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.