1. Checksum Calculations
Instructor: Anirban Mahanti
Office: ICT 745
Class Location: ICT 121
Lectures: MWF 12:00 – 12:50
Notes derived from “Computer Networking: A Top Down Approach Featuring the Internet”, 2005, 3rd edition, Jim Kurose, Keith Ross, Addison-Wesley.
Slides are adapted from the companion web site of the book, as modified by Anirban Mahanti (and Carey Williamson).
CPSC 441: Checksum

2. IP Checksum Calculation
At the sender
Set the value of the checksum field to 0.
Divide the header into 16-bit words
Add all segments using one’s complement arithmetic
The final result is complemented to obtain the checksum
At the receiver
Divide header into 16-bit words, adds them, and complement’s the results
All zero’s => accept datagram, else reject
CPSC 441: Checksum

3. Binary Checksum Example
001
0001 1
Carry from 16th column
Carry from 13th column
Carry from 12th column
Carry from 5th column
Carry from 4th column
Carry from 3rd column
Carry from 2nd column
Carry from 1st column
The UDP checksum technique provides weak error detection – why?
As an example, suppose that one bit is flipped from a 1 to 0 in a
16-byte segment, while another bit is flipped from a 0 to 1 in the next
16-byte segment (and that these errors align). Doesn’t detect error.
Why used? Easier to implement in software. Also, we hope that other stronger error correction techniques are used at lower layers.
CPSC 441: Checksum

4. UDP Checksum The pseudo-header Add pseudo-header
32-bit source IP address
32-bit destination IP address
The pseudo-header
0’s
protocol
length
Add pseudo-header
Fill checksum with 0’s
Divide into 16-bit words (adding padding if required)
Add words using 1’s complement arithmetic
Complement the result and put in checksum field
Drop pseudo-header and padding
Deliver UDP segment to IP
source port #
dest port #
length
checksum
data
(add padding to make
data a multiple of 16 bits)
32 bits
CPSC 441: Checksum

5. Checksum calculation at Receiver
Add pseudo-header to the UDP segment
Add padding, if needed
Divide into 16-bit words and add words using 1’s complement arithmetic
Complement result
If result is all 0’s
Drop pseudo-header and padding (if any)
Accept segment
Else
drop segment
CPSC 441: Checksum