Re: Cable Modem [really responsible engineering]

From: Fletcher E Kittredge (no email)
Date: Thu Jun 28 2001 - 16:41:10 EDT

On Thu, 28 Jun 2001 15:48:42 -0400 (EDT) Greg A. Woods wrote:
> Luckily not much of this thread has been polluting NANOG, so I'm not
> 100% sure what your goal is here, but I will point out that:

You restated my goal later in your message. In your words:

"there are no doubt ways to cache this information and maybe even
 refresh a cache intelligently."

That is exactly what we are after :) What is the best way to obtain
this information?

I have three goals for this information:

1) Customer traceability/accountability. For example, after the
   launch of a DDOS attack, child porn server, etc. Cable modems are
   attractive targets as launch pads.

2) Traffic accountability for statistical modeling, charging, etc.

3) Debug information.

> The client-IP addresses active on any CPE interface of any cable modem
> compliant with the DOCS-CABLE-DEVICE-MIB (rfc2669) can easily be
> discovered with SNMP.
> From there it's very simple to find the client
> MAC in your dhcpd.leases file (though I don't know why the IP#s wouldn't
> be sufficient for most needs).

This doesn't help in a number of common cases. For example, most
connectivity support problems manifest themselves as a failure to get
an IP address from the DHCP server in the first place. Another example
is tracing a MAC when a client, either through customer error
(usual case) or malice, gets an IP not allocated by a valid DCHP

So we really need to have that MAC in cases where the DHCP lease file
can not be counted on for accurate information.

> Note too that this same table should
> also act as a filter to lock static addresses to a given modem if its
> entries are assigned by the operator.


> The only problem of course is that you've got to SNMP-scan all your
> modems to find which customer's using a given IP# (I did that a couple
> of times just this morning! ;-). There are no doubt ways to cache this
> information, and maybe even refresh a cache intelligently. There may
> even be a better, proprietary, way do do such searches in well
> implemented head-end systems (doesn't seem to be in Terayon's though).

Exactly. I found doing such scans got old years ago. Not to mention,
it doesn't scale well :) Let n be number of customers, then finding a
particular MAC is order O(n) where "O is something unsavory" because
of individual queries of cable modems which may or may not time
out... People turn the damn things off!

Likewise, searching through different Cisco UBR tables to find which
MAC was associated with which IP, then trying to painfully trace that
MAC to a particular node, then customer, etc. really sucks.... Been
there, done that, rotted out the tee shirt at the armpits.


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