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Re: VPN over slow Internet connections
From: Steven Bellovin <smb () cs columbia edu>
Date: Thu, 21 Apr 2011 17:53:36 -0400


On Apr 21, 2011, at 5:28 46PM, Terry Baranski wrote:

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.

To the OS, OpenVPN is an application that uses the underlying TCP (or UDP)/IP
stack; it can't behave any differently than any other application.  Since
(as far as I know) Windows, Linux, NeBSD, FreeBSD, MacOS, and all of the other 
platforms that OpenVPN runs on just have normal TCPs, that's what OpenVPN
does.


                --Steve Bellovin, https://www.cs.columbia.edu/~smb







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