EEC-484/584 Computer Networks Lecture 8 Wenbing Zhao [email protected]

EEC-484/584EEC-484/584Computer NetworksComputer Networks

Lecture 8Lecture 8

Wenbing ZhaoWenbing Zhao

[email protected]@ieee.org

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Spring Semester 2008Spring Semester 2008 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao

OutlineOutline• Reminder:

– 3/3 (Monday): Lab #3– 3/5 (Wednesday): Quiz #2

• 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 AccessCarrier 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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• 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

Carrier Sense Multiple Access:Carrier Sense Multiple Access:Collision Still PossibleCollision Still Possible

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pp-persistent CSMA:-persistent CSMA:Reduce the Probability of CollisionReduce 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 CSMANon-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 CSMAPersistent and Nonpersistent CSMA

• Improves over ALOHA because they ensure no station to transmit when it senses channel is busy

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Manchester EncodingManchester 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 StructureEthernet 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

>= 64 bytesNot considered as part of the

header!

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Ethernet Frame StructureEthernet 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

1111


Ethernet MAC Sublayer ProtocolEthernet MAC Sublayer Protocol

• Uses 1-persistent CSMA/CD

• Binary exponential backoff

• Provides unreliable connectionless service

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CSMA with Collision DetectionCSMA 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 CollisionMinimum Time to Detect Collision

• To ensure the sender can detect collision– All frames must take more than 2 to send so that

transmission is still taking place when the noise burst gets back to the sender

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Randomization and Randomization and Binary Exponential BackoffBinary Exponential Backoff

• Time divided into slots– Length of slot = 2 = worst-case round-trip

propagation time– To accommodate longest path, slot time = 512 bit

times = 51.2 sec (10Mbps Ethernet)

• Binary exponential backoff

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Randomization and Randomization and Binary Exponential BackoffBinary 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 PerformanceEthernet 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– Network bandwidth increases– Cable length increases– Number of stations increases– Frame length decreases

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Ethernet PerformanceEthernet Performance

Efficiency of Ethernet at 10 Mbps with 512-bit slot times

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Switched EthernetSwitched 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 ARP –– Address Resolution Protocol Address Resolution Protocol

How do IP addresses get mapped onto data link layer addresses, such as Ethernet?

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ARP OptimizationARP 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: ARP: How to Handle Remote TrafficHow 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 – ExerciseARP – 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 RARP –– Reverse Address Resolution ProtocolReverse Address Resolution Protocol

• RARP - Allows a newly-booted diskless-workstation (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

32-bit Internet address

48-bit Ethernet address

ARP RARP

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Limitations of RARPLimitations 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 BOOTP –– Bootstrap Protocol 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 ProtocolDynamic 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)– 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 ScenarioDHCP Client-Server Scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

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DHCP Client-Server ScenarioDHCP Client-Server ScenarioDHCP server: 223.1.2.5 arriving

client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

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DHCP ReplayDHCP 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 AgentDHCP 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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ExerciseExercise

• 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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ExerciseExercise

• 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?

Documents

EEC-484/584 Computer Networks Lecture 8 Wenbing Zhao [email protected]