Transla ng Addresses Reading Sec on 4 1 and 9 1 COS 461 Computer Networks Spring 2009 MW 1 30 2 50 in COS 105 Michael Freedman hLp www cs princeton edu courses archive spring08 cos461 1 Goals of Today s Lecture Three di erent kinds of addresses Host names e g www cnn com IP addresses e g 64 236 16 20 MAC addresses e g 00 15 C5 49 04 A9 Protocols for transla ng between addresses Domain Name System DNS Dynamic Host Con gura on Protocol DHCP Address Resolu on Protocol ARP Two main topics Decentralized management of the name space Boot strapping an end host that aLaches to the net 2 Grades for assignment 1 3 Separa ng Names and IP Addresses Names are easier for us to remember www cnn com vs 64 236 16 20 IP addresses can change underneath Move www cnn com to 173 15 201 39 E g renumbering when changing providers Name could map to mul ple IP addresses www cnn com to mul ple replicas of the Web site Map to di erent addresses in di erent places Address of a nearby copy of the Web site E g to reduce latency or return di erent content Mul ple names for the same address E g aliases like ee mit edu and cs mit edu 4 Separa ng IP and MAC Addresses LANs are designed for arbitrary network protocols Not just for IP e g IPX Appletalk X 25 Though now IP is the main game in town Di erent LANs may have di erent addressing schemes Though now Ethernet address is the main game in town A host may move to a new loca on So cannot simply assign a sta c IP address Since IP addresses depend on host s posi on in topology Instead must recon gure the adapter To assign it an IP address based on its current loca on Must iden fy the adapter during bootstrap process Need to talk to the adapter to assign it an IP address 5 Three Kinds of Iden ers Host name e g www cnn com Mnemonic name appreciated by humans Provides liLle if any informa on about loca on Hierarchical variable of alpha numeric characters IP address e g 64 236 16 20 Numerical address appreciated by routers Related to host s current loca on in the topology Hierarchical name space of 32 bits MAC address e g 00 15 C5 49 04 A9 Numerical address appreciated within local area network Unique hard coded in the adapter when it is built Flat name space of 48 bits 6 Three Hierarchical Assignment Processes Host name www cs princeton edu Domain registrar for each top level domain e g edu Host name local administrator assigns to each host IP addresses 128 112 7 156 Pre xes ICANN regional Internet registries and ISPs Hosts sta c con gura on or dynamic using DHCP MAC addresses 00 15 C5 49 04 A9 Blocks assigned to vendors by the IEEE Adapters assigned by the vendor from its block 7 Mapping Between Iden ers Domain Name System DNS Given a host name provide the IP address Given an IP address provide the host name Dynamic Host Con gura on Protocol DHCP Given a MAC address assign a unique IP address and tell host other stu about the Local Area Network To automate the boot strapping process Address Resolu on Protocol ARP Given an IP address provide the MAC address To enable communica on within the Local Area Network 8 Domain Name System DNS Proposed in 1983 by Paul Mockapetris 9 Outline Domain Name System Computer science concepts underlying DNS Indirec on names in place of addresses Hierarchy in names addresses and servers Caching of mappings from names to from addresses DNS sokware components DNS resolvers DNS servers DNS queries Itera ve queries Recursive queries DNS caching based on me to live TTL 10 Strawman Solu on 1 Local File Original name to address mapping Flat namespace etc hosts SRI kept main copy Downloaded regularly Count of hosts was increasing moving from a machine per domain to machine per user Many more downloads Many more updates 11 Strawman Solu on 2 Central Server Central server One place where all mappings are stored All queries go to the central server Many prac cal problems Single point of failure High tra c volume Distant centralized database Single point of update Does not scale Need a distributed hierarchical collection of servers 12 Domain Name System DNS Proper es of DNS Hierarchical name space divided into zones Distributed over a collec on of DNS servers Hierarchy of DNS servers Root servers Top level domain TLD servers Authorita ve DNS servers Performing the transla ons Local DNS servers Resolver sokware 13 DNS Root Servers 13 root servers see hLp www root servers org Labeled A through M A Verisign Dulles VA C Cogent Herndon VA also Los Angeles D U Maryland College Park MD G US DoD Vienna VA K RIPE London Amsterdam Frankfurt H ARL Aberdeen MD J Verisign 11 locations I Autonomica Stockholm E NASA Mt View CA F Internet Software C Palo Alto CA and 17 other locations plus 3 other locations m WIDE Tokyo B USC ISI Marina del Rey CA L ICANN Los Angeles CA 14 TLD and Authorita ve DNS Servers Top level domain TLD servers Generic domains e g com org edu Country domains e g uk fr ca jp Typically managed professionally Network Solu ons maintains servers for com Educause maintains servers for edu Authorita ve DNS servers Provide public records for hosts at an organiza on For the organiza on s servers e g Web and mail Can be maintained locally or by a service provider 15 Distributed Hierarchical Database unnamed root com edu org generic domains bar uk ac zw arpa country domains ac inaddr west east cam 12 foo my usr 34 my east bar edu usr cam ac uk 56 12 34 56 0 24 16 Using DNS Local DNS server default name server Usually near the end hosts who use it Local hosts con gured with local server e g etc resolv conf or learn the server via DHCP Client applica on Extract server name e g from the URL Do gethostbyname to trigger resolver code Server applica on Extract client IP address from socket Op onal gethostbyaddr to translate into name 17 Example Host at cis poly edu wants IP address for gaia cs umass edu root DNS server 2 3 TLD DNS server 4 local DNS server 5 dns poly edu 1 8 requesting host 7 6 authoritative DNS server dns cs umass edu cis poly edu gaia cs umass edu 18 Recursive vs Itera ve Queries Recursive query root DNS server Ask server to get answer for you E g request 1 and response 8 local DNS server Itera ve query Ask server who to ask next E g all other request response pairs 2 3 4 TLD DNS server 5 dns poly edu 1 8 requesting host 7 6 authoritative DNS server dns cs umass edu cis poly edu 19 DNS Caching …
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