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 : terminal mobility problem AP Mobile Node L2 handoff: in case subnets are the same L3 handoff: in case the new AP is in different subnet Motivation Cooperative roaming Results Conclusion Source:

L2 handoff in Motivation Cooperative roaming Results Conclusion

L3 handoff in Motivation Cooperative roaming Results Conclusion

Handoffs due to mobility L2 handoff (~ ms) Scanning (>90%) Network authentication Re-association L3 handoff (~1000ms) 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 Store AP Info to CacheSelective Scanning Signal Low 1 Cache KeyBest Next A B C A Cache KeyBestNext A B C A B B C A 6 11 SSID, Channel, SubnetID (e.g. MAC(A), 1, ) Motivation Cooperative roaming Results Conclusion Source: www1.cs.columbia.edu/~ss2020/presentation/L2handoff-poster.ppt

Cooperative Roaming: AP caching Store AP Info to CacheSelective Scanning Signal Low 1 Cache KeyBest Next A B C A Cache KeyBestNext A B C A B B C A BC B CA CA Motivation Cooperative roaming Results Conclusion Source: www1.cs.columbia.edu/~ss2020/presentation/L2handoff-poster.ppt

L2 cooperation protocol Mobile node BMobile node A 2. InfoResp diff(cache A, cache B) 1. InfoReq (cache A) Random backoff Mobile node C 1. InfoReq (cache A) 2. InfoResp diff(cache A, cache C) Motivation Cooperative roaming Results Conclusion

L3 cooperation protocol Mobile node B (subnet 1) Mobile node A (subnet 2) 2. AmnResp (MAC(B), IP(B)) 1. AmnDiscover (subnet 1) 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)) Acquire IP, using MAC(A) from DHCP server Motivation Cooperative roaming Results Conclusion L2 handoff begins Cache: subnet1( IP(A), IP(router))

Cooperative authentication 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? Motivation Cooperative roaming 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 Motivation Cooperative roaming Results Conclusion Source:

Cooperative roaming vs Motivation Cooperative roaming Results Conclusion Source:

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 Room for improvement: mobility detection, application layer handoff … Motivation Cooperative roaming Results Conclusion

Thank you Questions? Comments?

Backup slides From: 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

ARP Req. NAK MNDHCPd DHCP Req. ARP Req. Router ARP Resp. CN SIP INVITE SIP OK SIP ACK RTP packets (TEMP_IP) 138 ms 22 ms 4 ms 29 ms Waiting time IP acquisition SIP signaling L2 handoff complete Detecting subnet change Processing overhead Experimental Results (1/2)

Handoff Scenarios Scenario 1 The MN enters in a new subnet for the first time ever. Scenario 2 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 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. Scenario 2 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.