Telstra ADSL questions (pppoe technical)

Martijn van Oosterhout kleptog at
Wed Dec 12 21:57:51 EST 2001

On Wed, Dec 12, 2001 at 09:40:11PM +1100, Eyal Lebedinsky wrote:
> I wrote a simple script that looks at the traffic at my firewall
> (pppstats) and tracks my usage.
> I use pppoe as in LEAF (Linux Router 3.1.0).
> I encountered a few uncertainties. If anyone has an idea... Some
> Qs may be stupid, but I will still appreciate your comments.
> 1) The AUP says the "free" limit is 3G. Elsewhere (on the usage page)
>    it says 3000M. And what is M anyway then?

I think it says somewhere that they consider 1M = 1000K.

> 2) I see ADSL traffic on eth0 and on ppp0. I expected to see the raw
>    volume on eth0, with the net on ppp0 but this is not what I see.
>    While the number of packets on both is identical, the volume on
>    ppp0 os higher. Can anyone explain?

The packets may be compressed, meaning that the traffic on eth0 will be less
than on ppp0.

> 3) What traffic is chargeable then? The ppp0 has extra headers
>    that only exist between me and the Telstra connection, but it
>    does not go into the external network. So I assume the net
>    volume is measured? How do I get at it?

I imagine they count everything including the IP headers but nothing below
that. Under that any encapulation would be variable length.

>    Also, an idle line has constant traffic showing on eth0 but not
>    on ppp0 (as I expected) which comes up to an hourly rate of:
> 	rcv bytes	34560
> 	rcv packets	  540
> 	xmt bytes	32400
> 	xmt packets	  540

Those would be LCP requests and ack. If you turn on pppd debug you can watch

>    BTW, I xfered a large file (15,751,285) and eth0 had a rcv of
>    16,644,696 while ppp0 shows 19,001,215. Reasonable? The xfer
>    rcvd 11,358 packets and sent 5,758.

20% overhead. Seems high. I would have thought more like 5-10%. Mind you,
who knows what else went over the link at the time.

> 4) I am on a 512/128K plan. I find that the best rcv rate I get is
>    51KB/s (looking at 'curl' running). How does this match the 512Kb
>    rate? 512Kb/8 -> 64KB, right? how does it go down all the way so
>    badly? Maybe I need better settings? Is the slower upstream slowing
>    it that much? I do appreciate there is some headers overhead but
>    this is more than that.

Well, there is no speed guarentee. And going by the 1K = 1000 byte rules,
51K is about right.

> 5) On the same subject, pppstats shows zero compressed packets. Is it
>    my setup or telstra that prevents header compression? My ifconfig
>    says:
> 	ppp0:  mtu 1492 qdisc pfifo_fast qlen 10
> What I keep is the above for both eth0 and ppp0 once an hour. This
> should give enough accuracy.

There are various types of compression. The actual bodies of packets can be
compressed and the headers can be compressed. They are independant. On the
link here it appears that there is VJ header compression but the bodies are

Martijn van Oosterhout <kleptog at>
> Terrorists can only take my life. Only my government can take my freedom.

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