12002 M. T. Harandi and J. HouStudent Notes Pages 2002, M. T. Harandi and J. Hou Lecture 14-1Lecture 14-1Computer Science 425Distributed SystemsComputer Science 425Distributed SystemsLecture 14NETWORKING – Part III, RPCs, etc. 2002, M. T. Harandi and J. Hou Lecture 14-2Lecture 14-2DNS: Domain Name SystemDNS: Domain Name SystemPeople: many identifiers:– SSN, name, Passport #Internet hosts, routers:– IP address (32 bit) - used for addressing datagrams– Resource “name”, e.g., URL sal.cs.uiuc.edu – human-readable formatQ: given a resource name, how does a client find out the IP address of the service/server?Domain Name System:• distributed databaseimplemented in a hierarchy of many name servers• application-layer protocol that is responsible for resolving names (address/name translation) 2002, M. T. Harandi and J. Hou Lecture 14-3Lecture 14-3DNS Name ServersDNS Name ServersAlternative• no server has all name-to-IP address mappings• Hierarchy of name serversauthoritative name server:– for a resource, stores the mapped IP address for that resourcelocal name servers:– each institution/company/ISP owns a local (default) name server– host DNS query first goes to local name server– local name server might be caching an answerWhy not have a central DNS server?• single point of failure• traffic volume• may be far• maintenance difficultdoesn’t scale!(WWW contains several billion pages today) 2002, M. T. Harandi and J. Hou Lecture 14-4Lecture 14-4DNS: Root Name ServersDNS: Root Name Servers• contacted by local name server that can not resolve name• root name server:– contacts authoritative name server if name mapping not known– gets mapping– returns mapping to local name server• ~ dozen root name servers worldwide (as of ’98) 2002, M. T. Harandi and J. Hou Lecture 14-5Lecture 14-5Simple DNS ExampleSimple DNS Examplehost surf.eurecom.frwants IP address of dragon.cs.uiuc.edu1. Contacts its local DNS server, dns.eurecom.fr2. dns.eurecom.fr contacts root name server, if necessary3. root name server contacts authoritative name server, dns.cs.uiuc.edu, if necessaryAnswer returned by first server that is caching the mapping tuplerequesting hostsurf.eurecom.frdragon.cs.uiuc.eduroot name serverauthoritative name serverdns.cs.uiuc.edulocal name serverdns.eurecom.fr123456 2002, M. T. Harandi and J. Hou Lecture 14-6Lecture 14-6DNS ExampleDNS ExampleRoot name server:• may not know the authoritative name server• may know intermediate name server:whom to contact to find authoritative name server• Hierarchy..edu .com .mil …uiuc.edu mit.edu…requesting hostsurf.eurecom.frdragon.cs.uiuc.eduroot name serverlocal name serverdns.eurecom.fr123456authoritative name serverdns.cs.uiuc.eduintermediate name serverdns.uiuc.edu7822002 M. T. Harandi and J. HouStudent Notes Pages 2002, M. T. Harandi and J. Hou Lecture 14-7Lecture 14-7DNS: Iterated QueriesDNS: Iterated Queriesrecursive query:• puts burden of name resolution on servers along the way• may fail if a server does not know next server to contactiterated query:• contacted server replies with name of server to contact• “I don’t know this resource name, but ask this other server”• takes longer (more replies) but gives client more controlrequesting hostsurf.eurecom.frdragon.cs.uiuc.eduroot name serverlocal name serverdns.eurecom.fr123456authoritative name serverdns.cs.uiuc.eduintermediate name serverdns.uiuc.edu78iterated query 2002, M. T. Harandi and J. Hou Lecture 14-8Lecture 14-8DNS: Caching and Updating RecordsDNS: Caching and Updating Records• once (any) name server learns mapping, it cachesmapping– cache entries timeout (disappear) after some time• update/notify mechanisms– RFC 2136– http://www.ietf.org/html.charters/dnsind-charter.html– Akamai uses this extensively 2002, M. T. Harandi and J. Hou Lecture 14-9Lecture 14-9Firewalls Firewalls A set of processes that monitor & control all comm. into and out of an intranet, for: Service control Behavior control User control Firewall filtering can be done at diff. levels of protocol stack IP packet filtering E.g., disallow packets from certain IP addresses TCP gateway filtering E.g., check format of TCP packets to avoid attacks using malformed TCP packets Application gateway filtering E.g., web proxy 2002, M. T. Harandi and J. Hou Lecture 14-10Lecture 14-10Firewall ConfigurationsFirewall ConfigurationsInternetRouter/Protected intraneta) Filtering routerInternetb) Filtering router and bastionfilterInternetR/filterc) Screened subnet for bastionR/filter BastionR/filter Bastionweb/ftpserverweb/ftpserverweb/ftpserverFor efficiency reasons, IP filtering -done in kernel-Separated from other filtering- Hosts in intranetneed not know IP addresses of hosts in external network,and vice-versa- Double-walled protectionTCP and appl filtering 2002, M. T. Harandi and J. Hou Lecture 14-11Lecture 14-11SummarySummaryStructure of the Internet• Protocol stacks (e.g., OSI 7 layer, 5 layer)• IP packet format• ARP, DNS • Routing algorithms – distance vector, link state, proactive, reactive• TCP• FirewallsStructure constantly evolving with new networks and standards (e.g., data over cellphones, IPv6) 2002, M. T. Harandi and J. Hou Lecture 14-12Lecture 14-12Computer Science 425Distributed SystemsComputer Science 425Distributed SystemsLecture 14Inter-process Communication & Distributed Objects (I)Sections 4.3-4.4, Chapter 532002 M. T. Harandi and J. HouStudent Notes Pages 2002, M. T. Harandi and J. Hou Lecture 14-13Lecture 14-13Search in ChordSearch in ChordN800Say m=7N32N45File bad.mp3 with key K42 stored hereAt node n, send query for key k to largest successor/finger entry < kif none exist, return successor(n) to requestorAll “arrows” are RPCsN112N96N16Who has bad.mp3?(hashes to K42)What are “RPCs”? 2002, M. T. Harandi and J. Hou Lecture 14-14Lecture 14-14Bank Database ExampleBank Database Example• Bank Database: Think of two simultaneous deposits of $10,000 into your bank account, each from one ATM. – Both ATMs read initial amount of $1000 concurrently from the bank server– Both ATMs add $10,000 to this amount (locally at the ATM)– Both write the final amount to the server– What’s wrong?• The ATMs need mutually
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