hi miki,
| 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.
The MTU limit is because bezeq uses a fast ATM backbone for the whole
ADSL operation. ATM works with little packets. Also, even in LANs, system
administrator rarly give a MTU larger than 8K since this tends to slow
down RTT (round trip time).
| Issue II
| --------
| What if the other side of our (above-described) tunnel session between ISP
| computer and home-linux-router (or redback and home-linux-router) frags
| packets?
|
| Does the ADSL modem handle fragmented packets from the ISP side correctly?
| My guess is "NO, it's broken here too", because this problem is
| ISP specific.
| Obviously this poses a problem from some ISP's and doesn't from
| others. (probbably those that also worked around this problem by limiting
| the MTU on the tunnel interface on THEIR side) to avoid sending too large
| packets to your DSL modem.
|
| What do we do then? notify them?
For this one of the reasons the ICMP protocol exsists. If a packet too
large comes to a router, it drops it, and send back a ICMP message to the
sender, with a "please fragment" request.
| Final comment, I don't know this issue that well, but can these ISP
| routers be convinced to send smaller packets by sending them ICMP
| source-quench requests? is this done automatically by some socket
| mechanism?
I doubt that. The limitation are for a reason.
Shlomi.
--
-------------------------------------------
Shlomo Matichin [EMAIL PROTECTED]
The Mosix Group www.mosix.org
=================================================================
To unsubscribe, send mail to [EMAIL PROTECTED] with
the word "unsubscribe" in the message body, e.g., run the command
echo unsubscribe | mail [EMAIL PROTECTED]
