Download DHCP for Wireless Ad

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
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)
|
|
|
|
|
+---------------------------------------------------------------+
DHCP Packet Fields
ciaddr
yiaddr
giaddr
chaddr
: The client’s previous IP address
: The address offered by the
server
: Relay address
: Client’s hardware address
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
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
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
0.0.0.0
DHCP Server
192.168.0.130
192.168.0.1
192.168.0.129
192.168.0.128
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 0.0.0.0
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