4.3.2.8 Rate Limiting

A router which sends ICMP Source Quench messages MUST be able to limit the rate at which the messages can be generated. A router SHOULD also be able to limit the rate at which it sends other sorts of ICMP error messages (Destination Unreachable, Redirect, Time Exceeded, Parameter Problem). The rate limit parameters SHOULD be settable as part of the configuration of the router. How the limits are applied (e.g., per router or per interface) is left to the implementor's discretion.

DISCUSSION

Two problems for a router sending ICMP error message are:

1. The consumption of bandwidth on the reverse path, and
2. The use of router resources (e.g., memory, CPU time)

To help solve these problems a router can limit the frequency with which it generates ICMP error messages. For similar reasons, a router may limit the frequency at which some other sorts of messages, such as ICMP Echo Replies, are generated.

IMPLEMENTATION

Various mechanisms have been used or proposed for limiting the rate at which ICMP messages are sent:

1. Count-based - for example, send an ICMP error message for every N dropped packets overall or per given source host. This mechanism might be appropriate for ICMP Source Quench, if used, but probably not for other types of ICMP messages.

2. Timer-based - for example, send an ICMP error message to a given source host or overall at most once per T milliseconds.

3. Bandwidth-based - for example, limit the rate at which ICMP messages are sent over a particular interface to some fraction of the attached network's bandwidth.