ICMP is an auxiliary protocol, which provides routing, diagnostic and error functionality for IP. It is described in [INTERNET:8]. A router MUST support ICMP.
ICMP messages are grouped in two classes that are discussed in the following sections:
ICMP error messages:
Destination Unreachable Section 4.3.3.1 Redirect Section 4.3.3.2 Source Quench Section 4.3.3.3 Time Exceeded Section 4.3.3.4 Parameter Problem Section 4.3.3.5 ICMP query messages: Echo Section 4.3.3.6 Information Section 4.3.3.7 Timestamp Section 4.3.3.8 Address Mask Section 4.3.3.9 Router Discovery Section 4.3.3.10General ICMP requirements and discussion are in the next section.