Latency, TCP ACKs and upload needs
eric.kuhnke at gmail.com
Wed Apr 20 02:01:10 UTC 2016
One of the things to consider is that geostationary satellite operators
operate based entirely on the economics of oversubscription.
If you were to purchase a full duplex 1 Mbps x 1 Mbps connection via VSAT
terminal in North America (whether C, Ku or Ka-band) you'd be looking at
$2000/month or more. You'd have a fixed FDD pipe at 495ms latency end to
32:1 oversubscription or more is normal. It costs close to $150 million to
build and launch a 5000 kilogram satellite into geostationary orbit before
you build any teleport infrastructure. The entire satellite has far less
aggregate data throughput capacity than two strands of singlemode.
Modern Ka-band satellites as used by consumer grade VSAT services in the
United States use dozens of individual spot beams. The FDD capacity in each
spot beam may be exhausted or significantly oversubscribed in one
geographical region, yet relatively unused in others. Compare, for example,
real world user reported speeds at 10pm on Exede service in western WA
state vs. somewhere in a very rural part of Wyoming.
Spot beam TDMA contention ratios are carefully managed by satellite
operators - they're very much aware of the issue you describe, and do their
best to mitigate it. Extensive massaging of TDMA parameters in spot beams
is the only way that it's economical to offer service for between $75 to
$150/month even with a 2 or 3 year contract.
There are a number of physics, OSI layer 1 and 2 issues to consider with
satellite before discussing anything TCP related.
On Tue, Apr 19, 2016 at 6:29 PM, Jean-Francois Mezei <
jfmezei_nanog at vaxination.ca> wrote:
> As part of the ongoing CRTC hearings, the incumbents' claim that
> continued implementation of the current 5/1 standard would make Canada a
> world leader for broadband in the future.
> A satellite company who currently can't even deliver its advertised 5/1
> now brags its next satellite will deliver 25/1.
> So I have a few questions:
> Considering a single download TCP connection. I am aware that modern TCP
> stacks will rationalize ACKs and send 1 ACK for every x packets
> received, thus reducing upload bandwidth requirements. Is this basically
> widespread and assumed that everyone has that ?
> Also, as you split available bandwidth between multiple streams, won't
> ack upload requirements increase because ACK rationalisation happens far
> less often sicne each TCP connection has its own context for ACKs?
> When one considers the added latency of satellite links, does 25/1 make
> sense ? (I need a sanity check to distinguish between marketing spin
> presented to the regulator and real life)
> I noticed that in the USA, EXEDE Satellite advertises 12/3 plans and
> they are also on a VIA Sat satellite, presumably the same vehicle that
> Xplornet tries to deliver its measly 5/1 on. Would all beams be
> identical on a satellite or can they be configured differently with a
> ISP adjustable rate of upload/download inside the same spectrum ?
> Also, when you establish a TCP connection, do most stacks have a default
> window size that gives the sender enough "patience" to wait long enough
> for the ACK ?
> If sender sends packet 457, doesn't get ACK in time and resends 457,
> doesn't that also result in reduction in window size (the very opposite
> of what would be needed in high latency links) ?
> And when the first ACK finally arrives, won't the sender assume this ACK
> was for the resent 457 ? Or is satellite latency low enough that
> stacks all have enough default "patience" to wait for ACKs and this is a
> non issue ?
> (Note Xplornet refused to answer questions on whether they operate
> special proxies at their gound stations to manage TCP connections to
> appear "close").
> What i am trying to get at here is whether 25/1 on satellite, in real
> life with a few apps exchanging data, would actually be able to make use
> of the 25 download speed or whether the limited 1mbps upload would choke
> the downloads ?
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