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Error Control in TCP

Error Control in TCP. Jean Walrand U.C. Berkeley www.eecs.berkeley.edu/~wlr. Outline. ARQ versus FEC Link vs. End-to-end ARQ TCP’s version of Go Back N. # | data | ED. Seq # | ACK | ED. data | EC. Network. ARQ versus FEC. FEC: Forward Error Correction

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Error Control in TCP

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  1. Error Control in TCP Jean Walrand U.C. Berkeley www.eecs.berkeley.edu/~wlr

  2. Outline • ARQ versus FEC • Link vs. End-to-end ARQ • TCP’s version of Go Back N

  3. # | data | ED Seq # | ACK | ED data | EC Network ARQ versus FEC • FEC: Forward Error Correction • ARQ: Automatic Repeat Request Network

  4. ARQ FEC ARQ versus FEC (c’d) • Comparison: Longer delays More extra bits

  5. ARQ versus FEC (c’d) • Tradeoff: Efficiency vs. Delay • FEC requires more extra bits than ARQ • FEC has smaller delays than ARQ • Example • PER = 1%, RTT = 40ms, R = 100Mbps • FEC = 10% extra bits • FEC = about 90% efficient, delay = 20ms • ARQ = 2% extra bits (e.g.) • ARQ = about 99% efficient, delay = 20ms (99%) or 60ms (1%)

  6. Link ARQ vs. End-to-End ARQ • Question: Should every link perform ARQ? • Link ARQ:

  7. Link ARQ vs. End-to-End ARQ (c’d) • End-to-End ARQ:

  8. Link ARQ vs. End-to-End ARQ (c’d) • Two Extreme Cases. Case (1): Noisy Links Link ARQ is faster than E-2-E ARQ when there are many errors

  9. Link ARQ vs. End-to-End ARQ (c’d) • Case (2): Reliable Links Link ARQ is slower than E-2-E ARQ when there are few errors

  10. TCP’s version of Go Back N • End-to-End ARQ • Uses Go Back N; N adjusted to control congestion • Numbering scheme:ACK number = seq. number of next byte expected in correct order

  11. data # 2000 | LEN = 200 | P1 P1 ack # 2200 # 2200 | LEN = 300 | P2 P2 # 2500 # 2500 | LEN = 100 | P3 # 2600 | LEN = 100 | P4 # 2500 # 2500 | LEN = 100 | P3 # 2700 P3 P4 TCP’s version of Go Back N (c’d) • Example Store P4

  12. TCP’s version of Go Back N (c’d) • Buffers: Last Sent Last ACK SENDER RECEIVER  MAX

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