Help needed: cannot send packets larger 230 bytes via vodafone umts

[Tobias Roth]

On Tue, Oct 11, 2005 at 02:19:42PM +0200, Rehsack Jens (ext) wrote:
> > > 
> > > I started some diagnostics why my web wont work, and detected
> > > that packets larger than 229 byte payload via ping will swallowed
> > > by the connection (I don't know where).
> > 
> > Did you try traceroute with a packetlength larger than your 229 bytes
> > to see where the packets get dropped and for what reason?
> > 
> > greets, t.
> 
> Now I did ;)
> 
> And horribly I had to see, that traceroute seems to transport even
> larger packets an get's the wanted answers. I also tried after that
> shock >traceroute -P TCP< and seen, that it works, too.

Initially, my theory was that you have a problem with pathMTU, but I
am not so sure anymore. If pathMTU is broken, telnetting to port 80
won't work, since the packets sent have the DF bit set and thus cannot
be fragmented. When tracerouting with udp or tcp, they don't have that
bit set, so they get fragmented and things work.

However, this does not explain why your icmp pings don't get through
if larger than 229 bytes.

One last idea: You may try to lower the mtu of your ppp interface
with ifconfig down to 229 and see if telnet then works. If so, you
might have an mtu problem after all, if not, something else is rotten.

good luck, t.