[GTALUG] Rogers and MTU size

D. Hugh Redelmeier hugh at mimosa.com
Mon Jan 5 04:42:14 UTC 2015


| From: James Knott <james.knott at rogers.com>

| One thing I noticed recently was that my firewall, running Linux, was
| getting a MTU of 576 bytes, instead of the normal 1500 bytes.

576 is the smallest legal MTU for IPv4.  I could imagine some system
defaulting to this in the absence of other knowledge.

In other messages you show that this is an explicit setting from Rogers' 
DHCP server.  That certainly sounds like a bad configuration at their end.

My Rogers connection has an MTU of 1500.

The normal (non-jumbo) MTU size for ethernet is 1500.

ADSL uses PPPoE encapsulation which eats 8 bytes off the MTU.

|   You can
| verify the MTU with a ping -s 1500 <destination>.  If that works, then
| you have a 1500 byte MTU along the entire path to the destination.

Not quite.  "ping -s 1500" on IPv4 generates a 1508-byte ICMP packet.
Plus any encapsulation that might be used (probably none with Rogers).

Elsewhere you mention PMTU discovery.  The folk wisdom (as of a few years 
ago) was that this is useless since so many routers and firewalls drop the 
necessary ICMP messages.  Sad.

| I have advised Rogers of this and the person I was speaking to agreed it
| should be 1500.  Let's see if they fix it.

Wow.

Years ago I tried to report DHCP RFC violations to Rogers customer
support folks and got nowhere.  There seemed to be no connection
between the customer support department and the relevant engineering
department.


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