[Gllug] Problem with new Virgin 50Mbps Modem

James Courtier-Dutton james.dutton at gmail.com
Tue Mar 27 19:11:53 UTC 2012

On 27 March 2012 18:52, Alain Williams <addw at phcomp.co.uk> wrote:
> You may be interested in the solution for a customer of mine who recently
> installed (upgraded to) the Virgin 50Mbps Super Hub.
> The symptoms were lost packets, this badly affected incoming emails (we run an
> MTA) especially, for some reason, those from google/gmail. There was (I think)
> also a problem with an SSL session.
> The fix is to set the MTU to 1460. We did that with an immediate improvement.
> The sad thing is that we have been beating our heads on the table for a week,
> with various Virgin support droids blaming our config.
> On a redhat system edit /etc/sysconfig/network-scripts/ifcfg-eth0 and
> insert the line:
>    MTU=1460
> After fixing it you will see 1460 as in below:
> # ifconfig eth0
> eth0      Link encap:Ethernet  HWaddr 00:0D:56:64:3A:C6
>          inet addr:  Bcast:  Mask:
>          inet6 addr: fe80::20d:56ff:fe64:3ac6/64 Scope:Link
>          RX packets:31171623 errors:4 dropped:13 overruns:0 frame:0
>          TX packets:19810324 errors:0 dropped:0 overruns:0 carrier:0
>          collisions:0 txqueuelen:1000
>          RX bytes:4150314049 (3.8 GiB)  TX bytes:3814195185 (3.5 GiB)
>          Interrupt:201


Just so you know, the cause is a problem with their network, not yours.
The problem you were running into is call "black hole" packets.
This is where, if you send packets of varying sizes, some will not get through.
What you did is work around the problem, not actually fix it.

What should happen is this:
For all packet sizes up to a fixed limit, packets with the DF bit set
should get through.
For any packets sizes larger than that fixed limit, you should get a
ICMP packet back saying the packet was too big.

The problem with their network is that there are black holes.
I.e. Packets of certain sized get lost, instead of "get through" or
"get ICMP response".

The problem packet sizes in your case, are somewhere between 1460 and 1500.
The problem could be anything on their network, the new home hub, or
some device in their network.

Kind Regards

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