ICMP Message Types

ICMP (Internet Control Message Protocol) is a network-layer (IP) companion protocol used to report errors and send operational/query messages (e.g., ping) between hosts and routers. ICMP messages are carried inside IP datagrams; the ICMP header starts with 1 byte Type, 1 byte Code, 2 bytes Checksum, followed by type-dependent data.

1. Error Messages (destination-unreachable / informational)

Type	Message	Typical Code values & meaning
3	Destination Unreachable	0 network, 1 host, 2 protocol, 3 port unreachable; 4 fragmentation needed & DF set
4	Source Quench	Congestion — sender should slow down (obsolete, deprecated)
5	Redirect	0 redirect for network, 1 for host, 2 for TOS+network, 3 TOS+host — used by a router to tell the source a better first hop
11	Time Exceeded	0 TTL expired in transit (traceroute), 1 fragment reassembly time exceeded
12	Parameter Problem	Error in the IP header field (pointer gives the offending byte)
3, 4, 5 (Type 3/9/10/13/14)	Fragmentation / Filtering problems	4 fragmentation needed, 9/10 admin prohibited (firewall filtering)

Error messages are never generated for ICMP error messages themselves (to avoid infinite loops). The IP header + 8 bytes of the original datagram that caused the error are included so the source can identify the offending packet.

2. Query Messages (informational / request–reply pairs)

Type	Message	Purpose
0 / 8	Echo Reply / Echo Request	ping — test reachability and RTT of a host
13 / 14	Timestamp Request / Reply	Measure round-trip time and synchronize clocks
17 / 18	Address Mask Request / Reply	Host asks subnet mask from a router
9 / 10	Router Advertisement / Solicitation	A host discovers available routers on a link

Summary: Error messages report problems (types 3, 4, 5, 11, 12), while query messages are request–reply pairs used for diagnostics (echo, timestamp, mask, router discovery). ICMP handles only error/control reporting, never delivery of user data.