ICMP

Although IPv4 is not a reliable protocol, it does provide for messages to be sent in the event of certain errors. These messages are sent using services of the Internet Control Messaging Protocol (ICMPv4). The purpose of these messages is to provide feedback about issues related to the processing of IP packets under certain conditions, not to make IP reliable. ICMP messages are not required and are often not allowed for security reasons

ICMP is the messaging protocol for the TCP/IP suite. ICMP provides control and error messages and is used by the ping and traceroute utilities. Although ICMP uses the basic support of IP as if it were a higher-level protocol ICMP, it is actually a separate Layer 3 of the TCP/IP suite

ICMP messages that may be sent include:

• Host confirmation
• Unreachable Destination or Service
• Time exceeded
• Route redirection
• Source quench

Host Confirmation

An ICMP Echo Message can be used to determine if a host is operational. The local host sends an ICMP Echo Request to a host. The host receiving the echo message replies with the ICMP Echo Reply, as shown in the figure. This use of the ICMP Echo messages is the basis of the ping utility.

Unreachable Destination or Service

The ICMP Destination Unreachable can used to notify a host that the destination or service is unreachable. When a host or gateway receives a packet that it cannot deliver, it may send an ICMP Destination Unreachable packet to the host originating the packet. The Destination Unreachable packet will contain codes that indicate why the packet could not be delivered. Among the Destination Unreachable codes are:

0 = net unreachable

1 = host unreachable

2 = protocol unreachable

3 = port unreachable

Codes for net unreachable and host unreachable are responses from a router when it cannot forward a packet. If a router receives a packet for which it does not have a route, it may respond with an ICMP Destination Unreachable with a code = 0, indicating net unreachable. If a router receives a packet for which it has an attached route but is unable to deliver the packet to the host on the attached network, the router may respond with an ICMP Destination Unreachable with a code = 1, indicating that the network is known but the host is unreachable.

The codes 2 and 3 (protocol unreachable and port unreachable) are used by an end host to indicate that the TCP segment or UDP datagram contained in a packet could not be delivered to the upper layer service.

When the end host receives a packet with a Layer 4 PDU that is to be delivered to an unavailable service, the host may respond to the source host with an ICMP Destination Unreachable with a code = 2 or code = 3, indicating that the service is not available. The service may not be available because no daemon is running providing the service or because security on the host is not allowing access to the service.


Time Exceeded

An ICMP Time Exceeded message is used by a router to indicate that a packet cannot be forwarded because the TTL field of the packet has expired. If a router receives a packet and decrements the TTL field in the packet to zero, it discards the packet. The router may also send an ICMP Time Exceeded message to the source host to inform the host of the reason the packet was dropped.

Route Redirection 

A router may use the ICMP Redirect Message to notify the hosts on a network that a better route is available for a particular destination. This message may only be used when the source host is on the same physical network as both gateways. If a router receives a packet for which it has a route and for which the next hop is attached to the same interface as the packet arrived, the router may send an ICMP Redirect Message to the source host. This message will inform the source host of the next hop contained in a route in the routing table.

Source Quench

The ICMP Source Quench message can be used to tell the source to temporarily stop sending packets. If a router does not have enough buffer space to receive incoming packets, a router will discard the packets. If the router has to do so, it may also send an ICMP Source Quench message to source hosts for every message that it discards.

A destination host may also send a source quench message if datagrams arrive too fast to be processed.

When a host receives an ICMP Source Quench message, it reports it to the Transport layer. The source host can then use the TCP flow control mechanisms to adjust the transmission.

Altre risorse:

• ICMP (wikipedia)