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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

Introduction To The ICMP Protocol

Posted in ICMP Protocol

The Internet Control Message Protocol (ICMP), is a very popular protocol and actually part of an Internet Protocol (IP) implementation. Because IP wasn't designed to be absolutely reliable, ICMP came into the scene to provide feedback on problems which existed in the communication environment.

ICMP is one of the most useful protocols provided to troubleshoot network problems like DNS resolutions, routing, connectivity and a lot more, however caution must be taken because you can easily end up spending half a day trying to figure out why you're not getting a 'ping reply' ('echo reply' is the correct term) from a web server when in fact its firewall is configured not to reply to 'pings' for security reasons! This usually leads most engineers to the incorrect conclusion that the remote host might be down.

Note
A few years ago there was a program released, which still circulates around the Internet, called Click (I got my hands on version 1.4). Click was designed to run on a Windows platform and work against MIRC users - Windows based program for the Internet Relay Chat (IRC) network. The program would utilise the different messages available within the ICMP protocol to send special error messages to Mirc users, making the remote user's program think it had lost connectivity with the IRC server, thus disconnecting them from the server ! The magic is not what the program can do, but how it does it!

 

The ICMP Protocol

ICMP is defined in RFC (Request For Comments) RFC792. Looking at its position in the OSI model we can see that it's sitting in the Network layer (layer 3) alongside IP. There are no ports used with ICMP, this is because of where the protocol sits in the OSI model. Ports are only used for protocols which work at the Session layer and above:

icmp-intro-1

 

The ICMP protocol uses different 'messages' to identify the purpose of an ICMP packet, for example, an 'echo' (ping) is one type of ICMP message.

I am going to break down the different message descriptions as they have been defined by the RFC792.

There is a lot of information to cover in ICMP so I have broken it down to multiple pages rather than sticking everything into one huge page that would bore you!

 

 

Also, I haven't included all the messages which ICMP supports, rather I selected a few of the more common ones that you're likely to come across. You can always refer to the RFC792 to get the details on all messages.

 

We will start with a visual example of where the ICMP header and information are put in a packet, to help you understand better what we are dealing with :)

icmp-header

The structure is pretty simple, not a lot involved, but the contents of the ICMP header will change depending on the message it contains. For example, the header information for an 'echo' (ping) message (this is the correct term) is different to that of a 'destination unreachable' message, also a function of ICMP.

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