Reliable Transmission. Spring 2018 CS 438 Staff - University of Illinois 1

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1 Reliable Trasmissio Sprig 2018 CS 438 Staff - Uiversity of Illiois 1

2 Reliable Trasmissio Hello! My computer s ame is Alice. Alice Bob Hello! Alice. Sprig 2018 CS 438 Staff - Uiversity of Illiois 2

3 Reliable Trasmissio Hello! My Computer s ame is Alice Bob Alice. is My ame Alice. Sprig 2018 CS 438 Staff - Uiversity of Illiois 3

4 Reliable Trasmissio Suppose error protectio idetifies valid ad ivalid packets How? Ca we make the chael appear reliable? Isure packet delivery Maitai packet order Provide reliability at full lik capacity Sprig 2018 CS 438 Staff - Uiversity of Illiois 4

5 Reliable Trasmissio Outlie Fudametals of Automatic Repeat request (ARQ) algorithms A family of algorithms that provide reliability through retrasmissio ARQ algorithms (simple to complex) stop-ad-wait cocurret logical chaels slidig widow go-back- selective repeat Alterative: forward error correctio (FEC) Sprig 2018 CS 438 Staff - Uiversity of Illiois 5

6 Termiology Ackowledgemet (ACK) Receiver tells the seder whe a frame is received Selective ackowledgemet (SACK) Specifies set of frames received Cumulative ackowledgemet (ACK) Have received specified frame ad all previous Negative ackowledgemet (NAK) Receiver refuses to accept frame ow, e.g., whe out of buffer space Sprig 2018 CS 438 Staff - Uiversity of Illiois 6

7 Termiology Timeout (TO) Seder decides the frame (or ACK) was lost Seder ca try agai Sprig 2018 CS 438 Staff - Uiversity of Illiois 7

8 Stop-ad-Wait Basic idea 1. Sed a frame 2. Wait for a ACK or TO 3. If TO, go to 1 4. If ACK, get ew frame, go to 1 Sprig 2018 CS 438 Staff - Uiversity of Illiois 8

9 Stop-ad-Wait: Success How log should the timeout be? Time Timeout Seder Frame ACK Receiver RTT What ca go wrog? How will it affect our protocol? Sprig 2018 CS 438 Staff - Uiversity of Illiois 9

10 Stop-ad-Wait: Lost Frame Seder Receiver Time Timeout Frame RTT Frame Timeout ACK Sprig 2018 CS 438 Staff - Uiversity of Illiois 10

11 Stop-ad-Wait: Lost ACK Seder Receiver Time Timeout Frame ACK RTT Frame Timeout ACK Sprig 2018 CS 438 Staff - Uiversity of Illiois 11

12 Stop-ad-Wait: Delayed Frame Seder Receiver How ca receiver distiguish betwee two frames? Time Timeout Frame ACK RTT Frame Timeout ACK How may bits do you eed for sequece umbers? Sprig 2018 CS 438 Staff - Uiversity of Illiois 12

13 Stop-ad-Wait Goal Guarateed, at-most-oce delivery Protocol Challeges Dropped frame/ack Duplicate frame/ack Requiremets 1-bit sequece umbers (if physical etwork maitais order) seder tracks frame ID to sed receiver tracks ext frame ID expected Sprig 2018 CS 438 Staff - Uiversity of Illiois 13

14 Stop-ad-Wait State Diagram Seder Receiver Receive ACK 1 Sed:? 0 Expect:? 0 Receive frame 0 Sed: 0 Expect: 1 Receive frame 0 Receive ACK 1 Receive ACK 0 Sed: 1 Expect: 0 Receive frame 1 Sed: 1 Expect: 1 Sprig 2018 CS 438 Staff - Uiversity of Illiois 14

15 Stop-ad-Wait We have achieved Frames delivered reliably ad i order Is that eough? Problem Oly allows oe outstadig frame Example Does ot keep the pipe full 100ms RTT Oe frame per RTT = 1KB 1024x8x10 = kbps Regardless of lik badwidth! Sprig 2018 CS 438 Staff - Uiversity of Illiois 15

16 Cocurret Logical Chaels Used i ARPANET IMP-IMP protocol Idea Multiplex logical chaels over a physical lik Iclude chael ID i header Use stop-ad-wait for each chael Result Each chael is limited to stop-ad-wait badwidth Aggregate badwidth uses full physical chael Supports multiple commuicatig processes Ca use more tha oe chael per process Sprig 2018 CS 438 Staff - Uiversity of Illiois 16

17 Cocurret Logical Chaels Problem Badwidth Use of a sigle chael per process may waste BW Orderig Use of multiple chael per process does ot maitai packet orderig across chaels! If applicatio has chaels, ad oe eeds a retrasmissio, it will always be oe packet behid the other chaels Sprig 2018 CS 438 Staff - Uiversity of Illiois 17

18 ARQ: Where are We? Goals for reliable trasmissio Make chael appear reliable Maitai packet order (usually) Impose low overhead/allow full use of lik Stop-ad-Wait Provides reliable i-order delivery Sacrifices performace Cocurret Logical Chaels Provides reliable delivery at full lik badwidth Sacrifices packet orderig Slidig Widow Protocol Achieves all three! Sprig 2018 CS 438 Staff - Uiversity of Illiois 18

19 Slidig Widow Protocol Most importat ad geeral ARQ algorithm Used by TCP Outlie Cocepts Termiology (from P&D) Details Code example Proof of evetual i-order delivery Classificatio scheme (go-back-, selective repeat) Sprig 2018 CS 438 Staff - Uiversity of Illiois 19

20 Keepig the Pipe Full Stop-ad-Wait Goal Seder Receiver Seder Receiver Frame ACK Frame Frame Frame Frame Frame Time ACK Frame Time ACK ACK Frame ACK Advatages: More frames i pipe Less time overall Piggybacked ACKs Sprig 2018 CS 438 Staff - Uiversity of Illiois 20

21 Cocepts Cosider a ordered stream of data frames Stop-ad-Wait Widow of oe frame Slides alog stream over time Time Sprig 2018 CS 438 Staff - Uiversity of Illiois 21

22 Cocepts Slidig Widow Protocol Multiple-frame sed widow Multiple frame receive widow Time Sprig 2018 CS 438 Staff - Uiversity of Illiois 22

23 Slidig Widow Sed Widow Fixed legth Starts at earliest uackowledged frame Oly frames i widow are active Time Set ad ackowledged Set ad ot ackowledged Available, outside sed widow Uavailable Sprig 2018 CS 438 Staff - Uiversity of Illiois 23

24 Slidig Widow Receive Widow Fixed legth (urelated to sed widow) Starts at earliest frame ot received Oly frames i widow accepted Time Received ad ackowledged Received ad ot ackowledged Received, outside receive widow Not yet received Sprig 2018 CS 438 Staff - Uiversity of Illiois 24

25 Slidig Widow Termiology Seder Parameters Sed Widow Size (SWS) Last Ackowledgemet Received (LAR) Last Frame Set (LFS) SWS = 4 Time LAR = 14 LFS = 18 Ivariat: LFS LAR SWS Sprig 2018 CS 438 Staff - Uiversity of Illiois 25

26 Slidig Widow Termiology Receiver Parameters Receive Widow Size (RWS) Next Frame Expected (NFE) Last Frame Acceptable (LFA) RWS = 6 Time NFE = 4 LFA = 9 Ivariat: LFA NFE + 1 RWS Sprig 2018 CS 438 Staff - Uiversity of Illiois 26

27 Slidig Widow Details Seder Tasks Assig sequece umbers O ACK Arrival Advace LAR Slide widow SWS = 4 SWS = 4 Time LAR = 14 LAR = 16 LFS = 18 LFS = 20 Receive ACK 16 Sprig 2018 CS 438 Staff - Uiversity of Illiois 27

28 Slidig Widow Details Receiver Tasks O Frame Arrival (N) Siletly discard if outside of widow N < NFE (NACK possible, too) N >= NFE + RWS Sed cumulative ACK if withi widow RWS = 6 Time NFE = 4 LFA = 9 Receive Frame 64 Sed ACK 37 Sprig 2018 CS 438 Staff - Uiversity of Illiois 28

29 Slidig Widow Details Receiver Tasks O Frame Arrival (N) Siletly discard if outside of widow N < NFE (NACK possible, too) N >= NFE + RWS Sed cumulative ACK if withi widow RWS = 6 Time NFE = 8 LFA = 13 Sprig 2018 CS 438 Staff - Uiversity of Illiois 29

30 Slidig Widow Details Sequece umber space Fiite umber, so wrap aroud Need space larger tha SWS (outstadig frames) Example I fact, eed twice as large 3-bit sequece umbers (0-7) RWS = SWS = 7 Sprig 2018 CS 438 Staff - Uiversity of Illiois 30

31 Slidig Widow Details Is log 2 (SWS+1) bits eough? No. Example: 3-bit sequece umbers (0-7) RWS = SWS = 7 Why is t 3 bits eough (ca you thik of a example where it does t work?) Sprig 2018 CS 438 Staff - Uiversity of Illiois 31

32 Slidig Widow Details Example of icorrect behavior 3-bit sequece umbers 0-7 RWS = SWS = 7 Timeout Sed(0) Sed(1) Sed(2) Sed(3) Sed(4) Sed(5) Sed(6) Seder trasmits 0-6 All arrive, but ACK s lost Seder retrasmits Receiver accepts as secod icaratio of 0-6 Sed(0) Sed(1) Sed(2) Sed(3) Sed(4) Sed(5) Sed(6) Sprig 2018 CS 438 Staff - Uiversity of Illiois 32

33 Slidig Widow Sequece Numbers How may sequece umbers are ecessary? Key questios Where ca the sed widow be? What frame ca be received ext? Sprig 2018 CS 438 Staff - Uiversity of Illiois 33

34 Slidig Widow Sequece Numbers Assume SWS = RWS (simplest, ad typical) Seder trasmits full SWS Two extreme cases: Noe received (waitig for 0...SWS - 1) All received (waitig for SWS...2 SWS - 1) All possible packets must have uique sequece umbers Sed Widow Sed Widow Sed Widow Sed Widow Sed Widow NFE Receive Widow Sprig 2018 CS 438 Staff - Uiversity of Illiois 34

35 Slidig Widow Sequece Numbers Extreme Locatios for SWS Requiremets If a received packet is ot i the receive widow with o wrap, the it must ot be i the receive widow with wrap! Correctess coditio: Number of Sequece Numbers ³ SWS + RWS Alterates betwee two halves of the sequece umber space Sed Widow Sed Widow NFE Receive Widow Sprig 2018 CS 438 Staff - Uiversity of Illiois 35

36 Slidig Widow Sequece Numbers Example If SWS = RWS = 8 At least 16 sequece umbers are eeded A 4-bit sequece umber space is eough Warig P&D sometimes uses the variable Max_Seq_Num for the umber of sequece umbers ad sometimes for the maximum sequece umber (these differ by oe!) Use Num_Seq_Num for the umber of sequece umbers: 0, 1,, Num_Seq_Num 1 Sprig 2018 CS 438 Staff - Uiversity of Illiois 36

37 Widow Sizes How big should we make SWS? Compute from delay x badwidth How big should we make RWS? Depeds o buffer capacity of receiver Sprig 2018 CS 438 Staff - Uiversity of Illiois 37

38 Delay x Badwidth Product - Revisited Amout of data i pipe chael = pipe delay = legth badwidth = area of a cross sectio badwidth x delay product = volume Delay Badwidth Sprig 2018 CS 438 Staff - Uiversity of Illiois 38

39 Delay x Badwidth Product Badwidth x delay product How may bits the seder must trasmit before the first bit arrives at the receiver if the seder keeps the pipe full Takes aother oe-way latecy to receive a respose from the receiver A B Sprig 2018 CS 438 Staff - Uiversity of Illiois 40

40 Slidig Widow Protocol Code Example Parameters last ackowledgemet received (LAR) last frame set (LFS) ext frame expected (NFE) last frame acceptable (LFA) Sprig 2018 CS 438 Staff - Uiversity of Illiois 41

41 Slidig Widow Protocol Code Example Costats Red/receive widow size (SWS/RWS) Maximum sequece umber (MAX_SEQ_NO) Frame size (FRAME_SIZE, costat for simplicity) Sprig 2018 CS 438 Staff - Uiversity of Illiois 42

42 Slidig Widow Protocol Code Example Data structures Next frame expected (a iteger) Oe frame buffer for each etry i receive widow Oe presece bit for each etry Receive widow cycles through Sequece umbers Data structures (thus RWS must divide MAX_SEQ_NO) Sprig 2018 CS 438 Staff - Uiversity of Illiois 43

43 Slidig Widow Protocol Code Example #defie RWS 8 /* receive widow size */ #defie MAX_SEQ_NO 16 /* max. sequece umber+1 */ /* (must be multiple of */ /* RWS for this code) */ #defie FRAME_SIZE 1000 /* costat for simplicity*/ char buf[rws][frame_size]; /* RWS frame buffers */ it preset[rws]; /* are frame buffers full?*/ /* (iitialized to 0 s) */ it NFE = 0; /* ext frame expected */ exter void sed_ack (it seq_o); exter void pass_to_app (char* data); void recv_frame (char* data, it seq_o); Sprig 2018 CS 438 Staff - Uiversity of Illiois 44

44 Slidig Widow Protocol Code Example void recv_frame (char* data, it seq_o) { it idx; /* idex ito data structures */ it i; /* loop idex */ /* Map sequece umbers NFE...predecessor (NFE) ito 0...MAX_SEQ_NO - 1, the see if seq_o falls withi the receive widow. */ if (((seq_o + (MAX_SEQ_NO - NFE)) % MAX_SEQ_NO) if (seq_o - NFE)< RWS) < RWS) { /* Frames outside the widow */ /* are igored. (but a ACK */ /* is set; why?) */ Sprig 2018 CS 438 Staff - Uiversity of Illiois 45

45 Slidig Widow Protocol Code Example /* Calculate idex ito data structures. */ idx = (seq_o % RWS); if (!preset[idx]) {/* frame is ot dup */ preset[idx] = 1;/* mark received */ memcpy (buf[idx], data, FRAME_SIZE); /* copy data ito buf */ Sprig 2018 CS 438 Staff - Uiversity of Illiois 46

46 Slidig Widow Protocol Code Example /* Got a ew frame; pass frames up to host? */ for (i = 0; i < RWS; i++) { idx = (i + NFE) % RWS; /* Re-use idx.*/ /* first missig frame becomes NFE */ /* after this loop termiates */ if (!preset[idx]) break; } /* Frame is preset sed it up! */ pass_to_app (buf[idx]); preset[idx] = 0; /* Mark buffer empty. */ } /* Advace NFE to first missig frame. */ NFE = (NFE + + i; i) % MAX_SEQ_NO; Sprig 2018 CS 438 Staff - Uiversity of Illiois 47

47 Slidig Widow Protocol Code Example /* Frame hadled (might have */ /* bee duplicate). */ } /* (Sed ACK for ay frame received */ } /* Now sed ackowledgemet for */ /* predecessor (NFE). */ sed_ack (NFE ((NFE-+ 1); MAX_SEQ_NO - 1) % MAX_SEQ_NO); Sprig 2018 CS 438 Staff - Uiversity of Illiois 48

48 Correctess Claim A slidig widow protocol leads to i-order delivery of all frames Assumptios All sequece umbers are differet Frames ca be lost Frames ca be delayed a arbitrarily fiite amout of time Frames are ot reordered Frames ca arrive with detectable errors Are these assumptio adequate? Sprig 2018 CS 438 Staff - Uiversity of Illiois 49

49 Slidig Widow Protocol Correctess Need oe more assumptio Ay give frame is received without errors after a fiite umber of retrasmissios Proof i two steps Establish correctess assumig ifiite sequece umber space Show that fiite sequece umber space does ot affect result as log as it has >= 2 max (SWS, RWS) possible umbers Sprig 2018 CS 438 Staff - Uiversity of Illiois 50

50 Slidig Widow Protocol Correctess Step 1: establish correctess assumig ifiite sequece umber space Use iductio o k with ivariat the k th frame is evetually received Step 2: show that fiite sequece umber space does ot affect result as log as it has >= 2 max (SWS, RWS) possible umbers RWS = 6 Time NFE = 4 LFA = 9 What frame ca arrive ext? Sprig 2018 CS 438 Staff - Uiversity of Illiois 51

51 ARQ Algorithm Classificatio Three Types: Stop-ad-Wait: SWS = 1 RWS = 1 Go-Back-N: SWS = N RWS = 1 Selective Repeat: SWS = N RWS = M Usually M = N Stop-Ad-Wait Go-Back-N Selective Repeat Sprig 2018 CS 438 Staff - Uiversity of Illiois 52

52 Slidig Widow Variatios: Go-Back-N SWS = N, RWS = 1 Receiver oly buffers oe frame If a frame is lost, the seder may eed to retrasmit up to N frames i.e., seder goes back N frames Variatios How log is the frame timeout? Does receiver sed NACK for out-of-sequece frame? Sprig 2018 CS 438 Staff - Uiversity of Illiois 53

53 Go-Back-N: Cumulative ACKs A Timeout for Packet 2 Packets 2,3,4,5 are retrasmitted Packet 1 Packet 0 Packet 2 Packet 5 Packet 4 Packet 3 Packet 5 Packet 4 Packet 3 Packet 2 B ACK 0 ACK 1 loss ACK 1 ACK 1 ACK 1 ACK 3 ACK 4 ACK 5 ACK 6 Sprig 2018 CS 438 Staff - Uiversity of Illiois 54

54 Slidig Widow Variatios: Selective Repeat SWS = N, RWS = M Receiver buffer M frames If a frame is lost, seder must oly resed Frames lost withi the receive widow Variatios How log is the frame timeout? Use cumulative or per-frame ACK? Does protocol adapt timeouts? Does protocol adapt SWS ad/or RWS? Sprig 2018 CS 438 Staff - Uiversity of Illiois 55

55 Selective Repeat A Packet 2 is retrasmitted Packet 1 Packet 0 Packet 2 Packet 5 Packet 4 Packet 3 Packet 8 Packet 7 Packet 6 Packet 2 B ACK 0 ACK 1 loss ACK 3 ACK 4 ACK 5 ACK 2 ACK 6 ACK 7 ACK 8 Sprig 2018 CS 438 Staff - Uiversity of Illiois 56

56 Roles of a Slidig Widow Protocol Reliable delivery o a ureliable lik Core fuctio Preserve delivery order Cotrolled by the receiver Flow cotrol Allow receiver to throttle seder Separatio of Cocers Must be able to distiguish betwee differet fuctios that are sometimes rolled ito oe mechaism Sprig 2018 CS 438 Staff - Uiversity of Illiois 57

57 Forward Error Correctio (FEC) Alterative to ARQ algorithms Idea Error correctio istead of error detectio Sed extra iformatio to avoid retrasmissio (i.e., fix errors first/forward rather tha afterward/backward) Why Very high latecy coectios Difficult for retrasmissio Sprig 2018 CS 438 Staff - Uiversity of Illiois 58