Identifier

Network 2 octets ICMP echo message unverified

Matches replies to requests. Echo messages only.

Reading a capture

Typically the sending process id, which is how several pings running at once on one host tell their replies apart. Doing the job a port number would do, in a protocol that has no ports.

Where it sits

ICMP Message > Identifier

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

Accuracy

Clause: RFC 792
Byte order: big-endian — network byte order
Verified: Not verified against the standard. Treat this as a claim, not a fact.