ICMP for IPv4 (RFC 792): error, diagnostic and echo messages.
Every ICMPv4 message opens with the same 4 bytes; the type field alone tells a receiver which of roughly a dozen message shapes follows.
Checksum shown as a placeholder here since a real one needs a full message body - see the other tabs for worked examples.
What every ping command actually sends. Identifier and sequence let a sender match replies to requests when several pings are in flight at once; the payload is arbitrary and usually just a byte-counting pattern.
A reply is byte-identical except type=0 and the checksum recomputed for that type change.
One type, sixteen meanings via the code field. Code 4 is the one with real teeth: "fragmentation needed but the Don't Fragment bit was set" - the message that drives IPv4 Path MTU Discovery, carrying the next hop's MTU in what would otherwise be unused bytes (RFC 1191).
A router telling the sender: your packet needed fragmenting, DF was set, so I dropped it - retry at 1480 bytes instead.
Sent when TTL hits zero in transit (code 0) or a fragment reassembly timer expires before all fragments arrive (code 1). Code 0 from every router along a path, one hop-count at a time, is literally what traceroute is built out of.
In 1981, this was IP's only congestion-control signal: a router sends Source Quench to tell a sender "slow down." Same byte shape as Time Exceeded - the interesting part is entirely what happened to it, not what's in it.