CS555. Spring /8/2005. Chapter 3: Transport Layer. Chapter 3 outline. TCP basics. WMU-CS, Dr. Gupta 1. Reliable, ordered delivery
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1 CS555. Spring 2005 Chapter 3: Transport Layer Our goals: understand principles behind transport layer services: multiplexing/demultipl exing reliable data transfer flow control congestion control learn about transport layer protocols in the Internet: UDP: connectionless transport TCP: connection-oriented transport TCP congestion control Source: Guizani; Kurose and Ross textbooks and Internet. Material from textbooks is copyrighted by appropriate authors. For example: All material copyright , J.F Kurose and K.W. Ross, All Rights Reserved Transport Layer 3-1 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 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 control 3.8 TCP fairness 3.9 Delay Modeling Transport Layer 3-2 TCP basics Reliable, ordered delivery uses sequence numbers, acknowledgements, timeouts and retransmissions End-to-end semantics (ACK after data recd) Provides flow and congestion control uses sliding window based buffers and feedback from receiver/network to adjust transmission rate Transport Layer 3-3 WMU-CS, Dr. Gupta 1
2 CS555. Spring 2005 TCP header Transport Layer 3-4 TCP reliable data transfer TCP creates rdt service on top of IP s unreliable service Pipelined segments Cumulative acks TCP uses single retransmission timer Retransmissions are triggered by: timeout events duplicate acks Initially consider simplified TCP sender: ignore duplicate acks ignore flow control, congestion control Transport Layer 3-5 TCP sender events: data rcvd from app: Create segment with seq # seq # is byte-stream number of first data byte in segment start timer if not already running (think of timer as for oldest unacked segment) expiration interval: TimeOutInterval timeout: retransmit segment that caused timeout restart timer Ack rcvd: If acknowledgement for previously unacked segments update what is known to be acked start timer if there are outstanding segments Transport Layer 3-6 WMU-CS, Dr. Gupta 2
3 CS555. Spring 2005 NextSeqNum = InitialSeqNum SendBase = InitialSeqNum loop (forever) { switch(event) event: data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP NextSeqNum = NextSeqNum + length(data) event: timer timeout retransmit not-yet-acknowledged segment with smallest sequence number start timer event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } TCP sender (simplified) Comment: SendBase-1: last cumulatively ack ed byte Example: SendBase-1 = 71; y= 73, so the rcvr wants 73+ ; y > SendBase, so that new data is acked } /* end of loop forever */ Transport Layer 3-7 TCP: retransmission scenarios Host A Host B Host A Host B Seq=92, 8 bytes data Seq=92, 8 bytes data timeout SendBase = 100 X loss ACK=100 Seq=92, 8 bytes data ACK=100 time lost ACK scenario Sendbase = 100 SendBase = 120 SendBase = 120 Seq=92 timeout Seq=92 timeout time Seq=100, 20 bytes data ACK=100 ACK=120 Seq=92, 8 bytes data ACK=120 premature timeout Transport Layer 3-8 TCP retransmission scenarios (more) Host A Host B Seq=92, 8 bytes data timeout Seq=100, 20 bytes data X loss ACK=100 SendBase = 120 ACK=120 time Cumulative ACK scenario Transport Layer 3-9 WMU-CS, Dr. Gupta 3
4 CS555. Spring 2005 TCP ACK generation [RFC 1122, RFC 2581] Event at Receiver Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Arrival of in-order segment with expected seq #. One other segment has ACK pending Arrival of out-of-order segment higher-than-expect seq. #. Gap detected Arrival of segment that partially or completely fills gap TCP Receiver action Delayed ACK. Wait up to 500ms for next segment. If no next segment, send ACK Immediately send single cumulative ACK, ACKing both in-order segments Immediately send duplicate ACK, indicating seq. # of next expected byte Immediate send ACK, provided that segment starts at lower end of gap Transport Layer 3-10 Fast Retransmit Time-out period often relatively long: long delay before resending lost packet Detect lost segments via duplicate ACKs. Sender often sends many segments back-toback If segment is lost, there will likely be many duplicate ACKs. If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost: fast retransmit: resend segment before timer expires Transport Layer 3-11 Fast retransmit algorithm: event: ACK received, with ACK field value of y if (y > SendBase) { SendBase = y if (there are currently not-yet-acknowledged segments) start timer } else { increment count of dup ACKs received for y if (count of dup ACKs received for y = 3) { resend segment with sequence number y } a duplicate ACK for already ACKed segment fast retransmit Transport Layer 3-12 WMU-CS, Dr. Gupta 4
5 CS555. Spring 2005 Timeouts and retransmission TCP manages four different timers for each connection retransmission timer: when awaiting ACK persist timer: keeps window size information flowing keepalive timer: when other end crashes or reboots 2MSL timer: for the TIME_WAIT state Transport Layer 3-13 RTT estimation Exponential Averaging Filter: Measure SampleRTT for segment/ack pair Compute weighted average of RTT EstimatedRTT = α EstimatedRTT + (1 α) samplertt Retransmit Time Out Interval RTO = β * EstimatedRTT Typically α = 0.9; β = 2 Transport Layer 3-14 Timeout: Exponential backoff Double RTO on each timeout Packet transmitted T1 T2 = 2 * T1 Timeout interval doubled Time-out occurs before ack received, packet retransmitted Transport Layer 3-15 WMU-CS, Dr. Gupta 5
6 CS555. Spring 2005 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 3.5 Connection-oriented transport: TCP segment structure reliable data transfer bit errors flow control connection management 3.6 Principles of congestion control 3.7 TCP congestion control 3.8 TCP fairness 3.9 Delay Modeling Transport Layer 3-16 Bit Errors Send ACK (duplicate) when checksum not matched, forcing sender to retransmit Transport Layer 3-17 TCP Flow Control receive side of TCP connection has a receive buffer: flow control sender won t overflow receiver s buffer by transmitting too much, too fast app process may be slow at reading from buffer speed-matching service: matching the send rate to the receiving app s drain rate Transport Layer 3-18 WMU-CS, Dr. Gupta 6
7 CS555. Spring 2005 TCP Flow control: how it works (Suppose TCP receiver discards out-of-order segments) spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] Rcvr advertises spare room by including value of RcvWindow in segments Sender limits unacked data to RcvWindow guarantees receive buffer doesn t overflow Transport Layer 3-19 Window based flow control Window size minimum of receiver s advertised window - determined by available buffer space at the receiver congestion window - determined by sender, based on network feedback Sender s window Acks received Not transmitted Transport Layer 3-20 Advertised window AdvertisedWindow = MaxRcvBuffer - (LastByteRcvd - NextByteRead) SendingWindow = AdvertisedWindow - (LastByteSent - LastByteAcked) Sender uses persist timer to probe receiver when AdvertisedWindow=0. Transport Layer 3-21 WMU-CS, Dr. Gupta 7
8 CS555. Spring 2005 Congestion window Limits amount of data in transit MaxWin = MIN (CongestionWindow, AdvertisedWindow) EffectiveWin = MaxWin - (LastByteSent - LastByteAcked) Transport Layer 3-22 Congestion control On detecting a packet loss TCP sender assumes that network congestion has occurred TCP sender drastically reduces the congestion window Reducing congestion window reduces amount of data that can be sent per RTT Transport Layer 3-23 AIMD: Additive increase Multiplicative decrease Source infers congestion upon RTO Increase CongestionWindow (linearly, by 1 segment per RTT) when congestion goes down Decrease CongestionWindow (multiplicatively, by factor of 1/2) when congestion goes up Transport Layer 3-24 WMU-CS, Dr. Gupta 8
9 CS555. Spring 2005 Slow start and Congestion avoidance AIMD may be too conservative CongestionWindow: cwnd Slow Start Increase cwnd exponentially upto a threshold (ssthresh) Congestion Avoidance Increase cwnd linearly after ssthresh Transport Layer 3-25 Typical TCP behaviour After timeout Congestion window (segments) cwnd = ssthresh = 8 ssthresh = 10 Time (round trips) Transport Layer 3-26 Fast retransmit and Fast recovery Waiting for TCP sender timeouts leads to idle periods Fast retransmit: use duplicate (triplicate) ACKs to trigger retransmission Fast recovery remove the slow start phase; go directly to half the last successful cwnd Transport Layer 3-27 WMU-CS, Dr. Gupta 9
10 CS555. Spring 2005 Fast retransmit and Fast recovery Window size (segm ents) Time (round trips) advertised window After fast recovery Transport Layer 3-28 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 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 control 3.8 TCP fairness 3.9 Delay Modeling Transport Layer 3-29 TCP Fairness Fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K TCP connection 1 TCP connection 2 bottleneck router capacity R Transport Layer 3-30 WMU-CS, Dr. Gupta 10
11 CS555. Spring 2005 Why is TCP fair? Two competing sessions: Additive increase gives slope of 1, as throughout increases multiplicative decrease decreases throughput proportionally R equal bandwidth share Connection 2 throughput Connection 1 throughput loss: decrease window by factor of 2 congestion avoidance: additive increase loss: decrease window by factor of 2 congestion avoidance: additive increase R Full bandwidth utilization line Transport Layer 3-31 Fairness (more) Fairness and UDP Multimedia apps often do not use TCP do not want rate throttled by congestion control Instead use UDP: pump audio/video at constant rate, tolerate packet loss Research area: TCP friendly Fairness and parallel TCP connections nothing prevents app from opening parallel connections between 2 hosts. Web browsers do this Example: link of rate R supporting 9 connections; new app asks for 1 TCP, gets rate R/10 new app asks for 11 TCPs, gets R/2! Transport Layer 3-32 HW 5? RDT protocol implementation using Java Ideas to be liberally borrowed from Get started even before I customize it to our environment HW 6? TBD FTP response time modeling using OPNET? UDP app versus TCP app response times? Transport Layer 3-33 WMU-CS, Dr. Gupta 11
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