[Gllug] ADSL modem -- WTF?

[Russell Howe]

On Thu, Jul 15, 2004 at 07:50:09PM +0100, Chris Bell wrote:
> On Thu 15 Jul, Russell Howe wrote:
> 
>    I am sure that any errors in the following will be corrected:

And all those perceived errors too, regardless of whether the correction
has any basis in fact or fiction :)

>    The BT main backbone system was designed for minimum delay on speech, so
> runs om ATM with a very small packet size. 1492 is a convenient multiple
> which does not exceed the standard 1500 maximum ethernet packet size, and
> will give a slightly better maximum rate. BT are upgrading their network so
> that it is better able to handle 1500, so there should soon be no advantage
> of using the smaller packet size.

1492 is the Ethernet MTU (1500) minus the 8-byte overhead of PPPoE, as
far as I know.

Since ATM allows larger frame sizes (up to 64KiB IIRC?), PPPoA packets
can be larger than 1500 bytes, and can therefore produce PPP connections
with an MTU of 1500. This matches ethernet nicely, and means that even
with pretty broken PMTU implementations, things should still work well
enough. PPPoE can't do this, since it has to run over ethernet, and
without using large frames (which some ethernet devices support - needed
for some forms of VLAN tagging, for example), your PPPoE packets have a
limit of 1500 bytes. Take off the 8 byte overhead, and you can only
squirt 1492 bytes down a PPPoE tunnel in one packet/datagram/avian
carrier/etc.

>    You must connect to BT using PPPoA, but some ADSL modems can be

Not necessarily. See http://www.sinet.bt.com/ (A wonderful link murble
told me about). I forget which one, but it's 3xx and has L2TP in the
name. It basically says that the BT equipment can do PPPoE and PPPoA,
and it's up to the ISP which it supports. I guess it's possible
different exchanges have different equipment and that only some do PPPoE
(whereas all do PPPoA), but I am talking PPPoE to Nildram's DSL boxen:

23:23:00.400385 PPPoE  [ses 0xae7a] IPCP, Conf-Request (0x01), id 1, IP-Addr lon1-adsl4.nildram.net, length 102

I somehow doubt there's a PPPoE -> PPPoA translator in there, more
likely the PPPoE is being bridged onto the ATM using some ethernet
emulation (LANE? AALsomething? I don't know much about ATM....)

> configured to act as a PPPoA - PPPoE converter, allowing you to run PPPoE on
> your final termination, i.e. your firewall.

I still think it's unlikely that the router's talking PPPoA to the ISP..
it'd have to convert every PPPoE packet, including all the negotiations,
into PPPoA and vice-versa. I don't know how similar the two are, but I
can't imagine it being a trivial thing to do (and experience shows that
DSL router manufacturers shy away from the non-trivial...)

>    I understand that if your firewall is configured to run PPPoE and the
> ADSL modem is just used to convert between PPPoE and PPPoA, then the ADSL
> modem can have a non-routable IP address, and the connecting ethernet
> interface on your firewall can carry the assigned IP address. The system
> should just work as well as if there was any other hardware, including an
> analogue modem, hub, switch, or simple router, also in the path.

The router in this case has no IP address. It doesn't do IP at all, in
fact. It talks DSL (and whatever extras are needed for that - ATM?),
ethernet and PPPoE. That's all it needs to know. The PPPoE packets are
opaque as far as the router's concerned.

-- 
Russell Howe       | Why be just another cog in the machine,
rhowe at siksai.co.uk | when you can be the spanner in the works?
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