What’s Web Management Message Protocol (ICMP) and the way does it work?

Web Management Message Protocol (ICMP) is a 3rd layer (community layer) protocol within the OSI seven-layer mannequin. The protocol diagnoses community connectivity or knowledge transmission points between gadgets. It helps to ship, obtain and course of ICMP messages to report connectivity points on the supply community system.

Due to this fact, the first objective of the ICMP protocol is to report errors on the community stage. Nevertheless, malicious actors can manipulate its performance to launch assaults resembling Distributed Denial of Service (DDoS) and Ping of Demise assaults. Right here, you’ll study concerning the ICMP protocol, its use circumstances, and necessary parameters to grasp ICMP messages.

How does ICMP work?

Routers use the ICMP protocol to carry out community administration and diagnostics points. When the community disables the supply of packets to the vacation spot system it generates and sends an error message to the sending system. These error messages embrace time exceeded, parameter drawback, vacation spot unreachable, community congestion, and many others.

For instance, when a sender sends knowledge from one system to the vacation spot system within the type of an IP datagram, it travels via a number of routers or middleman gadgets. Generally, there could also be an error forwarding that IPV4 datagram. Due to this fact, ICMP messages are divided into two broad classes: error messages and question messages.

error-reporting message

These are the messages {that a} router or middleman host might encounter in datagram processing:

  1. Time elapsed: An ICMP packet has a time-to-live parameter that when it reaches 0, the router or host discards the datagram and sends a timed out error message. The message is distributed even when the vacation spot has not obtained all the information fragments.
  2. vacation spot unreachable: When the router or intermediate host fails to ship it, the system initiating the connection receives an unreachable vacation spot error message. Consequently, the gadgets discard the datagram.
  3. supply quench: When gadgets fail to ship a datagram as a consequence of community congestion, they ship an ICMP “supply mitigation message” to the sender of the message. Error reporting provides circulate management to the community layer by notifying the message supply system about congestion within the path and slowing down the method of sending knowledge.
  4. Redirect: A router in a non-optimal path redirects the packet to a router alongside an optimum path between the supply and the vacation spot. As such, it updates the supply about modifications in its route.
  5. Parameter drawback: The supply system receives this message when the datagram header comprises a lacking worth. For instance, a distinction within the calculated checksum from supply to vacation spot causes this ICMP error message.

question message

Question messages happen in pairs and supply particular data from intermediate gadgets to the supply system.

  1. Echo-request and Echo-reply message: These messages assist in resolving community diagnostics points.
  2. Timestamp request and reply: These time stamps assist decide the spherical journey time between request and reply messaging gadgets.

Understanding ICMP Parameters

ICMP message knowledge is contained beneath the IP header, which signifies that the ICMP datagram header comes after the IPv4/v6 data. The primary a part of the ICMP packet header comprises parameters that assist community gadgets decide error or question messages.

Within the first 32 bits, the primary 8 bits decide the kind of message, the following 8 bits decide the code of the message, and the remaining bits include data associated to knowledge integrity. Due to this fact, there are three data fields: ICMP sort, ICMP code, and checksum.

ICMP sort

The kind determines the aim of the ICMP packet. Because it briefly explains why the supply system is receiving that message. There are 0-255 kinds of ICMP messages such that every sort gives completely different data. Some necessary ICMP sorts are:

0: echo request

3: vacation spot unreachable

4: extinguish the supply

5: Redirect

8: echo reply

11: Time handed

12: Parameter Drawback

30: traceroute

icmp code

ICMP codes present extra details about messages. For instance, ICMP Sort 3 with codes 0-15 specifies why the vacation spot is unreachable; That’s, if the code is 0, it’s as a result of the community is unavailable. Equally, for sort 3 code 1, the message specifies that the host is unreachable.

As well as, Sort 8 code 0 and Sort 0 code 0 symbolize echo-request and echo reply messages. Due to this fact, every message sort with the identical code worth returns completely different community data.


Checksum is an error detection mechanism that determines knowledge integrity. The checksum worth represents the bits within the transmission message. The sender calculates and joins the information checksum, and the receiver recalculates the worth and compares it to the unique to find out any modifications to the unique message throughout transmission.

The second a part of the header signifies the byte worth that’s inflicting the issue within the unique message. Lastly, the final a part of the ICMP packet is the precise datagram.

Examples of ICMP in use

Listed below are some working examples of the ICMP protocol in numerous community administration and administrative utilities.


Ping is a command-line utility that depends on ICMP echo-request and echo-reply messages to check community availability and latency between gadgets. The utility determines the supply of the vacation spot server by merely estimating the overall packets despatched or obtained and the share of packet loss. It additionally helps in figuring out the overall most transmission unit (MTU) measurement.

use ping To find out google.com availability:


traceroute is one other command-line utility that makes use of echo-request and echo-reply messages with TTL variables to map paths between supply and vacation spot gadgets. It is usually helpful in figuring out the delay in response and assures accuracy by querying every hop a number of occasions.

How does ICMP work with out ports?

From the above rationalization, ICMP is a connectionless protocol that sends knowledge independently as datagrams, and its header seems after the IPv4 description. Nevertheless, you may additionally discover the absence of a port quantity which raises the query of how do gadgets obtain ICMP knowledge or change into conscious of a connection?

The reply is sort of easy. The ICMP protocol was designed to function on the community layer of the OSI mannequin and talk between routers and hosts to share data. Port numbers are a part of the transport layer, and ICMP is neither a TCP nor a UDP protocol. Lastly, ICMP sort and code mixtures assist gadgets decide messages and obtain all details about the community.

Error and Move Management with ICMP

The above description concerning the ICMP protocol reveals the way it ensures the circulate management and error management of knowledge at layer three of the OSI mannequin. The protocol message and parameters within the first a part of the ICMP packet header cowl all facets of community administration and diagnostics points.

You will need to know that improper firewall settings can permit attackers to comb via a corporation’s community and make it vulnerable to DDoS, ping of demise, and ICMP flood assaults. You need to use Wireshark to seize packets from web and do detailed evaluation of ICMP packets for higher understanding.

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