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Chapter 3 outline r 3. 1 Transport-layer services r 3. 2 Multiplexing and demultiplexing 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 m segment structure reliable data transfer flow control connection management r 3. 6 Principles of congestion control r 3. 7 TCP congestion control Transport Layer 3 -1

TCP: Overview r end-to-end: m one sender, one receiver r reliable, in-order byte stream: TCP: Overview r end-to-end: m one sender, one receiver r reliable, in-order byte stream: m no “message boundaries” r pipelined: m TCP congestion and flow control set window size r send & receive buffers RFCs: 793, 1122, 1323, 2018, 2581 r full duplex data: m bi-directional data flow in same connection m MSS: maximum segment size r connection-oriented: m handshaking (exchange of control msgs) init’s sender, receiver state before data exchange r flow controlled: m sender will not overwhelm receiver Transport Layer 3 -2

TCP segment structure 32 bits URG: urgent data (generally not used) ACK: ACK # TCP segment structure 32 bits 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) source port # dest port # sequence number acknowledgement number head not UA P R S F len used checksum Receive window Urg data pnter Options (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept application data (variable length) Transport Layer 3 -3

TCP seq. #’s and ACKs Seq. #’s: m byte stream “number” of first byte TCP seq. #’s and ACKs Seq. #’s: m byte stream “number” of first byte in segment’s data ACKs: m seq # of next byte expected from other side m cumulative ACK Q: how receiver handles out-of-order segments m A: TCP spec doesn’t say, - up to implementor Host B Host A User types ‘C’ Seq=4 2, ACK = 79, da ta ta = 3, da 4 K= , AC q=79 Se host ACKs receipt of echoed ‘C’ = ‘C’ host ACKs receipt of ‘C’, echoes back ‘C’ Seq=4 3, ACK =80 simple telnet scenario Transport Layer time 3 -4

TCP Round Trip Time and Timeout Q: how to set TCP timeout value? r TCP Round Trip Time and Timeout Q: how to set TCP timeout value? r longer than RTT m but RTT varies r too short: premature timeout m unnecessary retransmissions r too long: slow reaction to segment loss Q: how to estimate RTT? r Sample. RTT: measured time from segment transmission until ACK receipt m ignore retransmissions r Sample. RTT will vary, want estimated RTT “smoother” m average several recent measurements, not just current Sample. RTT Transport Layer 3 -5

TCP Round Trip Time and Timeout Estimated. RTT = (1 - )*Estimated. RTT + TCP Round Trip Time and Timeout Estimated. RTT = (1 - )*Estimated. RTT + *Sample. RTT r Exponential weighted moving average r influence of past sample decreases exponentially fast r typical value: = 0. 125 Transport Layer 3 -6

Example RTT estimation: Transport Layer 3 -7 Example RTT estimation: Transport Layer 3 -7

TCP Round Trip Time and Timeout Setting the timeout r Estimted. RTT plus “safety TCP Round Trip Time and Timeout Setting the timeout r Estimted. RTT plus “safety margin” m large variation in Estimated. RTT -> larger safety margin r first estimate of how much Sample. RTT deviates from Estimated. RTT: Dev. RTT = (1 - )*Dev. RTT + *|Sample. RTT-Estimated. RTT| (typically, = 0. 25) Then set timeout interval: Timeout. Interval = Estimated. RTT + 4*Dev. RTT Transport Layer 3 -8

Chapter 3 outline r 3. 1 Transport-layer services r 3. 2 Multiplexing and demultiplexing 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 m segment structure reliable data transfer flow control connection management r 3. 6 Principles of congestion control r 3. 7 TCP congestion control Transport Layer 3 -9

TCP reliable data transfer r TCP creates rdt service on top of IP’s unreliable TCP reliable data transfer r TCP creates rdt service on top of IP’s unreliable service r Pipelined segments r Cumulative acks r TCP uses single retransmission timer r Retransmissions are triggered by: m m timeout events duplicate acks r Initially consider simplified TCP sender: m m ignore duplicate acks ignore flow control, congestion control Transport Layer 3 -10

TCP sender events: data rcvd from app: r Create segment with seq # r TCP sender events: data rcvd from app: r Create segment with seq # r seq # is byte-stream number of first data byte in segment r start timer if not already running (think of timer as for oldest unacked segment) r expiration interval: Time. Out. Interval timeout: r retransmit segment that caused timeout r restart timer Ack rcvd: r If acknowledges previously unacked segments m m update what is known to be acked start timer if there are outstanding segments Transport Layer 3 -11

Next. Seq. Num = Initial. Seq. Num Send. Base = Initial. Seq. Num loop Next. Seq. Num = Initial. Seq. Num Send. Base = Initial. Seq. Num loop (forever) { switch(event) event: data received from application above create TCP segment with sequence number Next. Seq. Num if (timer currently not running) start timer pass segment to IP Next. Seq. Num = Next. Seq. Num + 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 > Send. Base) { Send. Base = y if (there are currently not-yet-acknowledged segments) start timer } } /* end of loop forever */ TCP sender (simplified) Comment: • Send. Base-1: last cumulatively ack’ed byte Example: • Send. Base-1 = 71; y= 73, so the rcvr wants 73+ ; y > Send. Base, so that new data is acked Transport Layer 3 -12

TCP: retransmission scenarios Host A 2, 8 by tes da t Seq=92 timeout a TCP: retransmission scenarios Host A 2, 8 by tes da t Seq=92 timeout a 100 X = ACK loss Seq=9 2, 8 by tes da ta 100 Sendbase = 100 Send. Base = 120 = ACK Send. Base = 100 time Host B Seq=9 Send. Base = 120 lost ACK scenario 2, 8 by tes da ta Seq= 100, 2 0 byte s data 00 =1 20 CK CK=1 A A Seq=9 2, 8 by Seq=92 timeout Seq=9 timeout Host A Host B time tes da t a 20 K=1 AC premature timeout Transport Layer 3 -13

TCP retransmission scenarios (more) Host A Host B Seq=9 timeout 2, 8 by Send. TCP retransmission scenarios (more) Host A Host B Seq=9 timeout 2, 8 by Send. Base = 120 Seq=1 tes da t a 100 CK= A 00, 20 bytes data X loss 120 = ACK time Cumulative ACK scenario Transport Layer 3 -14

TCP ACK generation [RFC 1122, RFC 2581] Event at Receiver TCP Receiver action Arrival TCP ACK generation [RFC 1122, RFC 2581] Event at Receiver TCP Receiver action Arrival of in-order segment with expected seq #. All data up to expected seq # already ACKed Delayed ACK. Wait up to 500 ms for next segment. If no next segment, send ACK Arrival of in-order segment with expected seq #. One other segment has ACK pending Immediately send single cumulative ACK, ACKing both in-order segments Arrival of out-of-order segment higher-than-expect seq. #. Gap detected Immediately send duplicate ACK, indicating seq. # of next expected byte Arrival of segment that partially or completely fills gap Immediate send ACK, provided that segment starts at lower end of gap Transport Layer 3 -15

Fast Retransmit r Time-out period often relatively long: m long delay before resending lost Fast Retransmit r Time-out period often relatively long: m long delay before resending lost packet r Detect lost segments via duplicate ACKs. m m Sender often sends many segments back-toback If segment is lost, there will likely be many duplicate ACKs. r If sender receives 3 ACKs for the same data, it supposes that segment after ACKed data was lost: m fast retransmit: resend segment before timer expires Transport Layer 3 -16

Host A Host B timeout X resend 2 nd se gment time Figure 3. Host A Host B timeout X resend 2 nd se gment time Figure 3. 37 Resending a segment after triple duplicate ACK Layer Transport 3 -17

Fast retransmit algorithm: event: ACK received, with ACK field value of y if (y Fast retransmit algorithm: event: ACK received, with ACK field value of y if (y > Send. Base) { Send. Base = 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 -18

Chapter 3 outline r 3. 1 Transport-layer services r 3. 2 Multiplexing and demultiplexing 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 m segment structure reliable data transfer flow control connection management r 3. 6 Principles of congestion control r 3. 7 TCP congestion control Transport Layer 3 -19

TCP Flow Control r receive side of TCP connection has a receive buffer: flow TCP Flow Control r receive side of TCP connection has a receive buffer: flow control sender won’t overflow receiver’s buffer by transmitting too much, too fast r speed-matching r app process may be service: matching the send rate to the receiving app’s drain rate slow at reading from buffer Transport Layer 3 -20

TCP Flow control: how it works r Rcvr advertises spare (Suppose TCP receiver discards TCP Flow control: how it works r Rcvr advertises spare (Suppose TCP receiver discards out-of-order segments) r spare room in buffer room by including value of Rcv. Window in segments r Sender limits un. ACKed data to Rcv. Window m guarantees receive buffer doesn’t overflow = Rcv. Window = Rcv. Buffer-[Last. Byte. Rcvd Last. Byte. Read] Transport Layer 3 -21

Chapter 3 outline r 3. 1 Transport-layer services r 3. 2 Multiplexing and demultiplexing 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 m segment structure reliable data transfer flow control connection management r 3. 6 Principles of congestion control r 3. 7 TCP congestion control Transport Layer 3 -22

TCP Connection Management Recall: TCP sender, receiver establish “connection” before exchanging data segments r TCP Connection Management Recall: TCP sender, receiver establish “connection” before exchanging data segments r initialize TCP variables: m seq. #s m buffers, flow control info (e. g. Rcv. Window) r client: connection initiator Socket client. Socket = new Socket("hostname", "port number"); r server: contacted by client Socket connection. Socket = welcome. Socket. 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 server allocates buffers m specifies server initial seq. # Step 3: client receives SYNACK, replies with ACK segment, which may contain data m Transport Layer 3 -23

TCP Connection Management (cont. ) Closing a connection: client closes socket: client. Socket. close(); TCP Connection Management (cont. ) Closing a connection: client closes socket: client. Socket. close(); client close Step 1: client end system close FIN timed wait FIN, replies with ACK. Closes connection, sends FIN ACK sends TCP FIN control segment to server Step 2: server receives server ACK closed Transport Layer 3 -24

TCP Connection Management (cont. ) Step 3: client receives FIN, replies with ACK. m TCP Connection Management (cont. ) Step 3: client receives FIN, replies with ACK. m client closing Enters “timed wait” will respond with ACK to received FINs server FIN ACK Step 4: server, receives closing FIN Note: with small modification, can handle simultaneous FINs. timed wait ACK. Connection closed. ACK closed Transport Layer 3 -25

TCP Connection Management (cont) TCP server lifecycle TCP client lifecycle Transport Layer 3 -26 TCP Connection Management (cont) TCP server lifecycle TCP client lifecycle Transport Layer 3 -26

Chapter 3 outline r 3. 1 Transport-layer services r 3. 2 Multiplexing and demultiplexing 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 m segment structure reliable data transfer flow control connection management r 3. 6 Principles of congestion control r 3. 7 TCP congestion control Transport Layer 3 -27

Principles of Congestion Control Congestion: r informally: “too many sources sending too much data 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! Transport Layer 3 -28

Causes/costs of congestion: scenario 1 Host A r two senders, two receivers r one Causes/costs of congestion: scenario 1 Host A r two senders, two receivers r one router, infinite buffers r no retransmission Host B lout lin : original data unlimited shared output link buffers r large delays when congested r maximum achievable throughput Transport Layer 3 -29

Causes/costs of congestion: scenario 2 r one router, finite buffers r sender retransmission of Causes/costs of congestion: scenario 2 r one router, finite buffers r sender retransmission of lost packet Host A lin : original data lout l'in : original data, plus retransmitted data Host B finite shared output link buffers Transport Layer 3 -30

Causes/costs of congestion: scenario 2 (goodput) = l out in r “perfect” retransmission only Causes/costs of congestion: scenario 2 (goodput) = l out in r “perfect” retransmission only when loss: r always: l l > lout in r retransmission of delayed (not lost) packet makes (than perfect case) for same R/2 l lout R/2 in larger R/2 lin a. R/2 lout R/3 lin b. R/2 R/4 lin R/2 c. “costs” of congestion: r more work (retrans) for given “goodput” r unneeded retransmissions: link carries multiple copies of pkt Transport Layer 3 -31

Causes/costs of congestion: scenario 3 r four senders Q: what happens as l in Causes/costs of congestion: scenario 3 r four senders Q: what happens as l in and l increase ? r multihop paths in r timeout/retransmit Host A lin : original data lout l'in : original data, plus retransmitted data finite shared output link buffers Host B Transport Layer 3 -32

Causes/costs of congestion: scenario 3 H o st A l o u t H Causes/costs of congestion: scenario 3 H o st A l o u t H o st B Another “cost” of congestion: r when packet dropped, any “upstream transmission capacity used for that packet wasted! Transport Layer 3 -33

Approaches towards congestion control Two broad approaches towards congestion control: End-end congestion control: r Approaches towards congestion control Two broad 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 (SNA, DECbit, TCP/IP ECN, ATM) m explicit rate sender should send at Transport Layer 3 -34

Case study: ATM ABR congestion control ABR: available bit rate: r “elastic service” RM Case study: ATM ABR congestion control ABR: available bit rate: r “elastic service” RM (resource management) cells: r if sender’s path r sent by sender, interspersed “underloaded”: m sender should use available bandwidth r if sender’s path congested: m sender throttled to minimum guaranteed rate with data cells r bits in RM cell set by switches (“network-assisted”) m NI bit: no increase in rate (mild congestion) m CI bit: congestion indication r RM cells returned to sender by receiver, with bits intact Transport Layer 3 -35

Case study: ATM ABR congestion control r two-byte ER (explicit rate) field in RM Case study: ATM ABR congestion control r two-byte ER (explicit rate) field in RM cell m congested switch may lower ER value in cell m sender’ send rate thus maximum supportable rate on path r EFCI bit in data cells: set to 1 in congested switch m if data cell preceding RM cell has EFCI set, sender sets CI bit in returned RM cell Transport Layer 3 -36

Chapter 3 outline r 3. 1 Transport-layer services r 3. 2 Multiplexing and demultiplexing 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 m segment structure reliable data transfer flow control connection management r 3. 6 Principles of congestion control r 3. 7 TCP congestion control Transport Layer 3 -37

TCP congestion control: additive increase, multiplicative decrease r Approach: increase transmission rate (window size), TCP congestion control: additive increase, multiplicative decrease r Approach: increase transmission rate (window size), Saw tooth behavior: probing for bandwidth congestion window size probing for usable bandwidth, until loss occurs m additive increase: increase Cong. Win by 1 MSS every RTT until loss detected m multiplicative decrease: cut Cong. Win in half after loss time Transport Layer 3 -38

TCP Congestion Control: details r sender limits transmission: Last. Byte. Sent-Last. Byte. Acked Cong. TCP Congestion Control: details r sender limits transmission: Last. Byte. Sent-Last. Byte. Acked Cong. Win r Roughly, rate = Cong. Win Bytes/sec RTT r Cong. Win is dynamic, function of perceived network congestion How does sender perceive congestion? r loss event = timeout or 3 duplicate acks r TCP sender reduces rate (Cong. Win) after loss event three mechanisms: m m m AIMD slow start conservative after timeout events Transport Layer 3 -39

TCP Slow Start r When connection begins, Cong. Win = 1 MSS m m TCP Slow Start r When connection begins, Cong. Win = 1 MSS m m Example: MSS = 500 bytes & RTT = 200 msec initial rate = 20 kbps r When connection begins, increase rate exponentially fast until first loss event r available bandwidth may be >> MSS/RTT m desirable to quickly ramp up to respectable rate Transport Layer 3 -40

TCP Slow Start (more) r When connection m m double Cong. Win every RTT TCP Slow Start (more) r When connection m m double Cong. Win every RTT done by incrementing Cong. Win for every ACK received RTT begins, increase rate exponentially until first loss event: Host A Host B one segme nt two segme nts four segme nts r Summary: initial rate is slow but ramps up exponentially fast time Transport Layer 3 -41

Refinement: inferring loss r After 3 dup ACKs: m Cong. Win m window is Refinement: inferring loss r After 3 dup ACKs: m Cong. Win m window is cut in half then grows linearly r But after timeout event: m Cong. Win instead set to 1 MSS; m window then grows exponentially m to a threshold, then grows linearly Philosophy: q 3 dup ACKs indicates network capable of delivering some segments q timeout indicates a “more alarming” congestion scenario Transport Layer 3 -42

Refinement Q: When should the exponential increase switch to linear? A: When Cong. Win Refinement Q: When should the exponential increase switch to linear? A: When Cong. Win gets to 1/2 of its value before timeout. Implementation: r Variable Threshold r At loss event, Threshold is set to 1/2 of Cong. Win just before loss event Transport Layer 3 -43

Summary: TCP Congestion Control r When Cong. Win is below Threshold, sender in slow-start Summary: TCP Congestion Control r When Cong. Win is below Threshold, sender in slow-start phase, window grows exponentially. r When Cong. Win is above Threshold, sender is in congestion-avoidance phase, window grows linearly. r When a triple duplicate ACK occurs, Threshold set to Cong. Win/2 and Cong. Win set to Threshold. r When timeout occurs, Threshold set to Cong. Win/2 and Cong. Win is set to 1 MSS. Transport Layer 3 -44

TCP sender congestion control State Event TCP Sender Action Commentary Slow Start (SS) ACK TCP sender congestion control State Event TCP Sender Action Commentary Slow Start (SS) ACK receipt Cong. Win = Cong. Win + MSS, for previously If (Cong. Win > Threshold) unacked data set state to “Congestion Avoidance” Resulting in a doubling of Cong. Win every RTT Congestion Avoidance (CA) ACK receipt Cong. Win = Cong. Win+MSS * for previously (MSS/Cong. Win) unacked data Additive increase, resulting in increase of Cong. Win by 1 MSS every RTT SS or CA Loss event detected by triple duplicate ACK Threshold = Cong. Win/2, Cong. Win = Threshold, Set state to “Congestion Avoidance” Fast recovery, implementing multiplicative decrease. Cong. Win will not drop below 1 MSS. SS or CA Timeout Threshold = Cong. Win/2, Cong. Win = 1 MSS, Set state to “Slow Start” Enter slow start SS or CA Duplicate ACK Increment duplicate ACK count for segment being acked Cong. Win and Threshold not changed Transport Layer 3 -45

TCP throughput r What’s the average throughout of TCP as a function of window 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/2 RTT. r Average throughout: . 75 W/RTT Transport Layer 3 -46

TCP Futures: TCP over “long, fat pipes” r Example: 1500 byte segments, 100 ms TCP Futures: TCP over “long, fat pipes” r Example: 1500 byte segments, 100 ms RTT, want 10 Gbps throughput r Requires window size W = 83, 333 in-flight segments r Throughput in terms of loss rate: r ➜ L = 2·10 -10 Wow r New versions of TCP for high-speed Transport Layer 3 -47

TCP Fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, TCP 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 -48

Why is TCP fair? Two competing sessions: r Additive increase gives slope of 1, Why is TCP fair? Two competing sessions: r Additive increase gives slope of 1, as throughout increases r multiplicative decreases throughput proportionally equal bandwidth share Connection 2 throughput R loss: decrease window by factor of 2 congestion avoidance: additive increase Connection 1 throughput R Transport Layer 3 -49

Fairness (more) Fairness and UDP r Multimedia apps often do not use TCP m 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 m new app asks for 1 TCP, gets rate R/10 new app asks for 11 TCPs, gets R/2 ! Transport Layer 3 -50

Chapter 3: Summary r principles behind transport layer services: m multiplexing, demultiplexing m reliable 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” Transport Layer 3 -51