Networking III: The Transport Layer and Internet names and addressesAnnouncementsReview: Hierarchical NetworkingReview: OSI LevelsSlide 5Purpose of this layerUDPUDP FunctionalityMultiplexing/DemultiplexingImplementing PortsUDP ChecksumUDP DiscussionTCPHandling Packet LossLost AcksDelayed ACKsSlide 17Insertion of PacketsMessage IdentifiersTCP PacketsTCP Connection EstablishmentTCP UsageTCP timeoutsTCP WindowsSlide 25TCP Windows and Sequence NumbersTCP Congestion ControlTCP Window SizeTCP FairnessTCP Slow StartSlide 31TCP SummaryInternet: Names and AddressesNaming in the InternetLocating the resourcesLocating the Hosts?Mapping Not 1 to 1How to get a name?Domain name structureTop-level Domains (TLDs)How to get a domain name?How to get an IP Address?How to get lots of IP Addresses? Internet RegistriesAre there enough addresses?DHCP and NATsNATs in operationDNS: Domain Name SystemDNS name serversName Server Zone StructureName Servers (NS)Slide 52DNS: Root name serversSimple DNS exampleDNS exampleDNS ArchitectureDNS: query resolutionDNS records: More than Name to IP AddressnslookupPTR RecordsArpa top level domainWhy is it backwards?In-addr.arpa domainDNS protocol, messagesSlide 65Slide 66Networking III:The Transport Layer andInternet names and addresses2Announcements•Homework 5 due next Tuesday, November 11th•Prelim II will be Thursday, November 20th, in class–Topics: Everything after first prelim–Lectures 14-22, chapters 10-15 (8th ed)•Nazrul will teach next Tuesday, November 11th3Review: Hierarchical Networking •How can we build a network with millions of hosts (the Internet)?–Hierarchy! Not every host connected to every other one–Use a network of Routers to connect subnets togethersubnet1subnet2RouterOthersubnetsRouterRouterTranscontinentalLinksubnet3Othersubnets4Review: OSI Levels•Physical Layer–electrical details of bits on the wire•Data Link Layer–sending “frames” of bits and error detection•Network Layer–routing packets to the destination•Transport Layer–reliable transmission of messages, disassembly/assembly, ordering, retransmission of lost packets•Session Layer–really part of transport, typically Not implemented•Presentation Layer–data representation in the message•Application–high-level protocols (mail, ftp, etc.)5Review: OSI LevelsPresentationTransportNetworkData LinkPhysicalApplicationPresentationTransportNetworkData LinkPhysicalApplicationNode ANode BNetworkSessionSessionNetworkData LinkPhysicalRouter6Purpose of this layer•Interface end-to-end applications and protocols–Turn best-effort IP into a usable interface•Data transfer between processes:–Compared to end-to-end IP•We will look at 2:–UDP (Unreliable Datagram Protocol)–TCP (Transmission Control Protocol)applicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicallogical end-end transport7UDP•Unreliable Datagram Protocol•Best effort data delivery between processes–No frills, bare bones transport protocol–Packet may be lost, out of order•Connectionless protocol:–No handshaking between sender and receiver–Each UDP datagram handled independently8UDP Functionality•Multiplexing/Demultiplexing–Using ports•Checksums (optional)–Check for corruptionapplicationtransportnetworkMP2applicationtransportnetworkreceiverHtHnsegmentsegmentMapplicationtransportnetworkP1MMMP4segmentheaderapplication-layerdataP39Multiplexing/Demultiplexing•Multiplexing: –Gather data from multiple processes, envelope data with header –Header has src port, dest port for multiplexing •Why not process id?•Demultiplexing:–Separate incoming data in machine to different applications–Demux based on sender addr, src and dest portsource port # dest port #32 bitsApplicationdata (message)UDP segment formatlengthchecksumLength, inbytes of UDPsegment,includingheader10Implementing Ports•As a message queue–Append incoming message to the end–Much like a mailbox file•If queue full, message can be discarded•When application reads from socket –OS removes some bytes from the head of the queue•If queue empty, application blocks waiting11UDP Checksum•Over the headers and data–Ensures integrity end-to-end–1’s complement sum of segment contents•Is optional in UDP•If checksum is non-zero, and receiver computes another value:–Silently drop the packet, no error message detected12UDP Discussion•Why UDP?–No delay in connection establishment–Simple: no connection state–Small header size–No congestion control: can blast packets•Uses:–Streaming media, DNS, SNMP–Could add application specific error recovery13TCP•Transmission Control Protocol–Reliable, in-order, process-to-process, two-way byte stream•Different from UDP–Connection-oriented–Error recovery: Packet loss, duplication, corruption, reordering•A number of applications require this guarantee–Web browsers use TCP14Handling Packet LosssenderreceivertimemessageThere are a number of reasons why the packet may get lost:- router congestion, lossy medium, etc.How does sender know of a successful packet send?15Lost AckssenderreceivertimemessageackWhat if packet/ack is lost?timeout16Delayed ACKssenderreceivertimemessageacktimeoutWhat will happen here? Due to congestion, small timeout, …Delayed ACKs  duplicate packetsmessage17Delayed ACKssenderreceivertimem1acktimeoutm1timeoutm2ackHow to solve this scenario?18Insertion of Packetssenderreceivertimem1ack1m2ack2m2’m2’ could be from an old expired session!19Message Identifiers•Each message has <message id, session id>–Message id: uniquely identifies message in sender–Session id: unique across sessions•Message ids detect duplication, reordering•Session ids detect packet from old sessions•TCP’s sequence number has similar functionality:–Initial number chosen randomly–Unique across packets–Incremented by length of data bytes20TCP Packetssource port #dest port #32 bitsapplicationdata (variable length)sequence numberacknowledgement numberrcvr window sizeptr urgent datachecksumFSRPAUheadlennotusedOptions (variable length)URG: urgent data (generally not used)ACK: ACK #validPSH: push data now(generally not used)RST, SYN, FIN:connection estab(setup, teardowncommands)# bytes rcvr willingto acceptcountingby bytes of data(not segments!)Internetchecksum(as in UDP)21TCP