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Cooperation Between Stations in
Wireless Networks
Andrea G. Forte
Henning Schulzrinne
Department of Computer Science
Columbia University
October 17, 2007
VoIP and IEEE 802.11
Architecture
Internet
Router
Router
PBX
160.38.x.x
128.59.x.x
AP
AP
Mobile Node
2
VoIP and IEEE 802.11
Problems

Support for real-time multimedia

Handoff

L2 handoff


Authentication



SIP re-INVITE
Low capacity



Subnet change detection
IP address acquisition time
SIP session update


802.11i, WPA, WEP
L3 handoff


Scanning delay
Large overhead
Limited bandwidth
Quality of Service (QoS)

Inefficient support at MAC layer
3
VoIP and IEEE 802.11
Related Work




IEEE
IEEE
IEEE
IEEE
802.11k
802.11f
802.11r
802.11i
Requirements
 Change in the protocol
 Change in the infrastructure
4
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
5
Cooperative Roaming
Why Cooperation ?

Same tasks




Layer 2 handoff
 Same information
Layer
3 handoff
 Topology (failover)
Authentication
 Same goals
 DNS
Multimedia
 Low latency
session update
 Geo-Location
 QoS
 Services
 Load balancing


Admission and
congestion control
Service discovery
6
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 applicationlayer mobility and load balancing
7
Cooperative Roaming
Architecture
Internet
8
Cooperative Roaming
Mobile Node’s Cache

L2 + L3 information
Current AP (KEY)
Best AP
Second best AP
MAC A
MAC B
MAC C
Channel 1
Channel 11
Channel 6
Subnet ID 1
Subnet ID 2
Subnet ID 3
+
LEASE FILE
9
Cooperative Roaming
Layer 2 Cooperation (1/2)
R-MN
Stations
NET_INFO_REQ
NET_INFO_RESP

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
10
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)
11
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 RMN 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!
12
Cooperative Roaming
Layer 3 Cooperation (2/3)
R-MN
Stations
ASTA_DISCOV (m)
ASTA_RESP (u)


m: multicast
u: unicast
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.
13
Cooperative Roaming
Layer 3 Cooperation (3/3)
R-MN
A-STA
IP_REQ
(Client ID)
DHCP_OFFER
(client ID)
DHCP
Server
.
.
DHCP_ACK
IP_RESP
(New IP)

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.
14
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
15
Cooperative Roaming
Cooperative Authentication (2/3)
AP
RN data
packets
+
relayed data
packets
802.11i
authentication
packets
Relayed Data Packets
RN

R-MN
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
16
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
17
Cooperative Roaming
Measurement Results (1/2)
Handoff without authentication
1400
1210.0
1200
1000
867.0
800
ms
L2
L3
Total
600
400
343.0
200
4.2
11.4
15.6
0
CR
IEEE 802.11 Handoff
18
Cooperative Roaming
Measurement Results (2/2)
Handoff with authentication (IEEE 802.11i)
1800
1669.5
1579.8
1600
1531.6
1400
1200
967
1000
L2
ms
897
867
L3
772.4
800
Total
664.6
612.8
600
400
200
10 11.4 21.4
0
EAP-TLS (1024)
EAP-TLS (2048)
PEAP/MSCHAPv2
(1024)
CR
19
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)
20
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
21
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
22
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
23
Thank you.
Questions?
For more information:
•http://www.cs.columbia.edu/~andreaf
•[email protected]
24
BACKUP Slides
25
Cooperative Roaming
Application Layer Handoff 
SIP handshake


Problems
INVITE  200 OK  ACK
(Few hundred milliseconds)
User’s direction (next AP/subnet)


Not known before a L2 handoff
MN not moving after all
26
Cooperative Roaming
Application Layer Handoff - CR


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



After 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
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
27
Cooperative Roaming
Load Balancing 
Selection of new best AP

Used


Signal strength and SNR
Not used




Problems
Packet loss
Effective throughput
Number of collisions and retries
Load balancing today


Number of users connected (to an AP)
Actual available bandwidth not considered
28
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
29
Cooperative Roaming
Multicast & Scalability

Use different multicast groups


PROBLEM: Client A needs to know the
multicast address of Client B in order to
cooperate
SOLUTIONS

Clients cache their multicast address for a
particular location (e.g., subnet)


Share it with other clients
Each client computes its multicast address as
the hash of one or more values

Subnet ID, current AP’s BSSID, etc.
30
Layer 2 Handoff
Handoff delays
APs available
on all channels
Mobile Node
Probe Request (broadcast)
Probe Response(s)
Discovery Phase
Probe Delay
New AP
Authentication Request
Open Authentication Delay
Authentication Response
Association Request
Open Association Delay
Authentication Phase
Association Response
31
Layer 2 Handoff
Motivation

Handoff latency is too big for VoIP



Scanning



Seamless VoIP requires less than 90ms latency
Handoff delay is from 200ms to 400ms
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
32
Layer 3 Handoff
Subnet 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
33
Layer 3 Handoff
IP address acquisition
DHCP Server
MN
L2 Handoff
Complete
DHCP DISCOVER
DAD
DHCP OFFER
DHCP REQUEST
DHCP ACK
34
Layer 3 Handoff
Motivation

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
35