[geeks] Bandwidth education

[Mike Parson]

On Fri, Jan 07, 2005 at 08:29:20PM -0500, Jeff Cole wrote:
> Okay. Could someone give me a rundown of how exactly commercial 
> bandwidth works? Let's say I want to get a 10mb connection in a colo for 
> a reasonable price per mbit. What's the best way to get a reasonable 
> price? I assume that billing for the bandwidth is provider specific, but 
> is there some generally accepted way of billing for bandwidth? I've 
> heard the term 99th percentile tossed around. What is that exactly? I'm 
> assuming I'll have telco fees on top of the price of the bandwidth? Can 
> I max out the line for the entire month and expect to have the same 
> amount due each month, or will it change based on how much bandwidth I 
> actually use?

The percentile type billing is usually been 95th %tile, at least when
I've encountered it.  With this, you usually buy a minimum commit rate,
under which, you won't get billed extra, say your 10mb connection.  They
sample your usage every 5 minutes and collect the data.  At the end of
the month, they basically toss out the top 5% of usage and see where
you're at.  If you're under your minimum commit, you're golden, if
you're over, you pay whatever rate was negotiated for bits over.

This basically allows for normal usage, but if for some reason, someone
does a flood-type DoS agains you, as long as it lasts for < 5% of the
time (a day and a half-ish), you shouldn't see any increase in your bill
at the end of the month.

Somewhere out there is a guy that has patches to MRTG to graph the 95th
percentile.  Along with his patches, he has sample graphs, and a fairly
decent explaination.

Ah, quick with google:

http://www.seanadams.com/95/

> I've never dealt with anything like before, and would like to have a 
> fairly solid handle on it before I make any plans or decisions.

That's the rough and simple answer.

For my colo, they're charging me for 10 gigs transfered/month + a charge
for each gig over the amount my base is set to.  Now, I could just QoS
my interface to the rate that gives me my 10 gigs/month, but that would
mean my max transfer rate would be what I would have to sustain to get
anywhere near the commit.  Instead, I have my QoS max set to 1 mb/s,
then each service has a fraction of it so that a sudden hit on my web
server doesn't affect my interactive ssh sessions.

A sustained 1 mb/s transfer would be way over my commit, but most usage
is way under this, but I put the cap on to help slow down accidents so
they don't pile up too high before I can respond.

Some quick work in Excel^WOO.org's calc could give you an idea of how
much data you can transfer around with various bitrates.

My 10 gigs/month is in+out.  The 95th percentile stuff tends to be 95th
of whicever is larger, in or out, which is the ISPs real cost.

It's been a while since I was subscribed, but this came up fairly
periodically on the NANOG list when I was reading it (along with RFC1918
addy's on glue networks breaking path MTU discovery, and if it matters,
but that's another flamewar for another list ;).

-- 
Michael Parson
mparson at bl.org