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Principles of Reliable Data Transfer
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
Reliable Delivery: Service Model
Reliable, In-order Delivery
unreliable channel
Reliable Data TransferProtocol (Sender Side)
Reliable Data TransferProtocol (Receiver Side)
1234… 1234…
UDT_Send RDT_Receive
Deliver_DataRDT_Send
– Reliable, In-order delivery• Typically done when reliability is implemented at
the transport layer, e.g., TCP• Example application: File transfer
Reliable Delivery: AssumptionsWe’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
RDT over Unreliable Channel
Unreliable channel
Reliable Data TransferProtocol (Sender Side)
Reliable Data TransferProtocol (Receiver Side)
1234… 1234…
UDT_Send RDT_Receive
Deliver_DataRDT_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
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
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
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
Tim
eou
t
Sender Receiver
Packet
ACKPacket
Recovering from Error
Packet
ACK
Tim
eou
t
Packet
ACK
Tim
eou
t
Time
Packet
ACK
Tim
eou
t
Packet
ACK
Tim
eou
t
ACK lost Early timeout
PacketT
ime
out
Packet
ACK
Tim
eou
t
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
Stop & Wait with Seq #s
Pkt 0
ACK 0
Tim
eou
t
Pkt 0
ACK 0
Tim
eou
t
Time
Pkt 0
ACK 0
Tim
eou
t
Pkt 0
ACK 0T
ime
out
ACK lost Early timeout
Pkt 0T
ime
out
Pkt 0
ACK 0
Tim
eou
t
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 1Pkt 1Pkt 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 arriveslast packet bit arrives, send ACK
ACK arrives, send next packet, t = RTT + L / R
U sender
= .008
30.008 = 0.00027
microseconds
L / R
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 arriveslast packet bit arrives, send ACK
ACK arrives, send next packet, t = RTT + L / R
last bit of 2nd packet arrives, send ACKlast bit of 3rd packet arrives, send ACK
U sender
= .024
30.008 = 0.0008
microseconds
3 * L / R
RTT + L / R =
Increase utilizationby 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
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
ReceiverReceiverSenderSender
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-NReceiverReceiverSenderSender
… …
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
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
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
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
Selective RepeatReceiverReceiverSenderSender
… …
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 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
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 #
senderpkt 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
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-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
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
– …
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
