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8. Transport Protocol and UDP

8. Transport Protocol and UDP. 8.1 Transport protocol : End-to-end protocol IP: Host to host packet delivery Transport: Process to process communication Port number Multiplexing and de-multiplexing. End-to-End Protocols. Underlying best-effort network drop messages re-orders messages

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8. Transport Protocol and UDP

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  1. 8. Transport Protocol and UDP 8.1 Transport protocol : End-to-end protocol • IP: Host to host packet delivery • Transport: Process to process communication • Port number • Multiplexing and de-multiplexing

  2. End-to-End Protocols • Underlying best-effort network • drop messages • re-orders messages • delivers duplicate copies of a given message • limits messages to some finite size • delivers messages after an arbitrarily long delay • Common end-to-end services • guarantee message delivery • deliver messages in the same order they are sent • deliver at most one copy of each message • support arbitrarily large messages • support synchronization • allow the receiver to flow control the sender • support multiple application processes on each host

  3. 8.2 UDP (User Datagram Protocol) • Unreliable and unordered datagram service • Adds multiplexing • No flow control • Endpoints identified by ports • Servers may have well-known ports • see /etc/services on Unix • Header format • Checksum • psuedo header + UDP header + data 0 16 31 SrcPort DstPort Checksum Length Data

  4. 9. TCP: Reliable Byte-Stream Outline Overview Segment format Connection Establishment/Termination Flow Control (Sliding Window) Adaptive Timeout Congestion Control

  5. Application process Application process W rite Read bytes bytes … … TCP TCP Send buffer Receive buffer … Segment Segment Segment T ransmit segments 9.1 TCP Overview • Full duplex • Flow control: keep sender from overrunning receiver • Congestion control: keep sender from overrunning network • Reliable, Connection-oriented • Byte-stream • app writes bytes • TCP sends segments • app reads bytes

  6. Data Link Versus Transport • Potentially connects many different hosts • need explicit connection establishment and termination • Potentially different RTT • need adaptive timeout mechanism • Potentially long delay in network • need to be prepared for arrival of very old packets • Potentially different capacity at destination • need to accommodate different node capacity • Potentially different network capacity • need to be prepared for network congestion

  7. 9.2 Segment Format

  8. Data (SequenceNum) Sender Receiver Acknowledgment + AdvertisedWindow Segment Format (cont) • Each connection identified with 4-tuple: • (SrcPort, SrcIPAddr, DsrPort, DstIPAddr) • Sliding window + flow control • acknowledgment, SequenceNum, AdvertisedWinow • Checksum • pseudo header + TCP header + data

  9. Flags • URG: urgent pointer is valid • ACK: ack# is valid • PUSH: this segment requests a push • RST: reset the connection • SYN: synchronize sequence number • FIN: sender has reached end of its byte stream (no more data to send)

  10. 9.3 Connection Establishment and Termination Active participant Passive participant (client) (server) SYN, SequenceNum = x , y 1 + SYN + ACK, SequenceNum = x Acknowledgment = ACK, Acknowledgment = y + 1

  11. CLOSED Active open /SYN Passive open Close Close LISTEN SYN/SYN + ACK Send/ SYN SYN/SYN + ACK SYN_RCVD SYN_SENT ACK SYN + ACK/ACK Close /FIN ESTABLISHED Close /FIN FIN/ACK FIN_WAIT_1 CLOSE_WAIT FIN/ACK ACK Close /FIN ACK + FIN/ACK FIN_WAIT_2 CLOSING LAST_ACK Timeout after two ACK ACK segment lifetimes FIN/ACK TIME_WAIT CLOSED State Transition Diagram

  12. Connection Establishment: three-way handshake Active participant Passive participant (client) (server) CLOSED CLOSED LISTEN SYN, SequenceNum = x SYN-SENT , y 1 + SYN + ACK, SequenceNum = x Acknowledgment = SYN-RCVD ACK, Acknowledgment = ESTABLISHED y + 1 ESTABLISHED

  13. Termination • This side closed first: • ESTABLISHED  FIN-WAIT-1 FIN-WAIT-2 TIME-WAIT CLOSED • The other side closes first: • ESTABLISHED CLOSE-WAIT  LAST-ACK CLOSED • Both sides close at the same time: • ESTABLISHED  FIN-WAIT-1  CLOSING TIME-WAIT CLOSED

  14. Example of Simultaneous Close ESTABLISHED ESTABLISHED FIN, seq#= j FIN-WAIT-1 FIN-WAIT-1 k FIN, Seq#= ACK, j+1 CLOSING CLOSING TIME-WAIT CLOSED ACK, k+1 TIME-WAIT CLOSED

  15. 9.4 Flow Control • Sliding window • It guarantees the reliable delivery of data • It ensures that data is delivered in order • It enforces flow control between the sender and the receiver

  16. Variables Sending application Receiving application TCP TCP LastByteWritten LastByteRead LastByteAcked LastByteSent NextByteExpected LastByteRcvd

  17. Sending application Receiving application TCP TCP LastByteWritten LastByteRead LastByteAcked LastByteSent NextByteExpected LastByteRcvd Flow Control • Sending side • LastByteAcked < = LastByteSent • LastByteSent < = LastByteWritten • buffer bytes between LastByteAcked and LastByteWritten • Receiving side • LastByteRead < NextByteExpected • NextByteExpected < = LastByteRcvd +1 • buffer bytes between LastByteRead and LastByteRcvd

  18. Flow Control (cont.) • Send buffer size: MaxSendBuffer • Receive buffer size: MaxRcvBuffer • Receiving side • LastByteRcvd - LastByteRead < = MaxRcvBuffer • AdvertisedWindow = MaxRcvBuffer - (NextByteExpected - NextByteRead) • Sending side • LastByteSent - LastByteAcked < = AdvertisedWindow • EffectiveWindow = AdvertisedWindow - (LastByteSent - LastByteAcked) • LastByteWritten - LastByteAcked < = MaxSendBuffer • block sender if (LastByteWritten - LastByteAcked) + y > MaxSenderBuffer • Always send ACK in response to arriving data segment • Persist when AdvertisedWindow= 0

  19. Protection Against Wrap Around • 32-bit SequenceNum Bandwidth Time Until Wrap Around T1 (1.5 Mbps) 6.4 hours Ethernet (10 Mbps) 57 minutes T3 (45 Mbps) 13 minutes FDDI (100 Mbps) 6 minutes STS-3 (155 Mbps) 4 minutes STS-12 (622 Mbps) 55 seconds STS-24 (1.2 Gbps) 28 seconds

  20. Keeping the Pipe Full • 16-bit AdvertisedWindow Bandwidth Delay x Bandwidth Product T1 (1.5 Mbps) 18KB Ethernet (10 Mbps) 122KB T3 (45 Mbps) 549KB FDDI (100 Mbps) 1.2MB STS-3 (155 Mbps) 1.8MB STS-12 (622 Mbps) 7.4MB STS-24 (1.2 Gbps) 14.8MB

  21. TCP Extensions • Implemented as header options • Store timestamp in outgoing segments • Extend sequence space with 32-bit timestamp (PAWS) • Shift (scale) advertised window

  22. 9.5 Adaptive Retransmission(Original Algorithm) • Measure SampleRTT for each segment/ ACK pair • Compute weighted average of RTT • EstRTT = axEstRTT + bxSampleRTT • where a+b = 1 • a between 0.8 and 0.9 • b between 0.1 and 0.2 • Set timeout based on EstRTT • TimeOut=2xEstRTT

  23. Karn/Partridge Algorithm Sender Receiver Sender Receiver • Do not sample RTT when retransmitting • Double timeout after each retransmission Original transmission Original transmission TT TT ACK Retransmission SampleR SampleR Retransmission ACK

  24. Jacobson/ Karels Algorithm • New Calculations for average RTT • Diff = SampleRTT - EstRTT • EstRTT = EstRTT + (dx Diff) • Dev = Dev + d( |Diff| - Dev) • where d is a factor between 0 and 1 • Consider variance when setting timeout value • TimeOut = mxEstRTT + fxDev • where m = 1 and f = 4 • Notes • algorithm only as good as granularity of clock (500ms on Unix) • accurate timeout mechanism important to congestion control (later)

  25. Design Alternatives • Stream-oriented vs request-reply • Byte stream vs message stream • Explicit setup/teardown vs implicit setup/teardown • Window-based vs rate-based

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