Routing TCP/IP, Volume I (CCIE Professional Development)

ICMP

The Internet Control Message Protocol, or ICMP, described in RFC 792, specifies a variety of messages whose common purpose is to manage the internetwork. ICMP messages may be classified as either error messages or queries and responses. Figure 2.28 shows the general ICMP packet format. The packets are identified by type; many of the packet types have more specific types, and these are identified by the code field. Table 2.5 lists the various ICMP packet types and their codes, as described in RFC 1700.

Figure 2.28. The ICMP packet header includes a type field, a code field that further identifies some types, and a checksum. The rest of the fields depend on the type and code.

Table 2.5. ICMP packet types and code fields.

Type ...