CS162 Operating Systems and Systems Programming Lecture 22 Networking IIMultiple Access AlgorithmRandom Access Protocol: AlohaNetAloha Transmission StrategyEthernetEthernet’s CSMA/CDEthernet’s CSMA/CD (more)Minimum Packet Size (more)Goals for TodayReview: Point-to-point networksNetwork (IP) LayerIP RoutingSlide 13Packet ForwardingInternet Routing: Two Level HierarchyAdministriviaTransport LayerTransport Layer: DemultiplexingWhat is Cloud Computing?What is MapReduce?What is MapReduce Used For?MapReduce GoalsTypical Hadoop ClusterChallengesHadoop ComponentsHadoop Distributed File SystemMapReduce Programming ModelExample: Word CountWord Count ExecutionAn Optimization: The CombinerWord Count with CombinerMapReduce Execution DetailsConclusionCS162Operating Systems andSystems ProgrammingLecture 22Networking IIApril 13, 2010Ion Stoicahttp://inst.eecs.berkeley.edu/~cs162Lec 22.24/13/10 CS162 ©UCB Spring 2010Multiple 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 senderLec 22.34/13/10 CS162 ©UCB Spring 2010Random Access Protocol: AlohaNet•Norm Abramson left Stanford in search of surfing•Set up first radio-based data communication system connecting the Hawaiian islands–Hub at Alohanet HQ (Univ. Hawaii, Oahu)–Other sites spread among the islands•Had two radio channels:–Random access: sites sent data on this channel–Broadcast: only used by hub to rebroadcast incoming dataLec 22.44/13/10 CS162 ©UCB Spring 2010Aloha Transmission Strategy•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•If collision, site waited random delay before retransmitting•Problem: Stability: what if load increases?–More collisions less gets through more resent more load… More collisions…–Unfortunately: some sender may have started in clear, get scrambled without finishingLec 22.54/13/10 CS162 ©UCB Spring 2010Ethernet•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–Sensing is subject to time-lag–Only detect those sending a short while before•Monitor channel to detect collisions–Once sending, can tell if anyone else is sending tooLec 22.64/13/10 CS162 ©UCB Spring 2010Ethernet’s CSMA/CD•CSMA: Carrier Sense Multiple Access•CD: Collision detection•Sense channel, if idle–If detect another transmission»Abort, send jam signal»Delay, and try again–Else»Send frame•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)Lec 22.74/13/10 CS162 ©UCB Spring 2010Ethernet’s CSMA/CD (more)•Exponential back-off–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}•Minimum packet size–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?Lec 22.84/13/10 CS162 ©UCB Spring 2010Minimum Packet Size (more)propagation delay (d)a) Time = t; Host 1 starts to send frameHost 1 Host 2propagation delay (d)Host 1 Host 2b) Time = t + d; Host 2 starts to send a frame, just before it hears fromhost 1’s framepropagation delay (d)Host 1 Host 2c) Time = t + 2*d; Host 1 hears Host 2’s frame detects collisiond = 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 approxWhat about 100 mbps? 1 gbps? 10 gbps?Lec 22.94/13/10 CS162 ©UCB Spring 2010Goals for Today•Networking–Network layer–Transport layer (start)•MapReduce primer (project 4)Lec 22.104/13/10 CS162 ©UCB Spring 2010Review: Point-to-point networks•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.RouterInternetSwitchLec 22.114/13/10 CS162 ©UCB Spring 2010Network (IP) Layer•Deliver a packet to specified network destination–Packet forwarding & routing•Perform segmentation/reassemble•Others:–packet scheduling–buffer management•Packet forwarding: the process of selecting outgoing link (next hop) to forward a packet–Usually done based on destination address•Routing: the process of computing paths between end-points and building forwarding tables at routersLec 22.124/13/10 CS162 ©UCB Spring 2010IP Routing•Each packet is routed individually (like a letter)•Packets of same connection may take different pathsHost AHost BHost EHost DHost Crouter 1router 2router 3router 4router 5router 6router 7Lec 22.134/13/10 CS162 ©UCB Spring 2010IP Routing•Each packet is routed individually (like a letter)•Packets of same connection may take different pathsHost CHost AHost BHost EHost DHost Crouter 1router 2router 3router 4router 5router 6router 7Lec 22.144/13/10 CS162 ©UCB Spring 2010Packet ForwardingInternet…… 2128.15.11.xxx 3128.15.xxx.xxx1216.82.100.xxx 2RouterForwarding table16.25.31.10128.15.11.12data•IP v4 addresses (32b)–Quad notation (bytes separated by dots)–x: don’t care •At each router the packet destination address1. Is matched according to longest prefix matching rule2. Packet is forwarded to the corresponding output port
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