Latency, TCP ACKs and upload needs

Jean-Francois Mezei jfmezei_nanog at
Wed Apr 20 01:29:12 UTC 2016

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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