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ICMP - Time Exceeded Message Analysis

Posted in ICMP Protocol

The ICMP - Time exceeded message is one which is usually created by gateways or routers. In order to fully understand this ICMP message, you must be familiar with the IP header within a packet. Our readers can also visit the IP Protocol section which covers the IP protocol structure in great depth.

When looking at an IP header, you will see the TTL and Fragment Flag fields which play a big part in how this ICMP message works. Please make sure you check them out before attempting to continue!

The ICMP - Time exceeded message is generated when the gateway processing the datagram (or packet, depending on how you look at it) finds the Time To Live field (this field is in the IP header of all packets) is equal to zero and therefore must be discarded. The same gateway may also notify the source host via the time exceeded message.

The term 'fragment' means to 'cut to pieces'. When the data is too large to fit into one packet, it is cut into smaller pieces and sent to the destination. On the other end, the destination host will receive the fragmented pieces and put them back together to create the original large data packet which was fragmented at the source.

 

Analysis of the ICMP Time Exceeded Message

Let's have a look at the structure of an ICMP - Time exceeded message:

icmp-time-exceeded-1

ICMP - Redirect Message Analysis

Posted in ICMP Protocol

The ICMP - Redirect message is always sent from a gateway to the host and the example below will illustrate when this is used.

Putting it simply (before we have a look at the example) the ICMP - Redirect message occurs when a host sends a datagram (or packet) to its gateway (destination of this datagram is a different network), which in turn forwards the same datagram to the next gateway (next hop) and this second gateway is on the same network as the host. The second gateway will generate this ICMP message and send it to the host from which the datagram originated.

There are 4 different ICMP - Redirect message types and these are:

icmp-redirect-msgs

ICMP - Source Quench Message Analysis

Posted in ICMP Protocol

The ICMP - Source quench message is one that can be generated by either a gateway or host. You won't see any such message pop up on your workstation screen unless you're working on a gateway which will output to the screen all ICMP messages it gets. In short, an ICMP - Source quench is generated by a gateway or the destination host and tells the sending end to ease up because it cannot keep up with the speed at which it's receiving the data.

Analysis of the ICMP Source Quench Message

Now let's get a bit more technical: A gateway may discard internet datagrams (or packets) if it does not have the buffer space needed to queue the datagrams for output to the next network on the route to the destination network. If a gateway discards a datagram, it may send an ICMP - Source quench message to the internet source host of the datagram.

Let's have a look at the packet structure of the ICMP - Source quench message:

icmp-source-quench-packet1

ICMP - Destination Unreachable Message Analysis

Posted in ICMP Protocol

The 'ICMP Destination unreachable' message is quite interesting, because it doesn't actually contain one message, but infact six! This means that the ICMP Destination unreachable futher breaks down into 6 different messages.

This article will analyse all six destination unreachable messages and explain which occasions each message is used. The table below shows an brief summary of the available messages and their code value contain in the ICMP header:

icmp-dest-unreach-msgs

ICMP - Echo / Echo Reply (Ping) Message

Posted in ICMP Protocol

As mentioned in the previous page, an Echo is simply what we networking engineers call a 'ping'. The Echo Reply is, as most would guess,  the 'ping reply'. ICMP Echos are used mostly for troubleshooting. When there are 2 hosts which have communication problems, a few simple ICMP Echo requests will show if the 2 hosts have their TCP/IP stacks configured correctly and if there are any problems with the routes packets are taking in order to get to the other side.

The 'ping' command is very well known, but the results of it are very often misunderstood and for that reason I have chosen to explain all those other parameters next to the ping reply, but we will have a look at that later on.

Let's have a look at what an ICMP-Echo or Echo Reply packet looks like:

icmp-echo-header

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