The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Concepts and General Operation

ICMP General Operation 1 2 3 ICMP Message Creation and Processing Conventions and Rules

ICMP messages are used to allow the communication of different types of information between IP devices on an internetwork. The messages themselves are used for a wide variety of purposes, and are organized into general categories, as well as numerous specific types and subtypes.

At the highest level, ICMP messages are divided into two classes:

• Error Messages: These messages are used to provide feedback to a source device about an error that has occurred. They are usually generated specifically in response to some sort of action, usually the transmission of a datagram, as shown in the example of Figure 137. Errors are usually related to the structure or content of a datagram, or to problem situations on the internetwork encountered during datagram routing.
  
  
• Informational (or Query) Messages: These are messages that are used to let devices exchange information, implement certain IP-related features, and perform testing. They do not indicate errors and are typically not sent in response to a regular datagram transmission. They are generated either when directed by an application, or on a regular basis to provide information to other devices. An informational ICMP message may also be sent in reply to another informational ICMP message, since they often occur in request/reply or solicitation/advertisement functional pairs.

ICMP General Operation 1 2 3 ICMP Message Creation and Processing Conventions and Rules