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19:
Wireless
Last Modified:
7/7/2017 10:06:50 PM
5: DataLink Layer
5a-1
IEEE 802.11 Wireless LAN
 wireless LANs: untethered/mobile
networking
 IEEE 802.11 standard:
 MAC protocol
 unlicensed frequency spectrum
governed by pre-defined rules
vs. restricted allocation
 Basic Service Set (BSS) (a.k.a.
“cell”) contains:
 wireless hosts/stations (STA)
 access point (AP): base
station
 BSS’s combined to form
distribution system (DS) and
extended service set (ESS)
5: DataLink Layer
5a-2
IEEE 802.11 Architecture
 Distribution system (DS) – backbone
network
 Access point (AP) – bridge and relay point
 Basic service set (BSS)
 Stations
competing for access to shared
wireless medium
 Isolated or connected to backbone DS through
AP
 Extended service set (ESS)
 Two or more basic service sets interconnected
by DS
5: DataLink Layer
5a-3
5: DataLink Layer
5a-4
 Interesting aspects to 802.11 protocols
relative to what we’ve seen already
 Both a polling (contention-free) mode and
CSMA/CA mode for dealing with
contention
 CSMA/CA – collision avoidance rather than
CD collision detection
 Some reliable data transfer aspects
including ACKS (unlike Ethernet)
5: DataLink Layer
5a-5
Distributed vs Centralized
 802.11 working group considered 2
proposals for a MAC algorithm –
distributed access and centralized access
 Distributed access like CSMA in Ethernet
 Decision
to transmit distributed across nodes
 Makes sense especially for ad hoc network of
peer workstations
 Centralized access
 Decision when to transit controlled by
centralized decision maker, like base station/AP
 Good when network busy or when some data is
5: DataLink Layer
higher priority
5a-6
Distributed Foundation
Wireless MAC
 Compromise was Distributed Foundation
Wireless MAC (DFWMAC)
 Distributed Access control mechanism with
an optional centralized control layer on top
of that
Distributed Coordination Function (DCF) on top
of physical layer
 On top of that is optional Point Coordination
Function (PCF) that provides contention free
service

5: DataLink Layer
5a-7
Access Control
Contention Periods/
Contention-Free Periods
 The DCF and PCF respectively operate in
Contention Periods (CPs) and Contention
Free Periods (CFPs)
 In CPs, stations compete with each other
to win channel access (CSMA(
 In CFPs, an Access Point (AP) grants the
opportunity of transmission to stations by
polling
5: DataLink Layer
5a-9
Superframes
CPs and CFPs alternate in a superframe
A superframe is an interval between two
beacon frame transmissions.
A beacon frame is broadcasted by APs in
BSSs or random stations in IBSSs.
It carries management information to the
stations.
5: DataLink Layer 5a-10
IEEE 802.11 MAC Timing
PCF Superframe Construction
5: DataLink Layer 5a-11
Superframe
 Point coordinator would lock out asynchronous traffic by
issuing polls
 Superframe interval defined



During first part of superframe interval, point coordinator polls
round-robin to all stations configured for polling
Point coordinator then idles for remainder of superframe
Allowing contention period for asynchronous access
 At beginning of superframe, point coordinator may seize
control and issue polls for given period

Time varies because of variable frame size issued by responding
stations
 Rest of superframe available for contention-based access
 At end of superframe interval, point coordinator contends
for access using PIFS
 If idle, point coordinator gains immediate access



Full superframe period follows
If busy, point coordinator must wait for idle to gain access
5: DataLink
Results in foreshortened superframe period for next
cycle Layer
5a-12
IFS = interframe space
Medium Access Control Logic
Each time fail
increase time to
wait before send
Interframe Space (IFS) Values
 Actually three different IFS values
 Short IFS (SIFS)
 Shortest IFS
 Used for immediate response actions
 Point coordination function IFS (PIFS)
 Midlength IFS
 Used by centralized controller in PCF scheme when using polls
 Distributed coordination function IFS (DIFS)
 Longest IFS
 Used as minimum delay of asynchronous frames contending for
access
5: DataLink Layer 5a-14
Priority
 Stations using SIFS have “priority” over
others because they will test for idle
faster find and then start transmitting
 Others that wait longer will find the
channel busy when they listen after PIFS
or DIFSs
5: DataLink Layer 5a-15
IFS Usage
 SIFS
 Acknowledgment (ACK)
 Clear to send (CTS)
 Poll response( for PCF)
 PIFS
 Used by centralized controller in issuing polls
(for PCF)
 Takes precedence over normal contention
traffic
 DIFS
 Used for all ordinary asynchronous traffic
5: DataLink Layer 5a-16
Polling
 Since PIFS smaller than DIFS, coordinator
can seize coordinator and lock all traffic (
at least traffic that obeys the rules) while
it polls and receives responses
 When polling coordinator sends a poll to a
station, it can respond using SIFS ( beating
the next PIFS and any DIFS)
5: DataLink Layer 5a-17
Polling
 In a CFP, a PC polls the first station in its polling list,
and it may also piggyback some data to the polling frame.
 The polled station responds either with an ACK or a data
frame piggybacked to the ACK frame.
 An SIFS separates the polling and responding frames.
 Once the frame exchange sequence with the first
station is done, the PC waits for a PIFS and then polls
another station in its polling list.
5: DataLink Layer 5a-18
Reliable Data Delivery
 More efficient to deal with errors at the MAC level than
higher layer (such as TCP)

Transport layer timeouts can take seconds
 Two Frame exchange protocol
 Source station transmits data
 Destination responds with acknowledgment (ACK)
 If source doesn’t receive ACK, it retransmits frame
 Four frame exchange
 Source issues request to send (RTS)
 Destination responds with clear to send (CTS)
 Source transmits data
 Destination responds with ACK
5: DataLink Layer 5a-19
Clear To Send (CTS)
 Station can make it more likely its frame
will get though by first sending a small
Request to Sent (RTS frame)
 The recipient will then reply CTS
 Avoids the hidden terminal problem
5: DataLink Layer 5a-20
Hidden Terminal effect
 hidden terminals: A, C cannot hear each other
obstacles, signal attenuation
 collisions at B
 goal: avoid collisions at B
 CSMA/CA: CSMA with Collision Avoidance

5: DataLink Layer 5a-21
CSMA/CA
 DCF uses Carrier Sense Multiple Access (CSMA)
 CSMA means listen before you send to make sure the
medium is idle
 Collision Avoidance (CA) vs Collision Detection (CD)



CD based on listening while you send to make sure you hear only
your signal
Wireless HW not made to send and listen at same time
Large dynamic range of possible signals – cannot effectively
distinguish incoming weak signals from noise and the effects
of its own transmission
5: DataLink Layer 5a-22
Collision Avoidance: RTS-CTS
exchange
 CSMA/CA: explicit
channel reservation
 sender: send short
RTS: request to send
 receiver: reply with
short CTS: clear to
send
 CTS reserves channel for
sender, notifying (possibly
hidden) stations
5: DataLink Layer 5a-23
 RTS/CTS optional in a Contention Period
 RTS/CTS mechanism is activated when the
MAC frame length exceeds an RTS
threshold value.
 The
range of the RTS threshold is from 0 to
2347 bytes.
 However, it is usually set for the higher values
to avoid the RTS/CTS mechanism being used
for small frames, owing to the overhead of the
RTS and CTS frames.
5: DataLink Layer 5a-24
IEEE 802.11 MAC Protocol
802.11 CSMA Protocol:
others
 NAV: Network
Allocation
Vector
 802.11 frame has
transmission time field
 others (hearing data)
defer access for NAV
time units
5: DataLink Layer 5a-25
IEEE 802.11 MAC Protocol:
CSMA/CA
802.11 CSMA: sender
- if sense channel idle for
DIFS sec.
then transmit entire frame
(no collision detection)
-if sense channel busy
then binary backoff
802.11 CSMA receiver:
if received OK
return ACK after SIFS
5: DataLink Layer 5a-26
Acknowledgements
 When station received frame addressed directly
to it ( not broadcast or multicast) it replies with
an ACK after waiting SIFS
 ACKs allow for recovery from collision since no
collision detection
 Use of SIFS allows for efficient delivery of an
LLC data unit that requires multiple MAC frames


Just get SIFS between ACK and then next frame
No one else will gain control of the channel until the
entire LLC if over
5: DataLink Layer 5a-27
802.11 MAC Frame Format
All 802.11 when no security features enabled
5: DataLink Layer 5a-29
MAC Frame Fields (1)
 Frame Control ( 2 octets):
 Type of frame
 Control, management, or data
 Provides control information
• Includes whether frame is to or from DS, fragmentation
information, and privacy information
 Duration/Connection ID (2 octets):
 If used as duration field, indicates time (in s) channel will be
allocated for successful transmission of MAC frame
 In some control frames, contains association or connection
identifier
5: DataLink Layer 5a-30
MAC Addresses
 Addresses:
48-bit fields
 Number and meaning of each address field
depend on context
 Types include source, destination, transmitting
station, and receiving station

5: DataLink Layer 5a-31
MAC Address format
scenario
ad-hoc network
infrastructure
network, from AP
infrastructure
network, to AP
infrastructure
network, within DS
to DS from
DS
0
0
0
1
address 1 address 2 address 3 address 4
DA
DA
SA
BSSID
BSSID
SA
-
1
0
BSSID
SA
DA
-
1
1
RA
TA
DA
SA
DA: Destination Address, ultimate destination of frame
SA: Source Address, original source of frame
RA: Receiver Address, current receiver for this hop in a Distribution System (DS)
TA: Transmitter Address, current transmitter for this hop in a Distribution System (DS)
BSSID: Basic Service Set Identifier
for IBSS this is random number generated when LAN is formed
5: DataLink Layer 5a-32
MAC Frame Fields (2)
 Sequence Control ( 2 octets):
 4-bit fragment number subfield
• For fragmentation and reassembly
12-bit sequence number
 Number frames between given transmitter and
receiver

 Frame Body (0-2312 octets):
 MSDU (or a fragment of)
• LLC PDU or MAC control information
 Frame Check Sequence ( 4 octets):
 32-bit cyclic redundancy check
5: DataLink Layer 5a-33
MAC Frame Format
5: DataLink Layer 5a-35
Frame Control Fields
 Protocol version – 802.11 version
 Type – control, management, or data
 Subtype – identifies function of frame
 To DS – 1 if destined for DS
 From DS – 1 if leaving DS
 More fragments – 1 if fragments follow
 Retry – 1 if retransmission of previous
frame
5: DataLink Layer 5a-36
Frame Control Fields
 Power management – 1 if transmitting station is in
sleep mode

APs know the power management state for connected
clients, save frames for them and then periodically
announce that have frames waiting
 More data – Indicates that station has more data
to send
 WEP – 1 if wired equivalent protocol is
implemented
 Order – 1 if any data frame is sent using the
Strictly Ordered service
5: DataLink Layer 5a-37
5: DataLink Layer 5a-38
Management Frames
 Used to manage communications between
stations and APs
 E.g. management of associations
Association requests, association response
 Reassociation request and response
 Disassociation, authentication, deauthentication
 Probe request/probe response
 Beacon frame

• Timestamp, beacon interval, SSID, TIM…

Announcement traffic indication
5: DataLink Layer 5a-39
Association
 Association Request
 sent by a station to an AP to request an
association with this BSS
 Includes information on capability information
such as whether encryption is to be used and
whether this station is pollable
 Association Response
 Returned by the AP to the station to indicate
whether it is accepting this association request
 Disassociation
 Used by station to terminate an association
5: DataLink Layer 5a-40
Reassociation
 Reassociation Request
Sent by a station when it moves from one BSS
to another and needs to make an association
with the AP in the new BSS
 Uses reassociation rather than association so
that the new AP knows to negotiate with the old
AP for the forwarding of data frames

 Reassociation Response
 Returned by the AP to the station to indicate
whether it is accepting this reassociation
request
5: DataLink Layer 5a-41
Authentication
 Authentication
Used to authenticate one station to another in
order to set up secure communications
 In wired, physical connection implies authority
to connect
 Various authentication schemes in 802.11

 Deauthentication
 Sent by station to another station or AP to
indicate that it is terminating secure
communications
 Invoked when existing authentication is
terminated
 Privacy
 Prevents message contents from being read by
5: DataLink Layer
unintended recipient
5a-42
Other
 Probe request /response
 Used by station to obtain information from
another station or AP
 Beacon
 Transmitted periodically to allow mobile
stations to locate and identify a BSS
 Announcement Traffic Indication Message
 Sent by mobile station to alert other stations
that may have been in low power mode that this
station has frames buffered and waiting to be
delivered to the station addressed in the frame
5: DataLink Layer 5a-43
Control Frames Sub Types
 Assist in reliable data delivery
 Power Save-Poll (PS-Poll)
 Sent by any station to station that includes AP
 Request AP transmit frame buffered for this station while station in powersaving mode
 Request to Send (RTS)
 First frame in four-way frame exchange
 Clear to Send (CTS)
 Second frame in four-way exchange
 Acknowledgment (ACK)
 Contention-free (CF)-end
 Announce the end of a contention free period
 CF-end + CF-ack
 This frame ends the CFP and releases stations associated with that period
5: DataLink Layer 5a-44
Data Frame Subtypes
 Data-carrying frames
 Data – can be sent during Contention Free Period (CFP) or
Contention Period (CP)
 Only in CFP
• Data + CF-Ack ( data plus ack of previously received data)
• Data + CF-Poll ( sent by point coordinator deliver data plus request
buffered data from station)
• Data + CF-Ack + CF-Poll (BOTH)
 Other subtypes (don’t carry user data even though they
are data type)


Null Function, CF-Ack, CF-Poll, CF-Ack + CF-Poll
Null used only to carry the power management bit in the frame
control field to the AP to indicate station is changing to lower
power mode
5: DataLink Layer 5a-45
802.11 Physical Layer
Standards
Op. Freq.
Data Rate
Typical/Max
(Mbit/sec)
Range
Indoor/Outdoor
(meters)
Legacy
802.11-1997
2.4 GHz
1/2
?
802.11a (1999)
5 GHz
25/54
15-30
802.11b (1999)
2.4 GHz
5.5/11
45-90
802.11g(2003)
2.4 GHz
25/54
45-90
802.11n(2009)
5 and 2.4 GHz
144/600
91/182
5: DataLink Layer 5a-46
 802.11b was the first, followed by 802.11a (
higher BW, less popular)
 802.11g higher BW, directly compatible
with b
 802.11n – even higher BW, backwards
compatible with b and g
 Others
802.11ad (Using 2.4 GHz, 5 GHz and 60 GHz,
theoretical max throughput of up to 7Gbit/s;
2014?)
 802.11ac (high throughput in the 5 GHz band,
2014?)

5: DataLink Layer 5a-47
Outtakes
5: DataLink Layer 5a-48
Original Wired Equivalent Privacy
 Included in the security and privacy
features of the original 802.11
specification
 Unfortunately quite weak in several ways
 Does
not provide protection from other
legitimate users ( equivalent of a wired hub)
 Easy to break the encryption even if not a
legitimate user
5: DataLink Layer 5a-49
Progression
 Original WEP in 1999
 Longer key WEP
 Superseded by WiFi Protected Access
(WPA) in 2003
 Then by the full 802.11i security standard
(also known as WPA2) in 2004
5: DataLink Layer 5a-50
WEP keys
 Original 64 bit WEP used a 40 bit
encryption key
 Later revision, 128 bit WEP enabled use of
a 104 bit key (once US govt regulations
relaxed on export of crytoanalysis based
on key size)
 Some 256 bit WEP products
 WEP keys entered as a string of hex digits
 Each
digit represents 4 bits of the key
 Thus 26 hex digits for 104 bit key is 128 bit
WEP
5: DataLink Layer 5a-51
Avoid Repetition of Keys

WEP uses RC4 a stream cipher
 Stream ciphers are vulnerable to attack if the same key is used twice
(depth of two) or more.
 The purpose of the IV (which is sent in plain text) is to prevent
repetition of the traffic key
 WEP key concatenated with an 24 bit initialization vector to form
an RC4 traffic key
5: DataLink Layer 5a-52
Problems
 However 24 bits will wrap quickly on a busy
network
 If you capture enough packets you will get
repeated IVs (passive attack)
 Can also stimulate traffic (active attack)

Esp helpful for longer keys since cracking them
takes more traffic
5: DataLink Layer 5a-53
802.11i
 Addresses 3 main security areas
Authentication
 Key management
 Data Transfer Privacy

5: DataLink Layer 5a-54
802.11 architecture
 Already have stations and access point
(AP); Add a new entity, the authentication
server (AS)
 Exchange between station and AS provides
for secure authentication
 AS also responsible for key distribution to
the access point, which in turn manages and
distributes keys to stations
5: DataLink Layer 5a-55
Features
 Access control
 Authentication
 Privacy with message integrity
5: DataLink Layer 5a-56
Access Control
 Uses the IEEE 802.1X Port-Based Network
Access Control Standard
 In that language
Station = supplicant
 Access point = authenticator
 Authentication server = authentication server

5: DataLink Layer 5a-57
Access Control Process
 Before a supplicant is authenticated by the
AS using an authentication protocol, the
authenticator or AP just passes control or
authentication messages between the
supplicant and AS

Ie data channel is blocked
 Once supplicant is authenticated and keys
provided, then they can use the data
channel
5: DataLink Layer 5a-58
Authentication
 Open system authentication
 Exchange of identities, no security benefits
 Shared Key authentication
 Shared Key assures authentication
 Actual choice of protocols used for authentication
not defined in 802.11 (just ability to exchange
capabilities and agree on one of the choices)

Popular choices include: Remote Authentication Dial-In
User Service (RADIUS) and Extensible Authentication
Protocol (EAP)
5: DataLink Layer 5a-59
Privacy with message integrity
 Stronger encryption also used to protect
data transfer between the station and the
AP
 Three schemes
 Best
long-term uses Advanced Encryption
Standard (AES) with 128 bit keys
 Alternative schemes based on 104-bit RC4
because use of AES requires expensive
hardware upgrade to existing equipment
• Temporal Key Integrity Protocol (TKIP)
5: DataLink Layer 5a-60
WPA
 To prevent related-key attacks, a replacement for WEP, Wi-Fi




Protected Access (WPA)
Uses three levels of keys: master key, working key and RC4 key.
The working keys are then combined with a longer, 48-bit IV to
form the RC4 key for each packet.
This design mimics the WEP approach enough to allow WPA to be
used with first-generation Wi-Fi network cards, some of which
implemented portions of WEP in hardware.
However, not all first-generation access points can run WPA.

Upgrade firmware on wireless access points
5: DataLink Layer 5a-61
Michael
 In addition to authentication and encryption, WPA also provides
vastly improved payload integrity.
 The cyclic redundancy check (CRC) used in WEP is inherently
insecure; it is possible to alter the payload and update the message
CRC without knowing the WEP key.
 A more secure message authentication code (usually known as a
MAC, but here termed a MIC for "Message Integrity Code") is
used in WPA, an algorithm named "Michael".


The Michael algorithm was the strongest that WPA designers could
come up with that would still work with most older network cards. Due
to inevitable weaknesses of Michael, WPA includes a special
countermeasure mechanism that detects an attempt to break TKIP and
temporarily blocks communications with the attacker.
The MIC used in WPA includes a frame counter, which prevents replay
attacks being executed.
5: DataLink Layer 5a-62
WPA2
 More conservative approach is to employ a cipher
designed to prevent related-key attacks
altogether, usually by incorporating a strong key
schedule.
 WPA2, uses the AES block cipher instead of RC4,
in part for this reason.
 Some older network cards, however, cannot run
WPA2.
5: DataLink Layer 5a-63
WPA Personal vs Enterprise
 For Enterprise, you need to run an authentication
server like a RADIUS server


Standalone, built into access point, even third part
services
Every new session gets its own fresh random key, used
for a relatively short time.
 For Personal/Home, WPA-PSK (WPA with Pre-
Shared Key)


Uses a passphrase entered on all legitimate clients
Shared key used to generate a transient key
5: DataLink Layer 5a-64
Pairwise Transient Keys
 The AP and each station need an individual
Pairwise Transient Key (PTK) to protect
unicast communication between them.
To derive a different PTK for each AP/station
combo, a Pairwise Master Key (PMK) is fed into
an algorithm, along with MAC address (define)
and two values, ANonce and SNonce.
 AP and station derive the same PTK without
ever sending it over the air.

5: DataLink Layer 5a-65
Group Transient Key
 The AP also generates a Group Transient
Key (GTK) to protect all broadcast and
multicast communication.

Every station on the WLAN needs that same
GTK to decrypt broadcast/multicast frames,
the AP sends the current GTK to each station
encrypted with the PTK.
5: DataLink Layer 5a-66
Message Integrity Code
 To stop these handshake messages from
being forged, messages carry a Message
Integrity Code (MIC).
 Each MIC is generated by hashing a
specified part of the message, then
encrypting that hash with the PTK.
5: DataLink Layer 5a-67
Ways to crack
 Dictionary attack on shared passphrase or
exhaustive search on short 8 character
ones

When configuring a passphrase, the IEEE
802.11i standard strongly recommends using at
least 20 characters.
 Existence of precomputed WPA PSK lookup
tables for common SSIDs

Give your WLAN a unique SSID.
5: DataLink Layer 5a-68
802.11 Physical Layer
Standards
Op. Freq.
Data Rate
Typical/Max
(Mbit/sec)
Range
Indoor/Outdoor
(meters)
Legacy
802.11-1997
2.4 GHz
½
?
802.11a (1999)
5 GHz
25/54
15-30
802.11b (1999)
2.4 GHz
5.5/11
45-90
802.11g(2003)
2.4 GHz
25/54
45-90
802.11n(2009)
5 and 2.4 GHz
144/600
91/182
5: DataLink Layer 5a-69
 802.11b was the first, followed by 802.11a (
higher BW, less popular)
 802.11g higher BW, directly compatible
with b
 802.11n – even higher BW, backwards
compatible with b and g
5: DataLink Layer 5a-70
RC4
 WEP uses RC4 a stream cipher
Stream ciphers are vulnerable to attack if the same key is used
twice (depth of two) or more.
 Say we send messages A and B of the same length, both encrypted
using same key, K. The stream cipher produces a string of bits C(K)
the same length as the messages. The encrypted versions of the
messages then are:
 E(A) = A xor C
 E(B) = B xor C
 where xor is performed bit by bit.





Say an adversary has intercepted E(A) and E(B). He can easily
compute:
E(A) xor E(B)
However xor is commutative and has the property that X xor X
= 0 (self-inverse) so:
E(A) xor E(B) = (A xor C) xor (B xor C) = A xor B xor C xor C =
A xor B
5: DataLink Layer 5a-71