ICMP Message ICMP

An ICMP echo request or reply — the thing ping sends.

Reading a capture

The cheapest useful frame on a network, which is why it is the first thing anyone reaches for when something is wrong. It is also why an ICMP echo over a WLAN is a good airtime demonstration: almost none of the transmission is payload.

Subfields

• Type — What kind of message this is.
• Code — Narrows the type. Zero for echo; for Destination Unreachable it says whether the network, host, protocol or port was the problem.
• Checksum — Ones-complement checksum over the whole ICMP message, header and data.
• Identifier — Matches replies to requests. Echo messages only.
• Sequence Number — Increments per request. Echo messages only.

Where it sits

ICMP Message

• Open in the explorer — ICMP echo over IPv4, in a QoS Data frame

Accuracy

Clause

RFC 792

Verified

Not verified against the standard. Treat this as a claim, not a fact.