On 3/20/2011 9:28 PM, [email protected] wrote:
On Mon, 21 Mar 2011, Jonathan Morton wrote:
On 21 Mar, 2011, at 12:18 am, [email protected] wrote:
0) Buffering more than 1 second of data is always unacceptable.
what about satellite links? my understanding is that the four round
trips to geosync orbit (request up, down, reply up down) result in
approximatly 1 sec round trip.
That is true, but it doesn't require more than a full second of
buffering, just lots of FEC to avoid packet loss on the link. At those
timescales, you want the flow to look smooth, not bursty. Bursty is
normal at 100ms timescales.
What I've heard is that most consumer satellite links use split-TCP
anyway (proxy boxes at each end) thus relieving the Internet at large
from coping with an unusual problem. However, it also seems likely
that backbone satellite links exist which do not use this technique. I
heard something about South America, maybe?
I've heard that they do proxy boxes at each end for common protocols
like HTTP, but they can't do so for other protocols (think ssh for example)
Anyway, with a 1-second RTT, the formula comes out to max 1 second of
buffering because of the clamping.
and what if you have a 1 second satellite link plus 'normal internet
latency', or worse, both ends are on a satellite link, giving you a
2-second+ round trip time?
if you don't have large enough buffers to handle this, what happens?
Propagation delay to a geosynchronous relay is ~125 ms. Round-trip
propagation delay contributes ~500 ms, so it isn't quite as bad as you
think, though still long.
Many tricks are played with accelerator gateways to improve bulk
transfer throughput for TCP users (see e.g. RFC 3135).
There may be a challenge in debloating the devices that support such
links, as their buffers and the functions they serve optimize other
metrics (e.g. low packet loss rate, bulk transfer throughput, etc.).
--
Wes Eddy
MTI Systems
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