Ftp-WG: Updating the checksum

Hi All,
It's written in RFC1071 that updating the Internet Header Checksum after one 
field (m), had changed, can be carried out this way:
C' = C + (-m) + m' = C + (m' - m)
where C' is the new checksum, C is the old one, m is the old value of the 
field and m' is the new value of it.
However I used the following values as an example to apply the equation and 
found out that something is wrong.
asume that we are calculating the 16 bit checksum for 2 fields m1,m2.

Original values:
m1: 1234
m2: 5678
C, from the definition equation:  9753

After updating m1
m1':1235
m2 :the same
C' from the update equation: 9754
C', from the definition equation: 9752
may be I've some troubles in understanding the application of 1's complement 
summation and subtraction!
Can anyone help please?







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