> It is my complete guess that the Linux 'NAT' code forwards packets and
> only rewrites the source IP/port address on outgoing packets and does
> not modify the packet in any other way (like fragmenting it). Someone
> correct me if I am wrong ;)
You're both correct and incorrect.
You're looking at IP-layer modifications. True, NAT, under linux or not,
does not include any modifications besides the IP.
BUT
when ANY (linux or otherwise) implementation of TCP/IP wants to stuff a
10KB packet of a high-level layer into a lower layer that can only convey
1.5KB chunks, it does fragmentation. It happens between any 2 layers.
For instance, your Winsock wants to stuff a 4KB IP packet (Network
Layer) into an ethernet packet (Hardware layer) whose limited payload is
1500 bytes. So it fragments the 4KB into 3 1.5K packets.
Now, the ADSL:
Linux will want to frag anything that it has to stuff into the ppp
interface into 1452-byte chunks (which will get a header on top of them
and be conveyed to the adsl modem as 1500 byte chunks over the ethernet
inferface) - this is both if it gets the large packet from the LAN to
forward, or an application generated it on the linux box itself.
My initial two issues are still unanswered :-)
---= Miki Shapiro =------------------
---= Cell: (+972)-56-322433 =--------
---= ICQ: 3EE853 =-------------------
---= Windows Programmer in Rehab =---
-------------------------------------
"If at first you don't succeed...
.. Skydiving is probbably not for you."
On Mon, 11 Jun 2001, Cedar Cox wrote:
>
> On Mon, 11 Jun 2001, Miki Shapiro wrote:
>
> > Now to fragmentation problems:
> >
> > Fragmentation is done when data passes between layers, and, providing that
> > I trust my ISP's tunnel enterance and my linux router's tunnel exit (or
> > vice versa), fragmentation somewhere in the tunnel infrastructure is
> > botched up.
> >
> > Now, for my two Issues:
> >
> > I
> > ---
> > If I understood this correctly from Mulix, when a too-large packet
> > containing ppp-encapsulated stuff comes to the ADSL modem on
> > ethernet interface and wants to go on the DSL interface, the frarmentation
> > mechanism of the modem (I'm talking about my ATUR3) is broken.
> > Workaround: don't send packets larger than so-and-so from your linux box
> > to your ADSL modem.
> > The bottlenecking is done by the ppp interface (limited to MTU 1452) and
> > once we do that, We're completely sure that the packets that reach the
> > ADSL modem over the ethernet interface will be no larger than
> > (what the ppp driver constructed plus the 48 bytes it added) - 1500 bytes
> > and their ppp core will be no larger than 1452.
> > And if they're smaller than 1500, the modem doesn't need to frag them
> > before sending over the DSL. Problem Solved.
> >
> > Now can someonce PLEASE explain to me why we need a SECOND bottleneck by
> > limiting the MTU Win9x-client-to-linux-over-ethernet traffic, as this
> > traffic is bekol-mikre encapsulated in the ppp shell, and isn't seen by the ADSL
> > modem as IP traffic at all?
> > Why wouldn't an ethernet with 64K IP packets work? If I understand
> > correctly, it would.
>
> It is my complete guess that the Linux 'NAT' code forwards packets and
> only rewrites the source IP/port address on outgoing packets and does not
> modify the packet in any other way (like fragmenting it). Someone correct
> me if I am wrong ;)
>
> -Cedar
>
>
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