[Starlink] When Flows Collide?
Jack Haverty
jack at 3kitty.org
Fri Mar 8 18:44:07 EST 2024
It's great to see that latency is getting attention as well as action to
control it. But it's only part of the bigger picture of Internet
performance.
While performance across a particular network is interesting, most uses
of the Internet involve data flowing through several separate networks.
That's pretty much the definition of "Internet". The endpoints might be
some kind of LAN in a home or corporate IT facility or public venue.
In between there might be fiber, radio, satellite, or other (even
whimsically avian!?) networks carrying a users data. This kind of
system configuration has existed since the genesis of The Internet and
seems likely to continue. Technology has advanced a lot, with bigger and
bigger "pipes" invented to carry more data, but fundamental issues remain.
System configurations we used in the early research days were real
experiments to be measured and tested, or often just "thought
experiments" to imagine how the system would behave, what algorithms
would be appropriate, and what protocols had to exist to coordinate the
activities of all the components.
One such configuration was very simple. Imagine there are three very
fast computers, each attached to a very fast LAN. The computers and
LAN can send and receive data as fast as you can imagine, so that they
are not a limiting factor. The LANs are attached to some "ISP" which
isn't as fast (in bandwidth or latency) as a LAN. ISPs are
interconnected at various points, forming a somewhat rich mesh of
topology with several, or many, possible routes from any source to any
destination.
Now imagine a user configuration in which two of the computers send a
constant stream of data to the third computer at a predefined rate.
Perhaps it is a UDP datagram every N milliseconds, each datagram
containing a frame of video. If N=20 it corresponds to a 50Hz frame
rate, which is common for video.
Somewhere along the way to that common destination, those two data
streams collide, and there is a bottleneck. All the data coming in
cannot fit in the pipe going out. Something has to give.
Thought experiment -- What should happen? Does the bottleneck discard
datagrams it can't handle? How does it decide which ones to discard?
Does the bottleneck buffer the excess datagrams, hoping that the
situation is just temporary? Does the bottleneck somehow signal back
to the sources to reduce their data rate? Does th ebottleneck discard
datagrams that it knows won't reach the destination in time to be
useful? Does the bottleneck trigger some kind of network
reconfiguration, perhaps to route "low priority" data along some
alternate path to free up capacity for the video streams that requires
low latency?
Real experiment -- set up such a configuration and observe what happens,
especially from the end-users' perspectives. What kind of video does
the end-user see?
Second thought experiment -- Using the same configuration, send data
using TCP instead of UDP. This adds more mechanisms, but now in the
end-users' computers. How should the ISPs and TCPs involved behave?
How should they cooperate? What should happen? What mechanisms
(algorithms, protocols, etc.) are needed to make the system behave that way?
Second real Experiment -- How do the specific TCP implementations
actually behave? What kind of video quality do the end users
experience? What kind of data flows actually travel through the network
components?
Of course we all observe such real experiments every day, whenever we
see or participate in various kinds of videoconferences. Perhaps
someone has instrumented and gathered performance data...?
These questions were discussed and debated at great length more than 40
years ago as TCP V4 was designed. We couldn't figure out the
appropriate algorithms and protocols, and didn't have computer equipment
or communications capabilities to implement anything more than the
simplest mechanisms anyway. So the topic became an item on the "future
study" list.
But we did put various "placeholder" mechanisms in place in TCP/IP V4,
as a reminder that a "real" solution was needed for some future next
generation release. Time-to-live (TTL) would likely need to be based on
actual time instead of hops - which were silly but the best we could do
with available equipment at the time. Source Quench (SQ) needed to be
replaced by a more effective mechanism, and include details of how all
the components should act when sending or receiving an SQ. Routing
needed to be expanded to add the ability to send different data flows
over different routes, so that bulk and interactive data could more
readily coexist. Lots of such issues to be resolved.
In the meanwhile, the general consensus was that everything would work
OK as long as the traffic flows only rarely created "bottleneck"
situations, and such events would be short and transitory. There
wasn't a lot of data flow yet; the Internet was still an Experiment. We
figured we'd be OK for a while as the research continued and found
solutions.
Meanwhile, the Web happened. Videoconferencing, vlogs, and other
generators of high traffic exploded. Clouds have formed, with users now
interacting with very remote computers instead of the ones on their
desks or down the hall.
As Dorothy would say, "We're not in Kansas anymore".
Jack Haverty
On 3/8/24 12:31, Dave Taht via Nnagain wrote:
> I am deeply appreciative of everyones efforts here over the past 3
> years, and within starlink burning the midnight oil on their 20ms
> goal, (especially nathan!!!!) to make all the progress made on their
> systems in these past few months. I was so happy to burn about 12
> minutes, publicly, taking apart Oleg's results here, last week:
>
> https://www.youtube.com/watch?v=N0Tmvv5jJKs&t=1760s
>
> But couldn't then and still can't talk better to the whys and the
> problems remaining. (It's not a kernel problem, actually)
>
> As for starlink/space support of us, bufferbloat.net, and/or lowering
> latency across the internet in general, I don't know. I keep hoping a
> used tesla motor for my boat will arrive in the mail one day, that's
> all. :)
>
> It is my larger hope that with this news, all the others doing FWA,
> and for that matter, cable, and fiber, will also get on the stick,
> finally. Maybe someone in the press will explain bufferbloat. Who
> knows what the coming days hold!?
>
> 13 herbs and spices....
>
> On Fri, Mar 8, 2024 at 3:10 PM the keyboard of geoff goodfellow via
> Starlink<starlink at lists.bufferbloat.net> wrote:
>> it would be a super good and appreciative gesture if they would disclose what/if any of the stuff they are making use of and then also to make a donation :)
>>
>> On Fri, Mar 8, 2024 at 12:50 PM J Pan<Pan at uvic.ca> wrote:
>>> they benefited a lot from this mailing list and the research and even
>>> user community at large
>>> --
>>> J Pan, UVic CSc, ECS566, 250-472-5796 (NO VM),Pan at UVic.CA, Web.UVic.CA/~pan
>>>
>>>
>>> On Fri, Mar 8, 2024 at 11:40 AM the keyboard of geoff goodfellow via
>>> Starlink<starlink at lists.bufferbloat.net> wrote:
>>>> Super excited to be able to share some of what we have been working on over the last few months!
>>>> EXCERPT:
>>>>
>>>> Starlink engineering teams have been focused on improving the performance of our network with the goal of delivering a service with stable 20 millisecond (ms) median latency and minimal packet loss.
>>>>
>>>> Over the past month, we have meaningfully reduced median and worst-case latency for users around the world. In the United States alone, we reduced median latency by more than 30%, from 48.5ms to 33ms during hours of peak usage. Worst-case peak hour latency (p99) has dropped by over 60%, from over 150ms to less than 65ms. Outside of the United States, we have also reduced median latency by up to 25% and worst-case latencies by up to 35%...
>>>>
>>>> [...]
>>>> https://api.starlink.com/public-files/StarlinkLatency.pdf
>>>> via
>>>> https://twitter.com/Starlink/status/1766179308887028005
>>>> &
>>>> https://twitter.com/VirtuallyNathan/status/1766179789927522460
>>>>
>>>>
>>>> --
>>>> Geoff.Goodfellow at iconia.com
>>>> living as The Truth is True
>>>>
>>>> _______________________________________________
>>>> Starlink mailing list
>>>> Starlink at lists.bufferbloat.net
>>>> https://lists.bufferbloat.net/listinfo/starlink
>>
>> --
>> Geoff.Goodfellow at iconia.com
>> living as The Truth is True
>>
>> _______________________________________________
>> Starlink mailing list
>> Starlink at lists.bufferbloat.net
>> https://lists.bufferbloat.net/listinfo/starlink
>
>
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