The Checksum

Echo Honesty Boundary

The toy echo pair checks bytes; it does not model timing or network behavior.

highlighted = computed this step

What this tiny echo shows

This book checks one request and one reply message. The checksum is exact, but it is not a measurement.

messages=2\text{messages}=2
Echo checksum boundaryVerify mode sums the whole received ICMP message, including checksum.Request verify - verifyrowindexvalueword00x0800word10xe5caword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffffReply verify - verifyrowindexvalueword00x0000word10xedcaword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffff

The pair still matches

The identifier 0x1234 and sequence 1 match the reply. The checksums are 0xe5ca and 0xedca.

id=0x1234,seq=1\text{id}=0x1234,\quad \text{seq}=1
Echo checksum boundaryVerify mode sums the whole received ICMP message, including checksum.Request verify - verifyrowindexvalueword00x0800word10xe5caword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffffReply verify - verifyrowindexvalueword00x0000word10xedcaword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffff

What this does not model

NOTE: toy ICMP echo message only; not latency measurement, not routing, not firewall behavior, and not OS ping implementation.

toy message only=1\text{toy message only}=1
Echo checksum boundaryVerify mode sums the whole received ICMP message, including checksum.Request verify - verifyrowindexvalueword00x0800word10xe5caword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffffReply verify - verifyrowindexvalueword00x0000word10xedcaword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffff

Summary

ICMP echo checks this tiny message with the same one's-complement checksum rule, but this static book makes no network-behavior claim.

valid fold=0xffff\text{valid fold}=0xffff
Echo checksum boundaryVerify mode sums the whole received ICMP message, including checksum.Request verify - verifyrowindexvalueword00x0800word10xe5caword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffffReply verify - verifyrowindexvalueword00x0000word10xedcaword20x1234word30x0001raw sum0x0000fffffolded0xffffresult0xffff