Reliable Data Transfer

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Reliable Data Transfer

• by ??? 20000903

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Principles of Reliable data transfer

• top-10 list of important networking topics!
• important in app., transport, link layers
• Well
• incrementally develop sender, receiver sides of
  reliable data transfer protocol (rdt)
• consider only unidirectional data transfer

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Reliable data transfer getting started
send side
receive side
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Rdt1.0 reliable transfer over a reliable channel

• underlying channel perfectly reliable
• use finite state machines (FSM) to specify
  sender, receiver

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Rdt2.0 channel with bit errors

• underlying channel may flip bits in packet
• the question how to recover from errors
• acknowledgement (ACK) (OK)
• negative acknowledgement (NAK) (Please repeat
  that.)
• new mechanisms in rdt2.0
• error detection
• receiver feedback control msgs (ACK,NAK) rcvr ?
  sender
• retransmission

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rdt2.0 FSM specification
sender FSM
receiver FSM
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rdt2.0 in action (no errors)
sender FSM
receiver FSM
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rdt2.0 in action (error scenario)
sender FSM
receiver FSM
Critical Flaw What happens if ACK/NAK
corrupted? Handling duplicates sender adds
sequence to each pkt
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rdt2.1 discussion

• Sender
• seq added to pkt
• must check if received ACK/NAK corrupted
• twice as many states

• Receiver
• must check if received packet is duplicate
• state indicates whether 0 or 1 is expected pkt
  seq
• note receiver can not know if its last ACK/NAK
  received OK at sender

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rdt2.2 a NAK-free protocol

• instead of NAK, receiver sends ACK for last pkt
  received OK
• duplicate ACK at sender results in same action as
  NAK retransmit current pkt
• receiver now includes sequence of packet

sender FSM
!
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rdt3.0 channels with errors and loss

• New assumption underlying channel can also lose
  packets (data or ACKs)
• checksum, seq. , ACKs, retransmissions will be
  of help, but not enough
• Q how to deal with loss?
• sender waits until certain data or ACK lost, then
  retransmits
• drawbacks?

• Approach sender waits reasonable amount of
  time for ACK
• requires countdown timer
• retransmits if no ACK received in this time
• if pkt (or ACK) is delayed (not lost)
• retransmission leads to duplicate
• receiver must specify seq of pkt being ACKed

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rdt3.0 in action
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rdt3.0 in action
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Recap on simple retransmission protocol
stop-and-wait
sender transmit one frame, F0, set retrans.
timer, wait for ACK if ACK arrives, cancel the
timer, send next frame F1 receiver upon receiving
a frame, send an ACK
sender
receiver
time
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Stop-and-wait (cont)

• If timer expires before receiving ACK, retransmit
  F0
• if the timer set too short, results in a
  duplicate F0
• Need a unique identifier to distinguish each data
  frame
• a 1-bit sequence number, N, is enough for
  Stop-and-Wait

sender
receiver
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Pipelining for better performance

• rdt3.0 works, but too slow
• example 1 Gbps link, 30ms roundtrip delay, 1KB
  pkt

Server busy only 0.027 of the time! Pipelining
sending multiple, yet-to-be-acknowledged pkts
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Go-Back-N

• Sender
• k-bit seq in pkt header
• can send up to N consecutive, unacked pkts

• ACK(n) ACKs all pkts up to, including seq n
  (cumulative ACK)
• Set timer for each in-flight pkt
• timeout(n) retransmit pkt n and all higher seq
  pkts in window

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GBN receiver extended FSM

• receiver
• ACK-only always send ACK for correctly-received
  pkt with highest in-order seq
• out-of-order pkt
• discard (dont buffer)
• ACK pkt with highest in-order seq

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GBN inaction
Go-Back-N in action
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Selective Repeat

• 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
• sender window
• N consecutive seq s
• again limits seq s of sent, unACKed pkts

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Selective repeat sender, receiver windows
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Selective repeat

• data from above
• if next available seq in window, send pkt
• timeout(n)
• resend pkt n, restart timer
• ACK(n) in sendbase,sendbaseN
• mark pkt n as received
• if n smallest unACKed pkt, advance window base to
  next unACKed seq

• pkt n in rcvbase, rcvbaseN-1
• send ACK(n)
• out-of-order buffer
• in-order deliver (also deliver buffered,
  in-order pkts), advance window to next
  not-yet-received pkt
• pkt n in rcvbase-N,rcvbase-1
• ACK(n)
• otherwise
• ignore

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Selective repeat in action
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Selective repeat dilemma

• Example
• seq s 0, 1, 2, 3
• window size3
• receiver sees no difference in two scenarios!
• incorrectly passes duplicate data as new in (a)
• Q what is the relationship between seq size
  and window size?
• (Max. seq 1)/2 ? window-size

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Three basic components in reliable data delivery
by retransmission

• sequence number used to uniquely identify
  individual frames
• Acknowledgment (ACK) reception report from
  receiver
• Retransmission at the sender upon TIMEOUT (or
  NAK)
• must know how long to wait before retry