[c-nsp] rate limiting multicast

[John Kristoff]

On Wed, 16 Mar 2005 10:26:35 +0100 (CET)
"Mikael Carlander" <rip at kth.se> wrote:

> Most ISP's are now rate-limiting Multicast. My question is: When you limit
> Multicast, are you limiting CPU or bandwidth? and more importantly, are
> you limiting by percentage or a static number?

Can you provide references to the claim that most ISP's are rate
limiting multicast traffic?  I'd be curious in how specifically.  It
may be that certain types of multicast data and/or control traffic is
being limited, but it matters as to which one you are talking about.

Generally speaking, you may be limiting both CPU somewhere and link
usage (what you call bandwidth).  If you set rate limits on edge
interfaces for anything over UDP to 224/4 for example, you are potentially
saving capacity on links that multicast may have been forwarded onto.
In addition, you may also be limiting multicast state created on the
router with the rate limit, or ones further downstream, due to the sender
driven state mechanisms used in typical multicast configurations.  This
is often the case when worms have indiscriminately scanned 224/4.

Most rate limit knobs use a fixed number, but since you are often
talking about a rate limit of capacity on a fixed speed link, you
can think of it as a percentage if you'd like to.

John