Principles of Reliable Data Transfer 1 Reliable Delivery

Principles of Reliable Data Transfer 1

Reliable Delivery • Making sure that the packets sent by the sender are correctly and reliably received by the receiver amid network errors, i. e. , corrupted/lost packets – Can be implemented at LL, NL or TL of the protocol stack. Totally a design choice • When and why should this be used? – Link Layer • Rarely done over twisted-pair or fiber optic links • Usually done over lossy links (wireless) for performance improvement (versus correctness) in P 2 P links – Network/Transport Layers • Necessary if the application requires the data to be reliably delivered to the receiver, e. g. , file transfer 2

Reliable Delivery: Service Model … 4 3 2 … 4 1 3 2 1 Deliver_Data RDT_Send Reliable Data Transfer Protocol (Receiver Side) Reliable Data Transfer Protocol (Sender Side) UDT_Send RDT_Receive unreliable channel Reliable, In-order Delivery – Reliable, In-order delivery • Typically done when reliability is implemented at the transport layer, e. g. , TCP • Example application: File transfer 3

Reliable Delivery: Assumptions We’ll: • Consider only unidirectional data transfer – A sender sending packets to a receiver – Bidirectional communication is a simple extension, where there are 2 sender/receiver pairs • Start with simple a protocol and make it complex as we continue 4

RDT over Unreliable Channel … 4 3 2 … 4 1 3 2 1 Deliver_Data RDT_Send Reliable Data Transfer Protocol (Receiver Side) Reliable Data Transfer Protocol (Sender Side) UDT_Send RDT_Receive Unreliable channel • Channel may flip bits in packets/lose packets – The received packet may have been corrupted during transmission, or dropped at an intermediate router due to buffer overflow • The question: how to recover from errors? • ACKs, NACKs, Timeouts… Next 5

RDT over Unreliable Channel • Two fundamental mechanisms to accomplish reliable delivery over Unreliable Channels – Acknowledgements (ACK), Negative ACK (NACK) • Small control packets (header without any data) that a protocol sends back to its peer saying that it has received an earlier packet (positive ACK) or that it has not received a packet (NACK). • Sent by the receiver to the sender – Timeouts • Set by the sender for each transmitted packet • If an ACK is received before the timer expires, then the packet has made it to the receiver • If the timeout occurs, the sender assumes that the packet is lost (corrupted) and retransmits the packet 6

ARQ • The general strategy of using ACKs (NACKs) and timeouts to implement reliable delivery is called Automatic Repeat re. Quest (ARQ) • 3 ARQ Mechanisms for Reliable Delivery – Stop and Wait – Concurrent Logical Channels – Sliding Window 7

Stop and Wait • Simplest ARQ protocol • Sender: Timeout – Send a packet – Stop and wait until an ACK arrives – If received ACK, send the next packet – If timeout, Re. Transmit the same packet Sender Receiver Packe t ACK Packe t • Receiver: – When you receive a packet correctly, send an ACK 8

Packe t ACK Packet lost Packe t ACK Timeout t Packe t ACK lost t K C A Timeout Packe Timeout Recovering from Error t ACK Early timeout • Does this protocol work? • When an ACK is lost or a early timeout occurs, how does the receiver know whether the packet is a retransmission or a new packet? – Use sequence numbers: Both Packets and ACKs 9

Pkt 0 ACK 0 Pkt 1 Packet lost ACK 0 Timeout Pkt 0 ACK 0 Pkt 1 ACK lost Timeout Pkt 0 Timeout Stop & Wait with Seq #s Pkt 0 CK A 0 Pkt 0 ACK 0 Pkt 1 Early timeout – Sequence # in packet is finite -- how big should it be? • One bit – won’t send Pkt #1 until received ACK for Pkt #0 10

Performance of Stop and Wait • Can only send one packet per round trip • example: 1 Gbps link, 15 ms e-e prop. delay, 1 KB packet: sender first packet bit transmitted, t = 0 last packet bit transmitted, t = L / R RTT receive r first packet bit arrives last packet bit arrives, send ACK arrives, send next packet, t = RTT + L / R – 1 KB pkt every 30 msec -> 33 k. B/sec throughput over 1 Gbps link 11 – network protocol limits use of physical resources!

Pipelining: Increasing Utilization • Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledged pkts without waiting for first to be ACKed to keep the pipe full – Capacity of the Pipe = RTT * BW sender receiver first packet bit transmitted, t = 0 last bit transmitted, t = L / R RTT first packet bit arrives last packet bit arrives, send ACK last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK arrives, send next packet, t = RTT + L / R Increase utilization by a factor of 3! 12

Sliding Window Protocols • Reliable, in-order delivery of packets • Sender can send “window” of up to N, consecutive unack’ed packets • Receiver makes sure that the packets are delivered in-order to the upper layer • 2 Generic Versions – Go-Back-N – Selective Repeat 13

Sliding Window: Generic Sender/Receiver States Sender Receiver Next Packet Expected Last Packet Acceptable (LPA) (NPE) Last ACK Received Last Packet Sent (LAR) (LPS) … … Sender Window Size Sent & Acked Sent Not Acked OK to Send Not Usable … … Receiver Window Size Received & Acked Acceptable Packet Not Usable 14

Sliding Window- Sender Side • The sender maintains 3 variables – Sender Window Size (SWS) • Upper bound on the number of in-flight packets – Last Acknowledgement Received (LAR) – Last Packet Sent (LPS) – We want LPS – LAR <= SWS LAR LPS 15

Sliding Window- Receiver Side • The receiver maintains 3 variables – Receiver Window Size (RWS) • Upper bound on the number of buffered packets – Last Packet Acceptable (LPA) – Next Packet Expected (NPE) – We want LPS – NPE + 1 <= RWS NPE LPA 16

Go-Back-N Sender Receiver Next Packet Expected Last Packet Acceptable (LPA) (NPE) Last ACK Received Last Packet Sent (LAR) (LPS) … … SWS = N RWS = 1 packet Sent & Acked Sent Not Acked OK to Send Not Usable Received & Acked Acceptable Packet Not Usable • SWS = N: Sender can send up to N consecutive unack’ed pkts • RWS = 1: Receiver has buffer for just 1 packet • Always sends ACK for correctly-rcvd pkt with highest in-order seq # – Cumulative ACK • Out-of-order pkt: 17 – discard & re-ACK pkt with highest in-order seq #

Go-Back-N: Sender Actions • Data From Above: – Send packets as long as LPS-LAR <= SWS • ACK(k): An ACK with “seqno = k” arrives: – If k > LAR then, increase LAR until LAR hits a packet for which ACK has not arrived yet, or LAR == LPS – Send packet(s) as long as LPS-LAR <= SWS – Associate a timer with the oldest packet sent • Single timer for all packets in transit • Timeout: – Retransmit ALL packets that have been previously sent, but not yet ACKed • Therefore the name Go-Back-N 18

Go-Back-N: Receiver Actions • A packet with “seqno” arrives: – If seqno == NPE then // in-order packet • Deliver the packet to the upper layer • Send an ACK for pkt# = seqno • This is called “cumulative ACK” scheme – ACKs not only the current packet, but also all packets before it – If seqno != NPE then // out of order packet • Since sequence # of the last packet received is NPE – 1, send an ACK for pkt# = NPE-1 • Still using “cumulative ACK” scheme. 19

GBN in action (SWS = 4) 20

GBN: Last Word • Why use GBN? – Very simple receiver • Why NOT use GBN? – Throwing away out-of-order packets at the receiver results in extra transmissions, thus lowering the channel utilization: • The channel may become full of retransmissions of old packets rather than useful new packets – Can we do better? • Yes: Buffer out-of-order packets at the receiver and do Selective Repeat (Retransmissions) at the sender 21

Selective Repeat Sender Last ACK Received (LAR) Receiver Next Packet Expected Last Packet Acceptable (LPA) (NPE) Last Packet Sent (LPS) … … SWS = N RWS = N Sent & Acked Sent Not Acked OK to Send Not Usable Received & Acked Acceptable Packet Not Usable • SWS = RWS = N consecutive packets: Sender can send up to N consecutive unack’ed pkts, Receiver can buffer up to N consecutive packets • Receiver individually acknowledges all correctly received pkts – buffers pkts, as needed, for eventual in-order delivery to upper layer • Sender only resends pkts for which ACK not received – sender timer for each un. ACKed pkt 22

Selective repeat sender data from above : • if next available seq # in window, send pkt timeout(k): • resend pkt k, restart timer ACK(k) in [LAR+1, LPS] • Mark pkt “k” as received • if k == LAR +1 then, advance LAR to next un. ACKed pkt # receiver pkt k in [NPE, LPA] • send ACK(k) • out-of-order (k != NPE): – buffer • in-order (k == NPE): – deliver (also deliver buffered, in-order pkts), advance NPE to next notyet-received pkt • pkt k in [NPE-N, LPA-1] – Send ACK(k) otherwise: • ignore 23

Selective repeat in action 24

Selective Repeat: Sequence Numbers • How large do sequence numbers need to be? – Must be able to detect wrap-around – Depends on sender/receiver window size • E. g. – Assume SWS = RWS = 7. Also assume that we use 3 -bit sequence numbers, i. e. , 0. . 7 – Sender sends frames 0. . 6 • Assume receiver received all these frames successfully BUT all ACKs are lost • Receiver expects 7, 0. . 5 • Sender timeouts and retransmits old 0. . 6 • Receiver receives these but assumes these are new frames!! • It turns out that the sending window size can be no more than half as big as the number of available sequence numbers – WS <= (Max. Seq. No +1)/2 25

Sliding Window: Last Word • Go-Back-N and Selective Repeat are NOT the only sliding Window protocol alternatives • Other variations exist: – Let SWS = RWS = N and use cumulative ACKs • Simplifies the sender: Can have a single timer instead of a timer for each packet in transit • This is in fact what TCP does! – Let SWS = RWS = N, and use Negative ACKs • Can be used when the channel is pretty reliable, i. e. , packet loss is very rare • Only notify the sender when something goes wrong – … 26

SWS = RWS = 4 with cumulative ACKs RWS = 4 NPE = 5 LPA =8 • Assume NPE = 5 and RWS = 4 LPA = 8 • Assume frames 6 and 7 arrive. – They will be buffered, BUT no ACK will be sent since frame 5 is yet to arrive. – Frames 6 and 7 are said to arrive “out of order” – Receiver sends ACK for pkt #4 • If frame 5 now arrives because it may have been lost and retransmitted by the sender or it may have been simply delayed – The receiver will then ACK frame 7 (cumulative ACK) and sets NPE to 8 and LPA to 11 27