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

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

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

4. UCB ARQ versus FEC (c’d) Comparison: More extra bitsLonger delays ARQFEC

5. UCB 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. UCB Link ARQ vs. End-to-End ARQ Question: Should every link perform ARQ? Link ARQ:

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

8. UCB 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. UCB 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. UCB 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. UCB TCP’s version of Go Back N (c’d) Example # 2000 | LEN = 200 | P1 data # 2200 ack # 2200 | LEN = 300 | P2# 2500# 2500 | LEN = 100 | P3# 2600 | LEN = 100 | P4# 2500# 2500 | LEN = 100 | P3# 2700P1P2P3P4 Store P4

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