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Handoff in IEEE 802.11

Handoff in IEEE 802.11. Andrea G. Forte Sangho Shin Prof. Henning Schulzrinne. Handoff - Overview. L2 handoff Connectivity (Scanning, Auth., Assoc.) Fast MAC Layer Handoff Authentication (802.1x, 802.11i) L3 handoff Subnet detection IP address acquisition (DAD) Fast L3 handoff

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Handoff in IEEE 802.11

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

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

  3. Fast MAC Layer Handoff • Overview • Selective scanningWe do not need to scan all the channels. Some heuristics can be used to improve the scanning procedure. • CacheThe 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 • We use an extension of the L2 cache: + LEASE FILE

  5. DHCP - Overview • DHCP ServerAssigns 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.

  6. Fast Layer 3 Handoff (1/3) • Subnet detectionSend 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 acquisitionThis 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. • 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. • Normal DHCP procedure to acquire 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. CN MN New IP SIP Re-INVITE SIP OK RTP Data SIP ACK 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.

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

  10. 200 29 180 8 29 160 8 140 L3 handoff time (ms) 120 SIP Signaling Client processing 100 138 IP acquisition Detection of subnet change 80 138 60 40 29 20 22 8 0 Scenario 1 Scenario 2 Scenario 3 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. The delay introduced by DAD is in the order of seconds! • 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

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