ICMP — Message Format and Message Types

ICMP (Internet Control Message Protocol) is a network-layer companion protocol to IP, used for error reporting and query/diagnostic functions. ICMP messages are encapsulated inside IP datagrams (protocol field = 1), so they are delivered like ordinary packets — but ICMP never carries user data.

1. ICMP Message Format

Every ICMP message begins with a common 4-byte header, followed by type-dependent data:

+--------+--------+-----------------+
| Type | Code | Checksum |
+--------+--------+-----------------+
| Rest of Header (4 bytes) |
| (depends on Type/Code) |
+-----------------------------------+
| Data (variable length) |
| (first 8 bytes = IP header + |
| first 8 bytes of the data that |
| caused the error) |
+-----------------------------------+

• Type (1 byte): identifies the general message type (e.g., 3 = Destination Unreachable, 8 = Echo Request).
• Code (1 byte): refines the Type (e.g., for Type 3: 0 = network unreachable, 3 = port unreachable).
• Checksum (2 bytes): 16-bit one's-complement checksum over the whole ICMP message (calculated exactly like the IP header checksum).
• Rest of Header (4 bytes): meaning depends on type — e.g., unused/zero, pointer for Type 12, gateway address for Type 5, identifier + sequence number for Echo.
• Data: for error messages, ICMP embeds the IP header plus the first 8 bytes of the payload of the original datagram that caused the error, so the source can identify the offending process/port (since the first 8 bytes of TCP/UDP header contain source/destination ports).

2. Error Messages

Type	Code	Message	Meaning
3	0	Destination Unreachable — network	Router cannot find the network route
3	1	Destination Unreachable — host	Network reachable but host not found
3	2	Protocol unreachable	Destination host does not support the upper-layer protocol
3	3	Port unreachable	No process listening at the destination port (used by UDP)
3	4	Fragmentation needed & DF set	Router must fragment but Don't-Fragment bit is 1
4	0	Source Quench	Congestion — sender should slow down (deprecated)
5	0/1	Redirect (network/host)	Router tells the source a better first-hop route
11	0	Time Exceeded — TTL expired	Hop count exceeded (used by traceroute)
11	1	Time Exceeded — reassembly timeout	Fragments not all arrived within the reassembly window
12	0	Parameter Problem	Error in the IP header; pointer field gives offending byte

Rules: ICMP errors are never sent for ICMP error messages (prevents infinite loops), not for fragmented datagrams (except Type 3 Code 4), and never for multicast/broadcast destination addresses.

3. Query Messages (request–reply pairs)

Type (Req/Rep)	Message	Purpose
8 / 0	Echo Request / Echo Reply	ping — check host reachability and round-trip time
13 / 14	Timestamp Request / Reply	Measure RTT and synchronise clocks (carry originate, receive, transmit times)
17 / 18	Address Mask Request / Reply	A host obtains its subnet mask from a router
9 / 10	Router Advertisement / Solicitation	Host discovers active routers on its local network

4. Summary

• Format: Type–Code–Checksum (4 bytes) + Rest-of-Header + Data; checksum covers the entire ICMP message.
• Error messages (Types 3, 4, 5, 11, 12) report failures back to the source — they are one-way and always include part of the offending packet.
• Query messages (Echo, Timestamp, Mask, Router discovery) are request–reply pairs used for diagnostics and configuration.
• ICMP does not correct errors or retransmit data — it only informs; recovery is left to TCP or the application.