Rt-n16 pptp

[geoffrey]

I would be grateful for advice regarding PPTP streaming using a Asus RT-N16 router with AsusWRT-Merlin 3.0.0.4.374.40 beta2 firmware.
The router is set as a PPTP client using MPPE 128 encryption to a VPN provider in the UK. The radio is turned off.
Streaming TV suffers from intermittant stops and inadequate bandwidth warnings. However, Speedtest.net via the PPTP connection reports a ping time of 85 ms and a download speed of 13 MBPS; and BBC Speed test gives a final 5.6 kbps speed.
The Asus log does not show any events coinciding with the stops.
If I go back to my old Dlink DIR-130 router I get completely stable streaming despite a Speedtest.net speed of 2.8 MBPS and a BBC Speed test of 2.296 kbps via the PPTP connection.
Is there something going on with regards to AsusWRT-Merlin PPTP performance that I can alter to improve performance?

 

[geoffrey]

Rt-n16 pptp mtu

I am now wondering if this is something to do with the MTI settings that the RT-N16 is using for pptp connections. In both the Asus and the ASUSWRT-Merlin firmware there seems no way of directly setting MTU parameters.
What MTU settings are being used and how are the MTU setting being determined?
Can the MTU settings be manipulated using Linux commands in the pppd options field ?

 

[geoffrey]

screen shot of WAN setup screen

This is what I am trying:

# iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

 

[geoffrey]

The AsusWRT-Merlin firmware is supposed to fix PPTP MTU at 1400.

I have done some ping -f -l tests on my various routers to a UK address from Israel and have found:

The Asus RT-N16 with native firmware 3.0.0.4.374_4561 will only do unfragmented packets up to a MTU of 1372.

I get the same result with AsusWRT-Merlin firmware 3.0.0.4.374.40_beta2.

My old DLink DIR 130 (which unlike the Asus gives me a reliable PPTP connection) will do unfragmented packets up to a MTU of 1418.

Is the Merlin and Asus firmware somehow defective ? -

ADDED:
I now know that its not - its fixed at a maximum of 1400 including the header.

I tried modifying the MTU after installing Tomato 1.28.9054 firmware in the Asus RT-16.
This listed the MTU as defaulting to 1460 - but the ping test gave the same 1372 maximum unfragmented packet result as Asus and AsusWRT. Changing this field to anything else did not affect the result.

The ping test without the Asus router gives an MTU of 1464 so I presume that I should be able to get better PPTP performance if I can get somewhere near this.

 

[geoffrey]

Rather surprisingly just putting - mtu 1450 - in the pppd option field gives an unfragmented packet size of 1422 so if my theory regarding the poor performance of the Asus compared with the Dir 130 is correct then the Asus should now be as good as the Dir 130. I will test tomorrow.

ADDED
After testing performance is now as good as the DIR 130.
Remaining problem is that the dual wan does not work as it should and Mr Merlin says that its not his problem!