Topic: PPTP Masquerading problem...

[Franck]

Hi everybody.

I'm using an SME 5.1.2 box connected to internet via an ADSL ethernet modem (PPPoE protocol). I'm trying to connect to a PPTP server from a Windows XP client behind that gateway. Connection negociation is working fine, but it seems that "big" packets are lost. Does anyone has the same problem ? Is there an issue when using PPTP masquerading with PPPoE ?

Let's have a look to the following "ping" command with various packet size (ping from XP client to a remote host (192.168.16.1):

C:\>ping -f -l 1373 192.168.16.1

Pinging 192.168.16.1 with 1373 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 192.168.16.1:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\>ping -f -l 1372 192.168.16.1

Pinging 192.168.16.1 with 1372 bytes of data:

Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 192.168.16.1:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

C:\>ping -f -l 1368 192.168.16.1

Pinging 192.168.16.1 with 1368 bytes of data:

Reply from 192.168.16.1: bytes=1368 time=227ms TTL=255
Reply from 192.168.16.1: bytes=1368 time=222ms TTL=255
Reply from 192.168.16.1: bytes=1368 time=223ms TTL=255
Reply from 192.168.16.1: bytes=1368 time=238ms TTL=255

Ping statistics for 192.168.16.1:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 222ms, Maximum = 238ms, Average = 227ms

It seems there's a "black hole" for packet-size from 1369 to 1373 bytes. why ?

[pat]

Your ISP's routers may have default limits to packet sizes ie.:

"Packet needs to be fragmented but DF set."

for packets of 1373 bytes of data or larger on one of the routers,

and a black hole router for packets 1369 bytes and over.

The obvious solution would be to keep your packet size at 1368 bytes

or less.