Explanation of ICMP message types
- Echo Request,
Echo Reply
- Used to test destination accessibility and status. A host sends an
Echo Request and listens for a corresponding
Echo Reply. This is most commonly done using
the
ping command.
- Destination Unreachable,
Echo Reply
- Sent by a router when it cannot deliver an IP datagram. A datagram
is the unit of data, or packet, transmitted in a TCP/IP network.
- Source Quench
- Sent by a host or router if it is receiving data too quickly for it
to handle. The message is a request that the source reduce its rate of datagram
transmission.
- Redirect Message
- Sent by a router if it receives a datagram that should have been
sent to a different router. The message contains the address to which the
source should direct future datagrams. This is used to optimize the routing of
network traffic.
- Router Advertisement,
Router Solicitation
- Allow hosts to discover the existence of routers. Routers periodically broadcast their IP
addresses via Router Advertisement messages. Hosts may
also request a router address by broadcasting a Router
Solicitation message to which a router replies with a
Router Advertisement.
- Time Exceeded
- Sent by a router if the datagram has reached the maximum limit of
routers through which it can travel.
- Parameter Problem
- Sent by a router if a problem occurs during the transmission of a
datagram such that it cannot complete processing. One potential source of such
a problem is invalid datagram header.
- Timestamp Request,
Timestamp Reply
- Used to synchronize the clocks between hosts and to estimate
transit time.
- Information Request,
Information Reply
- Obsolete. These messages were used earlier by hosts to determine
their inter-network addresses, but are now considered outdated and should not
be used.
- Address Mask Request,
Address Mask Reply
- Used to find the mask of the subnet (i.e. what address bits define
the network). A host sends an
Address Mask Request to a router and receives
an
Address Mask Reply in return.