Angus Lees wrote:
>
> before everyone gets really excited about masquerading and adsl, i
> think i've uncovered a bug in telstra's end of the adsl
> connection.. more details later, i just want to confirm some things
> first:
Yes, ATM cells contain a 48 byte payload and a 5 byte header. But you don't
see this because packet data is transported across ATM adaption layer 5
(AAL5). AAL5 does all the magical fragmentation and assembly to permit layer
3 datagrams to be transported across ATM.
> i presume i'm right in saying that i should *NEVER* see a fragmented
> TCP packet? (cos the DF bit is set)
If the far end is sending an IP packet with DF=1 then that packet shouldn't
be fragmented.
There are some types of client equipment which do not support path MTU
discovery and they incorrectly set DF on client->network datagrams. There's
some hackery in the Lucent/Acsend dial gateways to overcome this, but I think
this only affects dial, not ADSL DSLAMS.
>
> specifically, what does atm do with (as a hypothetical example) a
> packet of 1500 bytes? (my two bits of atm knowledge imply it isn't
> going to do the normal IP fragmentation)
I suspect that your problem (which you haven't described yet...) is due to
PPPoE overhead. If you set your PPP MTU to greater than 1492 bytes, dem
packets won't fit into ethernet frames.
Play with your PPP MTU size.
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