Cooperation Between Stations in Wireless Networks

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Cooperation Between Stations in Wireless Networks

• Andrea G. Forte
• Henning Schulzrinne
• Department of Computer Science
• Columbia University

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VoIP and IEEE 802.11Architecture
Internet
Router
Router
PBX
160.38.x.x
128.59.x.x
AP
AP
Mobile Node
3
VoIP and IEEE 802.11 Problems

• Support for real-time multimedia
• Handoff
• L2 handoff
• Scanning delay
• Authentication
• 802.11i, WPA, WEP
• L3 handoff
• Subnet change detection
• IP address acquisition time
• SIP session update
• SIP re-INVITE
• Low capacity
• Large overhead
• Limited bandwidth
• Quality of Service (QoS)
• Inefficient support at MAC layer

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VoIP and IEEE 802.11 Related Work

• IEEE 802.11k
• IEEE 802.11f (Neighbor Graph)
• IEEE 802.11r
• IEEE 802.11i

Requirements

• Change in the protocol
• Change in the infrastructure

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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 wireless networks
• Working implementation available

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Cooperative Roaming Why Cooperation ?

• Same tasks
• Layer 2 handoff
• Layer 3 handoff
• Authentication
• Multimedia session update

• Same information
• Topology (failover)
• DNS
• Geo-Location
• Services

• Same goals
• Low latency
• QoS
• Load balancing
• Admission and congestion control
• Service discovery

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Cooperative RoamingOverview

• 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

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Cooperative RoamingArchitecture
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Cooperative RoamingMobile Nodes Cache

• L2 L3 information

LEASE FILE
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Cooperative Roaming Layer 2 Cooperation (1/2)

• Random waiting time
• Stations will not send the same information and
  will not send all at the same time
• The information exchanged in the NET_INFO
  multicast frames is

• APs BSSID, Channel
• SUBNET IDs

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Cooperative Roaming Layer 2 Cooperation (2/2)

• When a MN either than R-MN receives a
  NET_INFO_RESP it will perform two tasks
• Check if someone is lying (fix it!)
• Populate a temporary cache structure (cache
  chunks Bit Torrent)

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Cooperative Roaming Layer 3 Cooperation (1/3)

• Subnet detection
• Information exchanged in NET_INFO frames (Subnet
  ID)
• IP address acquisition time
• Other stations (STAs) can cooperate with the R-MN
  and acquire a new IP address for the new subnet
  on its behalf while the R-MN is still in the OLD
  subnet? Not delay sensitive!

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Cooperative Roaming Layer 3 Cooperation (2/3)

• R-MN has to discover the STAs that can help in
  this task (A-STA).

• R-MN builds a list of A-STAs for each possible
  next subnet.

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Cooperative Roaming Layer 3 Cooperation (3/3)

• R-MN can cooperate with A-STAs to acquire the L3
  information it needs.

R-MN builds a list of Subnet ID, IP address
pairs, one per each possible subnet it might move
to next.
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Cooperative Roaming Cooperative Authentication
(1/3)

• Cooperation in the authentication process itself
  is not possible as sensitive information such as
  certificates and keys are exchanged
• STAs can still cooperate in a mobile scenario to
  achieve a seamless L2 and L3 handoff regardless
  of the particular authentication mechanism used
• In IEEE 802.11 networks the medium is shared
• Each STA can hear the traffic of other STAs if on
  the same channel
• Packets sent by the non-authenticated STA will be
  dropped by the infrastructure but will be heard
  by the other STAs on the same channel/AP

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Cooperative Roaming Cooperative Authentication
(2/3)

• One selected STA (RN) can relay packets to and
  from the R-MN for the amount of time required by
  the R-MN to complete the authentication process

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Cooperative Roaming Cooperative Authentication
(3/3)

• The R-MN needs to
• Discover the available RNs for a given
  AP(Similar procedure to the one used for
  A-STAs)
• Select an RN and start the relaying of packets
  after the L2 handoff.

• In order to select an RN the R-MN sends a
  RELAY_REQ multicast frame
• RELAY_REQ contains
• MAC address of R-MN
• IP address of CN
• MAC and IP address of RN

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Cooperative Roaming Measurement Results (1/2)
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Cooperative Roaming Measurement Results (2/2)
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Cooperative Roaming Security Issues (1/2)

• A malicious MN might try to re-use the relaying
  mechanism over and over without ever
  authenticating
• Each RELAY_REQ allows an RN to relay packets for
  a limited amount of time (time required to
  authenticate)
• RELAY_REQ frames are multicast. All STAs can help
  in detecting a bad behavior and only nodes of the
  multicast group can send such frames
• RNs can detect if the R-MN is performing the
  normal authentication or not (Authentication
  failures can also be detected)

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Cooperative Roaming Security Issues (2/2)

• Countermeasures work only if we can be sure of
  the identity of a client (MAC spoofing)
• MAC spoofing is generally not possible if 802.11i
  or WPA are enabled
• To increase security, authentication and
  encryption at the multicast group level can be
  used
• Handoff from open to secure network

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Cooperative Roaming Application Layer Handoff -
Problems

• SIP handshake
• INVITE ? 200 OK ? ACK(Few hundred milliseconds)
• Users direction (next AP/subnet)
• Not known before a L2 handoff
• MN not moving after all

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Cooperative Roaming 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

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Cooperative Roaming Load Balancing - Problems

• Selection of new best AP
• Used
• Signal strength and SNR
• Not used
• Packet loss
• Effective throughput
• Number of collisions and retries
• Load balancing today
• Number of users connected (to an AP)
• Actual available bandwidth not considered

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Cooperative Roaming Load Balancing - CR

• Load balancing with CR
• MN gathers statistics about neighboring APs
• Asks other MNs to send such statistics
• Each MN collects statistics for its AP such as
  available throughput, packet loss, retry rate
• MNs send statistics to the MN that requested them
• The MN can now make a handoff to the less
  congested AP, or AP that can provide a certain
  QoS
• Even distribution of traffic flows among
  neighboring APs
• Even utilization of APs bandwidth

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Cooperative Roaming Other Applications

• In a multi-domain environment Cooperative Roaming
  (CR) can help with choosing AP/domain according
  to roaming agreements, billing, etc.
• CR can help for admission control and load
  balancing, by redirecting MNs to different APs
  and/or different networks. (Based on real
  throughput)
• CR can help in discovering services (encryption,
  authentication, bit-rate, Bluetooth, UWB, 3G)
• CR can provide adaptation to changes in the
  network topology (common with IEEE 802.11h
  equipment)
• CR can help in the interaction between nodes in
  infrastructure and ad-hoc/mesh networks

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Cooperative Roaming Conclusions

• Cooperation among stations allows seamless L2
  and L3 handoffs for real-time applications (15-21
  ms HO)

Completely independent from the authentication
mechanism used
It does not require any changes in either the
infrastructure or the protocol
It does require many STAs supporting the
protocol and a sufficient degree of mobility
Suitable for indoor and outdoor environments
Sharing information ? Power efficient
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Thank you.
Questions?

• For more information
• http//www.cs.columbia.edu/andreaf
• andreaf_at_cs.columbia.edu

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Layer 2 Handoff Handoff delays
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Layer 2 HandoffMotivation

• Handoff latency is too big for VoIP
• Seamless VoIP requires less than 90ms latency
• Handoff delay is from 200ms to 400ms

• Scanning
• Introduces more than 90 of the total handoff
  delay (open system)
• It is the most power consuming part of the
  handoff process
• Authentication
• WEP (broken)
• 802.11i, WPA

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Layer 3 HandoffSubnet Discovery

• 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
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Layer 3 HandoffIP address acquisition
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Layer 3 HandoffMotivation

• Problem
• When performing a L3 handoff, acquiring a new IP
  address using DHCP takes on the order of one
  second

The L3 handoff delay too big for
real-time multimedia sessions

• We optimize the layer 3 handoff time as follows
• Subnet discover
• IP address acquisition