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ICMP messages are delivered in IP packets, and includes a Message Type that specifies the format
of the ICMP message and a Code that is used to further qualify the message. For example, the
message type Destination Unreachable uses the Code parameter to specify the exact reason for the
error.
Either all ICMP message types can be accepted by a service (there are 256 possible types) or it is
possible to filter the types.
Specifying Codes
If a type is selected then the codes for that type can be specified in the same way that port numbers
are specified. For example, if the Destination Unreachable type is selected with the comma
deliminated code list 0,1,2,3 then this will filter Network unreachable, Host unreachable, Protocol
unreachable and Port unreachable.
When a message type is selected but no code values are given then all codes for that type is
assumed.
ICMP Message Types
The message types that can be selected are as follows:
Echo Request
Sent by PING to a destination in order to check connectivity.
Destination Unreachable
The source is told that a problem has occurred when delivering
a packet. There are codes from 0 to 5 for this type:
• Code 0: Net Unreachable
• Code 1: Host Unreachable
• Code 2: Protocol Unreachable
• Code 3: Port Unreachable
• Code 4: Cannot Fragment
• Code 5: Source Route Failed
Redirect
The source is told that there is a better route for a particular
packet. Codes assigned are as follows:
• Code 0: Redirect datagrams for the network
• Code 1: Redirect datagrams for the host
• Code 2: Redirect datagrams for the Type of Service and the
network
• Code 3: Redirect datagrams for the Type of Service and the
host
Parameter Problem
Identifies an incorrect parameter on the datagram.
Echo Reply
The reply from the destination which is sent as a result of the
Echo Request.
Source Quenching
The source is sending data too fast for the receiver, the buffer
has filled up.
Time Exceeded
The packet has been discarded as it has taken too long to be
delivered.
3.2.4. Custom IP Protocol Services Chapter 3. Fundamentals
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