The Checksum

After Header Verifies

The after header verifies when its recomputed checksum is included.

highlighted = computed this step

Verification includes the checksum

To verify the after header, the receiver sums every header word including checksum 0xb2e6.

valid fold=0xffff\text{valid fold}=0xffff
Verify after headerThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff

The fold is all ones

The full after header folds to 0xffff. That is the checksum verification identity.

folded=0xffff\text{folded}=0xffff
Verify after headerThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff

Before and after checksums

The before checksum was 0xb1e6; the after checksum is 0xb2e6.

0xb1e6→0xb2e60xb1e6\to0xb2e6
Verify after headerThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff

Summary

The after header is valid because its recomputed checksum makes the full header fold to all ones.

valid fold=0xffff\text{valid fold}=0xffff
Verify after headerThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff