[TriLUG] OT broadband question

[Ryan Leathers]

In a nutshell the rest of this post says "No DSL providers filter stuff
by IP address or port number"

I have provided technical consulting to several DSL hardware vendors and
service providers - and am feeling unexplainably enthusiastic over this
topic this morning.  This entire discussion gets a bit cloudy inasmuch
as a DSL provider may also be an ISP and vice versa.  For purposes of
clarity I discuss them as separate entities.  

To my knowledge, no DSL service providers make forwarding decisions on
anything but layer2 although certain hardware vendors (notably Cisco,
Paradyne) include the ability to do so at layer3 and to some extent at
layer4.  In simple terms DSL is a physical layer technology which
delivers some number of bits over copper wire.  It so happens that most
often the bits are organized as Ethernet frames and in many cases this
Ethernet frame encapsulates a PPP frame.   PPP provides for
authentication, and forwarding is sometimes based on this prior to frame
delivery to an ISP POP (Redback or Shasta for example).  Whether a
dynamic IP address is provided in response to a DHCP lease request over
an Ethernet frame or in response to a PPP authentication through PPPoE
the DSL service provider will probably not even see it let alone make
forwarding decisions on it.  Instead, forwarding will be handled at
layer2 in the DSLAM (or PPP concentrator as described above) where
either MAC or VCC addresses are associated with customer ports (wire
pairs).  Generally the DSLAM employs some sort of MUX forwarding which
eliminates the possibility of layer2 broadcast/discovery across its
ports channeling everything through an uplink interface.  This ensures
that customers will be subject to whatever security restrictions are
imposed by the ISP.  The DSL modem (ATU-R) speaks Layer1 on its provider
interface and layer2 on its customer interface.  The modem itself has no
ability to filter.  An ISP may very well decide to filter traffic (at
their POP or upstream) based on port number or address, but again, DSL
has nothing to do with this so a change of ISP's might yield the desired
capabilities rather than a change of technologies.  In some cases an ISP
might provide a combination modem/router.  These can be managed remotely
and traffic can be filterd, shaped, etc., once again without the
knowledge of the DSL provider.  

It may also be interesting to note that while the Telecom standards
bodies were deliberating over DSL standards that would give them
compatibility with the Q.931 family of standards the Cable industry got
together and whomped up a broadband protocol that paid no heed to the
Nx64 proponents.  Today, DOCSIS supports QoS suitable for voice
application and is every bit as viable as the DSL protocols for
residential applications.  DSL may yet provide multi-line Nx64 over a
single pair to business customers but the CLECs who tried this a couple
of years ago are all bankrupt now.  

Ryan 

           

-----Original Message-----
From: Chris Bullock [mailto:cgbullock at cox.net]
Sent: Wednesday, January 15, 2003 8:40 AM
To: trilug at trilug.org
Subject: [TriLUG] OT broadband question

Ben Pitzer wrote:

>DSL or cable?  Very important distinction in this instance.
>
Cable through cox communications.
--cgb

>
>Regards,
>Ben Pitzer
>
>---------------------------------------------
>
>"Those that can give up essential liberty to obtain a little temporary
>safety
> deserve neither liberty nor safety."
> --Ben Franklin--
>
>
>
>
>
>
> 
>
>>-----Original Message-----
>>From: trilug-admin at trilug.org [mailto:trilug-admin at trilug.org]On
Behalf
>>Of Chris Bullock
>>Sent: Tuesday, January 14, 2003 6:32 PM
>>To: trilug at trilug.org
>>Subject: [TriLUG] OT broadband question
>>
>>
>>I know we have some broadband employees in the group so helpfully
>>someone can answer my question.  I wanted to set up a small
>>mail/webserver at my house but my ISP blocks a lists of ports, 80
>>and 25 being the key ones I need.  They claim that the ports are
>>blocked at the modem not at the routers.  How is this?  Does the
>>modem receive a TFTP on the initial activation?  Just wanted to
>>know for curiosity.
>>
>>thanks
>>
>>Chris
>>
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>>   
>>
>
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> 
>



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