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09-networking - Networking Prof Sirer CS 4410 Cornell...

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Networking Prof. Sirer CS 4410 Cornell University
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Ethernet and Local Area Networking
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Ethernet 1976, Metcalfe & Boggs at Xerox w Later at 3COM Based on the Aloha network in Hawaii w Named after the “ luminiferous ether” Centered around a broadcast bus Can use different physical links Simple link-level protocol, scales well Simple algorithm for sharing the network well under load
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Ethernet Goals Connect local area networks w Few buildings, short distances (<1 km) Inexpensively w Low infrastructure costs Without bottlenecks w No expensive routers, bridges, switches etc. w No state in the network, no store-and-forward Tremendously successful Simple conceptual model still in use w Despite two orders of magnitude increase in bandwidth
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“CSMA/CD” Carrier sense n Listen before you speak Multiple access n Multiple hosts can access the network Collision detect n Detect and respond to cases where two hosts collide
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Ethernet basics An ethernet packet Destination Address Type Source Address …Data… Checksum
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Sending packets Carrier sense, broadcast if ether is available
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Addressing & ARP ARP is used to discover physical addresses w ARP = Address Resolution Protocol “What is the physical address of the host named 128.84.96.89” 128.84.96.90 128.84.96.89 128.84.96.91 “I’m at 1a:34:2c:9a:de:cc”
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Addressing & DHCP DHCP is used to discover network addresses “I just got here. My physical address is 1a:34:2c:9a:de:cc. What’s my IP?” 128.84.96.90 DHCP Server ??? 128.84.96.91 “Your IP is 128.84.96.89 for the next 24 hours”
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Collisions What happens if two people decide to transmit simultaneously ?
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Collision Detection & Retransmission The hosts involved in the collision stop data transmission, sleep for a while, and attempt to retransmit How long they sleep is determined by how many collisions have occurred before They abort after 16 retries, hence no guarantee that a packet will get to its destination Advantages: n Packet can be retransmitted at the link level immediately without high-level timeouts, n Packets are truncated early to avoid wasting bandwidth n Collision rates can be used to gauge net usage
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Collisions What happens if the packets are really short ?
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Odds & Ends Minimum packet size is 64 bytes, which is just right for the maximum length of an Ethernet wire for all hosts to detect a collision Truncated packets are filtered out of the network CRC is used to detect malformed packets, e.g. electrical interference, noise
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Ethernet Features Completely distributed w No central arbiter Inexpensive w No state in the network w No arbiter w Cheap physical links (twisted pair of wires)
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Ethernet Problems The endpoints are trusted to follow the collision-detect and retransmit protocol w Certification process tries to assure compliance w Not everyone always backs off exponentially Hosts are trusted to only listen to packets destined for them w But the data is available for all to see w Can place ethernet card in promiscuous mode and listen
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Gigabit Ethernet
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