ICMP - Internet control message protocol

ICMP basically provides a way to report the error messages which are encountered in network to the original source.

Do you know the protocol used in ping?

Well, the answer is ICMP. It is implemented using ICMP echo request and ICMP echo response. If a system receives an ICMP echo request, it will generate an ICMP echo response and sends it to the original source.

ICMP echo request is nothing but PING.

ICMP echo request packet

0                                                      7 8                                                      15  16                                                  31

TYPE

CODE

CHECKSUM

IDENTIFIER

SEQUENCE NUMBER

OPTIONAL DATA

ICMP echo reply packet

0                                                      7 8                                                      15  16                                                  31

TYPE

CODE

CHECKSUM

This is not used( So we can make bits set to zero)

OPTIONAL DATA

TYPE Field                                          ICMP Message Type

0                                                             Echo Reply

8                                                             Echo Response

CODE Value                                        Meaning

0                                                              Network Unreachable

CHECKSUM

 ICMP message header checksum (additive algorithm is used to calculate checksum) 

IDENTIFIER and SEQUENCE NUMBER

These fields are used to match echo requests to corresponding echo replies. 

DATA

 It is variable length; the data which is to be returned to sender will be present here.