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CIS 725

CIS 725. Data Link Layer. Flow Control. Producer-consumer problem Sliding window protocol - Go Back N - Selective retransmission. Go Back N. na. x. x. x. x. ns. S:: ns = 1; na = 1 do ns < na + W / avail( nl_buff ) / sbuff [ns] = null 

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CIS 725

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  1. CIS 725 Data Link Layer

  2. Flow Control • Producer-consumer problem • Sliding window protocol - Go Back N - Selective retransmission

  3. Go Back N na x x x x ns S:: ns = 1; na = 1 do ns < na + W /\ avail(nl_buff) /\ sbuff[ns] = null  sbuff[ns] = nl_buff; R!(sbuff[ns],ns); ns++ [] sbuff[ns] != null  R ! (sbuff[ns],ns); ns++ [] R ? ack(s)  if s > na then na = s [] timeout  ns = na od

  4. R:: nr = 1 do S ? (D,x)  if nr = x then deliver(D); nr++ S ! Ack(nr) od

  5. Acks sent but not yet received nr na ns ns <= na + W outstanding data packets ns na 1 1 nr W = 3 d1 2 1 1 3 1 d2 2 d3 4 1 3 a2 a3 4 2 5 2 d4 d5

  6. Cumulative Acks ns na 1 1 nr W = 3 d1 2 1 1 d2 3 1 2 d3 4 1 3 a2 a3 4 1 5 1 d4 d5

  7. d,1 d,1 d,2 d,2 d,3 a,2 d,3 start timer start timer a,2 a,2 a,3 start timer d,4 timeout d,1 start timer d,5 d,2 - Alternative: Timeout per message

  8. Go Back N S:: ns = 1; na = 1 do ns < na + W /\ avail(nl_buff) /\ sbuff[ns] = null  sbuff[n] = nl_buff; R!(sbuff[ns],ns); ns++; if ns = na + W then start timer [] sbuff[ns] != null  R ! (sbuff[ns],ns); ns++ [] R ? ack(s)  if s > na then na = s; cancel timer [] timeout  ns = na od

  9. d,1 • A ------------------B • Bandwidth =108 bytes/sec • RTD = 2 msecs • 2msecs ---- 2* 105 bytes • 1 packet = 1000 bytes • W = 200 • W will also depend on • buffer space at the receiver d,2 a,2 Timeout interval = 2msecs d,W

  10. Selective Retransmission • Retransmit only those packets the lost packets

  11. Selective Retransmission S:: ns = 1; na = 1 do ns < na + W /\ avail(nl_buff) /\ sbuff[ns] = null  sbuff[n] = nl_buff; R!(sbuff[ns],ns); ns++ [] R ? nack(s, bitmap)  na = s; for (0 <= i < W) if bitmap[i] then R! (sbuff[s+i], s+i); [] timeout  R ! (sbuf[na], na) od

  12. R:: nr = 1 do S ? (D,x)  rbuff[x] = D; recd[x] = true; if x != nr then S !nack(nr, bitmap) [] recd[nr]  deliver(rbuff[nr]); nr++ od

  13. R:: nr = 1 do S ? (D,x)  rbuff[x] = D; recd[x] = true; if x != nr then S !nack(nr, bitmap) [] recd[nr]  deliver(rbuff[nr]); nr++ od

  14. Receiver timeout • R:: nr = 1 do S ? (D,x)  rbuff[x] = D; recd[x] = true; [] timeout  S !nack(nr, bitmap) [] recd[nr]  deliver(rbuff[nr]); nr++ od

  15. Receiver based Windows • S:: ns = 1; na = 1 do : : [] R ? Ack(s,x)  W = x; if s > na then na = s od

  16. R:: nr = 1 do S ? (D,x)  if nr = x then deliver(D); nr++; compute W; S ! ack(nr, W); od

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