《计算机网络——自顶向下方法与Internet特色》幻灯片Lecture31(精品).ppt

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1、Chapter 3:Transport LayerOur goals:understand principles behind transport layer services:multiplexing/demultiplexingreliable data transferflow controlcongestion controllearn about transport layer protocols in the Internet:UDP:connectionless transportTCP:connection-oriented transportTCP congestion co

2、ntrolChapter 3 outline3.1 Transport-layer services3.2 Multiplexing and demultiplexing3.3 Connectionless transport:UDP3.4 Principles of reliable data transfer3.5 Connection-oriented transport:TCPsegment structurereliable data transferflow controlconnection management3.6 Principles of congestion contr

3、ol3.7 TCP congestion control3.1 Transport services and protocolsprovide logical communication between app processes running on different hoststransport protocols run in end systems send side:breaks app messages into segments,passes to network layerrcv side:reassembles segments into messages,passes t

4、o app layermore than one transport protocol available to appsInternet:TCP and UDPapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicallogical end

5、-end transport3.1.1 Transport work layernetwork layer:logical communication between hoststransport layer:logical communication between processes relies on,enhances,network layer servicesHousehold analogy:12 kids sending letters to 12 kidsprocesses=kidsapp messages=letters in envelopeshosts=housestra

6、nsport protocol=Ann and Billnetwork-layer protocol=postal service3.1.2 Internet transport-layer protocolsTCP(Transmission Control Protocol)and UDP(User Datagram Protocol)Segment and Datagram(also used in Network layer)Internets Network Layer protocol-IP(Internet Protocol).IP provides logical communi

7、cation between hosts.The IP service model is a best-effort delivery service(尽力而为交互服务).The most fundamental responsibility of UDP and TCP is to extend IPs delivery service between two end system to a delivery service between two processes running on the end systems.Extending host-to-host delivery to

8、process-to-process delivery is called transport layer multiplexing(多路复用)and demultiplexing(多路分解).3.1.2 Internet transport-layer protocolsreliable,in-order delivery(TCP)congestion control flow controlconnection setupunreliable,unordered delivery:UDPno-frills extension of“best-effort”IPservices not av

9、ailable:delay guaranteesbandwidth guaranteesapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicallogical end-end transportChapter 3 outline3.1 Tr

10、ansport-layer services3.2 Multiplexing and demultiplexing3.3 Connectionless transport:UDP3.4 Principles of reliable data transfer3.5 Connection-oriented transport:TCPsegment structurereliable data transferflow controlconnection management3.6 Principles of congestion control3.7 TCP congestion control

11、3.2 Multiplexing/demultiplexingapplicationtransportnetworklinkphysicalP1applicationtransportnetworklinkphysicalapplicationtransportnetworklinkphysicalP2P3P4P1host 1host 2host 3=process=socketdelivering received segmentsto correct socketDemultiplexing at rcv host:gathering data from multiplesockets,e

12、nveloping data with header(later used for demultiplexing)Multiplexing at send host:1、How demultiplexing workshost receives IP datagramseach datagram has source IP address,destination IP addresseach datagram carries 1 transport-layer segmenteach segment has source,destination port number host uses IP

13、 addresses&port numbers to direct segment to appropriate socketsource port#dest port#32 bitsapplicationdata(message)other header fieldsTCP/UDP segment format2、Connectionless demultiplexingCreate sockets with port numbers:DatagramSocket mySocket1=new DatagramSocket(99111);DatagramSocket mySocket2=new

14、 DatagramSocket(99222);UDP socket identified by two-tuple:(dest IP address,dest port number)When host receives UDP segment:checks destination port number in segmentdirects UDP segment to socket with that port numberIP datagrams with different source IP addresses and/or source port numbers directed t

15、o same socket2、Connectionless demux(cont)DatagramSocket serverSocket=new DatagramSocket(6428);ClientIP:BP2client IP:AP1P1P3serverIP:CSP:6428DP:9157SP:9157DP:6428SP:6428DP:5775SP:5775DP:6428SP provides“return address”3、Connection-oriented demuxTCP socket identified by 4-tuple:source IP addresssource

16、port numberdest IP addressdest port numberrecv host uses all four values to direct segment to appropriate socketServer host may support many simultaneous TCP sockets:each socket identified by its own 4-tupleWeb servers have different sockets for each connecting clientnon-persistent HTTP will have di

17、fferent socket for each request3、Connection-oriented demux(cont)ClientIP:BP1client IP:AP1P2P4serverIP:CSP:9157DP:80SP:9157DP:80P5P6P3D-IP:CS-IP:AD-IP:CS-IP:BSP:5775DP:80D-IP:CS-IP:B4、Connection-oriented demux:Threaded Web ServerClientIP:BP1client IP:AP1P2serverIP:CSP:9157DP:80SP:9157DP:80P4P3D-IP:CS

18、-IP:AD-IP:CS-IP:BSP:5775DP:80D-IP:CS-IP:BChapter 3 outline3.1 Transport-layer services3.2 Multiplexing and demultiplexing3.3 Connectionless transport:UDP3.4 Principles of reliable data transfer3.5 Connection-oriented transport:TCPsegment structurereliable data transferflow controlconnection manageme

19、nt3.6 Principles of congestion control3.7 TCP congestion control3.3 UDP:User Datagram Protocol RFC 768“no frills,”“bare bones”Internet transport protocol“best effort”service,UDP segments may be:lostdelivered out of order to appconnectionless:no handshaking between UDP sender,receivereach UDP segment

20、 handled independently of othersWhy is there a UDP?Finer app-level control over what data are sent,and when.no connection establishment(which can add delay).simple:no connection state at sender,receiversmall segment headerno congestion control:UDP can blast away as fast as desired3.3.1 UDP Segment s

21、tructureoften used for streaming multimedia appsloss tolerantrate sensitiveother UDP usesDNSSNMPreliable transfer over UDP:add reliability at application layerapplication-specific error recovery!source port#dest port#32 bitsApplicationdata(message)UDP segment formatlengthchecksumLength,inbytes of UD

22、Psegment,includingheader3.3.2 UDP checksumSender:treat segment contents as sequence of 16-bit integerschecksum:addition(1s complement sum)of segment contentssender puts checksum value into UDP checksum fieldReceiver:compute checksum of received segmentcheck if computed checksum equals checksum field

23、 value:NO-error detectedYES-no error detected.But maybe errors nonetheless?More later.Goal:detect“errors”(e.g.,flipped bits)in transmitted segmentInternet Checksum ExampleNoteWhen adding numbers,a carryout from the most significant bit needs to be added to the resultExample: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 11 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 11 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1wraparoundsumchecksum