Chapter 3 outline r 3 1 Transport-layer services

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: 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 # 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 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 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 + *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

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 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 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 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 (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 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. 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 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 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. 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 > 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 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 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 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 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 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(); 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 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

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 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 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 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 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 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 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 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 (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 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 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), 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. 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 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 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 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 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 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 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 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 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, 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, 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 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 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