Chapter 3 outline. 3.5 Connection-oriented transport: TCP. 3.6 Principles of congestion control 3.7 TCP congestion control

Transcription

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 Transport Layer 3-1

2 TCP: Overview point-to-point: one sender, one receiver reliable: all packets delivered pipelined: W packets are sent and ACKed full duplex data: bi-directional data flow in same connection connection-oriented: handshaking (exchange of control msgs) RFCs: 793, 1122, 1323, 2018, 2581 Transport Layer 3-2

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

4 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 Transport Layer 3-4

5 TCP reliable data transfer TCP creates rdt service on top of IP s unreliable service seq # and acks cumulative acks duplicate acks timers retransmissions pipelined segments sliding window -> retransmissions are triggered by: timeout events duplicate acks Transport Layer 3-5

6 TCP seq. # s and ACKs Seq. # s: byte stream number of first byte in segment s data Host A Host B ACKs: seq # of next byte expected from other side cumulative ACK Transport Layer 3-6

7 TCP: retransmission scenarios Host A Host B Host A Host B timeout loss Seq=92 timeout Seq=92 timeout lost ACK scenario premature timeout Transport Layer 3-7

8 TCP retransmission scenarios (more) Host A Host B timeout loss Cumulative ACK scenario Transport Layer 3-8

9 TCP Round Trip Time and Timeout Q: how to set TCP timeout value? longer than RTT but RTT varies too short: premature timeout unnecessary retransmissions too long: slow reaction to segment loss Q: how to estimate RTT? SampleRTT: measured time from segment transmission until ACK receipt SampleRTT will vary, want estimated RTT smoother average several recent measurements, not just current SampleRTT Transport Layer 3-9

10 Example RTT estimation: RTT: gaia.cs.umass.edu to fantasia.eurecom.fr RTT (milliseconds) time (seconnds) SampleRTT Estimated RTT Transport Layer 3-10

11 TCP Round Trip Time and Timeout EstimatedRTT = (1- α)*estimatedrtt α*samplertt Exponential weighted moving average influence of past sample decreases exponentially fast typical value: α = (RFC6298) Transport Layer 3-11

12 TCP Round Trip Time and Timeout Deviation of SampleRTT : DevRTT = (1-β)*DevRTT β* SampleRTT-EstimatedRTT typically, β = 0.25 (RFC6298) Then set timeout interval: TimeoutInterval = EstimatedRTT 4*DevRTT Transport Layer 3-12

13 Example RTT estimation: cont. Transport Layer 3-13

14 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 Transport Layer 3-14

15 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 3-15

16 TCP Flow control: how it works rcvr advertises spare room by including value of RcvWindow in segments RcvWindow - spare room in recv buffer sender limits unacked data to RcvWindow guarantees receive buffer doesn t overflow Transport Layer 3-16

17 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 Transport Layer 3-17

18 TCP Connection Management Recall: TCP sender, receiver establish connection before exchanging data segments initialize TCP variables: seq. #s buffers, flow control info (e.g. RcvWindow) client: connection initiator server: contacted by client Transport Layer 3-18

19 TCP Connection establishment Step 1: client host sends TCP SYN segment to server specifies initial seq # no data Step 2: server host receives SYN, replies with SYNACK segment server allocates buffers specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data Transport Layer 3-19

20 TCP Connection termination Step 1: client end system sends TCP FIN control segment to server Step 2: server receives FIN, replies with ACK. Closes connection, sends FIN. Step 3: client receives FIN, replies with ACK. Enters timed wait - will respond with ACK to received FINs Step 4: server, receives ACK. Connection closed. close timed wait closed client server close Transport Layer 3-20

21 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 Transport Layer 3-21

22 Principles of Congestion Control Congestion: informally: too many sources sending too much data too fast for network to handle different from flow control! manifestations: lost packets (buffer overflow at routers) long delays (queueing in router buffers) Results in unfairness and poor utilization of network resources: Resources utilized by dropped packets Retransmissions Transport Layer 3-22

23 Causes/costs of congestion: scenario 1 two senders, two receivers Host A λ in : original data λ out one router, infinite buffers Host B unlimited shared output link buffers no retransmission large delays when congested maximum achievable throughput Transport Layer 3-23

24 Causes/costs of congestion: scenario 2 one router, finite buffers sender retransmission of timed-out packet application-layer input = application-layer output: λ in = λ out transport-layer input includes retransmissions : λ in λ in Host B λ in : original data λ' in : original data, plus retransmitted data λ out Host A finite shared output link buffers Transport Layer 3-24

25 Congestion scenario 2: duplicates packets may get dropped at router due to full buffers sender times out prematurely, sending two copies, both of which are delivered λ out R/2 λ in R/2 when sending at R/2, some packets are retransmissions including duplicated that are delivered! costs of congestion: more work (retrans) for given goodput unneeded retransmissions: link carries multiple copies of pkt decreasing goodput Transport Layer 3-25

26 Approaches towards congestion control Two broad approaches towards congestion control: end-end congestion control: no explicit feedback from network congestion inferred from end-system observed loss, delay approach taken by TCP network-assisted congestion control: routers provide feedback to end systems single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM) explicit rate sender should send at Transport Layer 3-26

27 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 Transport Layer 3-27

28 TCP Congestion Control: Overview end-end control (no network assistance) Limit the number of packets in the network to window W Roughly, rate = W RTT Bytes/sec W is dynamic, function of perceived network congestion ACK-clocking mechanism 27 Transport Layer 3-28

29 TCP Congestion Control: details How does sender perceive congestion? loss event = timeout or 3 duplicate acks TCP sender reduces rate (cwnd) after loss event mechanisms: (RFC5681) AIMD slow start congestion avoidance fast retransmit Transport Layer 3-29

30 TCP AIMD: additive increase, multiplicative decrease approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs additive increase: increase window by 1 every RTT until loss detected multiplicative decrease: cut window in half after loss saw tooth behavior: probing for bandwidth cwnd: congestion window size 24 Kbytes 16 Kbytes 8 Kbytes congestion window time time Transport Layer 3-30

31 Slow Start Slow Start is used to reach the equilibrium state Initially: W = 1 (slow start) On each successful ACK: W = W 1 Exponential growth of W each RTT: W = 2 x W Enter CA when W >= ssthresh sender cwnd 1 data segment ACK receiver 30

32 Congestion avoidance Starts when W = ssthresh On each successful ACK W = W 1/W Linear growth of W each RTT W = W 1 (additive increase) 31 Transport Layer 3-32

33 TCP (initial version without loss) Window ssthresh Reached initial ssthresh value; switch to CA mode Slow Start Time 32

34 Detecting Packet Loss Assumption: loss indicates congestion Option 1: time-out Waiting for a time-out can be long! Option 2: duplicate ACKs How many? At least Sender Receiver 35

35 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 3-35

36 Host A Host B timeout Figure 3.37 Resending a segment after triple duplicate ACK Transport Layer 3-36

37 Fast Retransmit Immediately retransmits after 3 dupacks without waiting for timeout Adjusts ssthresh ssthresh = W/2 Enter Slow Start (Tahoe) W = 1 36

38 TCP Congestion Controls Tahoe (Jacobson 1988) Slow Start Congestion Avoidance Fast Retransmit Reno (Jacobson 1990) Fast Recovery SACK Vegas (Brakmo & Peterson 1994) Delay and loss as indicators of congestion 29

39 Refinement variable ssthresh on loss event, ssthresh is set to 1/2 of W just before loss event Transport Layer 3-39

40 TCP Reno: Fast Recovery Objective: prevent `pipe from emptying after fast retransmit each dup ACK represents a packet having left the pipe (successfully received) On 3 duplicate ACKs Fast retransmit and fast recovery On timeout Fast retransmit and slow start 46

41 Done with TCP congestion control mechanisms What type of pipelining is implemented in TCP?

42 Recall: Pipelined Protocols Go-back-N: N unacked packets in pipeline Window cumulative acks doesn t ack packet if there s a gap sender has timer for oldest unacked packet if timer expires, retransmit all unack ed packets Selective Repeat: sender can have up to N unack ed packets in pipeline rcvr sends individual ack for each packet sender maintains timer for each unacked packet when timer expires, retransmit only unack ed packet Transport Layer 3-42

43 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 3-43

44 Why is TCP fair? AIMD game for 2 competing sessions: additive increase gives slope of 1, as throughout increases multiplicative decrease decreases throughput proportionally R equal bandwidth share loss: decrease window by factor of 2 congestion avoidance: additive increase loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 1 throughput R Transport Layer 3-44

45 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 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 3-45

46 Chapter 3: Summary principles behind transport layer services: multiplexing, demultiplexing reliable data transfer flow control congestion control instantiation and implementation in the Internet UDP TCP Next: leaving the network edge (application, transport layers) into the network core Transport Layer 3-46

47 TCP: seq # plot There is a beautiful way to plot and visualize the dynamics of TCP behaviour Called a TCP Sequence Number Plot Plot packet events (data and acks) as points in 2-D space, with time on the horizontal axis, and sequence number on the vertical axis Example: Consider a 14-packet transfer 63

48 Key: Data Packet Ack Packet Time 64

49 TCP: seq n plot What can it tell you? Everything!!! 65

50 Key: Data Packet Ack Packet RTT Time 66

51 Key: Data Packet Ack Packet TCP Seg. Size Time 67

52 Key: Data Packet Ack Packet Time TCP Connection Duration 68

53 Key: Data Packet Ack Packet Num Bytes Sent Time 69

54 Key: Data Packet Ack Packet Bytes Sec Time 70

55 Key: Data Packet Ack Packet Access Network Bandwidth (Bytes/Sec) Time 71

56 Key: Data Packet Ack Packet Sender s Flow Control Window Size Time 72

57 Key: Data Packet Ack Packet TCP Slow Start Time 73

58 Key: Data Packet Ack Packet Delayed ACK Time 74

59 Key: Data Packet Ack Packet Packet Loss Duplicate ACK Time 75

60 Cumulative ACK Key: Data Packet Ack Packet Retransmit Time 76

61 Key: Data Packet Ack Packet Time RTO 77

62 TCP: seq # plot What happens when a packet loss occurs? Consider a 14-packet Web document For simplicity, consider only a single packet loss 78

63 Key: Data Packet Ack Packet Time 79

64 Key: Data Packet Ack Packet? Time 80

65 Key: Data Packet Ack Packet Time 81

66 Key: Data Packet Ack Packet Time 82

67 Key: Data Packet Ack Packet? Time 83

68 Key: Data Packet Ack Packet Time 84

69 Key: Data Packet Ack Packet Time 85

70 Key: Data Packet Ack Packet? Time 86

71 Key: Data Packet Ack Packet Time 87

72 Key: Data Packet Ack Packet Time 88

73 Key: Data Packet Ack Packet? Time 89

74 Key: Data Packet Ack Packet Time 90

75 TCP: seq # plot Main observation: Not all packet losses are created equal Losses early in the transfer have a huge adverse impact on the transfer latency Losses near the end of the transfer always cost at least a retransmit timeout Losses in the middle may or may not hurt, depending on congestion window size at the time of the loss 91