Transcript lecture8

EEC-484/584
Computer Networks
Lecture 8
Wenbing Zhao
[email protected]
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Outline
• Reminder:
– 3/3 (Monday): Lab #3
– 3/5 (Wednesday): Quiz #2
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CSMA protocols
Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Switched Ethernet
ARP and DHCP
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Carrier Sense Multiple Access
• When station has data to send, listens to channel
to see if anyone else is transmitting
• If channel is idle, station transmits a frame
• Else station waits for it to become idle
• If collisions occurs, station waits random amount
of time, tries again
• Also called 1-persistent CSMA
– With probability 1 station will transmit if channel is idle
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Carrier Sense Multiple Access:
Collision Still Possible
• After a station starts sending, it takes a while
before 2nd station receives 1st station’s signal
– 2nd station might start sending before it knows that
another station has already been transmitting
• If two stations become ready while third station
transmitting
– Both wait until transmission ends and start
transmitting, collision results
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p-persistent CSMA:
Reduce the Probability of Collision
• Sense continuously, but does not always send
when channel is idle
– Applicable for slotted channels
• When ready to send, station senses the channel
– If channel idle, station transmits with probability p,
defers to next slot with probability q = 1-p
– Else (if channel is busy) station waits until next slot
tries again
– If next slot idle, station transmits with probability p,
defers with probability q = 1-p
– …
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Non-Persistent CSMA
• Does not sense continuously, send if it senses
the channel is idle
• Before sending, station senses the channel
– If channel is idle, station begins sending
– Else station does not continuously sense, waits
random amount of time, tries again
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Persistent and Nonpersistent CSMA
• Improves over ALOHA because they ensure no
station to transmit when it senses channel is busy
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Manchester Encoding
• Binary encoding
– Hard to distinguish 0 bit (0-volt) from idle (0-volt)
– Requires clocks of all stations synchronized
• Manchester encoding and differential Manchester
encoding
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Ethernet Frame Structure
• Preamble: for clock synchronization
– First 7 bytes with pattern 10101010, last byte with
pattern 10101011
– The two consecutive 1’s indicate the start of a frame
• How can the receiver tell the end of the frame?
– No current on the wire
Not considered
as part of the
header!
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>= 64 bytes
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Ethernet Frame Structure
• Destination address: 6 bytes (48 bits)
– Highest order bit: 0 individual, 1 multicast;
all 1’s broadcast
– Frames received with non-matching destination
address is discarded
• Type: type of network layer protocol
• Pad – used to produce valid frame >= 64 bytes
• Checksum – 32-bit cyclic redundancy check
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Ethernet MAC Sublayer Protocol
• Uses 1-persistent CSMA/CD
• Binary exponential backoff
• Provides unreliable connectionless service
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CSMA with Collision Detection
• If two stations start transmitting simultaneously,
both detect collision and stop transmitting
• Minimum time to detect collision = time for signal
to propagate
• Monitor collision while sending
– Minimum time to detect collision => minimum frame
length
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Minimum Time to Detect Collision
• To ensure the sender can detect collision
– All frames must take more than 2t to send so that
transmission is still taking place when the noise burst
gets back to the sender
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Randomization and
Binary Exponential Backoff
• Time divided into slots
– Length of slot = 2t = worst-case round-trip
propagation time
– To accommodate longest path, slot time = 512 bit
times = 51.2 msec (10Mbps Ethernet)
• Binary exponential backoff
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Randomization and
Binary Exponential Backoff
• After 1st collision, station picks 0 or 1 at random, waits
that number of slots and tries again
• After 2nd collision, station picks 0,1,2,3 at random, waits
that number of slots and tries again
• ….
• After i-th collision, station picks 0,1,…,2i-1 at random, …
• If 10 <= i < 16, station picks 0,1,…,210-1 at random
• If i=16, controller reports failure to computer
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Ethernet Performance
• Binary exponential backoff results in
– Low delay when few stations collide
– Reasonable delay for collision resolution when many
stations collide
• When other factors are fixed, channel efficiency
decreases when
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Network bandwidth increases
Cable length increases
Number of stations increases
Frame length decreases
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Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
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Switched Ethernet
• Switch – contains a high-speed backplane and room for
typically 4 to 32 plug-in line cards, each containing 1-8
connectors
– Possibly each card forms its own collision domain, or
– Full-duplex operation if each input port is buffered
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ARP – Address Resolution Protocol
How do IP addresses get mapped onto data link layer
addresses, such as Ethernet?
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ARP Optimization
• ARP result is cached (step 5 in figure)
• When A wants to communicate with B, A includes its
IP-to-Ethernet mapping in the ARP packet so that B
knows the mapping right away (step 3 in figure)
• Have every machine broadcast its mapping when it
boots, so that everyone else knows the mapping
• To accommodate changes, entries in the ARP cache
time out after a few minutes
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ARP: How to Handle Remote Traffic
• Proxy ARP – A router is configured to answer ARP
requests on one of its networks for a host on another
network
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ARP – Exercise
• Node 1 wants to send a packet to node 3, what will be
returned by ARP?
• Node 1 wants to send a packet to node 2, what will be
returned by ARP?
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RARP –
Reverse Address Resolution Protocol
32-bit Internet address
ARP
RARP
48-bit Ethernet address
• RARP - Allows a newly-booted disklessworkstation (e.g., X terminal) to broadcast its
Ethernet address and ask for its IP address
– RARP server responds to a RARP request with the
assigned IP address
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Limitations of RARP
• RARP uses a link-layer broadcast, RARP
requests are not forwarded by routers, therefore,
an RARP server must be present on every
network
• The only thing returned by the RARP server is
the IP address
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BOOTP – Bootstrap Protocol
• BOOTP – uses UDP
– A client broadcasts to 255.255.255.255
– The source IP address is set to 0.0.0.0 if client does
not know its own IP address yet
– Port number: 67 for server, 68 for client
• BOOTP drawbacks
– Requires manual configuration of tables mapping IP
address to Ethernet address at the BOOTP server
• Replaced by DHCP
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Dynamic Host Configuration Protocol
• Allow host to dynamically obtain its IP address
from network server when it joins network
– IP address assignment is lease-based (to cope with
client failure, also enables reuse of addresses)
– Can renew its lease on address in use
• DHCP overview (UDP is used for communication)
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Host broadcasts “DHCP discover” msg
DHCP server responds with “DHCP offer” msg
Host requests IP address: “DHCP request” msg
DHCP server sends address: “DHCP ack” msg
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DHCP Client-Server Scenario
A
B
223.1.1.2
223.1.1.4
223.1.2.9
223.1.1.3 223.1.3.27
223.1.3.1
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223.1.2.1
DHCP
server
223.1.1.1
E
223.1.2.2
223.1.3.2
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arriving DHCP
client needs
address in this
network
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DHCP Client-Server Scenario
DHCP server: 223.1.2.5
DHCP discover
arriving
client
src : 0.0.0.0, 68
dest.: 255.255.255.255,67
yiaddr: 0.0.0.0
transaction ID: 654
DHCP offer
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddr: 223.1.2.4
transaction ID: 654
Lifetime: 3600 secs
DHCP request
time
src: 0.0.0.0, 68
dest:: 255.255.255.255, 67
yiaddr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
DHCP ACK
src: 223.1.2.5, 67
dest: 255.255.255.255, 68
yiaddr: 223.1.2.4
transaction ID: 655
Lifetime: 3600 secs
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DHCP Replay
• A DHCP relay agent can be configured on each LAN
• The agent stores the IP address of the DHCP server and
forward the request to the server
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DHCP with Replay Agent
• To find its IP address, a newly-booted machine
broadcasts a DHCP Discover packet
• The DHCP relay agent on its LAN receives all
DHCP broadcasts
• On receiving a DHCP Discover packet, the
agent sends the packet as a unicast packet to
the DHCP server, possibly on a distant network
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Exercise
• An IP packet to be transmitted by Ethernet
is 60 bytes long. Is padding needed in the
Ethernet frame, and if so, how many
bytes?
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Exercise
• Consider building a CSMA/CD network
running at 1 Gbps over a 1-km cable. The
signal speed in the cable is 200,000
km/sec. What is the minimum frame size?
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