Internet structure: network of networksInternet protocol stackEncapsulationChapter 2: applicationsArchitecturesHTTP overviewHTTP ReviewCookies: keeping “state” (cont.)Optimization example (cont)Scenario: Alice sends message to BobDistributed, Hierarchical DatabaseIterative Queries vs Recursive QueriesP2P: centralized directoryGnutella: protocolExploiting heterogeneity: KaZaAChapter 3: transportTransport Layer ReviewConnectionless demux (cont)Connection-oriented demux: Threaded Web ServerNotice:Internet Checksum ExampleReliable TransportPipelining: increased utilizationGo-Back-NSelective repeat: sender, receiver windowsTCP ACK generation [RFC 1122, RFC 2581]TCP Flow control: how it worksCauses/costs of congestion: scenario 3Conservative on TimeoutSummary: TCP Congestion ControlTCP sender congestion controlWhy is TCP fair?Fixed congestion window (2)TCP Delay Modeling (3)Chapter 4: Network LayerVirtual circuits: signaling protocolsDatagram networksComparisonFour sources of packet delayDelay in packet-switched networksHow does loss occur?Input Port QueuingOutput port queueingIP addressing: CIDRHierarchical addressing: route aggregationNAT: Network Address TranslationTunnelingSlide 48Dijkstra’s algorithmDijkstra’s algorithm, discussionSlide 51Slide 52Comparison of LS and DV algorithmsExample: Choosing among multiple ASesSlide 55Hierarchical OSPFBGP routing policyChapter 5: link layerParity CheckingCRC ExampleChannel Partitioning MAC protocols: TDMAChannel Partitioning MAC protocols: FDMARandom Access ProtocolsSlotted ALOHAPure (unslotted) ALOHASharing the MediumCSMA collisionsCSMA varieties“Taking Turns” MAC protocolsSlide 70DHCP client-server scenarioInstitutional networkChapter 6: wirelessReviewSlide 75Slide 76Slide 77Slide 78Mobility via Indirect RoutingMobility via Direct RoutingAccommodating mobility with direct routingMobile IP: indirect routingSlide 83Slide 84Chapter 8: SecuritySymmetric key cryptographySymmetric key crypto: DESPublic key cryptographyRSA: How?Authentication: another tryAuthentication: ap5.0ap5.0: security holeSlide 93Key Distribution Center (KDC)Certification AuthoritiesSlide 96Secure e-mail (continued)Introduction 1-1Internet structure: network of networksa packet passes through many networks!Tier 1 ISPTier 1 ISPTier 1 ISPNAPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPTier-2 ISPlocalISPlocalISPlocalISPlocalISPlocalISPTier 3ISPlocalISPlocalISPlocalISPIntroduction 1-2Internet protocol stackapplication: supporting network applicationsFTP, SMTP, HTTPtransport: process-process data transferTCP, UDPnetwork: routing of datagrams from source to destinationIP, routing protocolslink: data transfer between neighboring network elementsPPP, Ethernetphysical: bits “on the wire”applicationtransportnetworklinkphysicalIntroduction 1-3sourceapplicationtransportnetworklinkphysicalHtHnMsegmentHtdatagramdestinationapplicationtransportnetworklinkphysicalHtHnHlMHtHnMHtMMnetworklinkphysicallinkphysicalHtHnHlMHtHnMHtHnMHtHnHlMrouterswitchEncapsulationmessageMHtMHnframeIntroduction 1-4Chapter 2: applicationsIntroduction 1-5ArchitecturesClient-serverPeer-to-peerHybridIntroduction 1-6HTTP overviewHTTP: hypertext transfer protocolWeb’s application layer protocolclient/server modelclient: browser that requests, receives, “displays” Web objectsserver: Web server sends objects in response to requestsHTTP 1.0: RFC 1945HTTP 1.1: RFC 2068PC runningExplorerServer runningApache WebserverMac runningNavigatorHTTP requestHTTP requestHTTP responseHTTP responseLinux runningFirefoxHTTP requestHTTP responseIntroduction 1-7HTTP ReviewTCP“Stateless”Non-persistent  44 messages, 22 RTTPersistent  24 messagesNon-pipelined  12 RTTPipelined  3 RTTHTTP Commands (GET, POST, HEAD, etc)HTTP Fields (User-agent, Connection, etc)Telnet as a command-line TCP connectionIntroduction 1-8Cookies: keeping “state” (cont.)clientserverusual http request msgusual http response +Set-cookie: 1678 servercreates ID1678 for userentry in backend databaseusual http request msgcookie: 1678usual http response msgcookie-specificactionaccessCookie fileamazon: 1678ebay: 8734Cookie fileebay: 8734usual http request msgcookie: 1678usual http response msgcookie-spectificactionaccessCookie fileamazon: 1678ebay: 8734one week later:Introduction 1-9Optimization example (cont)Install cachesuppose hit rate is .4Consequence40% requests will be satisfied almost immediately60% requests satisfied by origin serverutilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secsoriginserverspublic Internetinstitutionalnetwork10 Mbps LAN1.5 Mbps access linkinstitutionalcacheIntroduction 1-10Scenario: Alice sends message to Bob1) Alice uses UA to compose message and “to” [email protected]) Alice’s UA sends message to her mail server; message placed in message queue3) Client side of SMTP opens TCP connection with Bob’s mail server4) SMTP client sends Alice’s message over the TCP connection5) Bob’s mail server places the message in Bob’s mailbox6) Bob invokes his user agent to read messageuseragentmailservermailserveruseragent123456Introduction 1-11Root DNS Serverscom DNS serversorg DNS servers edu DNS serverspoly.eduDNS serversumass.eduDNS serversyahoo.comDNS serversamazon.comDNS serverspbs.orgDNS serversDistributed, Hierarchical DatabaseClient wants IP for www.amazon.com; 1st approx:Client queries a root server to find com DNS serverClient queries com DNS server to get amazon.com DNS serverClient queries amazon.com DNS server to get IP address for www.amazon.comIntroduction 1-12Iterative Queries vs Recursive Queriesrequesting hostCs.virginia.edugaia.cs.umass.eduroot DNS serverlocal DNS serverCs.virginia.edu12456authoritative DNS serverdns.cs.umass.edu78TLDDNSserver3requesting hostCs.virginia.edugaia.cs.umass.eduroot DNS serverlocal DNS serverCs.virginia.edu123456authoritative DNS serverdns.cs.umass.edu78TLD DNS serverIntroduction 1-13P2P: centralized directoryoriginal “Napster” design1) when peer connects, it informs central server:IP addresscontent2) Alice queries for “Hey Jude”3) Alice requests file from Bobcentralizeddirectory serverpeersAliceBob111123Introduction 1-14Gnutella: protocolQueryQueryHitQueryQueryQueryHitQueryQueryQueryHitFile