Re: [Openvpn-users] Slow over satellite connection

[James Y. <ji...@yo...>]

On Tue, 14 Sep 2004, Dick St.Peters wrote:

> Eric Liew writes:
> > I have difficulties setting up VPN connections via satellite & ADSL connection.
> > Computer A (Win200) is connected via 512kbps / 256kbps satellite link. 
> > Computer B (Win200) is connected via 2Mbps / 256kbps ADSL and sit on an internal network.
> > 
> > I managed to connect the 2 computers but the connection was VERY slow. No abnormal errors in log file.
> > It took 8 mins to transfer a 4MB file (about 8kbps). Any configuration that I miss?
> 
> Your arithmetic is wrong.  4MB transferred in 8 minutes is a little
> under 70 kbps, not a mere 8 kbps.  So you're getting about 25% of the
> theoretical maximum for a 256 kbps uplink.
> 
> As for why only 25%, my guess is it has a lot to do with the latency
> of a satellite link.  Satellites in geo-synchronous orbit are a long
> ways out - about 22,000 miles (35000 km).  Each packet must go out and
> back, roughly equivalent to going around the Earth twice, which gives
> a megabit link the latency of a dialup connection.  This makes it hard
> for TCP, which must wait for acks, to make use of anything like the
> full bandwidth.

My understanding of TCP is that it measures the "window size" of the
connection, i.e. the number of bytes which sit in the "pipeline" between
sender and receiver, and uses this value to calibrate ACK timing.  So a
high-latency connection such as a satellite link shouldn't have too much
effect on usable bandwidth as long as the frequency of dropped packets is 
low.

James