The Checksum

TTL Honesty Boundary

The toy shows one exact hop, not live routing behavior.

highlighted = computed this step

What this toy hop shows

This book shows one exact header before a hop and one exact header after a hop. It does not model the route choice.

hops modeled=1\text{hops modeled}=1
TTL honesty boundaryThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff

The checked values

TTL drops from 64 to 63, and checksum changes from 0xb1e6 to 0xb2e6.

64→6364\to63
TTL honesty boundaryThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff

What this does not model

NOTE: toy single IP version 4 header and one hop only; not traceroute timing, not routing policy, not fragmentation, not NAT or firewall behavior, and not real capture timing.

toy hop only=1\text{toy hop only}=1
TTL honesty boundaryThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff

Summary

TTL is decremented by the hop, and the header checksum is recomputed so the after header still verifies.

after checksum=0xb2e6\text{after checksum}=0xb2e6
TTL honesty boundaryThe full after header verifies when the checksum field is included.After header verify - verifyrowindexvalueword00x4500word10x003cword20x1c46word30x4000word40x3f06word50xb2e6word60xac10word70x0a63word80xac10word90x0a0craw sum0x0002fffdfolded0xffffresult0xffff