Internet Control Message Protocol. What is the ICMP port? Is ICMP a ping? Ping is a computer network administration software utility used to test the reachability of a host on an Internet Protocol IP network.
What is ICMP redirect message? Redirects are used when a router believes a packet is being routed sub optimally and it would like to inform the sending host that it should forward subsequent packets to that same destination through a different gateway. Is Ping UDP? It uses ICMP. To be more precise ICMP type 8 echo message and type 0 echo reply message are used.
Does traceroute use ICMP? Check the All Programs radio button and click Next. How is ICMP used? ICMP Internet Control Message Protocol is an error-reporting protocol network devices like routers use to generate error messages to the source IP address when network problems prevent delivery of IP packets.
What is a ICMP attack? How big is an ICMP packet? For example, the destination unreachable message has 16 different codes. When you see code 0 it means that the destination network was unreachable while code 1 means that the destination host was unreachable. The third field are 2 bytes that are used for the checksum to see if the ICMP header is corrupt or not. What the remaining part of the header looks like depends on the ICMP message type that we are using. If you are interested, here is a full list with all ICMP codes and types.
The message above is the ICMP request, you can see it uses type 8 and code 0 for this. When R2 receives it will reply:. Another nice example to look at is the destination unreachable message. The ping fails and you can see the U unreachable messages on R1. Cisco IOS by default will send multiple probes. For this demonstration I only need one probe. The TTL and destination port will increase for every hop. Once R2 receives this packet it will reply like this:.
Once R1 receives this, it will send its second probe:. Above you can see that the TTL is now 2 and the destination port number has increased to This is a method for hosts to discover neighbouring routers without any manual configuring or DHCP support. The Solicitation type 10 message is sent by a host to multicast address Any routers that receive this message and support IRDP will reply with the Advertisement type 9 message to announce their IP address as available for routing.
Routers may also send the Advertisement message unsolicited on occasion as more of an advertisement than a response.
If more than one router on the local segment responds, the host will pick the first response. If the host makes a poor choice, ICMP redirects will be used to make routing more efficient.
The time exceeded message can be generated for two different errors; One is that the TTL Time To Live field value in the IP header has decremented to zero, and the packet had to be dropped.
The other is that a device could not reassemble a fragmented packed in the allocated time, and the packet was dropped. The code field is used to determine which one of these errors has been raised. Each layer-3 hop in the network will decrement this value by 1, and eventually the packet will either be delivered, or the TTL will drop to zero, and the packet will be discarded.
This is done for loop prevention, so a packet will be dropped if it loops around for too long. When a router decrements the TTL to zero, it creates the Time Exceeded message, and sends it to the source host. Of course, this can be caused if the TTL is set too low in the first place. This feature can be useful for security in some cases. Be aware that some security devices, even though they are layer-3, do not decrement the TTL of packets passing through them.
There are times that a packet has to be fragmented into smaller pieces. When the fragments arrive at the destination, they need to be reassembled into the original packet. The occasional problem here is that one of the fragments may go missing, resulting in the entire packet being discarded. To handle this, the IP stack starts a timer when the first fragment arrives. If the timer expires before all the fragments are reassembled, the packet is discarded, and the Time Exceeded message is created and sent to the source.
This will also be used if there is a corruption or missing data in the IP header. When the Parameter Problem message is sent, a pointer is included in the high 8-bits of the second part of the header.
This pointer contains the location of the problem in the original packet. In some cases, the pointer is not included. For example, if there is data missing codes 1 and 2 , there will be no pointer. Timestamps can be used to synchronise time between devices.
To achieve this, a device sends a Type 13 Timestamp-Request to another device. In the request, it includes the time that the packet was sent, measured in milliseconds past midnight in Universal time. The second four bytes of the header include an identifier and sequence number which are used to match requests and replies. In response, the recipient generates a type 14 Timestamp-Reply. The header format is the same as the request, but the payload includes the original timestamp, the timestamp when the packet was received, and the timestamp for when the reply packet was sent.
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