Principles of Reliable Data Transfer

Principles of Reliable DataTransfer

Reliable Delivery

•Making sure that the packets sent by the sender arecorrectly and reliably received by the receiver amidnetwork 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 performanceimprovement (versus correctness) in P2P links

–Network/Transport Layers

•Necessary if the application requires the data to be reliablydelivered to the receiver, e.g., file transfer

Reliable Delivery: Service Model

Reliable, In-order Delivery

unreliable channel

Reliable Data Transfer

Protocol (Sender Side)

Reliable Data Transfer

Protocol (Receiver Side)

…

UDT_Send

RDT_Receive

Deliver_Data

RDT_Send

–Reliable, In-order delivery

•Typically done when reliability is implemented at thetransport layer, e.g., TCP

•Example application: File transfer

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 complexas we continue

RDT over Unreliable Channel

Unreliable channel

Reliable Data Transfer

Protocol (Sender Side)

Reliable Data Transfer

Protocol (Receiver Side)

…

UDT_Send

RDT_Receive

Deliver_Data

RDT_Send

•Channel may flip bits in packets/lose packets

–The received packet may have been corrupted duringtransmission, or dropped at an intermediate router dueto buffer overflow

•The question: how to recover from errors?

•ACKs, NACKs, Timeouts… Next

RDT over Unreliable Channel

•Two fundamental mechanisms to accomplishreliable delivery over Unreliable Channels

–Acknowledgements (ACK), Negative ACK (NACK)

•Small control packets (header without any data) that aprotocol sends back to its peer saying that it has receivedan earlier packet (positive ACK) or that it has not received apacket (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 thepacket has made it to the receiver

•If the timeout occurs, the sender assumes that the packetis lost (corrupted) and retransmits the packet

ARQ

•The general strategy of using ACKs (NACKs)and timeouts to implement reliable delivery iscalled Automatic Repeat reQuest (ARQ)

•3 ARQ Mechanisms for Reliable Delivery

–Stop and Wait

–Concurrent Logical Channels

–Sliding Window

Stop and Wait

•Simplest ARQ protocol

•Sender:

–Send a packet

–Stop and wait until anACK arrives

–If received ACK, sendthe next packet

–If timeout, ReTransmitthe same packet

•Receiver:

–When you receive apacket correctly, sendan ACK

Time

Packet

ACK

Timeout

Sender

Receiver

Packet

ACK

Packet

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 thereceiver know whether the packet is a retransmission or a newpacket?

–Use sequence numbers: Both Packets and ACKs

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

Performance of Stop and Wait

first packet bit transmitted, t = 0

sender

receiver

RTT

last packet bit transmitted, t = L / R

first packet bit arrives

last packet bit arrives, sendACK

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!

Pipelining: Increasing Utilization

first packet bit transmitted, t = 0

sender

receiver

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 2nd packet arrives, send ACK

last bit of 3rd packet arrives, send ACK

Increase utilization

by a factor of 3!

•Pipelining: sender allows multiple, “in-flight”, yet-to-be-acknowledgedpkts without waiting for first to be ACKed to keep the pipe full

–Capacity of the Pipe = RTT * BW

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 aredelivered in-order to the upper layer

•2 Generic Versions

–Go-Back-N

–Selective Repeat

Receiver

Receiver

Sender

Sender

Sliding Window:Generic Sender/Receiver States

…

Sent & Acked

Sent Not Acked

OK to Send

Not Usable

…

Last Packet Acceptable

(LPA)

Receiver Window Size

Last ACK Received

(LAR)

Last Packet Sent

(LPS)

Received & Acked

Acceptable Packet

Not Usable

Sender Window Size

Next Packet Expected

(NPE)

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

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

Go-Back-N

Receiver

Receiver

Sender

Sender

…

Sent & Acked

Sent Not Acked

OK to Send

Not Usable

…

Last Packet Acceptable

(LPA)

RWS = 1 packet

Last ACK Received

(LAR)

Last Packet Sent

(LPS)

Received & Acked

Acceptable 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 #

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 packetfor 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 previouslysent, but not yet ACKed

•Therefore the name Go-Back-N

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.

GBN in action (SWS = 4)

GBN: Last Word

•Why use GBN?

–Very simple receiver

•Why NOT use GBN?

–Throwing away out-of-order packets at the receiverresults in extra transmissions, thus lowering thechannel utilization:

•The channel may become full of retransmissions of oldpackets rather than useful new packets

–Can we do better?

•Yes: Buffer out-of-order packets at the receiver and doSelective Repeat (Retransmissions) at the sender

Selective Repeat

Receiver

Receiver

Sender

Sender

…

Sent & Acked

Sent Not Acked

OK to Send

Not Usable

…

Last Packet Acceptable

(LPA)

RWS = N

Last ACK Received

(LAR)

Last Packet Sent

(LPS)

Received & Acked

Acceptable Packet

Not Usable

SWS = N

Next Packet Expected

(NPE)

•SWS = RWS = N consecutive packets: Sender can send up to Nconsecutive 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

Selective repeat

data from above :

•if next available seq # inwindow, send pkt

timeout(k):

•resend pkt k, restarttimer

ACK(k) in [LAR+1, LPS]

•Mark pkt “k” as received

•if k == LAR +1 then,advance LAR to nextunACKed pkt #

sender

pkt k in [NPE, LPA]

•send ACK(k)

•out-of-order (k != NPE):

–buffer

•in-order (k == NPE):

–deliver (also deliverbuffered, in-order pkts),advance NPE to next not-yet-received pkt

•pkt k in [NPE-N, LPA-1]

–Send ACK(k)

otherwise:

•ignore

receiver

Selective repeat in action

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-bitsequence numbers, i.e., 0..7

–Sender sends frames 0..6

•Assume receiver received all these frames successfully BUT allACKs 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 nomore than half as big as the number of availablesequence numbers

–WS <= (MaxSeqNo +1)/2

Sliding Window: Last Word

•Go-Back-N and Selective Repeat are NOT theonly 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 atimer 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., packetloss is very rare

•Only notify the sender when something goes wrong

–…

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 isyet 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 lostand retransmitted by the sender or it may have beensimply delayed

–The receiver will then ACK frame 7 (cumulative ACK) and setsNPE to 8 and LPA to 11