eec-484/584 computer networks
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EEC-484/584 Computer Networks. Lecture 10 Wenbing Zhao [email protected] (Part of the slides are based on materials supplied by Dr. Louise Moser at UCSB and Prentice-Hall). Outline. Manchester Encoding The Ethernet MAC Sublayer Protocol The Binary Exponential Backoff Algorithm - PowerPoint PPT PresentationTRANSCRIPT
EEC-484/584EEC-484/584Computer NetworksComputer Networks
Lecture 10Lecture 10
Wenbing ZhaoWenbing Zhao
[email protected]@ieee.org(Part of the slides are based on materials supplied by (Part of the slides are based on materials supplied by
Dr. Louise Moser at UCSB and Prentice-Hall)Dr. Louise Moser at UCSB and Prentice-Hall)
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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OutlineOutline
• Manchester Encoding• The Ethernet MAC Sublayer Protocol• The Binary Exponential Backoff Algorithm• Switched Ethernet• ARP and DHCP• Reminder: Quiz 2
– Oct 19 Thursday 4-6pm (TTh Session)– Oct 23 Monday 2-4pm (MW Session)
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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 bytes
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
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Minimum Frame LengthMinimum Frame Length• 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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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Ethernet MAC Sublayer ProtocolEthernet MAC Sublayer Protocol
• Uses 1-persistent CSMA/CD
• Binary exponential backoff
• Provides unreliable connectionless service
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
88Randomization 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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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Ethernet PerformanceEthernet Performance
• Efficiency of Ethernet at 10 Mbps with 512-bit slot times
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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ARP – Address Resolution ProtocolARP – 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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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?
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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BOOTP – Bootstrap ProtocolBOOTP – 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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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:– 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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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DHCP ReplayDHCP Replay• A DHCP relay agent is can be configured on each LAN• The agent stores the IP address of the DHCP server and
forward the request to the server
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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, including all its headers. Is padding needed in the Ethernet frame, and if so, how many bytes?
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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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?
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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ExerciseExercise
• A switch designed for use with fast Ethernet has a backplane that can move 10 Gbps. How many frames/sec can it handle? Assume there is an endless stream of 64-byte (512-bit) frames.
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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ExerciseExercise
• How long does a station, s, have to wait in the worst case before it can start transmitting its frame over a LAN that uses the basic bit-map protocol? (Assume N stations, each frame is d bits)
Spring Semester 2006Spring Semester 2006 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao
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ExerciseExercise
• Six stations, A through F, communicate using the MACA protocol. Is it possible that two transmissions take place simultaneously?