1Stan KurkovskyComputer NetworksTransport Layer ServicesMultiplexing/DemultiplexingBased on Computer Networking, 4thEdition by Kurose and RossStan KurkovskyTransport services and protocols• providelogical 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 linkphysical2Stan KurkovskyTransport vs. network layer•network layer:logical communication between hosts•transport layer:logical communication between processes • relies on, enhances, network layer servicesHousehold analogy:12 kids sending letters to 12 kids• processes = kids• app messages = letters in envelopes• hosts = houses• transport protocol = Ann and Bill• network-layer protocol = postal serviceStan KurkovskyInternet 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 linkphysical3Stan KurkovskyMultiplexing/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:Stan KurkovskyHow 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 • host uses IP addresses & port numbers to direct segment to appropriate socketsource port # dest port #32 bitsapplicationdata (message)other header fieldsTCP/UDP segment format4Stan KurkovskyConnectionless 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 socketEX: DatagramSocket serverSocket = new DatagramSocket(6428);ClientIP:BP2clientIP: AP1P1P3serverIP: CSP: 6428DP: 9157SP: 9157DP: 6428SP: 6428DP: 5775SP: 5775DP: 6428Stan KurkovskyConnection-oriented demultiplexing• 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 requestClientIP:BP1clientIP: AP1P2P4serverIP: CSP: 9157DP: 80SP: 9157DP: 80P5P6P3D-IP:CS-IP: AD-IP:CS-IP: BSP: 5775DP: 80D-IP:CS-IP: