Transcript Relationship between services - National Chiao Tung University

IEEE 802.11
Management Frames
Gast’s Book (Chapter 4)
Prof. Yu-Chee Tseng
CS, NCTU
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Introduction
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Establishing the identity of a network
station in a wired network is easy.
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just drag wires from a central office to stations
Wireless network must create
management feature to provide similar
functionality.
3 Steps of Management
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MS searches of connectivity
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Network authenticates the MS
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like finding a wired data jack on the wall
in wired network, this is provided by the cable
itself
MS associates with the AP to gain access
Structure of Management Frames
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header: similar
frame body: (two types)
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fixed fields: 10 types, fixed length
information elements: variable length, can be defined by
newer version of 802.11, appear in specific order
These fields are building blocks of management
frames (and will be assembled later).
Fixed Fields
These fields are building blocks of management frames
(and will be assembled later).
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Fixed Field:
Authentication Algorithm Number
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2 bytes to identify the type of authentication in
the authentication process
only 2 values are defined currently
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0: Open System authentication
1: Shared Key authentication
2 - 65,535: reserved for future use
Fixed Field: Authentication
Transaction Sequence Number
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authentication = multi-step process
consisting of challenges and responses
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2 bytes to track the progress of the auth.
exchange
1 to 65,535 (never uses 0)
Fixed Field:
Beacon Interval
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to indicate how frequent beacons sent
time unit (TU) = 1,024 us (about 1 ms)
beacon interval is commonly set to 100 TU
(about 100 ms = 0.1 sec)
Fixed Field:
Capability Information
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to advertise the network’s capability
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in Beacon
in Probe Request/Response
stations that do not implement all features
in the capability are not allowed to join
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ESS/IBSS: (mutually exclusive)
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Privacy:
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1 = binary convolution coding modulation for high-rate
DSS PHY in 802.11b
Channel Agility:
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1 = short preamble to support high-rate DSSS PHY in
802.11b
PBCC:
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1 = required to use WEP for confidentiality
Short Preamble:
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AP sets ESS = 1 and IBSS = 0 (infrastructure)
stations in IBSS sets ESS = 0 and IBSS = 1 (ad hoc
mode)
1 = use channel agility to support high-rate DSS PHY
in 802.11b
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CF Polling Bits:
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STA can set CF capability in
Association/Reassociation mgt. frame
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AP can set CF capability in Beacon, Probe Response,
Ass./Reass. Response,
Fixed Field:
Current AP Address
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to indicate the MAC address of AP
currently associated
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STA can transmit this address when
associating with a different AP to transfer the
association and retrieve buffered frames from
old AP.
Fixed Field:
Listen Interval
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To indicate under PS mode, how often a
STA will wake up to check buffered frames.
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From this, AP can determine:
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unit = one beacon interval
to estimate the resource required for buffering
may refuse to serve resource-intensive asso.
Fixed Field:
Association ID
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An associated STA is given an Asso. ID to
assist with control and mgt. functions.
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14 bits available (1-2,007 hosts)
to be compatible with
Duration/ID field
Fixed Field:
Timestamp
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the number of microseconds the BSS has
been active
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for synchronization purpose
64 bits (need > 580,000 years to wrap back)
Fixed Field:
Reason Code
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ex
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STA may send Disass. or Deauth. frames
in response to traffic when the sender has
not properly joined the network with
Reason Code.
Fixed Field:
Status Code
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ex
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to indicate whether an operation succeed
or fail, with proper Status Code
Information Elements
These fields are building blocks of management frames
(and will be assembled later).
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Information Elements
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generic format
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Element ID and meanings
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Information Element:
Service Set Identity (SSID)
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allow network manager to assign a logical ID to
the service set
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STA may scan and join the network with a specified
SSID
SSID is the same for all BSS composing an ESS
null SSID (known as broadcast SSID) is used in
Probe Request frames to discover all 802.11 APs
Information Element:
Supported Rate
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which rates are supported
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some mandatory, some optional
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indicated by the most significant bit
2 = 1 Mbps, 4 = 2 Mbps, 11 = 5.5 Mbps, 22 =
11 Mbps
Information Element:
FH Parameter Set
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dwell time: amount of time in a channel (in TU)
hop set: set of hopping patterns
hop pattern: which hopping pattern in the hop
set
hop index: current position in the hop sequence
Information Element:
DS Parameter Set
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which channel when DSSS is used
Information Element:
CF Parameter Set
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transmitted in Beacon by AP to advertise
the CFP parameters
optional
Information Element:
Traffic Indication Map (TIM)
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to indicate which low-power STAs have
buffered traffics waiting to be picked up
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partial virtual bitmap = 8 ~ 2,008 bits
each bit for one association ID
 1 = traffic buffered
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DTIM count:
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DTIM period:
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when will the next DTIM frame arrives
DTIM is for buffered broadcast/multicast
unit = beacon interval
period of DTIMs (unit = beacon interval)
Bitmap Control:
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bit 0 is for traffic indication of Asso.ID 0 (i.e., multicast
traffic)
the remaining 7 bits indicate the offset of the start of
Virtual Bitmap
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so we can only transmit a portion of the virtual bitmap
saving wireless bandwidth
Information Element:
IBSS Parameter Set
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to indicate the period of IBSS Beacons in
an ad hoc network
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unit = TU
the period is contained in ATIM (ATIM =
Announcement TIM)
Information Element:
Challenge Text
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Shared-key authentication requires STA
successfully decrypt an encrypted
challenge.
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to be filled in the Challenge Text
Management Frames
Fixed fields and information
elements will be used in
management frames
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Management Frames:
Introduction
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Fixed fields and information elements will
be used in the body of management
frames to convey information.
Frame types:
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Beacon, Probe Request, Probe Response,
ATIM, Disassociation, Deauthentication, Asso.
Request, Reasso. Request, Asso. Response,
Reasso. Response, Authentication
Mgt. Frame: Beacon
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FH and DS Parameter Sets are mutually
exclusive.
Beacon Generation by
APs and ad hoc networks
by AP:
Beacon
by contention
(D1 = random
backoff)
Busy
medium
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by IBSS:
Mgt. Frame: Probe Request
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SSID: to request a specific network
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if “broadcast SSID” is used, any network is
fine
Supported Rates
Mgt. Frame: Probe Response
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If a Probe Request encounters a network with
compatible parameters, the network sends
Probe Response.
In IBSS, beacon transmission is distributed. The
STA who sent the previous Beacon replies the
Probe Response.
Mgt. Frame: ATIM (for IBSS)
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When a STA has buffered frames for a
low-power receiver, it sends ATIM frame
during the delivery period to notify the
sleeping STA (in DA field).
Mgt. Frames:
Disassociation and Deauthentication
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Disassociation: to end an asso.
Deauthentication: to end an authentication
relationship
Mgt. Frame: Association Request
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Once a STA identifies a compatible
network, it may send an Asso. Req.
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The AP will verify the STA’s parameters
Mgt. Frame:
Reassociation Request
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When roaming between BSSs under the
same ESS, Reasso. Req. can be sent.
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Reasso. differs from Asso. in that it contains
the old AP’s address, so the new AP will
contact the old AP to pick up possible
buffered frames at the old AP.
Mgt. Frames:
Asso./Reasso. Response
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To respond to the earlier request
Mgt. Frame: Authentication
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exchanged between AP and STA for
authentication purpose.
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auth. algo. no:
auth. trans. seq. no: to track the progress
depending on what auth. algo. is used
Frame Transmission and
Asso./Authen. States
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Possible States
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A STA keeps two
state variables
for each STA:
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Authentication
state
Association state
Probe Request/Response
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Active scanning when missing APs
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infrastructure mode: probe response
generated by AP;
IBSS mode: probe response generated by the
STA which generated the last beacon.
Ad Hoc Network: Beacon vs. Probe
STA
STA
STA
STA
(first beacon) STA
Beacon
STA
STA
(second beacon)
STA
STA
(probe request
and response)
STA
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STA
Probe request
Probe response
STA
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For each channel, PROBE is sent.
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Probe Responses from all possible APs
are collected, until
Max_Probe_Response_Time.
Then the best quality AP may be chosen.
Summary
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Building blocks of management frames:
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fixed field
information elemenet
Management frames
A probe request/response example