TCP Internals. Spring 2018 CS 438 Staff, University of Illinois 1

Transcription

1 TCP Iterals Sprig 2018 CS 438 Staff, Uiversity of Illiois 1

2 TCP Usage Model Coectio setup 3-way hadshake Data trasport Seder writes data TCP Breaks data ito segmets Seds each segmet over IP Retrasmits, reorders ad removes duplicates as ecessary Receiver reads some data Teardow 4 step exchage Sprig 2018 CS 438 Staff, Uiversity of Illiois 2

3 TCP Coectio Establishmet 3-Way Hadshake Sequece Numbers J,K Message Types Sychroize (SYN) Ackowledge () Passive Ope Server listes for coectio from cliet Active Ope Cliet iitiates coectio to server Cliet Sychroize (SYN) J SYN K, ackowledge () J+1 K+1 Time flows dow Server liste Sprig 2018 CS 438 Staff, Uiversity of Illiois 3

4 Purpose of the hadshake Why use a hadshake before sedig / processig data? Suppose we do t wait for the hadshake sed data (e.g., HTTP request) alog with SYN deliver to applicatio sed some results (e.g., idex.html) alog with SYN What could go wrog? Hit: remember packets ca be delayed, dropped, duplicated, Sprig 2018 CS 438 Staff, Uiversity of Illiois 4

5 Purpose of the hadshake Why use a hadshake before sedig / processig data? Duplicated packet causes data to be set to applicatio twice Why does hadshake fix this? timeout Cliet Server request results request - 1 st Coectio closed - results Sprig 2018 CS 438 Staff, Uiversity of Illiois 5

6 Purpose of the hadshake If server receives request a secod time, it respods with SYN a secod time But seder will ot subsequetly respod with ( what is this garbage I just received?? ) Cliet timeout request results request results Server - 1 st Coectio closed - Sprig 2018 CS 438 Staff, Uiversity of Illiois 6

7 Aother purpose of the hadshake No hadshake == security hole Attacker seds request but spoofs source address, usig address of a victim (C) Server happily seds massive amouts of data to victim Attacker repeats for 10,000 web servers Massive deial of service attack, almost free ad aoymous for the attacker! Used i the largest distributed deial of service (DDoS) attacks i 2008, 2009, ad 2010 Use services that lack hadshake (e.g., DNS over UDP) Amplificatio factor 1:76 i 2008! Sprig 2018 CS 438 Staff, Uiversity of Illiois 7

8 Aother purpose of the hadshake Hadshake lets server verify source address is real SYN SYN Does t match a coectio iitiated by C: igore (or reply with reset) No received after timeout: drop coectio without sedig data Q: does this prevet reflectio attack? A: No, but at least it prevets amplificatio Sprig 2018 CS 438 Staff, Uiversity of Illiois 8

9 Hadshakig Iteret was ot desiged for accoutability Hard to tell where a packet came from ISPs filter suspicious packets: sometimes easy, sometimes hard, ad sometimes ot doe Ad the Iteret is ot secure util everyoe filters More geerally, Iteret was ot desiged for security Vulerabilities i most of the core protocols Eve with hadshake, early desigs are vulerable Had predictable Iitial Sequece Number (why s that bad?) Because security was ot iitial goal of the hadshake Sprig 2018 CS 438 Staff, Uiversity of Illiois 9

10 TCP Data Trasport Data broke ito segmets Limited by maximum segmet size (MSS) Defaults to 352 bytes Negotiable durig coectio setup Typically set to MTU of directly coected etwork size of TCP ad IP headers Three evets cause a segmet to be set ³ MSS bytes of data ready to be set Explicit PUSH operatio by applicatio Periodic timeout Sprig 2018 CS 438 Staff, Uiversity of Illiois 10

11 TCP Byte Stream Applicatio process Applicatio process Write bytes Read bytes TCP Sed buffer TCP Recv buffer TCP Segmet TCP Segmet TCP Segmet Sprig 2018 CS 438 Staff, Uiversity of Illiois 11

12 TCP Coectio Termiatio Two geerals problem Eemy camped i valley Two geerals hills separated by eemy Commuicatio by ureliable messegers Geerals eed to agree whether to attack or retreat Sprig 2018 CS 438 Staff, Uiversity of Illiois 12

13 Two geerals problem Ca messages over a ureliable etwork be used to guaratee two etities do somethig simultaeously? No, eve if all messages get through 11 am ok? Yes, 11 works So, 11 it is? Yeah, but what it you do t get this ack? No way to be sure last message gets through! Sprig 2018 CS 438 Staff, Uiversity of Illiois 13

14 TCP Coectio Termiatio Message Types Fiished (FIN) Cliet Server Ackowledge () Fiished (FIN) J Active Close Seds o more data Passive close J+1 FIN K Accepts o more data K+1 Time flows dow Sprig 2018 CS 438 Staff, Uiversity of Illiois 14

15 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios Sprig 2018 CS 438 Staff, Uiversity of Illiois 15

16 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios 16-bit source ad destiatio ports Sprig 2018 CS 438 Staff, Uiversity of Illiois 16

17 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios 32-bit sed ad sequece umbers Sprig 2018 CS 438 Staff, Uiversity of Illiois 17

18 ig ad Sequece Numbers Seder seds packet Data starts with sequece umber X Packet cotais B bytes X, X+1, X+2,.X+B-1 B bytes byte X byte X+B - 1 Sprig 2018 CS 438 Staff, Uiversity of Illiois 18

19 ig ad Sequece Numbers Upo receipt of packet, receiver seds a If all data prior to X already received: ackowledges X+B (because that is ext expected byte) B bytes byte X+B Sprig 2018 CS 438 Staff, Uiversity of Illiois 19

20 ig ad Sequece Numbers Upo receipt of packet, receiver seds a If highest byte already received is some smaller value Y ackowledges Y+1 Eve if this has bee ed before B bytes byte Y byte Y + 1 Sprig 2018 CS 438 Staff, Uiversity of Illiois 20

21 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios 4-bit header legth i 4-byte words Miimum 5 bytes Offset to first data byte Sprig 2018 CS 438 Staff, Uiversity of Illiois 21

22 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios Reserved Must be 0 Sprig 2018 CS 438 Staff, Uiversity of Illiois 22

23 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios 6 1-bit flags URG: Cotais urget data : Valid seq. umber PSH: Do ot delay data delivery RST: SYN: FIN: Reset coectio Sychroize for setup Fial segmet for teardow Sprig 2018 CS 438 Staff, Uiversity of Illiois 23

24 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios 16-bit advertised widow Space remaiig i receive widow Sprig 2018 CS 438 Staff, Uiversity of Illiois 24

25 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios 16-bit checksum Uses IP checksum algorithm Computed o header, data ad pseudo header (TDP) Source IP Address Destiatio IP Address TCP Segmet Legth Sprig 2018 CS 438 Staff, Uiversity of Illiois 25

26 TCP Segmet Header Format Source Port Destiatio Port Sequece Number Sequece Number Header Legth 0 Flags Advertised Widow TCP Checksum Urget Poiter Optios 16-bit urget data poiter If URG = 1 Idex of last byte of urget data i segmet Sprig 2018 CS 438 Staff, Uiversity of Illiois 26

27 TCP Optios Negotiate maximum segmet size (MSS) Each host suggests a value Miimum of two values is chose Prevets IP fragmetatio over first ad last hops Packet timestamp Allows RTT calculatio for retrasmitted packets Exteds sequece umber space for idetificatio of stray packets Negotiate advertised widow graularity Allows larger widows Good for routes with large badwidth-delay products Sprig 2018 CS 438 Staff, Uiversity of Illiois 27

28 TCP State Descriptios CLOSED LISTEN SYN_RCVD SYN_SENT ESTABLISHED CLOSE_WAIT LAST_ FIN_WAIT_1 FIN_WAIT_2 CLOSING TIME_WAIT Discoected Waitig for icomig coectio Coectio request received Coectio request set Coectio ready for data trasport Coectio closed by peer Coectio closed by peer, closed locally, await Coectio closed locally Coectio closed locally ad d Coectio closed by both sides simultaeously Wait for etwork to discard related packets Sprig 2018 CS 438 Staff, Uiversity of Illiois 28

29 TCP State Trasitio Diagram SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/ FIN/ FIN/ FIN + / CLOSED Close LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Sed/SYN Timeout Close SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ CLOSED Active ope/syn Sprig 2018 CS 438 Staff, Uiversity of Illiois 29

30 TCP State Trasitio Diagram SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/ FIN/ FIN + / Active Close FIN/ CLOSED Close LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Sed/SYN Timeout Close SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ CLOSED Active ope/syn Sprig 2018 CS 438 Staff, Uiversity of Illiois 30 Passive Close

31 TCP State Trasitio Diagram Message Evet from from local receiver/ applicatio respose set Passive ope SYN_RCVD Close/FIN SYN/SYN + FIN_WAIT_1 FIN_WAIT_2 Close/ FIN/ FIN/ FIN + / Evet from local CLOSED applicatio/ message Closeset LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Sed/SYN Timeout Close SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ CLOSED Active ope/syn Sprig 2018 CS 438 Staff, Uiversity of Illiois 31

32 TCP State Trasitio Diagram Reset after SYN/ was CLOSED set Passive ope Close SYN/SYN + LISTEN SYN_RCVD RST SYN/SYN + Close/FIN FIN_WAIT_1 FIN_WAIT_2 Close/ FIN/ FIN/ FIN + / ESTABLISHED CLOSING TIME_WAIT Sed/SYN Timeout Close SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ CLOSED Active ope/syn Sprig 2018 CS 438 Staff, Uiversity of Illiois 32

33 TCP State Trasitio Diagram Questios State trasitios Describe the path take by a server uder ormal coditios Describe the path take by a cliet uder ormal coditios Describe the path take assumig the cliet closes the coectio first Sprig 2018 CS 438 Staff, Uiversity of Illiois 33

34 TCP State Trasitio Diagram Establishmet uder ormal coditios SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/FIN FIN/ FIN/ FIN + / CLOSED Close LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Active ope/syn Close Sed/SYN SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ Timeout CLOSED Sprig 2018 CS 438 Staff, Uiversity of Illiois 34

35 TCP State Trasitio Diagram Lost from receiver? SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/FIN FIN/ FIN/ FIN + / CLOSED Close LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Active ope/syn Close Sed/SYN SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ Timeout CLOSED Sprig 2018 CS 438 Staff, Uiversity of Illiois 35

36 TCP State Trasitio Diagram Local sed whe i LISTEN SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/FIN FIN/ FIN/ FIN + / CLOSED Close LISTEN SYN/SYN + ESTABLISHED Never used CLOSING TIME_WAIT Active ope/syn Close Sed/SYN SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ Timeout CLOSED Sprig 2018 CS 438 Staff, Uiversity of Illiois 36

37 TCP State Trasitio Diagram Timeouts? Active CLOSED ope/syn Passive ope Close Close SYN/SYN + LISTEN Sed/SYN SYN_RCVD SYN_SENT SYN/SYN + SYN + / Close/FIN Close/FIN If o ESTABLISHED respose after multiple tries, retur FIN/ FIN_WAIT_1 FIN/ to CLOSED CLOSE_WAIT FIN_WAIT_2 FIN/ FIN + / CLOSING TIME_WAIT Timeout Close/FIN LAST_ CLOSED Sprig 2018 CS 438 Staff, Uiversity of Illiois 37

38 TCP State Trasitio Diagram Oe side closes first SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/FIN FIN/ FIN/ FIN + / CLOSED Close LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Sed/SYN Timeout Close SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ CLOSED Active ope/syn Sprig 2018 CS 438 Staff, Uiversity of Illiois 38

39 TCP TIME_WAIT State What purpose does the TIME_WAIT stae serve? Problem What happes if a segmet from a old coectio arrives at a ew coectio? Maximum Segmet Lifetime Max time a old segmet ca live i the Iteret TIME_WAIT State Coectio remais i this state from two times the maximum segmet lifetime Sprig 2018 CS 438 Staff, Uiversity of Illiois 39

40 TCP State Trasitio Diagram Both sides close at the same time SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/FIN FIN/ FIN/ FIN + / CLOSED Close LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Sed/SYN Timeout Close SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ CLOSED Active ope/syn Sprig 2018 CS 438 Staff, Uiversity of Illiois 40

41 TCP State Trasitio Diagram FIN_ received (rare) SYN_RCVD Close/FIN FIN_WAIT_1 FIN_WAIT_2 Passive ope SYN/SYN + Close/FIN FIN/ FIN/ FIN + / CLOSED Close LISTEN SYN/SYN + ESTABLISHED CLOSING TIME_WAIT Sed/SYN Timeout Close SYN_SENT SYN + / FIN/ CLOSE_WAIT Close/FIN LAST_ CLOSED Active ope/syn Sprig 2018 CS 438 Staff, Uiversity of Illiois 41

42 TCP Slidig Widow Protocol Sequece umbers Idices ito byte stream sequece umber Actually ext byte expected as opposed to last byte received Sprig 2018 CS 438 Staff, Uiversity of Illiois 42

43 TCP Slidig Widow Protocol Iitial Sequece Number Why ot just use 0? Practical issue IP addresses ad port #s uiquely idetify a coectio Evetually, though, these port #s do get used agai small chace a old packet is still i flight ad might be associated with ew coectio TCP requires (RFC793) chagig ISN Set from 32-bit clock that ticks every 4 microsecods oly wraps aroud oce every 4.55 hours To establish a coectio, hosts exchage ISNs Sprig 2018 CS 438 Staff, Uiversity of Illiois 43

44 TCP Slidig Widow Protocol Advertised widow Eables dyamic receive widow size Receive buffers Data ready for delivery to applicatio util requested Out-of-order data to maximum buffer capacity Seder buffers Uackowledged data Uset data out to maximum buffer capacity Sprig 2018 CS 438 Staff, Uiversity of Illiois 44

45 TCP Slidig Widow Protocol Seder Side LastByteAcked <= LastByteSet LastByteSet <= LastByteWritte Buffer bytes betwee LastByteAcked ad LastByteWritte Advertised widow Maximum buffer size Data available, but outside widow First uackowledged byte Last byte set Sprig 2018 CS 438 Staff, Uiversity of Illiois 45

46 TCP Slidig Widow Protocol Receiver Side LastByteRead < NextByteExpected NextByteExpected <= LastByteRcvd + 1 Buffer bytes betwee NextByteRead ad LastByteRcvd Maximum buffer size Advertised widow Buffered, out-of-order data Next byte expected ( value) Next byte to be read by applicatio Sprig 2018 CS 438 Staff, Uiversity of Illiois 46

47 Flow Cotrol vs. Cogestio Cotrol Flow cotrol Prevetig seders from overruig the capacity of the receivers Cogestio cotrol Prevetig too much data from beig ijected ito the etwork, causig switches or liks to become overloaded Which oe does TCP provide? TCP provides both Flow cotrol based o advertised widow Cogestio cotrol discussed later i class Sprig 2018 CS 438 Staff, Uiversity of Illiois 47

48 Advertised Widow Limits Rate W = widow size Seder ca sed o faster tha W/RTT bytes/sec Receiver implicitly limits seder to rate that receiver ca sustai If seder is goig too fast, widow advertisemets get smaller & smaller Sprig 2018 CS 438 Staff, Uiversity of Illiois 48

49 TCP Flow Cotrol: Receiver Receive buffer size = MaxRcvBuffer LastByteRcvd - LastByteRead < = MaxRcvBuf Advertised widow = MaxRcvBuf - (NextByteExp - NextByteRead) Shriks as data arrives ad Grows as the applicatio cosumes data Sprig 2018 CS 438 Staff, Uiversity of Illiois 49

50 TCP Flow Cotrol: Seder Sed buffer size = MaxSedBuffer LastByteSet - LastByteAcked < = AdvertWidow Effective buffer = AdvertWidow - (LastByteSet - LastByteAck) EffectiveWidow > 0 to sed data Relatioship betwee seder ad receiver LastByteWritte - LastByteAcked < = MaxSedBuffer block seder if (LastByteWritte - LastByteAcked) + y > MaxSederBuffer Sprig 2018 CS 438 Staff, Uiversity of Illiois 50

51 TCP Flow Cotrol Problem: Slow receiver applicatio Advertised widow goes to 0 Seder caot sed more data No-data packets used to update widow Receiver may ot spotaeously geerate update or update may be lost Solutio Seder periodically seds 1-byte segmet, igorig advertised widow of 0 Evetually widow opes Seder lears of opeig from ext of 1-byte segmet Sprig 2018 CS 438 Staff, Uiversity of Illiois 51

52 TCP Flow Cotrol Problem: Applicatio delivers tiy pieces of data to TCP Example: telet i character mode Each piece set as a segmet, retured as Very iefficiet Solutio Delay trasmissio to accumulate more data Nagle s algorithm Sed first piece of data Accumulate data util first piece d Sed accumulated data ad restart accumulatio Not ideal for some traffic (e.g., mouse motio) Sprig 2018 CS 438 Staff, Uiversity of Illiois 52

53 TCP Flow Cotrol Problem: Slow applicatio reads data i tiy pieces Receiver advertises tiy widow Seder fills tiy widow Kow as silly widow sydrome Solutio Advertise widow opeig oly whe MSS or ½ of buffer is available Seder delays sedig util widow is MSS or ½ of receiver s buffer (estimated) Sprig 2018 CS 438 Staff, Uiversity of Illiois 53

54 TCP Bit Allocatio Limitatios Sequece umbers vs. packet lifetime Assumed that IP packets live less tha 60 secods Ca we sed 2 32 bytes i 60 secods? Less tha a STS-12 lie Advertised widow vs. delay-badwidth Oly 16 bits for advertised widow Cross-coutry RTT = 100 ms Adequate for oly 5.24 Mbps! Sprig 2018 CS 438 Staff, Uiversity of Illiois 54

55 TCP Sequece Numbers 32-bit Badwidth Speed Time util wrap aroud T1 1.5 Mbps 6.4 hours Etheret 10 Mbps 57 miutes T3 45 Mbps 13 miutes FDDI 100 Mbps 6 miutes STS Mbps 4 miutes STS Mbps 55 secods STS Gbps 28 secods Sprig 2018 CS 438 Staff, Uiversity of Illiois 55

56 TCP Advertised Widow 16-bit Badwidth Speed Delay x Badwidth Product T1 1.5 Mbps 18 KB Etheret 10 Mbps 122 KB T3 45 Mbps 549 KB FDDI 100 Mbps 1.2 MB STS Mbps 1.8 MB STS Mbps 7.4 MB STS Gbps 14.8 MB Sprig 2018 CS 438 Staff, Uiversity of Illiois 56

57 Reasos for Retrasmissio Packet Packet Packet Timeout Packet Timeout Timeout Packet Timeout Timeout Packet Timeout Packet lost lost DUPLICATE PET Early timeout DUPLICATE PETS Sprig 2018 CS 438 Staff, Uiversity of Illiois 57

58 How Log Should Seder Wait? Seder sets a timeout to wait for a Too short wasted retrasmissios Too log excessive delays whe packet lost Sprig 2018 CS 438 Staff, Uiversity of Illiois 58

59 TCP Roud Trip Time ad Timeout How should TCP set its timeout value? Estimatig RTT SampleRTT Loger tha RTT Too short Too log But RTT varies Premature timeout Uecessary retrasmissios Slow reactio to segmet loss Measured time from segmet trasmissio util receipt Will vary Wat smoother estimated RTT Average several recet measuremets Not just curret SampleRTT Sprig 2018 CS 438 Staff, Uiversity of Illiois 59

60 TCP Adaptive Retrasmissio Algorithm - Origial Theory Estimate RTT Multiply by 2 to allow for variatios Practice Use expoetial movig average (α = 0.1 to 0.2) Estimate = (α) * measuremet + (1- α) * estimate Ifluece of past sample decreases expoetially fast Sprig 2018 CS 438 Staff, Uiversity of Illiois 60

61 TCP Adaptive Retrasmissio Algorithm - Origial Problem: What does a really? Was i respose to first, secod, etc trasmissio? A B A B Origial trasmissio Origial trasmissio Sample RTT retrasmissio Sample RTT retrasmissio Sprig 2018 CS 438 Staff, Uiversity of Illiois 61

62 TCP Adaptive Retrasmissio Algorithm Kar-Partridge Algorithm Exclude retrasmitted packets from RTT estimate For each retrasmissio Double RTT estimate Expoetial backoff from cogestio Sprig 2018 CS 438 Staff, Uiversity of Illiois 62

63 TCP Adaptive Retrasmissio Algorithm Kar-Partridge Problem Still did ot hadle variatios well Did ot solve etwork cogestio problems as well as desired At high loads roud trip variace is high Sprig 2018 CS 438 Staff, Uiversity of Illiois 63

64 Example RTT Estimatio 350 SampleRTT Estimated RTT RTT (millisecods) time (secods) Sprig 2018 CS 438 Staff, Uiversity of Illiois 64

65 TCP Adaptive Retrasmissio Algorithm Jacobso Algorithm Estimate variace of RTT Calculate mea iterpacket RTT deviatio to approximate variace Use secod expoetial movig average Dev = (β) * RTT_Est Sample + (1 β) * Dev β = 0.25, A = for RTT_est Use variace estimate as compoet of RTT estimate Next_RTT = RTT_Est + 4 * Dev Protects agaist high jitter Sprig 2018 CS 438 Staff, Uiversity of Illiois 65

66 TCP Adaptive Retrasmissio Algorithm Jacobso Notes Algorithm is oly as good as the graularity of the clock Accurate timeout mechaism is importat for cogestio cotrol Sprig 2018 CS 438 Staff, Uiversity of Illiois 66

67 Evolutio of TCP 1975 Three-way hadshake Raymod Tomliso I SIGCOMM TCP described by Vit Cerf ad Bob Kah I IEEE Tras Comm 1982 TCP & IP RFC 793 & BSD Uix 4.2 supports TCP/IP 1984 Nagel s algorithm to reduce overhead of small packets; predicts cogestio collapse 1986 Cogestio collapse observed 1987 Kar s algorithm to better estimate roud-trip time 1988 Va Jacobso s algorithms cogestio avoidace ad cogestio cotrol (most implemeted i 4.3BSD Tahoe) BSD Reo fast retrasmit delayed s Sprig 2018 CS 438 Staff, Uiversity of Illiois 67

68 TCP Through the 1990s 1996 S TCP (Floyd et al) Selective Ackowledgemet 1993 TCP Vegas (Brakmo et al) delay-based cogestio avoidace 1994 ECN (Floyd) Explicit Cogestio Notificatio 1996 Hoe NewReo startup ad loss recovery Ad beyod: TCP i challeged (e.g. wireless) coditios; faster flow completio; lower latecy; icast problem; Sprig 2018 CS 438 Staff, Uiversity of Illiois 68