ICMP type 16, Information reply

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

RFC 792, page 19:

This message may be sent with the source network in the IP header source and destination address fields zero (which means "this" network). The replying IP module should send the reply with the addresses fully specified. This message is a way for a host to find out the number of the network it is on.

The identifier and sequence number may be used by the echo sender to aid in matching the replies with the requests. For example, the identifier might be used like a port in TCP or UDP to identify a session, and the sequence number might be incremented on each request sent. The destination returns these same values in the reply.

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

RFC 1812, page 59:

The Information Request/Reply pair was intended to support self-configuring systems such as diskless workstations, to allow them to discover their IP network prefixes at boot time. However, these messages are now obsolete. The RARP and BOOTP protocols provide better mechanisms for a host to discover its own IP address.

MAC header

IP header

ICMP message 16

Data

Message format:

00	01	02	03	04	05	06	07	08	09	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31
Type								Code								ICMP header checksum
Identifier																Sequence number

Type. 8 bits. Set to 16.

Code. 8 bits. Always set to 0.

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 set 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.

Identifier. 16 bits.
If code is zero, an identifier is used to help match requests to the associated reply. May be cleared to zero.

Sequence number. 16 bits.
This value is used to help match requests to the associated reply. May be cleared to zero.

Glossary:

RFCs:

[IEN 212] IP - Local Area Network Addressing Issues.

[RFC 792] INTERNET CONTROL MESSAGE PROTOCOL.