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Cooperation between stations in wireless networks Andrea G. Forte, Henning Schulzrinne Department of Computer Science, Columbia University Presented by: Azbayar Demberel Duke University April 19, 2008 Agenda Motivation Cooperative roaming Results Conclusion VoIP and 802.11: terminal mobility problem AP AP Mobile Node L2 handoff: in case subnets are the same L3 handoff: in case the new AP is in different subnet Source: http://www.icnp2007.edu.cn/slides/04_aforte-cooperation.pdf Motivation Cooperative roaming Results Conclusion L2 handoff in 802.11 Motivation Cooperative roaming Results Conclusion L3 handoff in 802.11 Motivation Cooperative roaming Results Conclusion Handoffs due to mobility L2 handoff (~100-400 ms) L3 handoff (~1000ms) Scanning (>90%) Network authentication Re-association Subnet change discovery IP address acquisition (>90%) Application handoff Informing correspondent node of new IP address Motivation Cooperative roaming Results Conclusion Cooperative roaming: goals and solution Fast handoff for real-time multimedia in any network Different administrative domains Various authentication mechanisms No changes to protocol and infrastructure Fast handoff at all the layers relevant to mobility • Link layer • Network layer • Application layer New protocol: Cooperative Roaming Complete solution to mobility for real-time traffic in wireles networks Working implementation available Motivation Cooperative roaming Results Conclusion Cooperative roaming: overview Stations can cooperate and share information about the network (topology, services) Stations can cooperate and help each other in common tasks such as IP address acquisition Stations can help each other during the authentication process without sharing sensitive information, maintaining privacy and security Stations can also cooperate for application layer mobility and load balancing Motivation Cooperative roaming Results Conclusion Cooperative Roaming: AP caching Signal Low 1 A Selective Scanning Store AP Info to Cache 6 6 1 A B 11 C 6 1 A B 11 C 11 Cache Cache Key Best Next A Key BestNext B C A B SSID, Channel, SubnetID (e.g. MAC(A), 1, 160.39.5.0) Source: www1.cs.columbia.edu/~ss2020/presentation/L2handoff-poster.ppt Motivation Cooperative roaming Results Conclusion Cooperative Roaming: AP caching Signal Low 1 A Selective Scanning Store AP Info to Cache 6 6 1 A B 11 C 1 A B 11 C Cache Cache Key Best Next A B C B C A Key BestNext B C A B C A Source: www1.cs.columbia.edu/~ss2020/presentation/L2handoff-poster.ppt Motivation Cooperative roaming Results Conclusion L2 cooperation protocol Mobile node B Random backoff Motivation Mobile node A Mobile node C 1. InfoReq (cache A) 1. InfoReq (cache A) 2. InfoResp diff(cache A, cache B) 2. InfoResp diff(cache A, cache C) Cooperative roaming Results Conclusion L3 cooperation protocol Mobile node B (subnet 1) Mobile node A (subnet 2) 1. AmnDiscover (subnet 1) Acquire IP, using MAC(A) from DHCP server 2. AmnResp (MAC(B), IP(B)) Mobile node C (subnet 2) 1. AmnDiscover (subnet 1) Subnet1: nodeB( Mac(B), IP(B)) 3. IpReq (MAC(A)) 4. IpResp (MAC(A), IP(A), IP(router)) Motivation Cache: subnet1( IP(A), IP(router)) L2 handoff begins Cooperative roaming Results Conclusion Cooperative authentication Motivation Cooperative roaming Cooperation in the authentication itself not possible keys, certificates (sensitive info) Use relay node (RN) to relay packets during authentication No bridging delay Use timeout to achieve fairness What about RN mobility? Results Conclusion Experiment environment 2 subnets/AP’s 4 nodes (1 roamer, 1 helper, 2 sniffers) Roamer moved between two AP’s: perform L2, L3 handoff …i.e. extremely simple! Motivation Cooperative roaming Results Conclusion Experiment results Motivation Cooperative roaming Results Conclusion Cooperative roaming vs. 802.11 Source: http://www.icnp2007.edu.cn/slides/04_aforte-cooperation.pdf Motivation Cooperative roaming Results Conclusion Cooperative roaming vs. 802.11 Source: http://www.icnp2007.edu.cn/slides/04_aforte-cooperation.pdf Motivation Cooperative roaming Results Conclusion Discussion Too simple experiment: congestion and backoff might diminish all the benefits, in real life Assumes spatial locality / node “knows” what the next AP will be. No info on memory management policies: how often to ask neighbors In many places uses magic wand approaches (e.g. detect subnet change) CR might benefit from location routing Application layer mobility, load balancing left out Motivation Cooperative roaming Results Conclusion Summary Seamless/near-seamless handoff Requires cooperation of many other nodes to achieve the benefits Worst case scenario ~ current 802.11 Room for improvement: mobility detection, application layer handoff … Motivation Cooperative roaming Results Conclusion Thank you Questions? Comments? Backup slides From: http://www.cs.umd.edu/~waa/pubs/h andoff-lat-acm.pdf Subnet Discovery (1/2) Current solutions Router advertisements • Usually with a frequency on the order of several minutes. DNA working group (IETF) • Detecting network attachments in IPv6 networks only. No solution in IPv4 networks for detecting a subnet change in a timely manner. Subnet Discovery (2/2) Proposed approach Send bogus DHCP_REQUEST (using loopback address). DHCP server responds with a DHCP_NAK From the NAK extract subnet information such as default router IP address. The client saves the default router IP address in cache. If old AP and new AP have different default router, the subnet has changed. Application layer handoff MN builds a list of {RNs, IP addresses}, one per each possible next subnet/AP RFC 3388 Send same media stream to multiple clients All clients have to support the same codec Update multimedia session Before L2 handoff • Media stream is sent to all RNs in the list and to MN (at the same time) using a re-INVITE with SDP as in RFC 3388 • RNs do not play such streams After L2 handoff • Tell CN which RN to use, if any (re-INVITE) • After successful L2 authentication tell CN to send directly without any RN (re-INVITE) No buffering necessary Handoff time: 15ms (open), 21ms (802.11i) Packet loss negligible Experimental Results (1/2) MN DHCPd Router CN L2 handoff complete DHCP Req. ARP Req. 22 ms NAK Detecting subnet change 138 ms Waiting time IP acquisition ARP Req. 4 ms ARP Resp. 4 ms Processing overhead 29 ms SIP signaling SIP INVITE SIP OK RTP packets (TEMP_IP) SIP ACK Handoff Scenarios Scenario 1 Scenario 2 The MN enters in a new subnet for the first time ever. The MN enters in a new subnet it has been before and it has an expired lease for that subnet. Scenario 3 The MN enters in a new subnet it has been before and still has a valid lease for that subnet. IP Selection (1/3) Scenario 1 Scenario 2 Select random IP address starting from the router’s IP address (first in the pool). MN sends 10 ARP requests in parallel starting from the random IP selected before. Same than scenario 1 except that we start to send ARP requests to 10 IP addresses in parallel, starting from the IP we last used in that subnet. Scenario 3 We do not need TEMP_IP as we have a valid lease. We just renew the lease.