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:
A destination host may also send an ICMP - Source Quench message if datagrams arrive too fast to be processed. The ICMP - Source quench message is a request to the host to cut back the rate at which it is sending traffic to the internet destination. The gateway may send an ICMP - Source quench for every message that it discards. On receipt of an ICMP - Source quench message, the source host should cut back the rate at which it is sending traffic to the specified destination until it no longer receives ICMP - Source quench messages from the gateway. The source host can then gradually increase the rate at which it sends traffic to the destination until it again receives ICMP - Source quench messages.
The gateway or host may also send the ICMP - Source quench message when it approaches its capacity limit rather than waiting until the capacity is exceeded. This means that the data datagram which triggered the ICMP - Source quench message may be delivered.
Next - ICMP Protocol - Part 6: Redirect Message