Internet Core Protocols: The Definitive Guide

Name

Message Type

Synopsis

The Type field is used to specify the exact ICMP error message being sent.

Size

Eight bits.

Notes

ICMP essentially consists of a predefined dictionary of messages, allowing messages to be exchanged using numeric identifiers. The Type field is used to specify the major class of the message, while the Code field is used to specify the minor class, though some message Types do not have any subtype Codes.

Table 5.3 lists all of the ICMP error message Types used with IPv4, and their meanings.

Table 5.3. ICMP Error Message Types

Type	Message Description	Usage Notes
3	Destination Unreachable	The network, host, or port number specified by the original datagram is unreachable. This could be related to a variety of problems.
4	Source Quench	Either the destination system or an intermediary device is receiving more data than it can process, and is asking the sender to reduce the rate of transfer.
5	Redirect	This message will occur if a device tries to send a datagram to a destination through a router, but the router knows of a shorter path to the destination.
11	Time Exceeded	Either the datagram’s Time-to-Live timer expired, or some fragments were not received in time to be reassembled by the destination system.
12	Parameter Problem	Another problem occurred. This error is almost always related to problems with IP or TCP Options.

Capture Sample

In the capture shown in Figure 5.11, the Type field is set to 3, which is used for all Destination Unreachable errors.

Figure 5.11. The Message Type field ...