Re: brctl MTU problem over DSL

From: marian stagarescu (mstagarescu@telogy.com)
Date: Fri Jul 19 2002 - 11:32:41 EST


On Friday 19 July 2002 12:11 pm, Stephen Hemminger wrote:
> Bridging between networks with different MTU's is always going to be a
> problem. You can of course set a smaller mtu with ifconfig.

i tried this on the br0, eth0, nas0(atm) interfaces: nothing.

i don't think this is going to work anyway: the bridge is a passive
device and does not communicate the MTU to upstream/downstream.

> Ideally, the bridge would be smart enough to send back ICMP packets so
> MTU discovery would work, but the bridge probably doesn't understand IP.

that's exactly the problem. the bridge is just a bridge does not do IP.

I was trying to figure out the stack on this situation:

                                    
                           bridge (eth frame)
                                     ^
                                     |
                                     v
 br2684-add/strip LLC from eth frame (interface in promisc mode)
                                    ^
                                    |
                                    v
                                 driver
ROUTER<--->DLSAM< ---> ATM card (hw SAR)

If this works one way (from eth side toward ATM) why it breaks on the other
side ?

> On Thu, 2002-07-18 at 19:04, marian stagarescu wrote:
> > asymmetrici have a linux-bridge like below:
> >
> > br0
> >
> > |br2684 |
> >
> > --DSL line---|atm eth|------LAN
> >
> > I run a bridge on top of a regular eth interface and an
> > ATM interface which appears to the bridge as an ethernet interface (using
> > br2685 encapsulated ethernet over atm/aal5).
> >
> > ifconfig will show MTU 1500 on all devices (br0, eth0, nas0 (created by
> > br2685ctl)
> >
> > Now here is the problem: MTU
> >
> > from LAN if I do a ping with packet size over 1428 bytes doesn't do it.
> > an ftp get from lan will also break if the file is not too small to fit
> > under 1428. interesting an ftp-put from lan side will work with big files
> > ( MTU ?)
> >
> > how is this fixable ?
> >
> > moreover if this box runs in routed mode
> >
> > IP
> > ---internet---dsl---|br2684 eth0|---LAN
> >
> > this works in terms of data transfers both ways with large packet sizes
> > with pppoe over br2684 (over atm) or directly pppoa over ATM.
> >
> > i remember that this MTU was a problem with pppoe (1492 and not 1500
> > MTU).
> >
> > my device being a bridge will not participate in any path mtu discovery
> > or send ICMP fragmentation required messages back an originating server
> > (like the ftp server above). I wonder how it works in the ftp-put mode ?
> >
> > thanks,
> > marian
> >
> >
> >
> >
> > -
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