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WMU CS 5550 - Transport Layer

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Chapter 3 Transport Layer Transport Layer Our goals understand principles behind transport layer services multiplexing demult iplexing reliable data transfer flow control congestion control Chapter 3 outline Transport services and protocols 3 5 Connectionoriented transport TCP segment structure reliable data transfer flow control connection management 3 6 Principles of congestion control 3 7 TCP congestion control provide logical communication between 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 UDP application transport network data link physical network data link physical network data link physical network data link physical t or sp an tr UDP connectionless transport TCP connectionoriented transport TCP congestion control d en den al ic learn about transport layer protocols in the Internet g lo 3 1 Transport layer services 3 2 Multiplexing and demultiplexing 3 3 Connectionless transport UDP 3 4 Principles of reliable data transfer network data link physical network data link physical application transport network data link physical Transport vs network layer Internet transport layer protocols congestion control flow control connection setup application transport network data link physical services not available delay guarantees bandwidth guarantees Multiplexing demultiplexing Chapter 3 outline 3 1 Transport layer services 3 2 Multiplexing and demultiplexing 3 3 Connectionless transport UDP 3 4 Principles of reliable data transfer network data link physical network data link physical no frills extension of besteffort IP relies on enhances network layer services unreliable unordered delivery UDP network data link physical t or sp an tr network data link physical network data link physical d en den al ic network layer logical communication between hosts transport layer logical communication between processes application transport network data link physical g lo reliable in order delivery TCP 3 5 Connectionoriented transport TCP segment structure reliable data transfer flow control connection management 3 6 Principles of congestion control 3 7 TCP congestion control Multiplexing at send host gathering data from multiple sockets enveloping data with header later used for demultiplexing Demultiplexing at rcv host delivering received segments to correct socket socket application transport network link process P3 P1 P1 application transport network P2 P4 application transport network link link physical host 1 physical host 2 physical host 3 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 socket Connectionless demultiplexing 32 bits source port dest port other header fields application data message Connectionless demux cont P2 SP 6428 DP 9157 client IP A SP 9157 DP 6428 P1 P1 SP provides return address TCP socket identified by 4 tuple source IP address source port number dest IP address dest port number SP 5775 DP 6428 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 socket Connection oriented demux SP 6428 DP 5775 server IP C UDP socket identified by two tuple dest IP address dest port number P3 Create sockets with port numbers DatagramSocket mySocket1 new DatagramSocket 99111 DatagramSocket mySocket2 new DatagramSocket 99222 TCP UDP segment format DatagramSocket serverSocket new DatagramSocket 6428 When host receives UDP segment Client IP B 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 request Connection oriented demux cont P1 client IP A P4 SP 9157 DP 80 S IP A D IP C P5 server IP C P2 P6 Connection oriented demux Threaded Web Server P1P3 3 1 Transport layer services 3 2 Multiplexing and demultiplexing 3 3 Connectionless transport UDP 3 4 Principles of reliable data transfer P2 P4 SP 5775 DP 80 SP 5775 DP 80 S IP B D IP C S IP B D IP C SP 9157 DP 80 S IP B D IP C Client IP B Chapter 3 outline P1 client IP A SP 9157 DP 80 S IP A D IP C SP 9157 DP 80 server IP C S IP B D IP C P1P3 Client IP B UDP User Datagram Protocol RFC 768 3 5 Connectionoriented transport TCP segment structure reliable data transfer flow control connection management 3 6 Principles of congestion control 3 7 TCP congestion control 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 others Why 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 desired UDP checksum UDP more often used for streaming multimedia apps Length in bytes of UDP loss tolerant segment rate sensitive including other UDP uses DNS SNMP reliable transfer over UDP add reliability at application layer application specific error recovery 32 bits source port dest port length checksum header Application data message UDP segment format Internet Checksum Example Goal detect errors e g flipped bits in transmitted segment Sender Receiver Note When adding numbers a carryout from the most significant bit needs to be added to the result Example add two 16 bit integers 1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0 1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1 sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0 checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 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 field 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


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WMU CS 5550 - Transport Layer

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