[TriLUG] PoPToP VPN performance

[Scott Chilcote]

Jeremy Portzer wrote:
> There are some other things to consider besides blindly reducing the MTU, 
> though I don't know for certain what the best option is.  See 
> http://www.linux.org/docs/ldp/howto/IP-Masquerade-HOWTO/mtu-issues.html
> for a discussion.
> 
> --Jeremy

I hit the MTU problem when I attempted to combine PPTP with Earthlink 
DSL's PPPoE protocol requirement (2 x packet wrappers).  The symptom for 
this problem, at least in my case, was total loss of any sizable 
outgoing communication.

In effect, the connection would hang forever if it was dependent on 
something at the far end receiving what I was sending.  That would 
certainly hose an attempt to export a large file, as you were doing.

You can check this by ssh'ing to the remote system, sending a large text 
file to it, and running "tail -f <file>" to see if it's getting there.

If it's arriving at all, but slowly, I'd be more concerned about the 
connection bandwidth.  Does the remote VPN server have the bandwidth 
necessary to support the VPN fan-out?  You can certainly have a 
bottleneck there.

If not, it's easy to test the MTU theory.  "ifconfig mtu 1440 eth0" is a 
good place to start (if you're using eth0).

--
Scott C.