1. Principles of Reliable Data Transfer

2. 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 P2P links –Network/Transport Layers Necessary if the application requires the data to be reliably delivered to the receiver, e.g., file transfer

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

4. 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

5. RDT over Unreliable Channel Unreliable channel Reliable Data Transfer Protocol (Sender Side) Reliable Data Transfer Protocol (Receiver Side) 1 2 3 4 … 1 2 3 4 … UDT_Send RDT_Receive Deliver_Data RDT_Send 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

6. 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

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

8. Stop and Wait Simplest ARQ protocol Sender: –Send a packet –Stop and wait until an ACK arrives –If received ACK, send the next packet –If timeout, ReTransmit the same packet Receiver: –When you receive a packet correctly, send an ACK Time Packet ACK Timeout SenderReceiver Packet ACK Packet

9. Recovering from Error Packet ACK Timeout Packet ACK Timeout Time Packet ACK Timeout Packet ACK Timeout ACK lost Early timeout Packet Timeout Packet ACK Timeout Packet lost 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

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

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

12. Pipelining: Increasing Utilization first packet bit transmitted, t = 0 senderreceiver RTT last bit transmitted, t = L / R first packet bit arrives last packet bit arrives, send ACK ACK arrives, send next packet, t = RTT + L / R last bit of 2 nd packet arrives, send ACK last bit of 3 rd packet arrives, send ACK Increase utilization by a factor of 3! 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

13. 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

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

15. 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 <= SWS

16. 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 <= RWS

17. Go-Back-N Receiver Sender …… Sent & AckedSent Not Acked OK to SendNot Usable …… Last Packet Acceptable (LPA) RWS = 1 packet Last ACK Received (LAR) Last Packet Sent (LPS) Received & AckedAcceptable Packet Not Usable SWS = N Next Packet Expected (NPE) 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: –discard & re-ACK pkt with highest in-order seq #

18. 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

19. 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.

20. GBN in action (SWS = 4)

21. 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

22. Selective Repeat Receiver Sender …… Sent & AckedSent Not Acked OK to SendNot Usable …… Last Packet Acceptable (LPA) RWS = N Last ACK Received (LAR) Last Packet Sent (LPS) Received & AckedAcceptable Packet Not Usable SWS = N Next Packet Expected (NPE) 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 unACKed pkt

23. Selective repeat 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 unACKed pkt # sender 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 not- yet-received pkt pkt k in [NPE-N, LPA-1] –Send ACK(k) otherwise : ignore receiver

24. Selective repeat in action

25. 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 <= (MaxSeqNo +1)/2

26. 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 –…

27. SWS = RWS = 4 with cumulative ACKs NPE = 5 LPA =8 RWS = 4 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