Internet Core Protocols: The Definitive Guide

Source Quench error messages

The Source Quench error message is perhaps the simplest of all the ICMP error messages. Whenever a device is sending too much data for the destination host to process, the recipient can send an ICMP Source Quench error message back to the sender, suggesting that the sender throttle back on the rate at which it is sending data. If the sender does not slow down, then some packets are likely to be discarded by the congested device.

Source Quench is most often seen when a dial-up server connects a high-bandwidth network (such as a LAN) to a low-bandwidth device (such as a dial-up client). In this kind of scenario, it is easy for a high-powered system on the LAN to transmit more data than the dial-up server can feed to the end-point system. Eventually, the dial-up server will fill its transfer buffers and will have to start dropping packets if the sender doesn’t slow down. Source Quench allows the dial-up server to inform the sender of the congestion, effectively requesting it to please stop sending so much data.

According to RFC 1122, all Source Quench error messages must be reported to the transport layer that triggered the failure. In addition, RFC 1122 states that if the Source Quench error message is handed to TCP, then TCP should shrink the congestion window for that virtual circuit to “one segment” and implement the slow start recovery algorithm (as described in Section 7.1.6.6 in Chapter 7 ).

It is also important to note that RFC 792 states that either ...