Handling ICMP messages
ICMP messages can be classified into the following types:
Common ICMP messages—Without any extension information.
Extended ICMP messages with a length field—Carry extension information and a length field. The length field indicates the length of the original datagram that is encapsulated within the ICMP header and excludes the ICMP extension length. Such an ICMP message complies with RFC 4884.
Extended ICMP messages without a length field—Carry extension information but does not contain a length field. Such an ICMP message does not comply with RFC 4884.
ICMP/ICMPv6 messages that can carry extension information include only IPv4 redirect messages, IPv4/IPv6 time exceeded messages, and IPv4/IPv6 destination unreachable messages.
Based on how ICMP messages are handled, the device can operate in one of these modes: common mode, compliant mode, and non-compliant mode. Table 5 shows how ICMP messages are handled in different operating modes.
Table 5: Handling ICMP messages
Device mode | ICMP messages sent | ICMP messages received | Remarks |
|---|---|---|---|
Common mode | Common ICMP messages | Common ICMP messages | Extension information in extended ICMP messages will not be processed. |
Compliant mode | Common ICMP messages Extended ICMP messages with a length field | Common ICMP messages Extended ICMP messages with a length field | Extended ICMP messages without a length field are handled as common ICMP messages. |
Non-compliant mode | Common ICMP messages Extended ICMP messages without a length field | All three types of ICMP messages | N/A |