Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 186
More ICMP Functions

ICMP has added functions over the years as indicated by RFCs 1256 (Router Discovery) and 1393 (ICMP Traceroute). ICMP running in a router can respond to a host’s request to find the subnet address mask for its network. A host, upon startup, can request of a router the subnet mask assigned to the network.

Although not really used anymore (there are better methods for controlling traffic, such as the Slow Start algorithm discussed in a moment). Source quench is the endstation’s ability to indicate to the originator of a message that the host cannot accept the rate at which the sender is submitting the packets. A source quench packet is continually generated to the originator until the rate of data flow slows down. The intended recipient of a source quench will continue to slow down its data rate until it receives no more source quench packets. The station that was requested to slow down will then start to increase the data rate again. This is similar to a flow control, except that it is more like throttle control—the data is not stopped, merely slowed down and then increased again. It is generated by any network station on the internet to indicate that the node cannot handle the rate of the incoming data. This ICMP type was not included in ICMPv6. It was found that other protocols handle congestion better than forcing the routers to handle it.

There are many other uses of the ICMP protocol. When a router receives a datagram, it may determine a better router that can provide a shorter route to the destination network. This is an ICMP Redirect, and this message informs the sender of a better route. If the TTL field is 0, a router will inform the originator of this through an ICMP message (Time Exceeded). A user’s workstation can request a timestamp from a router, asking it to repeat the time when it received a packet. This is used for measuring delay to a destination.

Summary of Message Types