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

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Unformatted text preview: Networking Prof. Sirer CS 4410 Cornell University Ethernet and Local Area Networking 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 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 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 Ethernet basics An ethernet packet Destination Address Type Source Address Data Checksum Sending packets Carrier sense, broadcast if ether is available 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 Im at 1a:34:2c:9a:de:cc Addressing & DHCP DHCP is used to discover network addresses I just got here. My physical address is 1a:34:2c:9a:de:cc. Whats my IP? 128.84.96.90 DHCP Server ??? 128.84.96.91 Your IP is 128.84.96.89 for the next 24 hours Collisions What happens if two people decide to transmit simultaneously ? 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 Collisions What happens if the packets are really short ? 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 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) 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 Gigabit Ethernet...
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09-networking - Networking Prof. Sirer CS 4410 Cornell...

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