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Berkeley COMPSCI 162 - Lecture 22 Networking II

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Multiple Access Algorithm Single shared broadcast channel Avoid having multiple nodes speaking at once Otherwise collisions lead to garbled data Multiple access mechanism Distributed algorithm for sharing the channel Algorithm determines which node can transmit Classes of techniques Channel partitioning divide channel into pieces Taking turns scheme for trading off who gets to transmit Random access allow collisions and then recover Optimizes for the common case of only one sender CS162 Operating Systems and Systems Programming Lecture 22 Networking II April 13 2010 Ion Stoica http inst eecs berkeley edu cs162 4 13 10 Random Access Protocol AlohaNet When new data arrived at site send to hub for transmission Site listened to broadcast channel If it heard data repeated knew transmission was rec d If it didn t hear data correctly it assumed a collision Hub at Alohanet HQ Univ Hawaii Oahu Other sites spread among the islands If collision site waited random delay before retransmitting Problem Stability what if load increases Had two radio channels Random access sites sent data on this channel Broadcast only used by hub to rebroadcast incoming data CS162 UCB Spring 2010 Lec 22 2 Aloha Transmission Strategy Norm Abramson left Stanford in search of surfing Set up first radio based data communication system connecting the Hawaiian islands 4 13 10 CS162 UCB Spring 2010 More collisions less gets through more resent more load More collisions Unfortunately some sender may have started in clear get scrambled without finishing 4 13 10 Lec 22 3 Page 1 CS162 UCB Spring 2010 Lec 22 4 Ethernet Ethernet s CSMA CD CSMA Carrier Sense Multiple Access CD Collision detection Sense channel if idle Bob Metcalfe Xerox PARC visits Hawaii and gets an idea Shared medium coax cable Can sense carrier to see if other nodes are broadcasting at the same time If detect another transmission Abort send jam signal Delay and try again Else Send frame Sensing is subject to timelag Only detect those sending a short while before Receiver accepts Frames addressed to its own address Frames addressed to the broadcast address broadcast Frames addressed to a multicast address if it was instructed to listen to that address All frames promiscuous mode Monitor channel to detect collisions Once sending can tell if anyone else is sending too 4 13 10 CS162 UCB Spring 2010 4 13 10 Lec 22 5 Ethernet s CSMA CD more Host 1 a Time t Host 1 starts to send frame Goal adapt retransmission attempts to estimated current load Heavy load random wait will be longer First collision choose K from 0 1 delay is K x 512 bit transmission times After second collision choose K from 0 1 2 3 After ten or more collisions choose K from 0 1 2 3 4 1023 c Time t 2 d Host 1 hears Host 2 s frame detects collision Give a host enough time to detect collisions In Ethernet minimum packet size 64 bytes What is the relationship between minimum packet size and the length of the LAN Host 2 propagation delay d b Time t d Host 2 starts to send a frame just before it hears from host 1 s frame Minimum packet size CS162 UCB Spring 2010 Lec 22 6 Minimum Packet Size more Exponential back off 4 13 10 CS162 UCB Spring 2010 Host 1 Host 2 propagation delay d Host 1 Host 2 propagation delay d d LAN length ligh speed min frame size 2 bandwidth LAN length min frame size light speed 2 bandwidth 8 64b 2 5 108mps 2 107 bps 6400m approx What about 100 mbps 1 gbps 10 gbps 4 13 10 Lec 22 7 Page 2 CS162 UCB Spring 2010 Lec 22 8 Goals for Today Review Point to point networks Networking Switch MapReduce primer project 4 Router Internet Network layer Transport layer start Point to point network a network in which every physical wire is connected to only two computers Switch a bridge that transforms a shared bus broadcast configuration into a point to point network Hub a multiport device that acts like a repeater broadcasting from each input to every output Router a device that acts as a junction between two networks to transfer data packets among them 4 13 10 CS162 UCB Spring 2010 4 13 10 Lec 22 9 CS162 UCB Spring 2010 Network IP Layer IP Routing Each packet is routed individually like a letter Packets of same connection may take different paths Host C Deliver a packet to specified network destination Packet forwarding routing Perform segmentation reassemble Others Host D Host A packet scheduling buffer management router 1 router 5 Usually done based on destination address Host B Routing the process of computing paths between endpoints and building forwarding tables at routers CS162 UCB Spring 2010 router 2 router 3 Packet forwarding the process of selecting outgoing link next hop to forward a packet 4 13 10 Lec 22 10 router 4 4 13 10 Lec 22 11 Page 3 router 6 CS162 UCB Spring 2010 router 7 Host E Lec 22 12 IP Routing Packet Forwarding Each packet is routed individually like a letter Packets of same connection may take different paths Host C Host D Host A router 1 IP v4 addresses 32b Quad notation bytes separated by dots x don t care 1 Is matched according to longest prefix matching rule router 2 router 3 2 Packet is forwarded to the corresponding output port router 5 Internet At each router the packet destination address Forwarding table 128 15 xxx xxx 3 128 15 11 xxx 2 16 82 100 xxx 2 Router Host B router 7 router 6 router 4 Host E data 16 25 31 10 128 15 11 12 source address 4 13 10 CS162 UCB Spring 2010 destination address 4 13 10 Lec 22 13 Internet Routing Two Level Hierarchy 1 2 interconnection backplane CS162 UCB Spring 2010 interfaces Lec 22 14 Administrivia Autonomous system AS network owned by one admin entity e g ATT Comcast Intra domain routing within an AS I ll be away Wednesday Friday Eurosys Thursday s lecture will be taught by Ben No office hour on Thursday April 15 e g link state distance vector protocols Inter domain use across ASes Matei and Andy will be away as well Border Gateway Protocol BGP Ben will teach the discussion sections of both Matei and Andy No office hours for Andy and Matei next week Interior router BGP router AS 1 Project 4 Initial design Wednesday 4 21 will give you two discussion sections before deadline Code deadline Wednesday 5 5 two weeks later AS 3 AS 2 4 13 10 CS162 UCB Spring 2010 4 13 10 Lec 22 15 Page 4 CS162 UCB Spring 2010 Lec 22 16 Transport Layer Transport Layer Demultiplexing Demultiplex packets at the receiver decide to which process to deliver a packet Others Ports end points at receiver to which packets are delivered 16b in IPv4 Proc A


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Berkeley COMPSCI 162 - Lecture 22 Networking II

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