Echo or Echo 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+-

IP Fields:

Addresses

  The address of the source in an echo message will be the
  destination of the echo reply message.  To form an echo reply
  message, the source and destination addresses are simply reversed,
  the type code changed to 0, and the checksum recomputed.

IP Fields:

Type

  8 for echo message;

  0 for echo reply message.

Code

  0

Checksum

  The checksum is the 16-bit ones'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.
  If the total length is odd, the received data is padded with one
  octet of zeros for computing the checksum.  This checksum may be
  replaced in the future.

Identifier

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

Sequence Number: