CSE331:
Introduction to Networks
and Security
Lecture 6
Fall 2002
Announcements
• Project 1 will be handed out this Friday
– Form groups of two or three
– Mail group members to Aditya
achadha@gradient.cis.upenn.edu
– If you can’t find a partner, mail Aditya
– Groups should be formed before project is handed
out
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Recap
• Ethernet
– Exponential backoff algorithm
• 802.11
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Today
• Finish up link layer
– 802.11
– (briefly) Token Rings
• Packet Switching
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Multiple Access Collision Avoidance
• Sender transmits Request To Send (RTS)
– Includes length of data to be transmitted
– Timout leads to exponential backoff (like Ethernet)
• Receiver replies with Clear To Send (CTS)
– Echoes the length field
• Receiver sends ACK of frame to sender
• Any node that sees CTS cannot transmit for
durations specified by length
• Any node that sees RTS but not CTS is not
close enough to the receiver to interfere
– It’s free to transmit
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Wireless Access Points
Distribution System
A
AP1
B
AP3
AP2
D
C
• Distribution System – wired network infrastructure
• Access points – stationary wireless device
• Roaming wireless
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Selecting an Access Point
• Active scanning
– Node sends a Probe frame
– All AP’s within reach reply with a Probe Response
frame
– Node selects an AP and sends Association
Request frame
– AP replies with Association Response frame
• Passive scanning
– AP periodically broadcasts Beacon frame
– Node sends Association Request
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Node Mobility
Distribution System
A
AP1
B
AP3
AP2
B
D
C
• B moves from AP1 to AP2
• B sends Probes, eventually prefers AP2 to
AP1
• Sends Association Request
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Frame Format
16
16
48
48
48
16
48
32
Ctrl Length Addr1 Addr2 Addr3 Seq Addr4 Body CRC
• Ctrl: flags (CTS, RTS, or Data?)
• Body up to 2312 bytes
• 4 addresses
3
2
1
4
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802.11 Security Issues
• Packet sniffing is worse
–
–
–
–
–
No physical connection needed
Long range (6 blocks)
Current encryption standards (WEP) not that good
WEP = Wired Equivalent Privacy
http://www.nakedwireless.ca/winudcol.htm
• Denial of service
– Association (and Disassociation) Requests are not
authenticated
• We’ll talk more about these issues in the security part
of the course.
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Token Rings
• IBM Token Ring (IEEE 802.5)
– Support 4Mbps or 16Mbps over twisted pair for about 250
nodes.
• FDDI = Fiber Distributed Data Interface
– It supports 100Mbps for as much as 200km of fiber and 500
nodes (with at most 2km between nodes).
Data always flows
one direction
around the ring.
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Token Ring MAC
• The token is a special
bit pattern
– Sender gets the token
– Inserts a frame
– Waits for the frame to
return
– Forwards the token
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Token Ring Issues
• THT = Token Hold Time
– Prevent one node from hogging the network
– Higher THT = better utilization, but not as fair
– Typical THT = 10ms for IBM Token Ring
• What happens when a node fails?
– Must ensure that ring is unbroken.
– What happens if the token is lost?
• Nodes elect a monitor station
– Periodically sends “status OK” message
– Ensures that there is always one token.
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OSI Reference Model
Application
Presentation
Session
Transport
Network
Next: Packet switching, IP
Data Link
Covered so far: Ethernet, 802.11, Token Rings
Physical
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Packet Switching
• A switch
– Has many inputs and many outputs
– Takes packets that arrive on an input and forwards
them to the right output
Switch
• Key problem: finite output bandwidth
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Star Topology
• Scalability
– Large networks can be
built by interconnecting
switches.
– Can connect via high
bandwidth point-to-point
links = large distances.
– Adding a new host to a
switch doesn’t
necessarily degrade
performance.
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Switching Issues
• Contention
– Arrival rate of packets going to the same output
exceeds output capacity
– Switch buffers packets
• Congestion
– Switch runs out of buffer space
– Forces packets to be dropped
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Forwarding Decision
• How does the switch know where to forward a
packet?
– Looks at the packet header to make the decision
• Common approaches
– Datagram (or connectionless)
e.g. IP
– Virtual Circuit (or connection-oriented)
e.g. Frame Relay, ATM
– (Less common) Source routing
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Datagram approach
• Every packet contains a complete destination
address
– Enough information so that any switch can decide
where the packet goes.
• Features of datagram approach
– Packets can be sent at anywhere at any time
– Sender doesn’t know if network can deliver the
packet (or if destination host is available)
– Each packet is forwarded independently (two
packets may take different routes)
– Possible to route around switch or link failures
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Forwarding Tables
• Provide route information.
• Easy to determine if network
is known (and unchanging)
Forwarding table
for switch 2.
Dest.
Port
A
3
B
2
C
3
D
3
E
0
F
1
G
2
H
2
C
D
3
0
1
E
1
3
2
0
2
1
F
2
A
G
3
0
3
1
B
2
H
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Port
numbers
20
Virtual Circuit Switching
• VCI = Virtual Circuit Identifier
• Incoming port + VCI uniquely
identify virtual circuit
• Setup phase constructs
circuit table entries at
C
each switch
D
0
3
1
E
1
3
11
2
5
0
2
1
F
2
7
A
Switch In Port In VCI Out Port Out VCI
1
2
5
1
11
2
3
11
2
7
3
0
7
1
8
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G
3
0
3
1
4
B
2
H
21
Virtual Circuits
• Setup phase
– Initial setup message contains complete
destination address
– Intermediate switches (outgoing pass)
• Allocate an entry in the table
• Record In Port, Out Port
• Generate incoming VCI
– Intermediate switches (return pass)
• Get the outgoing VCI from next hop
• Reply to previous hop with the incoming VCI
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Virtual Circuit Switching Features
• Sender must wait for 1 RTT (minimum) before
first data is sent
• Per-packet overhead reduced
– After setup, only port # & VCI needed (small)
– Compare to full address in datagrams (big)
• If a switch or link fails, connection is broken
– Also, must deallocate old entries to free up space
• Can allocate resources to the virtual circuit
– Buffer space for reliable, in order delivery
– Percentage of outgoing bandwidth (QoS)
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Source Routing
• Sender knows net
topology
• Indicates sequence
of ports as part of
packet
D
C
3
0
1
1
3
2,1
2
1,2,1
– (many implementations)
E
0
2
1
F
2
1
A
• Headers of variable
(unbounded?) length
G
3
0
3
1
B
2
H
• IP includes source
route option
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Bridges and LAN Switches
A
B
C
• Bridge accepts LAN frames
on one port, outputs them on
another.
• Optimization: only forward
appropriate frames
1
Bridge
2
X
Y
Z
• Learning bridges
– watch incoming source address A at port number X
– add entry to forward address A to port X
– if no entry, broadcast to all ports
– doesn’t work if there are loops!
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