Transcript lecture8

EEC-484/584
Computer Networks
Lecture 8
Wenbing Zhao
[email protected]
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Outline
• Reminder
– Lab#3: Wednesday
– Quiz#2: Oct.15 (next Wed)
– Oct.13 (next Mon): No class, Columbus Day
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Manchester Encoding
The Ethernet MAC Sublayer Protocol
The Binary Exponential Backoff Algorithm
Switched Ethernet
ARP and DHCP
Fall Semester 2008
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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
• 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) => min frame
length: 51.2 msec X 10 Mbps = 512 b = 64 byte
• 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 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?
Fall Semester 2008
EEC-484/584: Computer Networks
Wenbing Zhao