Internet Control Message Protocol

Purpose

• ICMP is mainly designed for a host/router to report the error status of an IP packet.

• IP is not a reliable transfer protocol. If an packet is damaged, it is discarded. In fact, if a packet is damaged, the receiving router or host cannot trust any field in the packet, including the address field.

Then why do we need the ICMP protocol?

• Source quench

The router runs out of buffer space When a router run out of buffer space, it sends back to the source an ICMP message indicating the situation. The sending host then goes into a congestion control phase.
 
• Time exceeded

Time to Live reaches 0 or the reassembly timer expires in the destination IP.
 
• Destination Unreachable

the destination host is down or the router has failed.
 
• Redirect

better route for the destination host or network
 
• Fragmentation required

the fragamentation is required but not allowed.
 
• Parameter Problem

• Echo Request/Reply
• Address Mask Request/Reply

IP uses ICMP to report errors or to make request information but ICMP messages are sent through IP, i.e., encapsulated in IP datagrams.

ICMP is created in response to a specific IP datagram or an ICMP request.
If an IP datagram carrying the ICMP error message causes an error, no ICMP message is created. (NO error about error)
 

Applications of ICMP

• ping

 Use ICMP Echo Request/Response to test whether a host is up. It retransmits the request at most twice.(*)

• traceroute

 Send an UDP message to a nonexistent port of the destination. Starting from one, increasing the Time To Live
 by one until a destination unreachable ICMP message comes back. All the Time Exceded ICMP received contains the addresses of the intermediated routers. To implement this program, transmission errors and retransmission must be taken cared of. Dynamic routes may not be appropriate.

• Path MTU discovery

By specifying the corresponding bit in Flag fiels disallows the fragamentation of an IP datagram. Sending a sequence of messages with decreasing size until one suceeds.

ICMP message format and the IP datagram carrying ICMP message

Source quench message

Type = 4, code = 0;

Type = 11

Code
0 = time to live exceeded in transit; (from gateway)
1 = fragment reassembly time exceeded. (rom host only)

Type = 3

Code
0 = net unreachable;
1 = host unreachable;
2 = protocol unreachable; (from host only)
3 = port unreachable; (from host only)
4 = fragmentation needed and DF set;
5 = source route failed.

Type = 12

Code
0 = pointer indicate the octet in error

ICMP redirect message

Type=5

Code
0 = Redirect datagrams for the Network.
1 = Redirect datagrams for the Host.
2 = Redirect datagrams for the Type of Service and Network.
3 = Redirect datagrams for the Type of Service and Host.

Type = 8 for echo message;
Type = 0 for echo reply message.
code = 0 (from gateway or host)

For error report, ICMP includes the header of the datagram and and the first 64 bits of the data portion (of the target datagram). These 64 bits have the information needed by the source to associate errors with a particular protocol.

A router may not report all of the errors using ICMP. ex:

the destination is connected to an Ethernet but is powered off. The router to this LAN can not identify this pronblem because Ethernet provides no acknowledgment.

ICMP provides the mechanism to report network problems. But it does not specify how to react to the problems.

For inf req/res ICMP messages, the identification and sequence number are used to identify a particular ICMP message.

Routing change may be useful only between the sending host and the attached router. Routing between intermediate routers must be handled by other routing protocols.