1. Lecture 7: Reliable Packet Transmission

2. 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) HeaderBody 816 8 CRC Beginning sequence Ending sequence

3. CRC strategy Sender and Receiver know the “divisor polynomial” –e.g. C(x)=x 3 +x 2 +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  10011010 XYZ 1101 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=10011010  xor) 101

7. C(x) Examples CRC-8 x 8 +x 2 +x 1 +1=100000111 CRC-1011000110011 CRC-12110000000101 CRC-16 11000000000000101 CRC-CCITT10001000000100001 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