1. Passive DAD Henning Schulzrinne Columbia University

2. Problems of 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 decided between 0 ~ 1 sec randomly.

3. Passive DAD - Architecture Address Usage Collector (AUC)DHCP server Router/Relay Agent SUBNET AUC collects all broadcast and ARP packets. AUC builds IP:MAC 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. IPMACExpire IP1MAC1570 IP2MAC2580 IP3MAC3590 IP4MAC4 Broadcast/ARP ARP checking AUC scans unused IPs using ARP query periodically. Silent node can be detected.

4. Passive DAD – Example AUC DHCP server Router IP:128.59.19.46 MAC:AA:BB:CC:DD:EE IP:128.59.19.46 MAC:AA:BB:CC:FF:GG IP:128.59.19.46 MAC:AA:BB:CC:DD:EE IP:128.59.19.46 MAC:AA:BB:CC:DD:EE Lease info IP MACExp ARP query xxxxxxxxxxxx ARP query xxxxxxxxxxxx IP:128.59.19.46 MAC:AA:BB:CC:FF:GG Force Renew Web server Block AA:BB:CC:FF:GG Forward HTTP traffic

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. IPMACExpire IP1MAC1540 IP2MAC2550 IP3MAC3560 IP4MAC4580 IP5MAC5590 IP3MAC6600 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 honamsun DHCP server AUC AP ARP, broadcast Ethernet Switch Columbia Wireless Network CS Network

10. Experiment Results Convergence time 0 100 200 300 400 500 600 700 05000100001500020000250003000035000 Time (s) Number of IPs used

11. Experiment Results Packet arrival rate at the DHCP server 0 5 10 15 20 25 30 35 050001000015000200002500030000 Time (s) Number of packets (p/s)