1. Handoff in IEEE 802.11 Andrea G. Forte Sangho Shin Prof. Henning Schulzrinne

2. Handoff - Overview L2 handoff L2 handoff Connectivity (Scanning, Auth., Assoc.) Connectivity (Scanning, Auth., Assoc.) Fast MAC Layer Handoff Fast MAC Layer Handoff Authentication (802.1x, 802.11i) Authentication (802.1x, 802.11i) L3 handoff L3 handoff Subnet detection Subnet detection IP address acquisition (DAD) IP address acquisition (DAD) Fast L3 handoff Fast L3 handoff Passive DAD Passive DAD Multimedia Session update (SIP) Multimedia Session update (SIP)

3. Fast MAC Layer Handoff Overview Overview Selective scanning We do not need to scan all the channels. Some heuristics can be used to improve the scanning procedure. Selective scanning We do not need to scan all the channels. Some heuristics can be used to improve the scanning procedure. Cache The AP’s information is saved in the client in a cache so to avoid unnecessary scans in the future handoffs. Cache The AP’s information is saved in the client in a cache so to avoid unnecessary scans in the future handoffs. Changes in the client ONLY.

4. Fast Layer 3 Handoff Spatial locality  Cache Spatial locality  Cache We use an extension of the L2 cache: We use an extension of the L2 cache: Current AP (KEY) Best AP Second best AP MAC A MAC B MAC C Channel 1 Channel 11 Channel 6 Gateway D Gateway E Gateway F + LEASE FILE

5. DHCP - Overview DHCP Server Assigns IP addresses to clients that request them via the DHCP protocol. It directly serve clients in its subnet while it needs the Relay Agent in order to server clients in a different subnet than its own. DHCP Server Assigns IP addresses to clients that request them via the DHCP protocol. It directly serve clients in its subnet while it needs the Relay Agent in order to server clients in a different subnet than its own. Relay Agent (RA) We usually have one RA per subnet and usually the RA is located on the router/gateway of that subnet. The RA needs to relay DHCP packets between its network and the DHCP server. The server will know to which subnet a client belongs to (and which IP address to assign) according to which RA the packets came from. Relay Agent (RA) We usually have one RA per subnet and usually the RA is located on the router/gateway of that subnet. The RA needs to relay DHCP packets between its network and the DHCP server. The server will know to which subnet a client belongs to (and which IP address to assign) according to which RA the packets came from.

6. Fast Layer 3 Handoff (1/3) Subnet detection Send a bogus DHCP REQUEST packet so to acquire the gateway/router IP address. We then compare the new gateway IP address with the one contained in our L2 cache. If they match the subnet is the same and no other action is needed; if they do not match, we have a subnet change and a L3 handoff has to be performed. Subnet detection Send a bogus DHCP REQUEST packet so to acquire the gateway/router IP address. We then compare the new gateway IP address with the one contained in our L2 cache. If they match the subnet is the same and no other action is needed; if they do not match, we have a subnet change and a L3 handoff has to be performed.

7. Fast Layer 3 Handoff (2/3) IP address acquisition This is the most time consuming part of the L3 handoff process. DAD takes most of the time. We optimize the IP address acquisition time as follows: IP address acquisition This is the most time consuming part of the L3 handoff process. DAD takes most of the time. We optimize the IP address acquisition time as follows: Checking Lease file for a valid IP. Checking Lease file for a valid IP. Temporary IP (“Lease miss”)  The client “picks” a candidate IP using particular heuristics. Temporary IP (“Lease miss”)  The client “picks” a candidate IP using particular heuristics. SIP re-invite  The CN will update its session with the TEMP_IP. SIP re-invite  The CN will update its session with the TEMP_IP. Normal DHCP procedure to acquire the final IP. Normal DHCP procedure to acquire the final IP. SIP re-invite  The CN will update its session with the final IP. SIP re-invite  The CN will update its session with the final IP. While acquiring a new IP address via DHCP, we do not have now any disruption regardless of how long the DHCP procedure will be. We can use the TEMP_IP as a valid IP for that subnet until the DHCP procedure ends.

8. Fast Layer 3 Handoff (3/3) Multimedia session update (SIP) After a change in IP address, we have to inform the Correspondent Node (CN) about it. This is usually done with a re-Invite. The data stream will be resumed right after the 200 OK has been received. Multimedia session update (SIP) After a change in IP address, we have to inform the Correspondent Node (CN) about it. This is usually done with a re-Invite. The data stream will be resumed right after the 200 OK has been received. MN SIP Re-INVITE RTP Data SIP ACK New IP CN SIP OK

9. Fast Layer 3 - Implementation SIP client (mca) Wireless card driver (HostAP driver) DHCP client User Space Kernel Space kernel 2.4.20 Red Hat 9.0  MCA: SIP client for PDAs by SIPquest Inc.  DHCP client by Internet System Consortium (ISC)  HostAP wireless driver

10. 22 138 8 29 138 8 29 8 0 20 40 60 80 100 120 140 160 180 200 L3 handoff time (ms) Scenario 1Scenario 2Scenario 3 SIP Signaling Client processing IP acquisition Detection of subnet change Fast Layer 3 Handoff - Results

11. Passive DAD Duplicate Address Detection (DAD) Before the DHCP server decides to assign an IP address, it has to be sure that such address is not already in use. In order to do this, the DHCP server sends ICMP Echo requests and waits for ICMP Echo replies. Duplicate Address Detection (DAD) Before the DHCP server decides to assign an IP address, it has to be sure that such address is not already in use. In order to do this, the DHCP server sends ICMP Echo requests and waits for ICMP Echo replies. The delay introduced by DAD is in the order of seconds! Passive DAD (P-DAD) We introduce a new agent, namely (AUC), which collects information about the IP addresses in use in its subnet. The AUC will then inform the DHCP server about IP addresses already in use in a particular subnet. Passive DAD (P-DAD) We introduce a new agent, namely Address Usage Collector (AUC), which collects information about the IP addresses in use in its subnet. The AUC will then inform the DHCP server about IP addresses already in use in a particular subnet.

12. Thank You! For more information: Web: http://www.cs.columbia.edu/~andreaf http://www.cs.columbia.edu/IRT E-mail: andreaf@cs.columbia.edu