[Gllug] ADSL modem -- WTF?

[Chris Bell]

On Thu 15 Jul, Russell Howe wrote:
> 

> 
> > All I want is a box with two leads -- one into my phone socket and
> > off to my ISP, and one into my firewall (and on to the rest of my
> > network). It needs to be able to route my IP range frmo the outside
> > world and on into my network. It looks like the D-Link DSL-502T
> > should do that. Does that sound reasonable?
> 
> Needs to route your IP range, or needs to NAT your LAN? big big
> difference.
> 
> See the end of my email for a verbatim cut-n-paste from an email I sent
> to support at nildram.net
> 
> > Incidentally, has anyone else noticed how hard it is to buy a
> > non-wireless ADSL router these days?
> 
> A DSL router with an IP address is a dangerous, dangerous thing. I
> wouldn't trust these devices one little bit.
> 
> For the diagrams:
> 
> My router does support PPPoA, but I don't want to terminate the PPP on
> the router. If I use PPPoA, I have to do this: (view using a
> fixed-width font)
> 
>     [ Internet ]
>          |
>       DSL line
>        PPPoA
>          |
>   /---------------\
>   | 82.133.120.56 |
>   |               |
>   |  DSL router   |
>   | PPPoA client  |
>   |               |
>   |  82.133.8.9   |
>   \---------------/
>           |
>    ethernet (x-over)
>         cable
>           |
> /---------------------\
> |    82.133.8.10      |
> |                     |
> | Firewall PC (Linux) |
> |                     |
> |    82.133.8.11      |
> \---------------------/
>           |
>     ethernet cable
>           |
>          LAN
> 
> 
> As you can see, this has wasted 2 IP addresses - the router takes two,
> but doesn't need one - I only have 3 usable addresses remaining!
> 82.133.8.12-14. A bit wasteful considering that I have 7 addresses at my
> disposal.
> 
> What running PPPoE allows me to do is this:
> 
>     [ Internet ]
>          |
>       DSL line
>        PPPoE
>          |
>   /---------------\
>   |               |
>   |  DSL router   |
>   |  bridging DSL |
>   |  to ethernet  |
>   |               |
>   \---------------/
>           |
>         PPPoE
>      data stream
>           |
>    ethernet (x-over)
>         cable
>           |
> /---------------------\
> |   82.133.120.56     |
> |                     |
> | Firewall PC (Linux) |
> |    PPPoE client     |
> |                     |
> |    82.133.8.9       |
> \---------------------/
>           |
>     ethernet cable
>           |
>          LAN
> 
> This leaves me with 82.133.8.10 through .14 usable - a whole 5
> addresses, surely more than anyone could ever need!
> 
> However, because of MTU issues, it is unreliable.
> 
> The ideal (full 1500-byte MTU, plus fewer encapsulation layers) would be
> to run ATM between the DSL router and the firewall. That would allow an
> incoming PPPoA data stream to be bridged onto the ATM (can't bridge
> PPPoA onto ethernet) and have a PPPoA client on the firewall, listening
> on an ATM interface on the firewall.
> 


   I have just received an unofficial reply from a Demon staff member giving
the following information:

   The slight disadvantage of using standard 1500 frame sizes with BT has
been sorted, the problem went away when they upgraded their system, probably
last year. Any possible remaining differences would not be noticed.
   BT made changes some time ago that allowed Demon to offer their customers
connections using PPPoA or PPPoE, arranged so that a greater range of
customer equipment (mainly the cheaper end) is then able to connect, but
they recommend PPPoA if available because PPPoE is inherently less efficient
and some implementations have problems negotiating the correct MTU. In both
cases the data is carried over ATM between the customer provided equipment
and the DSLAM, so ethernet frames are encapsulated in ATM, and PPP inside
the ethernet frames. The PPP can be terminated on something behind the modem
over the ethernet connection, dependant on the customer's available
facilities.
   If anyone has any additional information I would be delighted to hear it.

-- 
Chris Bell

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