A checksum is a compact test over packet bytes. This lesson separates detection from correction before the arithmetic starts.
highlighted = computed this step
Why a checksum exists
A checksum is a small value carried with bytes so a receiver can detect many changes. It detects corruption; it does not correct the packet.
word bits=16
Start with exact words
The first checksum word here is 0x4500. The later sum uses exact 16-bit words like this one.
0x4500
What it can and cannot catch
A checksum catches every single-bit error and many multi-bit errors. It is not a proof that the packet was never changed, because compensating changes can cancel.
example word=0x4500
Summary
A checksum is error detection over exact words, not error correction. Error detection over the exact bytes; timing/throughput is not modeled here.