VPN over slow Internet connections

Thu Apr 21 21:28:46 UTC 2011

On Apr 21, 2011, at 4:20PM, Steven Bellovin wrote:

> For your application or for the VPN?  For the VPN, I *strongly*
> suggest you use UDP, or you're going to get dueling retransmissions
> and spend a lot of time sending many copies of the same thing. Consider:
> if a packet is dropped, either due to line noise or queuing delay for
> the slow link, the sending TCP will resend.  If you're using TCP for
> OpenVPN, that session's TCP will resend.  Of course, the TCP running
> on top of it will resend as well, so you'll get two copies of the data
> sent to the application's TCP, wasting precious bandwidth.  

Is this actually how OpenVPN's TCP encapsulation works? I'd be curious to
know. It isn't how Cisco's TCP/10000 encapsulation works, at least not with
the IOS devices I have experience with.

Cisco's TCP/10000 looks like TCP to a firewall, but it really isn't. There
is no reliability -- no retransmits, etc. It's pretty close to UDP behavior
but with a TCP header, which was confusing to troubleshoot at first but
quickly made perfect sense to me for the reasons you state above.

-Terry