May 2006
doc.: IEEE 802. 11-06-0629
Network Discovery Mechanism
Date: 2006-05-15
Authors:
Name
Company Address
Dave
Philips
Cavalcanti
Amjad
Philips
Soomro
345 Scarbourough Rd.,
Briarcliff Manor, NY
345 Scarbourough Rd.,
Briarcliff Manor, NY
Phone
email
1-914-9456083
[email protected]
1-914-9456319
[email protected]
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Submission
Slide 1
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Abstract
This submission proposes a mechanism to enable STA
to discover multiple SSPNs supported by an AN, and to
identify the services provided.
– Addresses requirements N1, N3 and N4 of the Network Selection
Cluster.
Submission
Slide 2
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Outline
•
•
•
•
Requirements overview
Analysis of possible solutions
Proposed network discovery mechanism
Conclusions
Submission
Slide 3
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Requirements Overview
• N1: Define functionality by which a STA can determine
whether its subscription to an SSPN would allow it to
access a particular 802.11AN before actually joining a
BSS within that 802.11 AN. Proposals must describe
their consideration of scalability.
– It’s not acceptable for a STA to be required to attempt IEEE
802.1X authentication with all available networks until it finds one
that works. Equally a solution is not practical if it requires every
possible credential supplier to be listed in a beacon (due to
scalability problems).
Submission
Slide 4
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Requirements Overview (cont.)
• N3: Define functionality to support authentication with
multiple SSPNs through a single AP.
• N4: Define functionality by which a STA can determine
which interworking services are available before
joining a BSS.
Submission
Slide 5
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Key Issues
• Access Network (AN):
– How AP advertises SSPNs and supported internetworking service
types?
• Wireless Station (STA):
– How STA gets information to enable efficient selection of one or
Supported
Services
more SSPN to authenticate to?
SSPNs
available
AP
Home SSPN
Foreign
SSPNs with
roaming
agreements
SSPN 1
SSPN 2
How to advertise the
supported SSPNs and
services?
STA
SSPN 1
A
SSPN 2
B
SSPN 3
C
Which SSPNs are supported?
Which services are available?
Which SSPN or SSPNs to select for authentication?
Submission
Slide 6
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Summary of Existing Solutions
• Information Elements (IE) have been proposed to carry
network discovery related information
• The IEs would be transmitted in
– Beacons (passive discovery)
– Probe responses (active discovery)
– Combine beacons and probe response to advertise the whole
information
• Network discovery related information may include:
– SSPN Id
– Service types
– …
Submission
Different proposals define distinct
information fields, but in general, the IE
carries information relevant to network
selection and authentication
Slide 7
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Analysis of Existing Solutions
•
Passive discovery
– AP broadcasts all network discovery information in a single IE added in every
beacon
• STA listens to beacons and discovers supported SSPNs and services
– Not practical, too much overhead
•
Active discovery
– To check availability of each SSPN, a STA probes AP and AP sends info related to
that SSPN
– Less overhead, but STA has to contend for channel to transmit probe and wait for
response, which consumes more power
– Efficient if STA is searching for a specific SSPN,
• but what happens if STA can use several SSPNs and/or wants to discover all options
available (SSPNS and services)?
–
STA has to probe the AP several times, resulting in higher power consumption and overhead
– Use Case: Search for best option (on-the-spot best SSPN selection/pay per use
scenario)
Submission
Slide 8
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Analysis of Existing Solutions (Cont.)
• Combined passive and active discovery
– AP transmits a small (partial) list of SSPNs supported in beacons
– To get a complete list, a STA has to probe AP
• Less fixed overhead, but STA is forced to use active probes if it wants
to discover all possibilities
Submission
Slide 9
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Proposed Network Discovery Mechanism
•
Objectives:
1. Provide a solution that is applicable to different scenarios:
•
•
Users trying to discover if a specific SSPN or service type is
supported by a AN
Users trying to discover all connectivity options (SSPNs and interworking services) available
2. Allow the AN to control the overhead that can be introduced by
network discovery related information
3. Allow the STA to decide how to collect network discovery
information
Submission
Slide 10
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Overview of the proposal
• The AN fully supports both passive and active
discovery
– All network discovery related information can be obtained using
• Passive discovery (only),
• Active discovery (only), or
• Combined Passive/Active discovery
• A new passive discovery mechanism is proposed in
which the network discovery related information is
advertised over a period of time, called Network
Discovery (ND) interval
• The ND interval is controlled by the AN, and may span
across multiple beacon frames
Submission
Slide 11
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Overview of the proposal
• A STA decides which method to use, for example,
– If a STA wants to discover all SSPNs supported by the AN, it may
choose to use only passive discovery, and it is able to get all
information by just listening to beacon frames
– If a STA is searching for a particular SSPN it may choose to
directly use active discovery and probe the AN
– If a STA is searching for a particular SSPN it may try to use
passive discovery, but if the desired information is not received
within a certain period of time, it may use active discovery
• Decision algorithm is out of the scope of the standard
Submission
Slide 12
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
New Passive Discovery Mechanism
• The overall ND information is advertised using one or more ND
Information Elements (ND IE)
– Each beacon carries one ND IE
• The time needed to advertise the whole ND information is called
ND interval
– AN controls the ND interval and the amount of information per ND IE
Submission
Slide 13
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Network Discovery IE (ND IE)
SSPN Information (Variable)
Element ID
Octets:
1
Length
1
ND
Counter
1
SSPN
Counter
1
SSPN Record 1
…
SSPN Record n
?
…
?
ND Counter : Number of ND IEs (beacons) expected until the beginning of the next ND interval
SSPN Counter : Number of different SSPNs advertised in this ND IE
SSPN Record: Includes the SSPN information to be advertised
Submission
Slide 14
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
SSPN Record
• Information in this record may be used to address
– Network selection cluster requirements: N1, N2, and N4
– Online enrolment cluster requirements: E1 and E4
Octets:
SSPN ID
SSPN_Info
2
2
B0
SSPN_Info
Bits:
Submission
B3 B4
Service Type
4
B5
B6
B7
Charge
Method
Enrollment
Method
2
2
Slide 15
B8
B9
Emergency
service
1
B15
Reserved
7
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
SSPN Info
• Service type field could combine information about:
– Type of high level connectivity available,
• Internet or local access
– Type of MAC service supported,
• Distributed access (DCF or EDCA)
• Controlled access (HCCA)
• Distributed and controlled access (EDCA and HCCA)
Submission
Code
Description
0000
Open Internet access with distributed MAC access
0001
Internet access with enrollment required and controlled MAC
access
0010
Internet and Private access with enrolment required and
distributed and controlled MAC access
0011
Private local access with enrollment required with distributed
MAC access
Slide 16
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Charge /Enrolment/Emergence services
Charge Method
Enrolment Method
Code
Description
Code
Description
00
Free
00
Method 1
01
Charged per time
01
Method 2
10
Charged per transferred bytes
10
…
11
Reserved
11
…
• Emergency Service field
– 0 : No support to emergence services from this AN
– 1 : Emergence service supported by SSPN from this AN
Submission
Slide 17
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
New attributes for the 802.11 MAC MIB
Submission
Attribute
Description
dot11NDThreshold
Indicates the maximum number of octets that
can be used to advertise ND Information
within a beacon period
dot11SSPNTable
A table of attributes for all supported SSPN
that needs to be advertised in the SSPN
Records
dot11SSPNNumber
The number entries in the dot11SSPNTable
dot11NDInterval
This is the number of beacon periods (or ND
IEs) used to advertise all SSPN Records at
least once
Slide 18
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
ND Interval
• The dot11NDInterval can be computed as:
– Assuming 4 bytes per SSPN Record and 4 bytes of fixed overhead per ND
IE
 dot11SSPNNumber  4 
dot11NDInterval  
 dot11BeaconPeriod

 (dot11NDThreshold  4) 
• By adjusting the dot11NDThreshold the AP can control how the
ND information is advertised
• Under low traffic loads, AP can use a higher dot11NDThreshold and add more
information per beacon
• Under higher loads, AP can reduce dot11NDThreshold, send less information
added in each beacon and increases the ND interval
Submission
Slide 19
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
dot11SSPNTable
Submission
dot11SSPNTableEntry
Type
dot11SSPNTableIndex
Integer
dot11SSPNTableSSPNId
Octet String
dot11SSPNTableServiceType
Integer
dot11SSPNTableChargeMode
Integer
dot11SSPNTableEnrolment
TruthValue
dot11SSPNTableEmergenceServ
TruthValue
Slide 20
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Processing a ND IE
• (Scenario 1) STA is searching for a specific SSPN:
– Checks whether the desired SSPN is advertised in the ND IE (STA
uses SSPN Id field in SSPN Records)
• If found desired SSPN, STA reads SSPN Info field from the
corresponding SSPN Record
• If SSPN was not found, STA can decide to wait for the next ND IE or
it can use Active probe
– In the first case, STA could enter the power save mode until next beacon
– In the second case, STA contends for the channel to transmit a probe
request
• Decision algorithm can use the ND Counter field in the ND IE,
– but algorithm is out of the scope of the standard
Submission
Slide 21
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Processing a ND IE
• (Scenario 2) STA can use several SSPNs
– STA reads the first ND IE and based on the ND Counter and on the
received SSPN Records, it decides whether to select one of the
SSPNs in the ND IE
• If STA selects a SSPN from the first ND IE, it tries to authenticate
with it
• Otherwise, STA decides whether it can wait for the next ND IE or it
should use active probe
– If ND Counter is small and delay is not a main concern, the STA could wait
for the next ND IE
– If delay is a major requirement, depending on the ND counter, the STA
may choose to probe the AN.
• If STA decides to wait for the next ND IE, it can enter Power Save
mode until next TBT
Submission
Slide 22
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Advantages of the proposed solution
• Provides flexibility to AP and STAs
– AP can control advertisement of ND information
– STA can choose which method to use (active or passive)
• Active scanning can still be used, if STAs doesn’t want to wait for several
beacons to discover all possibilities
• Provides a compromise between discovery delay and channel
utilization
– Small dot11NDThreshold  large discovery delay, but less overhead
(higher channel utilization)
– Large dot11NDThreshold  smaller discovery delay, but higher overhead
(less channel utilization)
• dot11NDThreshold and dot11NDInterval can be configured at the
network set up or they can be dynamically adjusted
– How to configure and/or adjust these attributes is out of the scope of the
standard
Submission
Slide 23
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Analysis of power requirement (G1)
• Operation can be adjusted to different scenarios and power saving
requirements
• If STA chooses to obtain all information from ND IEs sent in
beacons (passive scanning only)
– Usually, STAs are required to wakeup to receive beacons, therefore the
passive discovery does not significantly affect power consumption
– Passive discovery consumes less power than if STA has to probe the AN
for every connectivity option available
• If STA chooses to use active scanning, power consumption will
depend on the channel conditions and number of probe requests
generated
Submission
Slide 24
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Analysis of security requirements (G2)
• As in other proposed solutions, advertised information
(SSPN IDs and supported services) could be protected
– For example, using Hash functions to define SSPN ID and service
type code
– STAs would hash the names of the SSPNs in their supported list
and compare the result with the SSPN ID in the ND IE
– Only STAs that know the service types would be able to detect the
supported service types
– Still vulnerable to attacks, but it is not worse than current solutions
Submission
Slide 25
Dave Cavalcanti and Amjad Soomro, Philips
May 2006
doc.: IEEE 802. 11-06-0629
Conclusions
• Proposed network discovery mechanism addresses requirements
N1, N2 and N4
– Information provided in the ND IE can also be used to address
requirement N3
• Proposed mechanism can also be used to advertise
– Charge method (N5)
– Support to online enrolment (E1, E2 and E4)
– Support to emergency services (I1)
• This is a flexible solution that can be adjusted to achieve a
compromise between overhead in beacons, discovery time and
channel utilization
– Specific algorithms for STA and AN are not provided (out of the scope)
Submission
Slide 26
Dave Cavalcanti and Amjad Soomro, Philips