.0.130

0.0.0.0

Client NodeWaits for a beacon from an APOnce the IP address of the DHCP server is obtained:

Add an entry in routing table specifying same next-hop as for the associated APGenerate a DHCPREQUEST messageUnicast towards DHCP server

Neighboring NodesNext hop towards DHCP server receives the DHCP request from sourceThe giaddr field is filled in with the node’s IP addressPacket is forwarded to DHCP server

Intermediate NodesNodes between the designated relay node and the DHCP server

Are unaware of the DHCP messages and simply forwards packets back and forth

DHCP ServerAccess Point must know the IP address of the DHCP server (for beacons)DHCP server uses giaddr as the destination for responsesSource address must remain 0.0.0.0

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

Alternate ModelClient broadcasts DHCP messages

Do not need to wait for a beacon

Multiple neighbors serve as relaysClient might receive multiple responses from the same serverClient will select best offerRelays unicast to the DHCP server

Other AlternativesThe Access Point knows the location of the DHCP server

Client could unicast DHCP messages to AP

IssuesDHCP server probes for used addresses

ICMP ping packets will be forwarded by nodes in the ad hoc networkWhat 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

ConclusionDHCP fundamentally works as-is in a multi-hop environmentFuture Work

Run more scenariosAP could broadcast ping packets several times for the DHCP server, rather than just onceIntegration with a Internet gateway