EEC-484/584 Computer Networks Lecture 8 Wenbing Zhao wenbing@ieee.org

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

Lecture 8Lecture 8

Wenbing ZhaoWenbing Zhao

wenbing@ieee.orgwenbing@ieee.org

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

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

• 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

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– …

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

Persistent and Nonpersistent CSMAPersistent and Nonpersistent CSMA

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

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

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!

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

Ethernet MAC Sublayer ProtocolEthernet MAC Sublayer Protocol

• Uses 1-persistent CSMA/CD

• Binary exponential backoff

• Provides unreliable connectionless service

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

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

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

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

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

Ethernet PerformanceEthernet Performance

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

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

ARP ARP –– Address Resolution Protocol Address Resolution Protocol

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

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

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

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?

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

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

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

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

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

DHCP server

arriving DHCP client needsaddress in thisnetwork

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

client

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

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

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

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?

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?

EEC-484/584 Computer Networks Lecture 8 Wenbing Zhao wenbing@ieee.org

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