Transport LayerChapter 3: Transport LayerOur goals: understand principles behind transport layer services: multiplexing/demultiplexing reliable data transfer flow control congestion control learn about transport layer protocols in the Internet: UDP: connectionless transport TCP: connection-oriented transport TCP congestion controlChapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion controlTransport services and protocols provide logical communicationbetween app processes running on different hosts transport protocols run in end systems  send side: breaks app messages into segments, passes to network layer rcv side: reassembles segments into messages, passes to app layer more than one transport protocol available to apps Internet: TCP and UDPapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicallogical end-end transportTransport vs. network layer network layer: logical communication between hosts transport layer: logical communication between processes  relies on, enhances, network layer servicesInternet transport-layer protocols reliable, in-order delivery (TCP) congestion control  flow control connection setup unreliable, unordered delivery: UDP no-frills extension of “best-effort” IP services not available:  delay guarantees bandwidth guaranteesapplicationtransportnetworkdata linkphysicalapplicationtransportnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicalnetworkdata linkphysicallogical end-end transportChapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion controlMultiplexing/demultiplexingapplicationtransportnetworklinkphysicalP1applicationtransportnetworklinkphysicalapplicationtransportnetworklinkphysicalP2P3P4P1host 1host 2host 3= process= socketdelivering received segmentsto correct socketDemultiplexing at rcv host:gathering data from multiplesockets, enveloping data with header (later used for demultiplexing)Multiplexing at send host:How demultiplexing works host receives IP datagrams each datagram has source IP address, destination IP address each datagram carries 1 transport-layer segment each segment has source, destination port number (recall: well-known port numbers for specific applications) host uses IP addresses & port numbers to direct segment to appropriate socketsource port # dest port #32 bitsapplicationdata (message)other header fieldsTCP/UDP segment formatConnectionless demultiplexing Create sockets with port numbers:DatagramSocket mySocket1 = new DatagramSocket(99111);DatagramSocket mySocket2 = new DatagramSocket(99222); UDP socket identified by two-tuple:(dest IP address, dest port number) When host receives UDP segment: checks destination port number in segment directs UDP segment to socket with that port number IP datagrams with different source IP addresses and/or source port numbers directed to same socketConnectionless demux (cont)DatagramSocket serverSocket = new DatagramSocket(6428);ClientIP:BP2clientIP: AP1P1P3serverIP: CSP: 6428DP: 9157SP: 9157DP: 6428SP: 6428DP: 5775SP: 5775DP: 6428SP provides “return address”Connection-oriented demux TCP socket identified by 4-tuple:  source IP address source port number dest IP address dest port number recv host uses all four values to direct segment to appropriate socket Server host may support many simultaneous TCP sockets: each socket identified by its own 4-tuple Web servers have different sockets for each connecting client non-persistent HTTP will have different socket for each requestConnection-oriented demux(cont)ClientIP:BP1clientIP: AP1P2P4serverIP: CSP: 9157DP: 80SP: 9157DP: 80P5P6P3D-IP:CS-IP: AD-IP:CS-IP: BSP: 5775DP: 80D-IP:CS-IP: BConnection-oriented demux: Threaded Web ServerClientIP:BP1clientIP: AP1P2serverIP: CSP: 9157DP: 80SP: 9157DP: 80P4P3D-IP:CS-IP: AD-IP:CS-IP: BSP: 5775DP: 80D-IP:CS-IP: BChapter 3 outline 3.1 Transport-layer services 3.2 Multiplexing and demultiplexing 3.3 Connectionless transport: UDP 3.4 Principles of reliable data transfer 3.5 Connection-oriented transport: TCP segment structure reliable data transfer flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion controlUDP: User Datagram Protocol [RFC 768] “no frills,” “bare bones”Internet transport protocol “best effort” service, UDP segments may be: lost delivered out of order to app connectionless: no handshaking between UDP sender, receiver each UDP segment handled independently of othersWhy is there a UDP? no connection establishment (which can add delay) simple: no connection state at sender, receiver small segment header no congestion control: UDP can blast away as fast as desiredUDP: more often used for streaming multimedia apps loss tolerant rate sensitive other UDP uses DNS SNMP reliable transfer over UDP: add reliability at application layer application-specific error recovery!source port # dest port #32 bitsApplicationdata (message)UDP segment formatlengthchecksumLength, inbytes of UDPsegment,includingheaderUDP checksumSender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum fieldReceiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless?More later ….Goal: detect “errors” (e.g., flipped bits) in transmitted segmentInternet Checksum Example