ICMP type 12, Parameter problem

Protocol suite:	TCP/IP.
Protocol type:	Transport layer control protocol.
Base protocol:	ICMP, Internet Control Message Protocol.
Related ICMP message:
Host implementation:	Mandatory.
Router implementation:	Mandatory.
Links:	IANA: ICMP parameters.

The ICMP Parameter problem message is generated as a response for any error not specifically covered by another ICMP message.

The IP header plus the first 8 bytes of the original datagram's data is returned to the sender. This data is used by the host to match the message to the appropriate process. If a higher level protocol uses port numbers, they are assumed to be in the first 64 data bits of the original datagram's data.

This message is not generated in response to a datagram destined for a multicast address.

RFC 792, page 9:

If the gateway or host processing a datagram finds a problem with the header parameters such that it cannot complete processing the datagram it must discard the datagram. One potential source of such a problem is with incorrect arguments in an option. The gateway or host may also notify the source host via the parameter problem message. This message is only sent if the error caused the datagram to be discarded.

The pointer identifies the octet of the original datagram's header where the error was detected (it may be in the middle of an option). For example, 1 indicates something is wrong with the Type of Service, and (if there are options present) 20 indicates something is wrong with the type code of the first option.

Code 0 may be received from a gateway or a host.

Host Implementation:

RFC 1122, page 42:

A host SHOULD generate Parameter Problem messages. An incoming Parameter Problem message MUST be passed to the transport layer, and it MAY be reported to the user.

The ICMP Parameter Problem message is sent to the source host for any problem not specifically covered by another ICMP message. Receipt of a Parameter Problem message generally indicates some local or remote implementation error.

Router Implementation:

RFC 1812, page 58:

A router MUST generate a Parameter Problem message for any error not specifically covered by another ICMP message. The IP header field or IP option including the byte indicated by the pointer field MUST be included unchanged in the IP header returned with this ICMP message.

RFC 2003, pages 6 and 8:

After an encapsulated datagram has been sent, the encapsulator may receive an ICMP message from any intermediate router within the tunnel other than the tunnel exit point. The action taken by the encapsulator depends on the type of ICMP message received. When the received message contains enough information, the encapsulator MAY use the incoming message to create a similar ICMP message, to be sent to the originator of the original unencapsulated IP datagram (the original sender). This process will be referred to as "relaying" the ICMP message from the tunnel.

If the Parameter Problem message points to a field copied from the original unencapsulated datagram, the encapsulator MAY relay the ICMP message to the sender of the original unencapsulated datagram; otherwise, if the problem occurs with an IP option inserted by the encapsulator, then the encapsulator MUST NOT relay the ICMP message to the original sender. Note that an encapsulator following prevalent current practice will never insert any IP options into the encapsulated datagram, except possibly for security reasons.

MAC header

IP header

ICMP header

Data :::

ICMP type 12, Parameter problem message:

00	08	16
Type	Code	ICMP header checksum
Pointer	unused
IP header + the first 8 bytes of the original datagram's data.

Type. 8 bits.
Set to 12.

Code. 8 bits.
Specifies the reason for the error.

Code	Description
0	Invalid IP header.
1	A required option is missing.

ICMP Header Checksum. 16 bits.
The 16-bit one's complement of the one's complement sum of the ICMP message, starting with the ICMP Type field. When the checksum is computed, the checksum field should first be cleared to 0. When the data packet is transmitted, the checksum is computed and inserted into this field. When the data packet is received, the checksum is again computed and verified against the checksum field. If the two checksums do not match then an error has occurred.

Pointer. 8 bits.
In the case of an invalid IP header (Code 0), this field indicates the byte offset of the error in the header.

unused. 32 bits.
Cleared to 0.

Internet Header + 64 bits of Original Data Datagram.
The internet header plus the first 64 bits of the original datagram's data. This data is used by the host to match the message to the appropriate process. If a higher level protocol uses port numbers, they are assumed to be in the first 64 data bits of the original datagram's data.

Glossary:

RFCs:

[RFC 792] INTERNET CONTROL MESSAGE PROTOCOL.