MTU on BT Wholesale ADSL

[pctech]

My connection has been on a steady march upwards for the past few weeks so I decided to see if I could wring some more performance out of it by upping the MTU from 1430 to 1458 on the router.

Since doing that its been flapping a bit so my question is, what MTU value have people found works best?

[Steve]

I've normally left the router wide open and set the MTU on the adaptor i.e. ping google.com -f -l 1472 gives an MTU setting of 1500.

Or for a Mac it's ping -c 2 -D -s 1464 example.com since I'm on PPPOE as well.

[Lance]

Changing the MTU should change the physical performance of the line, only throughput performance.

[Rik]

I'm with Lance and Steve, I use 1500 and changing it should not affect stability.

[wecpcs]

I'm with Lance and Steve, I use 1500 and changing it should not affect stability.

I also use 1500 and have always done so with no problems whatsoever.

Colin

[pctech]

Cheers will adjust it back up, need to do it on the router as also have my xbox connected now.

[Steve]

Just find the size that packets fragment and add 28

[pctech]

fair enough, moved it back to 1500, will see how it goes, if not will move back to 1430, just wanted to see if I could wring some more speed out.

[psp83]

Just found this :-

MSS - Maximum Segment Size:
The maximally allowed size of a segment. MSS is closely related to MTU.

MTU - Maximum Transmit Unit:
The maximally allowed size of a network packet in bytes. This is normally 1500, but must be set to 1492 or lower for DSL.
(The correct setting should be a maximum of 1492, but actually it needs to be 8 bytes less than any router you pass through on the way to the site.)

MRU - Maximum Receive Unit:
Must be set to 1492 or lower for DSL

Rwin - Receive Window:
Should be set to 32767 for DSL.

TTL - Time To Live:
Normally 32, should be set to 64 for DSL (Chip 12/03 p.248 and Chip 10/02 p.250)

[psp83]

My 7800n is set to 1492 by default, but in the logs I see it tries to set the MTU & MRU to 1500 automatically but fails.

  Feb 10 08:44:01  daemon  pppd[604]: Couldn't increase MTU to 1500.
  Feb 10 08:44:01  daemon  pppd[604]: Couldn't increase MRU to 1500

So I'm leaving it at 1492

[pctech]

Cheers Paul.

[Steve]

My 7800n is set to 1492 by default, but in the logs I see it tries to set the MTU & MRU to 1500 automatically but fails.

So I'm leaving it at 1492

Paul your on FTTC so the max is 1492 for PPPOE , adsl you should get 1500 with IDNet

[pctech]

still experiencing resyncs every so often so not sure what is going on.

I shall put it back to 1430 and see how I get on, I say resyncs but not sure whether they are, just basing that on the WAN uptime count resetting.

[Lance]

MTU is to do with packet sizes and wouldn't cause a resync.

[armadillo]

still experiencing resyncs every so often so not sure what is going on.

I shall put it back to 1430 and see how I get on, I say resyncs but not sure whether they are, just basing that on the WAN uptime count resetting.

You could keep Router Stats Lite running and its graph will show you resyncs if they occur.

On the MTU point, it could be worth running the TCP/IP Analyzer tool
http://www.speedguide.net/sg_tools.php
to see what your MTU actually is. It might not be what you set it to. The last time I tried setting mine to 1500, the tool told me it was not 1500.

FWIW, my MTU (on idnet) is now 1458 according to both my router maintenance WAN setup and the TCP/IP Analyzer tool. I consistently get the maximum possible throughput for my sync speed, just over 7mb/s so I see no point in messing with my MTU

[edit] I should also add that I found RWIN had a hugely greater effect than MTU on throughput. The tool I linked to will tell you your RWIN too. If your version of Windows lets you alter RWIN, that might be a more fruitful avenue to explore.

[Lance]

Also worth remembering that your router will have one setting whilst later versions of windows dynamically adjust theirs.

[nowster]

Many operating systems now automatically work out the optimal MTU for a path to a server (Path MTU Discovery).

The "win" for ADSL is to use an MTU/MRU which allows you to fit the maximum size IP packet exactly into a whole number of ATM frames with no remainder. However, the gain in throughput is slight. I forget the exact figures.

[pctech]

Am running XP so it can be adjusted manually but Vista and 7 automatically manage it.

I'll look at the tools but all seems to be well after setting it back to 1430