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Lecture 7: Reliable Packet Transmission

Lecture 7: Reliable Packet Transmission. 8. 16. 16. 8. Beginning. Ending. Header. Body. CRC. sequence. sequence. Cyclic Redundancy Check. Add k bits of extra data (the CRC field) to an n -bit message to provide error detection function For efficiency, want k << n

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Lecture 7: Reliable Packet Transmission

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  1. Lecture 7: Reliable Packet Transmission

  2. 8 16 16 8 Beginning Ending Header Body CRC sequence sequence Cyclic Redundancy Check • Add k bits of extra data (the CRC field) to an n-bit message to provide error detection function • For efficiency, want k << n • e.g., k = 32 for Ethernet and n = 12,000 (1500 bytes)

  3. CRC strategy • Sender and Receiver know the “divisor polynomial” • e.g. C(x)=x3+x2+1=1101 • Send Message÷ CRC with CRC chosen so that the whole thing is evenly divisible by C(x) • Receiver calculates the remainder of Message÷ CRC/C(x) There’s an error if it is not zero

  4. CRC example • Message=10011010 C=1101(3 rd order) • Message÷ CRC=10011010XYZ • Choose XYZ so that remainder is zero for Message÷ CRC/C

  5. CRC Example cont. • 11111001 1101÷10011010XYZ1101 1001 1101 1000 1101 X=1 1011 Y=0 1101 Z=1 1100 1101 1xyz 1101 • Message÷ CRC=10011010÷ 101

  6. CRC Example cont. • 11111001 1101÷10011010000 1101 1001 1101 1000 1101 1011 1101 1100 1101 1000 1101 101 • Message÷ CRC=10011010000 - (xor) 101

  7. C(x) Examples • CRC-8 x8+x2+x1+1=100000111 • CRC-10 11000110011 • CRC-12 110000000101 • CRC-16 11000000000000101 • CRC-CCITT 10001000000100001 • CRC-32 1000001001100000010001110110110111

  8. 1’s complement addition • 0=0000 -0=1111 • 1=0001 -1=1110 • 2=0010 -2=1101 • 2+3=0010+0011=0101=5 • 2+(-0)=0010+1111+0001=0010=2 • 3+(-2)=0011+1101+0001=0001

  9. Internet Checksum Strategy • 0 1010 0001 1001 1100 =0000A19C • 0 0110 1010 1111 0010 =00006AF2 • 1 0000 1010 1000 1110 =00010A8E • =>00000A8E • =>0000 1010 1000 1111=00000A8F • =>0A8F • =>F670

  10. Internet Checksum Algorithm • View message as a sequence of 16-bit integers; sum using 16-bit ones-complement arithmetic; take ones-complement of the result. u_short cksum(u_short *buf, int count) { register u_long sum = 0; while (count--) { sum += *buf++; if (sum & 0xFFFF0000) { /* carry occurred, so wrap around */ sum &= 0xFFFF; sum++; } } return ~(sum & 0xFFFF); }

  11. Reliable Transmission • Problem: • To detect packet loss • To retransmit lost packets • Delete duplicates. • Assure proper order

  12. Stop and Wait Sequence numbers: 0,1,0,1 etc. Resend if no Ack before timeout + guarantees arrival of all frames in order -lots of downtime, particularly for long RTTs

  13. Frame sequencing for Stop-and-Wait protocol

  14. Time line for sliding window algorithm

  15. Sliding Window Algorithm-sender • LAR=last acknowledgement received • LFS=last frame sent • SWS=send window size • LFS-LAR<SWS

  16. Sliding Window Algorithm-receiver • NFE=next frame expected • LFA=last frame accepted • RWS=receive window size • LFA-NFE<RWS

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