On 26 September 2013 14:36, James Harper <james.harper@bendigoit.com.au> wrote:
On 26 September 2013 14:13, James Harper <james.harper@bendigoit.com.au> wrote:
However for some reason path MTU discovery has completely broken when using the newer modem. The primary Linux box is fine, but all clients (whether wired, wireless *or even virtual*) fail to work unless I manually set their interfaces to an mtu of 1492.
Is it outbound or inbound MTU path discovery that isn't working? Or both?
Hmm, possibly inbound, now you mention it.. I was testing by attempting to curl web pages or images, and didn't try sending anything outbound at the time.
I've had to set MRU explicitly before to get things working properly, although I don't see why the different modem would make a different, unless it was using baby jumbo frames or something.
Both models are 10/100 ethernet only.
What does 'ip link show dev ppp0' say? Is MTU definitely 1492 in there?
Yes, definitely 1492 mtu listed there.
What happens when a client computer tries to send a larger packet? Does your Linux router send a 'frag required' response?
If I'm reading the tcpdump correctly (see reply to myself with it this morning) then yes, there is a frag-required response.
Right, so your ISP router isn't sending frag required packets properly by the sounds of it, possibly because it thinks that you can accept such packets.
Are the modem settings exactly the same, including the VC-MUX / LLC settings? This can throw things out if it makes assumptions based on those settings.
As close as they could be, given differences in modem UI and options. However both old and new are set to LLC, yes.
Can you try setting the mru explicitly to 1492 in your ppp config? This will tell the other end that you can't accept packets bigger than 1492 bytes.
OK. I've just tried that now -- does NOT fix the problem. T