mtu 1500 FTTC RFC RFC 4638 - asus routers

Merlin has kindly implemented changes at my request to allow this to be easily configured in the gui now.

changelog here.

for routers ac66u/ac68u/rt-n16u/rt-n66u.

http://www.lostrealm.ca/asuswrt-merlin/changelog.txt

?
It seems to be easy in the standard ASUS GUI, in that I can edit the field?

Mine on Plusnet is 1492, so I didn't Apply the change. Do you think it should be 1500?

1500 will work if you enable jumbo frames as well.

But this is a merlin firmware only change I dont think it will work in standard asuswrt as the gui as a failsafe will block a 1500 value been entered, traditional pppoe cannot do 1500 byte mtu.

Can you still ping bbc.co.uk with a 1500 packet?

ie. ping bbc.co.uk -l 1500

Maximum I can ping BBC is with a packet size of 1464, above that it fails.

I can however ping my isp with 1492.

Just out of interest would setting the MTU at 1500 cause packets to certain websites to fragment and cause inefficiency?

See my pings below...

C:\Users\Paul >ping bbc.co.uk -l 1464

Pinging bbc.co.uk [212.58.246.104] with 1464 bytes of data
Reply from 212.58.246.104: bytes=1464 time=30ms TTL=54
Reply from 212.58.246.104: bytes=1464 time=31ms TTL=54
Reply from 212.58.246.104: bytes=1464 time=32ms TTL=54
Reply from 212.58.246.104: bytes=1464 time=31ms TTL=54

Ping statistics for 212.58.246.104:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 30ms, Maximum = 32ms, Average = 31ms

C:\Users\Paul >ping bbc.co.uk -l 1465

Pinging bbc.co.uk [212.58.246.104] with 1465 bytes of data
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 212.58.246.104:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\Users\Paul >ping zen.co.uk -l 1492

Pinging zen.co.uk [212.23.0.100] with 1492 bytes of data:
Reply from 212.23.0.100: bytes=1492 time=28ms TTL=120
Reply from 212.23.0.100: bytes=1492 time=26ms TTL=120
Reply from 212.23.0.100: bytes=1492 time=26ms TTL=120
Reply from 212.23.0.100: bytes=1492 time=26ms TTL=120

Ping statistics for 212.23.0.100:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 26ms, Maximum = 28ms, Average = 26ms

I can when this is done properly yes. (I am on older firmware with my mtu manually configured).

bear in mind I havent tested this new firmware.

on older firmwares (using pppoe 3.11). One has to set the mtu manually in the command line, also jumbo frames has to be enabled as it requires the network device to have a mtu of 1508 to allow the extra 8 bytes overhead of pppoe. I am not sure if merlin accounted for this on his firmware changes.

so it may still need a mtu command in the command line to set the parent device to 1508 to work.

if you have flashed this firmware.
have enabled jumbo frames on the router.
have set 1500 bytes mtu

and cannot ping with unfragmented 1460 bytes then its not working as it should.

ping -f -l 1460 bbc.co.uk

if that fails set mtu back to 1492.

Edited by Chrysalis (Tue 04-Feb-14 22:59:41)

In reply to a post by Chrysalis:

Merlin has kindly implemented changes at my request to allow this to be easily configured in the gui now.

Excellent! Will have to give this a go.

In reply to a post by Chrysalis:

cannot ping with unfragmented 1460 bytes then its not working as it should.

ping -f -l 1460 bbc.co.uk

if that fails set mtu back to 1492.

Shouldn't the 1460 above be 1465 or more? 1492 MTU -28 = 1464 packet size.

If your MTU is actually 1492:
then ping -f -l 1465 bbc.co.uk should give 'Packet needs to be fragmented but DF set.'
but ping -f -l 1464 bbc.co.uk should be OK.

For an actual MTU of 1500 the biggest unfragmented packet is 1472.

Edited by XRaySpeX (Tue 04-Feb-14 23:48:00)

I assume you have read the current SIN 498, which includes:-

For example, if the reported downstream VDSL2 data rate is 40,000 kbit/s and the IP packet size is 1500 bytes (i.e. Ethernet frame size at End User LAN is 1514 bytes

and

The maximum supported Ethernet frame size is 1530 bytes (excluding IFG and pre-amble and single/double tag – see 2.1.3).

correct, sorry for misinformation, so 1472 bytes for a 1500 byte mtu. make sure the -f flag is on the ping command.

C:\windows\system32>ping -f -l 1472 bbc.co.uk

Pinging bbc.co.uk [212.58.246.104] with 1472 bytes of data:
Reply from 212.58.246.104: bytes=1472 time=17ms TTL=56
Reply from 212.58.246.104: bytes=1472 time=17ms TTL=56
Reply from 212.58.246.104: bytes=1472 time=17ms TTL=56
Reply from 212.58.246.104: bytes=1472 time=17ms TTL=56

Ping statistics for 212.58.246.104:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 17ms, Maximum = 17ms, Average = 17ms

Edited by Chrysalis (Wed 05-Feb-14 04:23:47)