ICMPv6
Internet Control Message Protocol for IPv6 (ICMPv6) is a basic IPv6 protocol that uses error or informational messages to report errors and information generated during packet processing. Figure 11-5 shows the ICMPv6 message format.
Each field in an ICMPv6 message is described as follows:
Type field: message type. Values from 0 to 127 indicate an error message, and values from 128 to 255 indicate an informational message.
Code field: specific message type.
Checksum field: checksum of an ICMPv6 message.
ICMPv6 Error Message Classification
ICMPv6 error messages are classified as follows:
Destination Unreachable message
When an IPv6 node forwards IPv6 packets and finds that the destination address of the packets is unreachable, it sends an ICMPv6 Destination Unreachable message to the source node of the packets. The message carries specific causes for the error. Destination Unreachable messages are classified into the following types:
No Route to Destination
Address Unreachable
Port Unreachable
Datagram Too Big message
When an IPv6 node forwards IPv6 packets and finds that the size of the packets exceeds the outbound interface path MTU, it sends an ICMPv6 Datagram Too Big message to the source node of the packets. The message carries the outbound interface path MTU. Path MTU discovery is implemented based on Datagram Too Big messages.
Time Exceeded message
During IPv6 packet transmission, when a router receives a packet with the hop limit 0 or reduces the hop limit to 0, it sends an ICMPv6 Time Exceeded message to the source node of the packets. During the processing of a packet to be fragmented and reassembled, an ICMPv6 Time Exceeded message is also generated when the reassembly time is longer than the specified period.
Parameter Problem message
When a destination node receives an IPv6 packet, it checks its validity. If it detects any of the following errors, it sends an ICMPv6 Parameter Problem message to the source node of the packet:
A field in the IPv6 basic header or extension header is incorrect.
The NextHeader in the IPv6 basic header or extension header cannot be identified.
Unknown options exist in the extension headers.
ICMPv6 Informational Message Classification
ICMPv6 informational messages are classified as Echo Request or Echo Reply messages. ICMPv6 messages can be used for network fault diagnosis, path MTU discovery, and neighbor discovery. During interworking detection between two nodes, after a node receives an Echo Request message, it sends an Echo Reply message to the source node. Packets are subsequently transmitted between the two nodes.
