3. Transport Layer - UMass Amherst

3. Transport LayerComputerNetworking: A TopDown Approach6th editionJim Kurose, Keith RossAddison-WesleyMarch 2012All material copyright 1996-2012J.F Kurose and K.W. Ross, All Rights ReservedTransport Layer 3-1

3. Transport Layer: Goalsour goals:v understandprinciples behindtransport layerservices:§ multiplexing,demultiplexing§ reliable data transfer§ flow control§ congestion controlv learn about Internettransport layer protocols:§ UDP: connectionlesstransport§ TCP: connection-orientedreliable transport§ TCP congestion controlTransport Layer 3-2

3. Transport Layer: Outline3.1 transport-layerservices3.2 multiplexing anddemultiplexing3.3 connectionlesstransport: UDP3.4 principles of reliabledata transfer3.5 connection-orientedtransport: TCP§ § § § segment structurereliable data transferflow controlconnection management3.6 principles of congestioncontrol3.7 TCP congestion controlTransport Layer 3-3

Transport services and protocolsv v v provide logical communicationbetween app processesrunning on different hoststransport protocols run inend systems§ send side: breaks appmessages into segments,passes to network layer§ recv side: reassemblessegments into messages,passes to app layermore than one transportprotocol available to apps§ Internet: TCP and UDPapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalTransport Layer 3-4

Transport vs. network layerv v network layer: logicalcommunicationbetween hoststransport layer: logicalcommunicationbetween processes§ relies on and enhancesnetwork layer serviceshousehold analogy:12 kids in Ann’s house sendingletters to 12 kids in Bill’shouse:v hosts housesv processes kidsv app messages letters inenvelopesv transport protocol Annand Bill who demux to inhouse siblingsv network-layer protocol postal serviceTransport Layer 3-5

Internet transport-layer protocolsv reliable, in-orderdelivery (TCP)§ congestion control§ flow control§ connection setupv unreliable, unordereddelivery: UDP§ no-frills extension of“best-effort” IPv services not available:applicationtransportnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalapplicationtransportnetworkdata linkphysical§ delay guarantees§ bandwidth guaranteesTransport Layer 3-6

3. Transport Layer: Outline3.1 transport-layerservices3.2 multiplexing anddemultiplexing3.3 connectionlesstransport: UDP3.4 principles of reliabledata transfer3.5 connection-orientedtransport: TCP§ § § § segment structurereliable data transferflow controlconnection management3.6 principles of congestioncontrol3.7 TCP congestion controlTransport Layer 3-7

Multiplexing/demultiplexingmultiplexing at sender:handle data from multiplesockets, add transport header(later used for demultiplexing)demultiplexing at receiver:use header info to deliverreceived segments to lTransport Layer 3-8

How demultiplexing worksv host receives IP datagrams§ each datagram has sourceand destination IP address§ each datagram carries onetransport-layer segment§ each segment has sourceand destination port numberv host uses IP addresses &port numbers to directsegment to right socket32 bitssource port #dest port #other header fieldsapplicationdata(payload)TCP/UDP segment formatTransport Layer 3-9

Connectionless demultiplexingv recall: created socket hashost-local port #:v DatagramSocket mySocket1 new DatagramSocket(12534);v when host receives UDPsegment:§ checks destination IP andport # in segment§ directs UDP segment tosocket bound to that(IP,port)recall: when creatingdatagram to send intoUDP socket, must specify§ destination IP address§ destination port #IP datagrams with samedest. (IP, port), but differentsource IP addresses and/or source port numberswill be directed to samesocketTransport Layer 3-10

Connectionless demux: exampleDatagramSocketmySocket2 newDatagramSocket(9157);DatagramSocketserverSocket gramSocketmySocket1 sicallinkphysicalphysicalsource port: 6428dest port: 9157source port: 9157dest port: 6428source port: ?dest port: ?source port: ?dest port: ?Transport Layer 3-11

Connection-oriented demuxv TCP socket identifiedby 4-tuple:§ § § § v source IP addresssource port numberdest IP addressdest port numberdemux: receiver usesall four values to directsegment to right socketv server host has manysimultaneous TCP sockets:§ each socket identified by itsown 4-tuplev web servers have differentsocket each client§ non-persistent HTTP willhave different socket foreach requestTransport Layer 3-12

Connection-oriented demux: exampleserver socket, also port t: IPaddress Atransporttransportserver: IPaddress Bsource IP,port: B,80dest IP,port: A,9157source IP,port: A,9157dest IP, port: B,80three segments, all destined to IP address: B,dest port: 80 are demultiplexed to different socketsphysicalsource IP,port: C,5775dest IP,port: B,80host: IPaddress Csource IP,port: C,9157dest IP,port: B,80Transport Layer 3-13

Connection-oriented demux: examplethreaded serverserver socket, also port knetworklinknetworklinkphysicallinkphysicalhost: IPaddress Atransporttransportserver: IPaddress Bsource IP,port: B,80dest IP,port: A,9157source IP,port: A,9157dest IP, port: B,80physicalsource IP,port: C,5775dest IP,port: B,80host: IPaddress Csource IP,port: C,9157dest IP,port: B,80Transport Layer 3-14

3. Transport Layer: Outline3.1 transport-layerservices3.2 multiplexing anddemultiplexing3.3 connectionlesstransport: UDP3.4 principles of reliabledata transfer3.5 connection-orientedtransport: TCP§ § § § segment structurereliable data transferflow controlconnection management3.6 principles of congestioncontrol3.7 TCP congestion controlTransport Layer 3-15

UDP: User Datagram Protocol [RFC 768]v v no frills, bare bonestransport protocol for“best effort” service,UDP segments may be:§ lost§ delivered out-of-orderconnectionless:§ no sender-receiverhandshaking§ each UDP segmenthandled independentlyv UDP uses:§ streaming multimediaapps (loss tolerant, ratesensitive)§ DNS§ SNMPv reliable transfer overUDP:§ add reliability atapplication layer§ application-specific errorrecovery!Transport Layer 3-16

UDP: segment header32 bitssource port #dest port #lengthchecksumapplicationdata(payload)length, in bytes ofUDP segment,including headerwhy is there a UDP?v v v UDP segment formatv no connectionestablishment (which canadd delay)simple: no connectionstate at sender, receiversmall header sizeno congestion control:UDP can blast away asfast as desiredTransport Layer 3-17

UDP checksumGoal: detect “errors” (flipped bits) in segmentssender:receiver:v v v v treat segment contents,including header fields,as sequence of 16-bitintegerschecksum: addition(one’s complementsum) of segmentcontentssender puts checksumvalue into UDPchecksum fieldv compute checksum ofreceived segmentcheck if computedchecksum equals checksumfield value:§ NO - error detected§ YES - no error detected.But maybe errorsnonetheless? More later .Transport Layer 3-18

Internet checksum: exampleexample: add two 16-bit integers1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1Note: when adding numbers, a carryout from the mostsignificant bit needs to be added to the resultTransport Layer 3-19

Q1: Sockets and multiplexingv TCP uses more information in packet headers inorder to demultiplex packets compared to UDP.A. TrueB. FalseTransport Layer 3-20

Q2: Sockets UDPv Suppose we use UDP instead of TCP underHTTP for designing a web server where allrequests and responses fit in a single packet.Suppose a 100 clients are simultaneouslycommunicating with this web server. How manysockets are respectively at the server and at eachclient?A. 1,1B. 2,1C. 200,2D. 100,1E. 101, 1Transport Layer 3-21

Q3: Sockets TCPv Suppose a 100 clients are simultaneouslycommunicating with (a traditional HTTP/TCP)web server. How many sockets are respectivelyat the server and at each client?A. 1,1B. 2,1C. 200,2D. 100,1E. 101, 1Transport Layer 3-22

Q4: Sockets TCPv Suppose a 100 clients are simultaneouslycommunicating with (a traditional HTTP/TCP)web server. Do all of the sockets at the serverhave the same server-side port number?A. YesB. NoTransport Layer 3-23

Q5: UDP checksumsv Let’s denote a UDP packet as (checksum, data)ignoring other fields for this question. Suppose asender sends (0010, 1110) and the receiverreceives (0011,1110). Which of the following istrue of the receiver?A. Thinks the packet is corrupted and discardsthe packet.B. Thinks only the checksum is corrupted anddelivers the correct data to the application.C. Can possibly conclude that nothing is wrongwith the packet.D. A and CTransport Layer 3-24

3. Transport Layer: Outline3.1 transport-layerservices3.2 multiplexing anddemultiplexing3.3 connectionlesstransport: UDP3.4 principles of reliabledata transfer3.5 connection-orientedtransport: TCP§ § § § segment structurereliable data transferflow controlconnection management3.6 principles of congestioncontrol3.7 TCP congestion controlTransport Layer 3-25

Principles of reliable data transferv important in application, transport, link layers§ top-10 list of important networking topics!v characteristics of unreliable channel will determinecomplexity of reliable data transfer protocol (rdt)Transport Layer 3-26

Principles of reliable data transferv important in application, transport, link layers§ top-10 list of important networking topics!v characteristics of unreliable channel will determinecomplexity of reliable data transfer protocol (rdt)Transport Layer 3-27

Principles of reliable data transferv important in application, transport, link layers§ top-10 list of important networking topics!v characteristics of unreliable channel will determinecomplexity of reliable data transfer protocol (rdt)Transport Layer 3-28

Reliable data transfer: getting startedrdt send(): called from above,(e.g., by app.). Passed data todeliver to receiver upper layersendsideudt send(): called by rdt,to transfer packet overunreliable channel to receiverdeliver data(): called byrdt to deliver data to upperreceivesiderdt rcv(): called when packetarrives on rcv-side of channelTransport Layer 3-29

Reliable data transfer: getting startedwe’ll:v incrementally develop sender, receiver sides ofreliable data transfer protocol (rdt)v consider only unidirectional data transfer§ but control info will flow on both directions!v use finite state machines (FSM) to specify sender,receiverevent causing state transitionactions taken on state transitionstate: when in this“state” next stateuniquely determinedby next eventstate1eventactionsstate2Transport Layer 3-30

rdt1.0: reliable transfer over a reliable channelv underlying channel perfectly reliable§ no bit errors§ no loss of packetsv separate FSMs for sender, receiver:§ sender sends data into underlying channel§ receiver reads data from underlying channelWait forcall fromaboverdt send(data)packet make pkt(data)udt send(packet)senderWait forcall frombelowrdt rcv(packet)extract (packet,data)deliver data(data)receiverTransport Layer 3-31

rdt2.0: channel with bit errorsv underlying channel may flip bits in packet§ checksum to detect bit errorsv v the question: how to recover from errors:§ acknowledgements (ACKs): receiver explicitly tells senderthat pkt received OK§ negative acknowledgements (NAKs): receiver explicitly tellssender that pkt had errors§ senderpkt onreceipt fromof NAKHowretransmitsdo humansrecover“errors”new mechanisms in rdt2.0 (beyond rdt1.0):during conversation?§ error detection§ receiver feedback: control msgs (ACK,NAK) rcvr senderTransport Layer 3-32