Passive DAD

1
Passive DAD

• Sangho Shin
• Andrea Forte
• Henning Schulzrinne
• Columbia University

2
Problems with the current DAD

• In wireless networks, it takes long time to get
  ICMP echo response, or even the response can be
  lost when the channel is very congested.
• Windows XP SP2 activates the firewall, and the
  firewall blocks incoming ICMP echo by default.
• ISC DHCP software has the bug in the DAD timer,
  and the timer value is randomly decided between 0
  1 sec.

3
Passive DAD - Architecture
Address Usage Collector (AUC)
DHCP server
Broadcast/ARP
Router/Relay Agent
SUBNET

• AUC collects all broadcast and ARP packets.
• AUC builds IPMAC pair table.
• Whenever a new pair is added to the table, the
  AUC sends the pair to the DHCP server.
• DHCP server checks if the pair is correct or not.

• ARP checking
• AUC scans unused IPs using ARP query
  periodically.
• Silent node can be detected.

4
Passive DAD Example
xxxxxxxxxxxx
DHCP server
AUC
xxxxxxxxxxxx
Lease info
IP128.59.19.46 MACAABBCCDDEE
IP128.59.19.46 MACAABBCCDDEE
Web server
IP128.59.19.46 MACAABBCCFFGG
Router
Block AABBCCFFGG
Force Renew
Forward HTTP traffic
ARP query
IP128.59.19.46
ARP query
IP128.59.19.46 MACAABBCCDDEE
IP128.59.19.46 MACAABBCCFFGG
5
Conclusions

• It takes long time to get an IP from DHCP server
  mostly because of DAD.
• The current DAD does not work because of Windows
  XP SP2.
• Passive DAD performs DAD without any overhead.
• Passive DAD detects IP address collision and
  illegally used IPs.
• When a address collision is detected, Passive DAD
  resolves the duplicate IP problem by using DHCP
  Force Renew (or VLAN banning).

6
Backup slides
7
Passive DAD Expiration timer

• The DHCP server does not know if an IP is still
  used or not before the lease is expired.
• An illegal IP address does not have the lease
  information
• The DHCP server can check if IPs are used or not,
  periodically by introducing the expiration timer
  at the table of AUC.

IP
MAC
Expire
IP1
MAC1
540
IP2
MAC2
550
IP3
MAC3
560
IP4
MAC4
580
IP5
MAC5
590
IP3
MAC6
600
AUC
8
What to change

• New agent AUC
• A new packet between AUC and DHCP server
• DHCP server logics
• No changes in DHCP clients

Subnet Identifier (4)
MAC Address (6)
IP Address (4)
9
Experiments
Columbia Wireless Network
AP
DHCP server
AUC
ARP, broadcast
honamsun
Ethernet Switch
CS Network
10
Experiment Results

• Convergence time

11
Experiment Results

• Packet arrival rate at the DHCP server

35
30
25
20
Number of packets (p/s)
15
10
5
0
0
5000
10000
15000
20000
25000
30000
Time (s)