I have a linux server running potop.

I am tunnelling private data through an untrusted network (which I do not control) to a Zyzel router Prestige 334 to be exact. The Zyxel has our clients plugged into it.

The tunnel is dropping in and out frequently. When I turned on logging at the linux server end; I found that GRE packets were missing.

The log would say something like:

GRE : accepting packet #3440
GRE : accepting packet #3441
GRE : buffering packet #3443 (expecting #3442, lost or reordered)
GRE : buffering packet #3444 (expecting #3442, lost or reordered)
GRE : buffering packet #3445 (expecting #3442, lost or reordered)
GRE : buffering packet #3446 (expecting #3442, lost or reordered)

I can see packet 3442 did not arrive. It is lost in the etha somewhere. The LAN I am tunnelling though has a switch on each flor of the building and a fiber connecting the switches.

The people who run this network deny that packet loss occurs on their network, and probably have no idea.

I have changed the MTU and MRU inside the pptp server to 1300 with no improvement.

Does PPTP and GRE have any knid of retry mechanism?
Typically are packets only sent once and expected to arrive?
Is there any way to set up PPTP to tolerate Packet LOSS?

I would like to add, that In this case the untrusted network is not open internet, and I am not running any encryption. So stateful encryption issues wont bother me here.

I was thinking, of changing some PPP option to tolerate and accept packets out of order, rather than buffering and waiting for the right one. I can not find such an option.

According to the RFC, some GRE loss could be tolerated by a decryptor provided that a higher layer stateful encryption is not used.

To be 100% clear, The packet loss is the GRE raw packets themself, not data inside the GRE tunnel.

The entire link fails and reconnects, resetting sequence to zero, and re negotiating.

Man, I wish I had an answer to this one. I'm having a similar issue with GRE as well using PPTP.