Lecture 7: Reliable Packet Transmission

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)

Header Body

8 16 16 8

CRCBeginningsequence

Endingsequence

CRC strategy

• Sender and Receiver know the “divisor polynomial”– e.g. C(x)=x3+x2+1=1101

• Send MessageCRC with CRC chosen so that the whole thing is evenly divisible by C(x)

• Receiver calculates the remainder of

MessageCRC/C(x) There’s an error if it is not zero

CRC example

• Message=10011010 C=1101(3 rd order)

• MessageCRC=10011010XYZ

• Choose XYZ so that remainder is zero for MessageCRC/C

CRC Example cont.

• 11111001 110110011010XYZ

1101 1001 1101

1000 1101

X=1 1011 Y=0 1101 Z=1 1100

1101 1xyz1101

• MessageCRC=10011010 101

CRC Example cont.• 11111001

110110011010000 1101 1001 1101 1000

1101 1011 1101 1100

1101 1000 1101 101

• MessageCRC=10011010xor) 101

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

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

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

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_shortcksum(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);}

Reliable Transmission

• Problem:

• To detect packet loss

• To retransmit lost packets

• Delete duplicates.

• Assure proper order

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

Frame sequencing for Stop-and-Wait protocol

Time line for sliding window algorithm

Sliding Window Algorithm-sender

• LAR=last acknowledgement received• LFS=last frame sent• SWS=send window size• LFS-LAR<SWS

Sliding Window Algorithm-receiver

• NFE=next frame expected• LFA=last frame accepted• RWS=receive window size• LFA-NFE<RWS