Review Point to point networks Switch Networking II Router Internet CS162 Operating Systems and Systems Programming Lecture 22 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 November 18 2009 Prof John Kubiatowicz http inst eecs berkeley edu cs162 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Review Address Subnets Lec 22 2 Goals for Today Subnet A network connecting a set of hosts with related destination addresses With IP all the addresses in subnet are related by a prefix of bits Networking Mask The number of matching prefix bits Expressed as a single value e g 24 or a set of ones in a 32 bit value e g 255 255 255 0 Routing DNS Routing TCP IP Protocols A subnet is identified by 32 bit value with the bits which differ set to zero followed by a slash and a mask Example 128 32 131 0 24 designates a subnet in which all the addresses look like 128 32 131 XX Same subnet 128 32 131 0 255 255 255 0 Difference between subnet and complete network range Subnet is always a subset of address range Once subnet meant single physical broadcast wire now less clear exactly what it means virtualized by switches 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Lec 22 3 Note Some slides and or pictures in the following are adapted from slides 2005 Silberschatz Galvin and Gagne Gagne Many slides generated from my lecture notes by Kubiatowicz 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Lec 22 4 Simple Network Terminology Local Area Network LAN designed to cover small geographical area Multi access bus ring or star network Speed 10 1000 Megabits second Broadcast is fast and cheap In small organization a LAN could consist of a single subnet In large organizations like UC Berkeley a LAN contains many subnets Wide Area Network WAN links geographically separated sites Point to point connections over long haul lines often leased from a phone company Speed 1 544 45 Megabits second Broadcast usually requires multiple messages 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Lec 22 5 Routing Routing the process of forwarding packets hop by hop through routers to reach their destination Need more than just a destination address Need a path Post Office Analogy Destination address on each letter is not sufficient to get it to the destination To get a letter from here to Florida must route to local post office sorted and sent on plane to somewhere in Florida be routed to post office sorted and sent with carrier who knows where street and house is Internet routing mechanism routing tables Each router does table lookup to decide which link to use to get packet closer to destination Don t need 4 billion entries in table routing is by subnet Could packets be sent in a loop Yes if tables incorrect Routing table contains Destination address range output link closer to destination Default entry for subnets without explicit entries 11 18 09 Setting up Routing Tables Kubiatowicz CS162 UCB Fall 2009 Naming in the Internet How do you set up routing tables Internet has no centralized state Name No single machine knows entire topology Topology constantly changing faults reconfiguration etc Need dynamic algorithm that acquires routing tables Ideally have one entry per subnet or portion of address Could have default routes that send packets for unknown subnets to a different router that has more information Possible algorithm for acquiring routing table Routing table has cost for each entry Includes number of hops to destination congestion etc Entries for unknown subnets have infinite cost Neighbors periodically exchange routing tables If neighbor knows cheaper route to a subnet replace your entry with neighbors entry 1 for hop to neighbor In reality Internet has networks of many different scales Different algorithms run at different scales 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Lec 22 6 Address How to map human readable names to IP addresses E g www berkeley edu 128 32 139 48 E g www google com different addresses depending on location and load Why is this necessary IP addresses are hard to remember IP addresses change Say Server 1 crashes gets replaced by Server 2 Or google com handled by different servers Mechanism Domain Naming System DNS Lec 22 7 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Lec 22 8 Domain Name System edu 169 229 131 81 com MIT berkeley berkeley edu Is DNS Secure Definitely a weak link What if response returned from different server than original query Get person to use incorrect IP address eecs berkeley edu www Attempt to avoid substitution attacks 128 32 139 48 DNS is a hierarchical mechanism for naming Name divided in domains right to left www eecs berkeley edu Each domain owned by a particular organization Top level handled by ICANN Internet Corporation for Assigned Numbers and Names Subsequent levels owned by organizations Resolution series of queries to successive servers Caching queries take time so results cached for period of time 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Lec 22 9 Performance Considerations Before we continue need some performance metrics 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 In July 2008 hole in DNS security located Dan Kaminsky security researcher discovered an attack that broke DNS globally One person in an ISP convinced to load particular web page then all users of that ISP end up pointing at wrong address High profile highly advertised need for patching DNS Big press release lots of mystery Security researchers told no speculation until patches applied 11 18 09 Kubiatowicz CS162 UCB Fall 2009 Lec 22 10 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
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