1. Transport Layer3-1 Chapter 3 Transport Layer Computer Networking: A Top Down Approach 5 th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009. All material copyright 1996-2009 J.F Kurose and K.W. Ross, All Rights Reserved

2. Transport Layer3-2 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP r 3.4 Principles of reliable data transfer r 3.5 Connection-oriented transport: TCP m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

3. Transport Layer3-3 TCP segment structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number Receive window Urg data pnter checksum F SR PAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP)

4. Transport Layer3-4 TCP Connection Management Recall: TCP sender, receiver establish “connection” before exchanging data segments r initialize TCP variables: m seq. #s  buffers, flow control info (e.g. RcvWindow ) r client: connection initiator Socket clientSocket = new Socket("hostname","port number"); r server: contacted by client Socket connectionSocket = welcomeSocket.accept(); Three way handshake: Step 1: client host sends TCP SYN segment to server m specifies initial seq # m no data Step 2: server host receives SYN, replies with SYNACK segment m server allocates buffers m specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data

5. Three-Way Handshake in Action Transport Layer3-5

6. Transport Layer3-6 TCP Connection Management (cont.) Closing a connection: client closes socket: clientSocket.close(); Step 1: client end system sends TCP FIN control segment to server Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN. client FIN server ACK FIN close closed timed wait

7. Transport Layer3-7 TCP Connection Management (cont.) Step 3: client receives FIN, replies with ACK. m Enters “timed wait” - will respond with ACK to received FINs Step 4: server, receives ACK. Connection closed. Note: with small modification, can handle simultaneous FINs. client FIN server ACK FIN closing closed timed wait closed

8. Transport Layer3-8 TCP Connection Management (cont) TCP client lifecycle TCP server lifecycle

9. Transport Layer3-9 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP r 3.4 Principles of reliable data transfer r 3.5 Connection-oriented transport: TCP m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

10. Transport Layer3-10 Principles of Congestion Control Congestion: r informally: “too many sources sending too much data too fast for network to handle” r different from flow control! r manifestations: m lost packets (buffer overflow at routers) m long delays (queueing in router buffers) r a top-10 problem!

11. Transport Layer3-11 Causes/costs of congestion r two senders, two receivers r one router, infinite buffers r no retransmission r large delays when congested r maximum achievable throughput unlimited shared output link buffers Host A in : original data Host B out

12. Transport Layer3-12 Approaches towards congestion control End-end congestion control: r no explicit feedback from network r congestion inferred from end-system observed loss, delay r approach taken by TCP Network-assisted congestion control: r routers provide feedback to end systems m single bit indicating congestion m explicit rate sender should send at Two broad approaches towards congestion control:

13. Transport Layer3-13 Chapter 3 outline r 3.1 Transport-layer services r 3.2 Multiplexing and demultiplexing r 3.3 Connectionless transport: UDP r 3.4 Principles of reliable data transfer r 3.5 Connection-oriented transport: TCP m segment structure m reliable data transfer m flow control m connection management r 3.6 Principles of congestion control r 3.7 TCP congestion control

14. Transport Layer3-14 TCP Congestion Control: details r sender limits transmission: LastByteSent-LastByteAcked  CongWin r Roughly,  CongWin is dynamic, function of perceived network congestion How does sender perceive congestion? r loss event = timeout or 3 duplicate acks  TCP sender reduces rate ( CongWin ) after loss event three mechanisms: m AIMD m slow start m conservative after timeout events rate = CongWin RTT Bytes/sec

15. Transport Layer3-15 TCP Slow Start  When connection begins, CongWin = 1 MSS m Example: MSS = 500 bytes & RTT = 200 msec m initial rate = 20 kbps r available bandwidth may be >> MSS/RTT m desirable to quickly ramp up to respectable rate r When connection begins, increase rate exponentially fast until first loss event

16. Transport Layer3-16 TCP Slow Start (more) r When connection begins, increase rate exponentially until first loss event:  double CongWin every RTT  done by incrementing CongWin for every ACK received r Summary: initial rate is slow but ramps up exponentially fast Host A one segment RTT Host B time two segments four segments

17. Transport Layer3-17 Refinement: inferring loss r After 3 dup ACKs:  CongWin is cut in half m window then grows linearly r But after timeout event:  CongWin instead set to 1 MSS; m window then grows exponentially m to a threshold, then grows linearly  3 dup ACKs indicates network capable of delivering some segments  timeout indicates a “more alarming” congestion scenario Philosophy:

18. Transport Layer3-18 Summary: TCP Congestion Control  When CongWin is below Threshold, sender in slow-start phase, window grows exponentially.  When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly.  When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold.  When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.

19. Transport Layer3-19 Refinement Q: When should the exponential increase switch to linear? A: When CongWin gets to 1/2 of its value before timeout. Implementation: r Variable Threshold r At loss event, Threshold is set to 1/2 of CongWin just before loss event

20. Transport Layer3-20 TCP congestion control: additive increase, multiplicative decrease r Approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs m additive increase: increase CongWin by 1 MSS every RTT until loss detected m multiplicative decrease: cut CongWin in half after loss time congestion window size Saw tooth behavior: probing for bandwidth

21. Transport Layer3-21 TCP sender congestion control StateEventTCP Sender ActionCommentary Slow Start (SS) ACK receipt for previously unacked data CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance” Resulting in a doubling of CongWin every RTT Congestion Avoidance (CA) ACK receipt for previously unacked data CongWin = CongWin+MSS * (MSS/CongWin) Additive increase, resulting in increase of CongWin by 1 MSS every RTT SS or CALoss event detected by triple duplicate ACK Threshold = CongWin/2, CongWin = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS. SS or CATimeoutThreshold = CongWin/2, CongWin = 1 MSS, Set state to “Slow Start” Enter slow start SS or CADuplicate ACK Increment duplicate ACK count for segment being acked CongWin and Threshold not changed

22. Transport Layer3-22 TCP throughput r What’s the average throughout of TCP as a function of window size and RTT? m Ignore slow start r Let W be the window size when loss occurs. r When window is W, throughput is W/RTT r Just after loss, window drops to W/2, throughput to W/2RTT. r Average throughout:.75 W/RTT

23. Transport Layer3-23 TCP Futures: TCP over “long, fat pipes” r Example: 1500 byte segments, 100ms RTT, want 10 Gbps throughput r Requires window size W = 83,333 in-flight segments r Throughput in terms of loss rate:  ➜ L = 2·10 -10 Wow r New versions of TCP for high-speed

24. Transport Layer3-24 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

25. Transport Layer3-25 Why is TCP fair? Two competing sessions: r Additive increase gives slope of 1, as throughout increases r multiplicative decrease decreases 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

26. Transport Layer3-26 Fairness (more) Fairness and UDP r Multimedia apps often do not use TCP m do not want rate throttled by congestion control r Instead use UDP: m pump audio/video at constant rate, tolerate packet loss r Research area: TCP friendly Fairness and parallel TCP connections r nothing prevents app from opening parallel connections between 2 hosts. r Web browsers do this r Example: link of rate R supporting 9 connections; m new app asks for 1 TCP, gets rate R/10 m new app asks for 11 TCPs, gets R/2 !

27. Transport Layer3-27 Chapter 3: Summary r principles behind transport layer services: m multiplexing, demultiplexing m reliable data transfer m flow control m congestion control r instantiation and implementation in the Internet m UDP m TCP Next: r leaving the network “edge” (application, transport layers) r into the network “core”