Information Request or Information Reply Message

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |      Code     |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Identifier          |        Sequence Number        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Fields

Addresses

The address of the source in an information request message will be the destination of the information reply message. To form an information reply message, the source and destination addresses are simply reversed, the type code changed to 16, and the checksum recomputed.

ICMP Fields

Type

• 15 = information request message;
• 16 = information reply message.

Code

0

Checksum

The checksum is the 16-bit one's complement of the one's complement sum of the ICMP message starting with the ICMP Type. For computing the checksum, the checksum field should be zero. This checksum may be replaced in the future.

Identifier

If code = 0, an identifier to aid in matching request and replies, may be zero.

Sequence Number

If code = 0, a sequence number to aid in matching request and replies, may be zero.

Description

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.

Note: Information Request/Reply messages are obsolete. Modern networks typically use DHCP (RFC 2131) or other mechanisms to obtain network configuration information.