Книга: Iptables Tutorial 1.2.2
A source quench packet can be sent to tell the originating source of a packet or stream of packets to slow down when continuing to send data. Note that gateway or destination host that the packets traverses can also be quiet and silently discard the packets, instead of sending any source quench packets.
This packet contains no extra header except the data portion, which contains the internet header plus 64 bits of the original data datagram. This is used to match the source quench message to the correct process, which is currently sending data through the gateway or to the destination host.
All source quench packets have their ICMP types set to 4. They have no codes except 0.
Note Today, there are a couple of new possible ways of notifying the sending and receiving host that a gateway or destination host is overloaded. One way for example is the ECN (Explicit Congestion Notification) system.