Frustrating new Network issue

[colinjones]

By correct I simply mean is the correct standard for the Internet. In reality, however, that's usually not used any more and numbers closer/same as the Ethernet standard are used. That's why I said "correct" not correct

I only mention it because I wanted to point out that 576 wasn't just a random number picked off a heap somewhere, it is actually the exact Internet standard MTU. Point being, that it probably came from an Internet component somewhere like the modem or provider... but come to think of it, I wonder whether the kernel did it automatically because it received an IP lease not in the 10. 192.168 or 176 ranges??

[colinjones]

hari - here we go. http://en.wikipedia.org/wiki/Maximum_transmission_unit

Note in the table the Internet MTU size is 576.

Btw, yes I realise that the path itself doesn't have an MTU as such, it is actually the "minimum MTU" over the path based on the "weakest link in the chain". But fundamentally, there is a standard MTU for "the Internet" which essentially means what you're supposed to set on all router/host interfaces sitting on the Internet. As I say, in practice this isn't used any more, but perhaps the kernel was setting it to that for this reason based on the lease it got..?

[hari]

here we go? Guy that was best practice 20 years ago :-)

[colinjones]

I realise that hari, but the reality is, it was set to that number, so I'm just suggesting that that is why it was chosen by something (broadband router? kernel?) Why else would it suddenly change to that number? Perhaps it is intended to be a "fail safe" in case the kernel gets connected to an ISP that still insists on this? Ironically, of course, that is more likely to break things, as it did in this case!

As I say, in practice this isn't used any more,

[Afkpuz]

I know that I cannot bypass the router as I use comcast for my internet.  I needed to use a router to get the internet to LMCE.  I don't know what a router does to the internet signal, but I couldn't get LMCE to replicate that.

[tschak909]

I use comcast too. I use a standard cable modem, any will do. If you do not have just a standard modem, then Buy one, tell comcast you're using one, give them the mac address..Let them activate it.. connect it to a laptop to verify.. once it works... then connect it to the LMCE core. Done.

-Thom

[hari]

@colin: the default MTU for ethernet is 1500, so I'd assume it was announced with DHCP (as I wrote in that other post). Would be interesting to see the payload of the DHCP reply.

best regards,
Hari

[colinjones]

@colin: the default MTU for ethernet is 1500, so I'd assume it was announced with DHCP (as I wrote in that other post). Would be interesting to see the payload of the DHCP reply.

best regards,
Hari

Absolutely agreed - I would like to see that payload too... Certainly, I have never set the MTU DHCP option in the past, but who knows what that modem might be doing (would linux even obey it?) Either way it could be really important to 0810. If, for instance, the kernel is deciding on its own to set 576 simply because it got a public IP address, especially if this is new behaviour in 0804/0810, then that is something we probably want to override isn't it?

[PeteK]

Guys--

I'm definitely getting this MTU setting from the ISP through DHCP.  Calling dhclient -r; dhclient gets the new MTU.  For now I've set it to 1492 manually by adding the line:

Code: [Select]

pre-up /sbin/ifconfig $IFACE mtu 1492to /etc/network/interfaces

and removing mtu from the list of requested items in /etc/dhcp3/dhclient.conf

I can't explain what was different under 0710 but I definitely did not see this issue then.

[hari]

the q is, why does the ISP announce that MTU? And even with that MTU it should work when ICMP is not filtered. Maybe the ISP is doing stupid things here...

br, Hari