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North American Network Operators Group

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Re: Cable Modem [really responsible engineering]

  • From: Miquel van Smoorenburg
  • Date: Tue Jun 26 19:01:17 2001
  • Distribution: cistron
  • Newsgroups: lists.nanog

In article <>,
Fletcher E Kittredge  <> wrote:
>> I tend to agree because:
>> The mac addresses of the computers in my house may change quite a bit, but
>> my external IP addresses will remain the same (and have to, since only
>> those IPs are being routed to me).
>Do you have any actual experience with designing or operating such a
>public access network?  If so, please explain how to get the "port and
>switch number" for a user's PC on a cable network as I was unaware of
>this functionality.

I have a bit of experience in designing and operating such a network.
We are an ADSL provider, and we use RC1483 bridging. The modem at
the customer has an ether over ATM over ADSL connection to the DSLAM
and then that connection is forwarded to a central BRAS - a Redback
in our case.

When the BRAS requests config info when the circuit goes up (using
radius) or when it acts as a DHCP relay, it includes the VPI/VCI
of the ATM channel in the request. That means that you can assign
IP addresses based on the physical connection rather than the MAC
address, and this is what we do [well, will do soon anyway ;)]

The ATM-based COM21 modems operate in much the same way (ethernet
over ATM over cable using a BRAS to terminate the connection)
so if you're designing a new network it's very well possible to
get the "port and switch number".

I'm not sure about the DOCSIS system, if the modems have a unique
identifier that cannot be changed by the enduser and the headend
somehow puts this info in a DHCP option when it acts as a DHCP
relay you can probably do something similar. This is pure speculation

>We haven't had a problem explaining  to users how to get their MAC

Well, we have, and what's more if you're doing your accounting
purely based on the mac addresses in the DHCP logs how do you
ever know for sure who was using a certain IP address on a
certain time? That's important, did customer A send out spam
or was it his neighbor spoofing A's MAC address when A was
asleep ?

>> > 3) ARIN has sent the strong message that they expect IP over E public
>> >    network providers to use dynamic IP allocation in order to conserve
>> >    IPv4 addresses.
>> And the intelligent public has sent an equally strong message that dynamic
>> IPs are not acceptible.  Most people I know with DSL or similar service
>> make sure to use static IPs that are usable for server purposes.  Wether
>> static or dynamic IPs are used, the same _number_ of IPs is required, we
>> aren't talking about dial-up here where most of the users will be offline
>> most of the time.
>I disagree with all of the above.  Since it nothing more than your
>opinion and anecdotal evidence, mere contradiction suffices.

It's true though. With a cable or DSL network you must assume the
worst, that is all users online at the same time. It will happen.
And then your dynamic pool needs to be just as big as the number
of subscribers.


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