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Ping principle and ICMP Protocol

In the past, when I first looked at it, I always thought that the network protocol could only be encapsulated layer by layer. Although looking for information, it was said that there were IP, arp, ICMP and IGMP protocols on the network at the same time, but I just couldn't make a detour. Now I think, alas.

The principle of ping

The ping program is used to detect whether the host can communicate with the host. If it is not possible to ping to a host, it means that a connection cannot be established with that host. Ping uses the ICMP protocol, which sends an icmp echo request message to the destination host. The ICMP protocol stipulates that the destination host must return an ICMP echo reply message to the source host. If the source host receives a reply within a certain period of time, the host is considered reachable.

The ICMP protocol is sent over the IP protocol, which is a connectionless, unreliable packet protocol.

ICMP protocol packet in actual transmission: 20-byte IP header + 8-byte ICMP header + 1472 bytes 38 bytes

ICMP header: 8-bit type + 8-bit code + 16-bit checksum + (different types and codes, different formats)

By grabbing the packets, the icmp protocol discovers an ordered alphabet of random segments or segments.

ICMP is (Internet Control Message Protocol) Internet control message protocol, a connectionless protocol. The control message refers to the message of the network itself, such as whether the network is unreachable, whether the host is reachable, whether the route is available, and so on. These control messages themselves do not transmit user data.

Grab a packet to analyze a set of data:

For request packet capture, you can see that the type is 8 and the code is 0, which represents the request echo (ping request) and has a 16-bit checksum.

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For the grab packet of reply, you can see that the type is 0 and the code is 0, which represents the echo reply (ping reply)

Attach a specific type of ICMP message

ICMP protocols are roughly divided into two categories, one is query message, the other is error message. The query message has the following uses:

Ping query

Subnet mask query (used to initialize the subnet mask when the diskless workstation initializes itself)

Timestamp query (can be used to synchronize time)

The error message is generated when there is an error in data transmission. I won't repeat it.

Application of 2.ICMP-- ping

Ping is arguably the most famous application of ICMP when we don't go to one of our websites. Usually ping this website. Ping will show some useful information back. The general information is as follows:

Reply from 10.4.24.1: bytes=32 time

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