COSC 3213: Communication Networks Chapter 5: Handout #6

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1 OS 323: ommunication Networks hapter 5: Handout #6 Instructor: Dr. Marvin Mandelbaum Department o omputer Science York University F8 Section E Topics:. Peer-to-peer and service models 2. RQ and how to provide a reliable transer o inormation over a data link 3. Stop-and-Wait RQ 4. Go-Back N RQ 5. Selective Repeat RQ Garcia: Section 5.2

2 n+ entity n+ entity SP n-sdu n-sdu SP2 n-sdu H n entity n entity H n-sdu n-pdu SE323 E F8 Peer^2 & RQ 2

3 utomatic Repeat Request (RQ) Protocol. combines error detection and retransmission to ensure accurate data delivery despite transmission errors and PDU losses. 2. Basic goals: Error ee, sequence-ordered delivery o PDU. No duplication o PDU. Flow control when one o the host is slower. 3. Basic elements o the protocol: Error-detecting code with high probability o error detection. Positive cknowledgment () to indicate correct reception o a PDU. Negative cknowledgment (N) to indicate error in a PDU or loss o a PDU. Timeout mechanism to cover scenarios where PDU is lost. 4. Error detection perormed by the data-link and transport layers. Here we reer to the implementation o the RQ protocol in the data-link layer. The same procedure can be used at the transport layer. 5. nalyze three dierent RQ protocols: Stop-and-Wait RQ Go-Back N RQ Selective Repeat RQ SE323 E F8 Peer^2 & RQ 3

4 Basic Elements o RQ Transmitter Network Packet Data-link Inormation ame Ino ame ontrol ame Receiver B Network Error-ee packet Data-link ontrol ame: s Heade Packet R r. Network layer passes the packet on to the datalink layer. 2. Data-link layer encapsulates packet into an inormation ame adding: (i) a header containing the sequence number o the ame and (ii) a trailer containing the R code. 3. The physical layer is responsible or the delivery o the ame Heade R r 4. Physical layer passes the ino ame to data-link. 5. The data-link extracts the sequence number om the header and the R code om the trailer. 6. I no error is detected, the ino packet is passed to the network layer and an ame is transmitted to the transmitter SE323 E F8 Peer^2 & RQ 4

5 Stop and Wait RQ (). Transmitter transmits an inormation ame to the receiver and starts a timer. 2. Transmitter stops and waits or the ame om Receiver B. 3. I an ame is received om Receiver B beore the timer expires, Transmitter starts transmission o the next inormation ame. 4. I the timer expires without receiving an ame, Transmitter retransmits the inormation ame. Scenario I: Inormation Frame gets lost. Time-out Transmitter Frame Frame Frame Frame 2 Time Receiver B SE323 E F8 Peer^2 & RQ 5

6 Stop and Wait RQ (2) Scenario II: o Frame gets lost Time-out Transmitter Frame Frame Frame Receiver B. Loss o ame leads to duplication o Frame at Receiver B. Frame 2 Time 2. To avoid duplication in cases where ames are lost, we add a sequence number (indicating the current inormation ame number) to the header o the inormation ame. Scenario III: orrect Operation or Scenario II Time-out Transmitter Frame Frame Frame Frame 2 Time Receiver B. Despite duplicate reception o Frame, Receiver B recognizes that same ame is received twice and ignores the second copy o Frame. SE323 E F8 Peer^2 & RQ 6

7 Stop and Wait RQ (3) Scenario IV: Timer expires prematurely Time-out Transmitter Receiver B Frame Frame Frame Frame 2 Time. Transmitter assumes that Frame is received correctly and proceeds with transmission o Frame To avoid loss o ames because o premature time-outs, we add a sequence number R next (indicating the ame number o the next inormation ame) to the ames. Scenario V: orrect Operation or Scenario IV Time-out Transmitter Frame Frame Frame Frame Receiver B. Despite duplicate reception o Frame, Receiver B ignores the second copy o Frame. 2. Transmitter retransmits Frame since it does not receive 2. SE323 E F8 Peer^2 & RQ Time 7

8 Stop and Wait RQ : Protocol Description Initialization: Transmitter in Ready mode. Set transmitter-send-sequence number = and receiver-sequence-number R next =. Transmitter: Ready Mode: Transmitter prepares the requested inormation ame with the sequence number [( ) in the header, the packet data, and the R error detection code.] Transmitter then enters the wait mode and waits or acknowledgment R next with R next = ( + ) mod 2. Wait Mode: During the wait stage, Transmitter does not accept packets om the upper layer. I an acknowledgement is not received or i it is incorrectly received, Transmitter stays in the wait mode till the timer expires out. I the timer times-out, inormation ame is retransmitted with the same sequence number ( ). I correct acknowledgement R next is received, then is set to R next and the transmitter returns to ready mode and the start o step. SE323 E F8 Peer^2 & RQ 8

9 Stop and Wait RQ : Protocol Description cont d Receiver: Receiver B is always in the ready mode. When a ame arrives, the ame is checked or errors. I no error is detected and the received ame number is the expected number = R next, then the inormation ame is passed on to the higher layer, R next is updated to (Rnext + ) mod 2, and R next sent. I the received inormation ame has errors, the ame is discarded with no action taken. I the received inormation ame has no errors but incorrect sequence number, R next is sent SE323 E F8 Peer^2 & RQ 9

10 Stop and Wait RQ: Window Operation State (,): Transmitter ready to transmit ame. Timer R next State (,): Frame transmitted but not acknowledged Timer R next State (,): Transmitter gets or Frame beore timer expires Timer R next SE323 E F8 Peer^2 & RQ Successul operation: Frame correctly received and acknowledged

11 Stop and Wait RQ (5): Window Operation State (,): Transmitter ready to transmit ame. Timer R next State (,): Frame transmitted but not acknowledged Timer R next State (,): Timer expires Timer R next Unsuccessul operation: Frame not received SE323 E F8 Peer^2 & RQ

12 Stop and Wait RQ (7): State Diagram Unsuccessul operation: Frame incorrectly received Global State: (, R next ) (,) Error-ee ame (,) arrives at receiver or ame arrives at transmitter Error-ee ame (,) arrives at receiver (,) or ame arrives at transmitter Unsuccessul operation: Frame incorrectly received State Diagram SE323 E F8 Peer^2 & RQ 2

13 Stop and Wait RQ (8): Perormance Transmitter First ame bit enters channel Last ame bit enters channel First bit enters channel Last bit enters channel Receiver B Inormation Frame (n bits) Frame (n a bits) t prop Propagation delay t Transmission delay t proc Processing delay at Rx t ack t prop Propagation delay t proc Processing delay at Tx ase : No transmission errors occur Time o transmission: Number o inormation bits transmitted: Eective transmission rate : Transmission Eiciency: t = total time to transmit ame t ( n n o ) R e = R e R = 2t + 2t + t + t == 2t + 2t SE323 E F8 Peer^2 & RQ n R prop proc ack prop proc + + ( n no ) to = ( ) ( ( ) ) n n 2 t + t R + + n o n a prop n proc na R Eect o Frame overhead Eect o overhead Eect o Delay-BW prod 3

14 Stop and Wait RQ (9): Perormance -st ame bit enters channel Last ame bit enters channel First bit enters channel Last bit enters channel Transmitter Receiver B Inormation Frame (n bits) Frame (n a bits) t prop Propagation delay t Frame delay t proc Processing delay at Rx t ack t prop Propagation delay t proc Processing delay at Tx t = total time to transmit ame ase 2: Transmission errors occur with probability P The transmission eiciency is reduced by a actor o ( P ) to Transmission Eiciency: R R = e P ( )( ) ( ( ) ) n n 2 t + t R + + ctivity : ssume that the inormation ames are 25 bytes long with 25 bytes o overhead. The ames are assumed to be 25 bytes long. alculate the transmission eiciency o the Stop and Wait RQ in a Mbps transmission system assuming: (a) no transmission errors; and (b) transmission errors with a bit error rate o 6 and SE ssume E F8 that Peer^2 the & reaction RQ time 2(t prop + t proc ) = ms. n o n a prop n proc 4

15 Go-Back-N RQ (). Stop-and-Wait RQ is ineicient at high bit error rates and multiple small ames 2. Go-back-N RQ improves Stop-and-Wait RQ by not waiting and allowing multiple inormation ames to be transmitted without acknowledgment. 3. Optimistic based on high prob. o success in Tx, & Pipeline 4. Frames to (W s ) are transmitted without any acknowledgement. Window size W S 5. I or ame arrives beore window is exhausted, transmitter continues to transmit. 6. I or ame is not received, transmitted goes back by N = W s ames and begin retransmitting. Go-Back-4: 4 ames are outstanding; so go back Time B 2 3 out o sequence ames SE323 E F8 Peer^2 4 & RQ

16 Stop-and-Wait vs. Go-Back N RQ Stop-and-Wait RQ: Time-out expires Time B Go-Back-N RQ: Receiver is looking or R next = Four ames are outstanding; so go back Time B Receiver is Out-o-sequence looking SE323 or E ames F8 Peer^2 & RQ R next =

17 Go-Back N RQ (3) Modiications. Go-Back N RQ ails i the total number o ames to be transmitted are less than the window size W s and one o the ames get lost. 2. Retransmissions will not be triggered since the window size at the transmitter is not exceeded. 3. To resolve the problem, we associate a timer with each transmitted ame. 4. Go-back-N is triggered as soon as one o the timers expires. Frames transmitted and ed Transmitter Send Window:... S recent +W s Receiver Frames received R next Receive Window: Timer Timer Timer Buers: +... S recent... +W s oldest un-ed ame most recent transmission SE323 E F8 Peer^2 & RQ max Seq # allowed Receiver will only accept a ame that is error-ee and that has sequence number R next When such ame arrives R next is incremented by one, so the receive window slides orward by one 7

18 Go-Back-N RQ (4): Protocol Description Initialization: ssume window size = W s Transmitter: in Ready mode with = S recent =. Receiver: R next =. Transmitter: Ready Mode:. Transmitter prepares ame S recent with the sequence number (S recent ) in the header, the packet data, and the R error detection code. timer or the sent ame is started and S recent = (S recent + ) modulo W s. 2. I S recent = ( + W s ) modulo W s, Transmitter enters the wait mode. 3. I S recent ( + W s ) modulo W s, Transmitter goes to the ready mode. Wait Mode:. I, correct acknowledgement R next is received with R next between and S recent, beore timer expires then is set to R next, and the transmitter returns to the ready mode (step ). 2. I timer o expires, S recent is reset to and the transmitter returns to the ready mode (step ) Receiver:. Receiver B is always in the ready mode. When a ame arrives, the ame is checked or errors. I no error is detected and the received ame number equals R next, then the inormation ame is passed on to the higher layer. R next is updated to (R next + ) mod W s, and R next sent. I the received inormation ame SE323 has errors, E F8 the Peer^2 ame & is RQ discarded with no action taken. 8 I the received inormation ame has no errors but incorrect sequence number, R next is sent.

19 Go-Back N RQ (5): Maximum Window Size For m = log 2 (M) bits sequence no. ield in the header, maximum window size is: (N < 2 m ). Frames are still labeled om to 2 m. M = 2 2 = 4, Go-Back-4: 2 3 Transmitter goes back Time Depends on the # o bits m in Header B M = 2 2 = 4, Go-Back-3: Transmitter goes back 3 3 Receiver has R next =, but it does not know whether its or ame was received, so it does not know whether this is the old ame or a new ame 3 Time O i window is smaller B 2 3 SE323 Receiver E has F8 Peer^2 R next = 3, so it rejects the old & RQ 3 ame 3 9

20 Piggybacking. In most communications, data is transerred in both directions. Such links are called bidirectional. 2. ssume host wants to transmit data to host B. Host B itsel have some data to transmit to host. 3. Many o the ames are eliminated by piggybacking acknowledgements into the header o the inormation ames. 4. When the receiver gets an error-ee inormation ame: It starts an timer and waits I data is available to be sent, the is inserted in the header o the inormation ame and transmitted I timer expires then an ame is transmitted. No inormation ame is sent. 5. The reception step o the Go-back-N RQ algorithm is slightly modiied with piggybacking: ny ame received out-o-sequence is irst checked or errors. I error has occurred in the out-o-sequence ame, the ame is discarded. I no error has occurred in the out-o-sequence ame, the ield o the inormation ame is extracted beore discarding the inormation ame. SE323 E F8 Peer^2 & RQ 2

21 Go-Back N RQ (6): alculation o Time-out. The time-out value o the inormation ame should exceed the duration required to receive a ame acknowledgement once an inormation ame is transmitted. T out T prop Ino ame T T T prop T proc ame 2. ssume piggybacking between and B then the value o Time-out: out prop max T = 2T + 2T + T proc SE323 E F8 Peer^2 & RQ 2

22 Go-Back N RQ (7): Perormance Issues. I there were no time-out because o lost or erroneous inormation or ames, the time to transmit an inormation ame in steady state will approach T, the ame time. 2. Suppose that the probability o losing a ame due to error or packet losses = P, then the ollowing table or successul transmission o the inormation ame can be constructed No. o Transmissions + W s + 2W s + 3W s + 4W s + 5W s Probability P P ( P ) (P ) 2 ( P ) (P ) 3 ( P ) (P ) 4 ( P ) (P ) 5 ( P ) ctivity 2: Show that the average no. o attempts or successul delivery o an inormation ame is verage no. o attempts = + S ( P ) with the average time required to transmit an inormation ame in Go-back-N RQ is P W and the eiciency o Go-back-N RQ is η t GBN = + WS t ( P ) n n n R e t n GBN GBN SE323 = = E F8 = Peer^2 & RQ R R + WS P ( ) P ( P ). 22

23 Go-Back N RQ (8): Perormance Issues ctivity 3: Repeat ctivity or Go-back-NRQ. ssume that the inormation ames are 25 bytes long with 25 bytes o overhead. The ames are assumed to be 25 bytes long. alculate the transmission eiciency o the Go-back-N RQ in a Mbps transmission system with reaction time o ms assuming a bit error rate o 6 and 4. ssume that the window size W s is larger than the delay bandwidth product deined as the product o the reaction time with transmission rate. ompare the results with stop-and-wait RQ. SE323 E F8 Peer^2 & RQ 23

24 Selective Repeat RQ. Go-back-N RQ is ineicient because o the need to transmit all ames subsequent to an error ame. 2. n alternative to Go-back-N RQ is the selective repeat RQ. 3. Selective repeat RQ has three dierences om Go-back RQ. Receiver window size is increased so that out o order error-ee ames can be accepted Retransmission mechanism is modiied so that individual ames can be retransmitted In addition to command, the receiver can also send a N command corresponding to the ame not received in a sequence o ames. Whenever an out o sequence ame is received, an N ame is transmitted Time B 2 N 2SE E F8 2 Peer^2 7 8 & RQ

25 Selective Repeat RQ (2) Transmitter Send Window... Receiver Receive Window Frames transmitted and ed S recent + W s Frames received R next R next + W r Timer Buers Buers R next + Timer + R next + 2 Timer... S recent R next + W r max Seq # accepted + W s SE323 E F8 Peer^2 & RQ 25

26 Selective Repeat RQ (3) Maximum Window Size : verage Transmission Time : t Eiciency: ctivity 4: Repeat ctivity or Selective Repeat RQ. W η S SR SR = WR = 2 t = P = n n m ( P ) ssume that the inormation ames are 25 bytes long with 25 bytes o overhead. The ames are assumed to be 25 bytes long. alculate the transmission eiciency o the Selective Repeat RQ in a Mbps transmission system with reaction time o ms assuming a bit error rate o 6 and 4. omparison on Eiciency based on ctivities, 3, and 4: P Stop-and-wait Go-back-N RQ Selective Repeat RQ 6 8.8% 88.2% 97% 5 8.% 45.4% 89% 4 3.3% 4.9% 36% SE323 E F8 Peer^2 & RQ 26

Peer-to-Peer Protocols and Data Link Layer

Peer-to-Peer Protocols and Data Link Layer Peer-to-Peer Protocols ad Data Lik Layer Lecture Sprig 9 Flow otrol utomatic Repeat Request (RQ) Purpose: to esure a sequece o iormatio packets is delivered i order ad without errors or duplicatios despite