Shulou(Shulou.com)06/01 Report--

One: the role of ICMP

Internet Control message Protocol: it is an important protocol in the network layer. All kinds of error and control information are transmitted between ICMP devices, and it plays an important role in collecting all kinds of network information, diagnosing and troubleshooting various network faults. To sum up, ICMP is used to transmit error, control, query and other information.

Two: ICMP redirection

ICMP Redirect messages are used to support routing capabilities. As shown in the figure, host A wants to send a message to server A, so it sends a message to the gateway RTB according to the configured default gateway address. After receiving the message, the gateway RTB examines the message and finds that the message should be forwarded to another gateway device RTA in the same network segment as the source host, because the secondary forwarding path is a better path. So RTB sends a message to the host informing the host to send a message directly to another gateway RTA. After the host receives the Redirect message, it sends the message to RTA, and RTA forwards the message to server A.

Summary:

The 1.ICMP redirect message is generated by the router.

two。 The host handles the redirect message.

Three: ICMP error report

ICMP defines various error messages that are used to diagnose network connectivity problems; based on these error messages, the source device can determine the cause of the data transmission failure. As shown in the figure above, when the network device cannot access the destination, it will automatically send the ICMP destination unreachable message to the terminal device.

Four: ICMP packet format

In the figure above, Type represents the ICMP message type, and code represents different information in the same message type.

Five: apply 1.ping command to ICMP protocol

There are usually several kinds of message feedback when using the PING command to test interoperability:

1 、 Request Time Out

2 、 Destination Unreachable

3 、 TTL Expired in transit

Case 1: when the source machine PING a host machine, this situation occurs when the source machine fails to receive the ICMP response message within a period of time (after the source machine sends the ICMP request message, it starts timer 0). The reason for the above problems is that the routing from the source to the destination is normal, and there is no available path from the destination to the source.

Case 2: this reason occurs when there is no available access from the source machine to the hostel.

Case 3: when the source machine sends the IP packet (ICMP is directly encapsulated in the IP packet), the TTL (Time to Live) time of the packet will be added. Each time the packet passes through the router, the router will reduce the TTL time of the packet by 1. If the TTL of the packet is 0 before the ICMP request message reaches the host machine, the corresponding gateway discards the message and sends the ICMP timeout message to the source machine. The TTL Expired in transit message should be displayed on the source machine. The main reason for this problem is that the packet cannot reach the host machine because of the routing cycle within the network. Tracert tracking can be used to determine the source of the fault (it is best done on the host when using this command).

Tracter command

Tracert displays each of the packets that pass through the network.

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