Review Networking Network physical connection that allows two computers to communicate CS162 Operating Systems and Systems Programming Lecture 22 Packet sequence of bits carried over the network Broadcast Network Shared Communication Medium Transmitted packets sent to all receivers Arbitration act of negotiating use of shared medium Ethernet Carrier Sense Multiple Access Collision Detect Point to point network a network in which every physical wire is connected to only two computers Networking II Switch a bridge that transforms a shared bus broadcast configuration into a point to point network Internet Protocol IP unreliable packet service Used to route messages across globe 32 bit destination addresses November 16 2005 Prof John Kubiatowicz http inst eecs berkeley edu cs162 Routing the process of forwarding packets hop byhop through routers to reach their destination Internet has networks of many different scales LANs Autonomous Systems AS etc Different algorithms run at different scales 11 14 05 Review Hierarchical Networking The Internet How can we build a network with millions of hosts Hierarchy Not every host connected to every other one Use a network of Routers to connect subnets together Other subnets subnet1 Router subnet2 Example system calls are the protocol between the operating system and application Networking examples many levels Protocols on today s Internet NFS subnet3 Transport RPC Physical Link Kubiatowicz CS162 UCB Fall 2005 Lec 22 3 11 14 05 WWW UDP Network 11 14 05 Lec 22 2 Review Network Protocols Protocol Agreement between two parties as to how information is to be transmitted Transcontinental Link Router Kubiatowicz CS162 UCB Fall 2005 Physical level mechanical and electrical network e g how are 0 and 1 represented Link level packet formats error control for instance the CSMA CD protocol Network level network routing addressing Transport Level reliable message delivery Router Other subnets Border Gateway Protocol BGP at large scales Variants of Distance Vector DV protocols at short scales e mail ssh TCP IP Ethernet ATM Kubiatowicz CS162 UCB Fall 2005 Packet radio Lec 22 4 Goals for Today Network Layering Layering building complex services from simpler ones Networking Reliable Messaging Each layer provides services needed for higher layers by utilizing services provided by lower layers TCP windowing and congestion avoidance Two phase commit Our goal in the following is to show how to construct a secure ordered arbitrary sized message service routed to anywhere Note Some slides and or pictures in the following are adapted from slides 2005 Silberschatz Galvin and Gagne 11 14 05 Kubiatowicz CS162 UCB Fall 2005 Lec 22 5 Basic Networking Limitations The physical link layer is pretty limited Maximum Transfer Unit MTU often 200 1500 bytes Packets can get lost or garbled Hardware routing limited to physical link or switch Physical routers crash links get damaged Famous Baltimore tunnel fire July 2001 cut Internet half Datagram an independent self contained network message whose arrival arrival time and content are not guaranteed Need resilient routing algorithms to send messages on wide area Multi hop routing mechanisms Redundant links Ability to route around failed links Handling Arbitrary Sized Messages Must deal with limited physical packet size Split big message into smaller ones called fragments Must be reassembled at destination May happen on demand if packet routed through areas of reduced MTU e g TCP Checksum computed on each fragment or whole message Kubiatowicz CS162 UCB Fall 2005 Abstraction Messages Limited Size Unordered sometimes Unreliable Machine to machine Only on local area net Asynchronous Insecure Arbitrary Size Ordered Reliable Process to process Routed anywhere Synchronous Secure 11 14 05 Kubiatowicz CS162 UCB Fall 2005 Lec 22 6 Performance Considerations Before continuing need some performance metrics Packets of limited size 11 14 05 Physical Reality Packets Lec 22 7 Overhead CPU time to put packet on wire Throughput Maximum number of bytes per second Depends on wire speed but also limited by slowest router routing delay or by congestion at routers Latency time until first bit of packet arrives at receiver Raw transfer time overhead at each routing hop Router LW1 LR1 Router LW2 LR2 Lw3 Contributions to Latency Wire latency depends on speed of light on wire about 1 5 ns foot Router latency depends on internals of router Could be 1 ms for a good router Question can router handle full wire throughput 11 14 05 Kubiatowicz CS162 UCB Fall 2005 Lec 22 8 Sample Computations E g Ethernet within Soda Latency speed of light in wire is 1 5ns foot which implies latency in building 1 s if no routers in path Throughput 10 1000Mb s Throughput delay packet doesn t arrive until all bits So 4KB 100Mb s 0 3 milliseconds same order as disk E g ATM within Soda Latency same as above assuming no routing Throughput 155Mb s Throughput delay 4KB 155Mb s 200 IP Packet Format Internet Protocol IP Sends packets to arbitrary destination in network Deliver messages unreliably best effort from one machine in Internet to another Since intermediate links may have limited size must be able to fragment reassemble packets on demand Includes 256 different sub protocols built on top of IP Examples ICMP 1 TCP 6 UDP 17 IPSEC 50 51 IP Packet Format E g ATM cross country Latency assuming no routing 3000miles 5000ft mile 15 milliseconds IP Ver4 How many bits could be in transit at same time 15ms 155Mb s 290KB Time to Live hops In fact Berkeley MIT Latency 90ms Implies 1 7MB in flight if routers have wire speed throughput Type of transport protocol Requirements for good performance Local area minimize overhead improve bandwidth Wide area keep pipeline full 11 14 05 Kubiatowicz CS162 UCB Fall 2005 Lec 22 9 Process to process communication UDP Process to process communication 0 IP Header Length Size of datagram header data 15 16 31 4 IHL ToS Total length 16 bits 16 bit identification flags 13 bit frag off TTL protocol 16 bit header checksum 32 bit source IP address 32 bit destination IP address options if any 11 14 05 Data Kubiatowicz CS162 UCB Fall 2005 IP header 20 bytes Lec 22 10 Administrivia Basic routing gets packets from machine machine What we really want is routing from process process My office hours Several IP protocols include notion of a port which is a 16 bit identifiers used in addition to IP addresses Project 4 design document Example ssh email ftp web browsing A
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