EECS 122: Introduction to Computer Networks MulticastBarriers to MulticastApplication Layer Multicast (ALM)Narada: End System MulticastAlgorithmic ChallengeAdvantages of ALMPerformance ConcernsSlide 8Single Sender MulticastExampleWhat’s Wrong with SSM?What Do You Need to Know?EECS 122: Introduction to Computer Networks DNS and WWWInternet Names & AddressesDNS: HistoryBasic DNS FeaturesNaming HierarchyHost names are administered hierarchicallyServer HierarchyDNS Name ServersDNS: Root Name ServersSimple DNS ExampleExample of Recursive DNS QueryExample of Iterated DNS QueryDNS RecordsDNS Records (cont’d)DNS as Indirection ServiceSpecial TopicsImportant Properties of DNSThe WebUniform Record LocatorWeb and DNSHyper Text Transfer ProtocolBig PictureHyper Text Transfer Protocol CommandsResponse CodesClient RequestServer ResponseExample (from Kurose and Ross)HTTP/1.0 ExampleHHTP/1.0 PerformanceHTTP/1.0 CachingWeb ProxiesHTTP/1.1 (1996)Persistent ConnectionsPipelined Requests/ResponsesWhat You Need to KnowWhat’s the Moral of this Story?Katz, Stoica F04EECS 122: Introduction to Computer Networks MulticastComputer Science DivisionDepartment of Electrical Engineering and Computer SciencesUniversity of California, BerkeleyBerkeley, CA 94720-17762Katz, Stoica F04Barriers to MulticastHard to change IP-multicast means change to IP-details of multicast were very hard to get rightNot always consistent with ISP economic model-charging done at edge, but single packet from edge can explode into millions of packets within networkTroublesome security model-Anyone can send to a group-Denial-of-service attacks on known groups3Katz, Stoica F04Application Layer Multicast (ALM)Let the hosts do all the “special” work-only require unicast from infrastructureBasic idea:-hosts do the copying of packets-set up tree between hostsExample: Narada [Yang-hua et al, 2000]-Small group sizes <= hundreds of nodes-Typical application: chat4Katz, Stoica F04Narada: End System MulticastStanfordCMUStan1Stan2Berk2“Overlay” TreeGatechBerk1BerkeleyGatechStan1Stan2CMUBerk1Berk25Katz, Stoica F04Algorithmic ChallengeChoosing replication/forwarding points among hosts-how do the hosts know about each other-and know which hosts should forward to other hosts6Katz, Stoica F04Advantages of ALMNo need for changes to IP or routersNo need for ISP cooperationEnd hosts can prevent other hosts from sendingEasy to implement reliability-use hop-by-hop retransmissions7Katz, Stoica F04Performance ConcernsStretch-ratio of latency in the overlay to latency in the underlying networkStress-number of duplicate packets sent over the same physical link8Katz, Stoica F04Performance ConcernsDuplicate Packets:Bandwidth WastageCMUStan1Stan2Berk2GatechBerk1Delay from CMU to Berk1 increasesStanfordBerkeleyGatechStan1Stan2CMUBerk1Berk29Katz, Stoica F04Single Sender MulticastMany problems with IP multicast disappear if each group is associated with a single sourceHosts joining multicast group can send join messages to source-this sets up delivery tree-no worry about “root” being in wrong placeThis solves several problems:-better security and charging model-simple algorithm10Katz, Stoica F04ExampleGroup members: M1, M2, M3sourceM1M2M3control (join) messagesdata11Katz, Stoica F04What’s Wrong with SSM?Multiple sources?-can set up group per source, or...-source can serve as relay for other sendersAlgorithm?-trivialSo, why isn’t SSM the answer?-multicast no longer serves as “rendezvous”-ok for “broadcast” apps, not good for “meeting” apps12Katz, Stoica F04What Do You Need to Know?DVRMPCBTSSMHow they compareKatz, Stoica F04EECS 122: Introduction to Computer Networks DNS and WWWComputer Science DivisionDepartment of Electrical Engineering and Computer SciencesUniversity of California, BerkeleyBerkeley, CA 94720-177614Katz, Stoica F04Internet Names & AddressesNames: e.g., ariachne.berkeley.edu-human-usable labels for machines-conforms to “organizational” structureAddresses: e.g., 169.229.131.109-router-usable labels for machines-conforms to “network” structureHow do you map from one to another?-Domain Name System (DNS)15Katz, Stoica F04DNS: HistoryInitially all host-addess mappings were in a file called hosts.txt (in /etc/hosts)-Changes were submitted to SRI by email-New versions of hosts.txt ftp’d periodically from SRI-An administrator could pick names at their discretionAs the internet grew this system broke down because:-SRI couldn’t handled the load-Names were not unique-Many hosts had inaccurate copies of hosts.txtInternet growth was threatened!-Domain Name System (DNS) was born16Katz, Stoica F04Basic DNS FeaturesHierarchical namespace-As opposed to original flat namespaceDistributed storage architecture-As opposed to centralized storage (plus replication)Client--server interaction on UDP Port 53-But can use TCP if desired17Katz, Stoica F04Naming Hierarchy“Top Level Domains” are at the topDepth of tree is arbitrary (limit 128)Domains are subtrees-E.g: .edu, berkeley.edu, eecs.berkeley.eduName collisions avoided-E.g. berkeley.edu and berkeley.com can coexist, but uniqueness is job of domainrooteducomgov milorgnet uk frberkeleymiteecssimsargusetc.18Katz, Stoica F04Host names are administered hierarchicallyrooteducomgov milorgnet uk frberkeleymiteecssimsargusrooteducomgov milorgnet uk frberkeleyeecssimsA zone corresponds to an administrative authority that is responsible for that portion of the hierarchyeecs controls names: x.eecs.berkeley.eduberkeley controls names: x.berkeley.edu and y.sims.berkeley.edu19Katz, Stoica F04Server HierarchyEach server has authority over a portion of the hierarchy-A server maintains only a subset of all namesEach server contains all the records for the hosts in its zone-might be replicated for robustnessEach server needs to know other servers that are responsible for the other portions of the hierarchy-Every server knows the root-Root server knows about all top-level domains20Katz, Stoica F04DNS Name ServersLocal name servers:-Each ISP (company) has local default name server-Host DNS query first goes to local name serverAuthoritative name servers:-For a host: stores that host’s (name, IP address)-Can perform name/address translation for that host’s name Can also do IP to name translation, but won’t discuss21Katz, Stoica F04DNS: Root Name
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