[Gllug] Anyone having broadband (probably PlusNet) problems today?

[Richard Jones]

On Wed, Mar 23, 2005 at 02:35:55PM +0000, Russell Howe wrote:
> On Wed, Mar 23, 2005 at 01:35:29PM +0000, Richard Jones wrote:
> > 
> > Woke up today to find that my PlusNet internet connection behaving
> > very oddly indeed.  It's not that it doesn't work, just that certain
> > types of connections hang.
> 
> This reeks of MTU breakage, where either ICMP time-exceeded messages are
> being ignored, being blocked or not being sent at all.
> 
> Try pinging a remote host with ever-increasing ping sizes. You want to
> be making sure you are setting the DF (don't fragment) bit on the IP
> datagrams when you do so.

Interesting:

For packet sizes < 1432:

  $ ping -M do -s 1432 www.bbc.co.uk
  PING www.bbc.net.uk (212.58.224.116) 1432(1460) bytes of data.
  1440 bytes from www16.thdo.bbc.co.uk (212.58.224.116): icmp_seq=1 ttl=249 time=87.8 ms
  1440 bytes from www16.thdo.bbc.co.uk (212.58.224.116): icmp_seq=2 ttl=249 time=86.0 ms

For packet sizes >= 1433, <= 1472:

  rich at arctor:~$ ping -M do -s 1472 www.bbc.co.uk
  PING www.bbc.net.uk (212.58.224.123) 1472(1500) bytes of data.
  ^C
  --- www.bbc.net.uk ping statistics ---
  3 packets transmitted, 0 received, 100% packet loss, time 1999ms


For package sizes >= 1473:

  rich at arctor:~$ ping -M do -s 1473 www.bbc.co.uk
  PING www.bbc.net.uk (212.58.224.123) 1473(1501) bytes of data.
  From arctor.home.annexia.org (10.0.0.249) icmp_seq=1 Frag needed and DF set (mtu = 1500)
  From arctor.home.annexia.org (10.0.0.249) icmp_seq=1 Frag needed and DF set (mtu = 1500)

So it's a large packet / MTU problem, as suspected.

> If you are running PPPoA on a DSL link, your MTU is likely 1500.

These are the interfaces I've got on the ADSL router:

Interface       GRP MTU   RX       TX       TX-DROP  STATUS       HWADDR
0  loop         1   1500  2199     0        0          UP   
1  eth0         2   1500  2105743  10232918 0          UP   00:90:d0:02:20:7f
2  pppoa        0   1500  10033378 2001108  0          UP   


Do I suspect that the ADSL router is broken?  It seems to be throwing
away packets between 1460 - 1500 bytes in size ...

Note that nothing changed at all, configuration-wise, between
yesterday and today.

Rich.

-- 
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com
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