79 Sidevõrgud IRT 4060/ IRT 0020 vooruloeng 8 / 3. nov 2004 Vooülekanne Avo Ots telekommunikatsiooni õppetool, TTÜ raadio- ja sidetehnika inst.

80 ApplicationData lossBandwidthTime sensitive Application layer protocol Transport protocol File transferNoElasticNoFTPTCP NoElasticNoSMTPTCP Web documents NoElastic, KbpsNoHTTPTCP Real-time audio/video YesKbps, MbpsYes, 100s of Msec ProprietaryTCP or UDP Stored audio/video YesKbps, MbpsYes, secProprietary, NFS TCP or UDP Interactive games YesKbpsYes, 100s of Msec ProprietaryTCP or UDP Financial applications NoElasticBothProprietaryTCP TCP: guarantee: deliver all data; no guarantee: rate, delay; connection; congestion control UDP: no guarantee: deliver all data, order, delay; no connection; no congestion control

81 Web cache – proxy server Network entity that satisfies HTTP requests on behalf of a client. Keeps copies of recently requested objects in its own disk storage. Content distribution. HTTP request HTTP response HTTP request HTTP response client proxy server is also client of origin origin server

82 Transport protocols Transform host-to-host communication, offered by network layer, to process-to-process communication Deliver important services to application layer: 1.Guaranteed message delivery 2.In-order delivery 3.Detect/eliminate message replication 4.Support arbitrarily large messages 5.Support synchronization between sender & receiver 6.Allow receiver to apply flow control to sender 7.Support multiple application processes on each host 8.Other services? (congestion control, quality-of-service, …) Overview: –UDP: unreliable message delivery protocol –TCP: reliable stream transfer protocol

83 User Datagram Protocol (UDP) Connectionless, unreliable, message delivery –But, optional checksum provides limited error detection Computed over UDP header, message, and IP pseudoheader Pseudoheader: IP src & dst addresses, protocol #, and UDP length If source does not want to compute checksum, it sets it to zero Allows process demultiplexing using ports –16 bits per port: possible channels per host No flow control: sender can overrun receiver’s buffers

84 Application demultiplexing with UDP Application process Application process Application process UDP Packets arrive Ports Queues Packets demultiplexed

85 Transmission Control Protocol (TCP) Reliable, connection-oriented, byte-stream delivery service –Full duplex connection –Supports flow control (and congestion control; to be covered later) –Supports application layer demultiplexing using ports –Segments vs application-layer “writes”: when does TCP transmit a segment? Maximum Segment Size (MSS), Push operation, Send-Timer Application process Write bytes TCP Send buffer Segment Transmit segments Application process Read bytes TCP Receive buffer … ……

86 Sokkel TCP with buffers, variables process socket TCP with buffers, variables process socket Internet controlled by application developer controlled by operating system

87 Congestion window Round-trip times Slow start Congestion avoidance Congestion occurs Threshold TCP Congestion Control

88 TCP Window Size Over Time Long-lived TCP connection

89 Slow Start Sequence Plot Time Sequence No Window doubles every round

90 The big picture Time cwnd Timeout Slow Start Congestion Avoidance

91 Fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K TCP connection 1 bottleneck router capacity R TCP connection 2 TCP Fairness

92 Why is TCP fair? Two competing sessions: Additive increase gives slope of 1, as throughout increases multiplicative decrease drops throughput proportionally R R equal bandwidth share Connection 1 throughput Connection 2 throughput congestion avoidance: additive increase loss: decrease window by factor of 2 congestion avoidance: additive increase loss: decrease window by factor of 2

93 TCP Connection Management TCP client lifecycle TCP server lifecycle