An ICMP source quench message is intended as a congestion control mechanism in IP. Source quench messages are used when a network gateway cannot forward a message because its message buffers are full. The gateway transmits a source quench message back to the source host machine to request that the source reduce it's transmission rate until it no longer receives source quench messages from the gateway. Thus, this effectively throttles back the source's transmission rate.
The gateway can transmit multiple source quench messages, one for each packet it receives from a source. The source machine is not required to respond to these source quench messages.