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Fundamentals of Computer Networks ECE 478/578

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Fundamentals of Computer Networks. ECE 478/578. Lecture #7: Reliable Transmission ... Dept of Electrical and Computer Engineering. University of Arizona ... – PowerPoint PPT presentation

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Title: Fundamentals of Computer Networks ECE 478/578


1
Fundamentals of Computer Networks ECE 478/578
  • Lecture 7 Reliable Transmission
  • Instructor Loukas Lazos
  • Dept of Electrical and Computer Engineering
  • University of Arizona

2
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

3
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
. . .
4
Simplex Protocol Lossy Channel
  • Stop-and-wait

5
Failure of the Stop-and-Wait
  • ACK is lost or arrives after timeout

6
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
7
(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

8
Go-back-N Protocol
  • Main idea Leave up to N frames unacknowledged at
    any given time

9
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

10
Sliding Window Depicted
  • At the sender
  • At the receiver

11
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

12
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

13
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
14
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)

15
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
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