Cooperation between stations in wireless networks

Andrea G. Forte, Henning SchulzrinneDepartment of Computer Science, Columbia University

Presented by: Azbayar Demberel

Duke UniversityApril 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 sameL3 handoff: in case the new AP is in different subnet

Motivation Cooperative roaming Results Conclusion

Source: http://www.icnp2007.edu.cn/slides/04_aforte-cooperation.pdf

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) 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

1

611

Signal Low

1

Cache

KeyBestNext

A

B

C

A

1

6

11

Cache

KeyBestNext

A

B

C

A BB

C

A 6

11

SSID, Channel, SubnetID(e.g. MAC(A), 1, 160.39.5.0)

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

1

611

Signal Low

1

Cache

KeyBestNext

A

B

C

A

1

6

11

Cache

KeyBestNext

A

B

C

A BB

C

A

B CB C A C A

Motivation Cooperative roaming Results Conclusion

Source: www1.cs.columbia.edu/~ss2020/presentation/L2handoff-poster.ppt

L2 cooperation protocol

Mobile node B Mobile 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. 802.11

Motivation Cooperative roaming Results ConclusionSource: http://www.icnp2007.edu.cn/slides/04_aforte-cooperation.pdf

Cooperative roaming vs. 802.11

Motivation Cooperative roaming Results ConclusionSource: http://www.icnp2007.edu.cn/slides/04_aforte-cooperation.pdf

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/handoff-lat-acm.pdf

Subnet Discovery (1/2)

Current solutionsRouter 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

MN DHCPd

DHCP Req.

ARP Req.

Router

ARP Resp.

CN

SIP INVITE

SIP OK

SIP ACK

RTP packets (TEMP_IP)

138 ms

22 ms

4 ms

4 ms

29 ms

Waiting timeIP acquisition

SIP signaling

L2 handoffcomplete

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.