Fundamentals of Computer Networks ECE 478/578

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Fundamentals of Computer NetworksECE 478/578

• Lecture 7 Reliable Transmission
• Instructor Loukas Lazos
• Dept of Electrical and Computer Engineering
• University of Arizona

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Reliable Transmission

• Transmission strategies under a lossless channel
• Simplex connection
• Simplex Stop-and-wait
• Transmission Strategies under a lossy channel
• Process of retransmitting frames that cannot be
  corrected by CRC codes
• Stop-and-wait
• Go-back-N
• Selective repeat

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Simplex Protocol Lossless Channel
Sender
Receiver

• Data flows one direction only
• Communication channel never loses frames
• Receiver is always ready to receive packets
• Problem receiver can be flooded if it does not
  process packets fast enough
• Solution Introduce delay at sender

request
reply
negotiate
ack
. . .
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Simplex Protocol Lossy Channel

• Stop-and-wait

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Failure of the Stop-and-Wait

• ACK is lost or arrives after timeout

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Solution based on Sequence No

• Use a one-bit sequence number on the header

Sender
Receiver
Frame 0
ACK
Frame 0
Frame 1
ACK
Frame 0
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(In)Efficiency of Stop-and-Wait

• Example Consider 1.5Mbps link, 45ms RTT
• Delay x Bandwidth 67.5kb 8KB
• If frames are 1KB long then max rate
• 1024 x 8 /0.045 182Kbps
• Only 1/8th of links capacity
• Goal keep the pipe full, i.e. have maximum of
  bits unacknowledged at any given time
• Our example, we could have 8 frames
  unacknowledged

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Go-back-N Protocol

• Main idea Leave up to N frames unacknowledged at
  any given time

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Go-Back-N

• Each frame is assigned a SeqNum
• Variables at the sender
• SWS Send Window Size
• LAR Last Acknowledgment Received
• LFS Last Frame Sent
• Rule LFS LAR ? SWS (at most SWS frames
  unACKed)
• Variables at the receiver
• RWS Receive Window Size
• LAF Largest Acceptable Frame (in seq )
• LFR Last Frame Received
• Rule LAF LFR ? RWS

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Sliding Window Depicted

• At the sender
• At the receiver

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Updating Variables
Si
SWS
RWS
Receiver Variables LFRi LAFi
Sender Variables LSFi LARi

• Si LFRi1 (Go-back-N only accepts packets in
  order)
• Si ? LAFi (which is guaranteed in our case, for
  RWS ? 1)
• LSFi1 LSF 1, LFRi1 LFRi 1, LAFi1 LAFi
  1
• Window at sender advanced only if ACK received

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Example of Go-Back-N
Sender
Receiver
SWS 4
RWS 1
0
1
LFR 0 LAF 1
2
Timeout
3
LFS 4 LAR 0
4
discarded
1
2

• Max Sequence Number ? SWS 1
• Link does not re-arrange packets

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Piggypacking

• Duplex communication, attach ACK on the reply
  frame
• Asymmetry in frame size can cause timeouts
• Go-Back-N can be turned into Stop-and-Wait

Sender
Receiver
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Selective Repeat

• Window size can be very large for nets with large
  delay x bandwidth
• Inefficient to retransmit all N frames if one is
  lost
• Selective repeat allows the re-transmission of
  only the lost packets
• Accepts out-of-order packets
• Simply increase the RWS up to SWS (does not make
  sense to allow for RWS gt SWS)

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Example of Selective Repeat
Sender
Receiver
SWS 4
RWS 4
0
1
LFR 0 LAF 4
2
Timeout
3
LFS 4 LAR 0
4
Buffered
1
LFS 4 LAR 4
5
6
7

• Max Sequence Number ? 2 SWS