VPN Problem: Extra TCP ACK in a VPN communication.

[Sam]

Hi,

I have the following strange problem:

I have a VPN server setup behind a firewall (Linksys in this case). When I
connect to this server from the internal network everything works fine. When
I try to connect to this server from the outside the connecting client
responds with an ACK packet instead of a PPP LCP packet. Eventually the
client times out with error 721.

Here is the packet trace:
WHO PROT. INFO
---------------------------------------------------------
1. Client: TCP PPTP [SYN]
2. Server: TCP PPTP [SYN, ACK]
3. Client: TCP PPTP [ACK]
4. Server: PPTP START-CONTROL-REQUEST
5. Client: PPTP START-CONTROL-REPLY
6. Server: PPTP OUTGOING-CALL-REQUEST
7. Client: PPTP OUTGOING-CALL-REPLY
8. Server: PPTP SET-LINK
9. Client: TCP pptp [ACK] <-- This is wrong, should be a PPP LCP
packet.

Any idea what's going on here? Why would the client send an ACK instead of
the PPP LCP?

Thanks
-Sam

 

[Carl DaVault [MSFT]]

PPTP PPP Packets are carried in GRE, not TCP.

If you're just seeing an empty ACK, the PPTP call setup is probably over so
there's no more data to send along with the ack.