ICMP

Internet Control Message Protocol

IP relies on several protocols to performs control and routing functions:

- Control functions (ICMP)

- Multicast Signalling (IGMP)

- Setup routing tables (RIP, OSPF, BGP, PIM...)

ICMP is a helper protocol, that supports IP with facilities for:

==Error Reporting

==Simple Queries

ICMP messages are encapsulated as IP datagrams.

----------------------------------------

|  IP Header  |  ICMP payload message  |

----------------------------------------

4-byte header:

*Type (1 byte).

*Code (1 byte).

*Checksum (2 bytes).

If there is no additional data, 4-bytes are set as zero.

So Each ICMP is at least 8 bytes long.

ICMP query:

Request sent by host to a router or host.

Reply sent back to querying host.

Type/Code: 8/0: Echo Request

Type/Code: 0/0: Echo Reply

The ping command uses Echo Request / Echo Reply.

Ping's are handled directly by the kernel.

Each Ping is translated into an ICMP Echo Request.

The Ping'ed host responds with an ICMP Echo Reply.