[Starlink] Starlink hidden buffers

[Ulrich Speidel]

Hi Sebastian,

Yes and no. But yes, not completely, and I'm not sure whether anyone has 
ever looked at this, in fact. People who build mobile networks tend to 
regard their job as complete the moment they can get an IP packet from a 
mobile device to the Internet and vice versa, and mobile users tend to 
be a bit more tolerant if things slow down for a moment or three.

There are a few differences, though. One is that cells are (or at least 
can be) fibre connected, and that is something you would do along a 
high-speed train line. So there is less of a bottleneck than having to 
use RF for downlinking. I'd also imaging total user numbers to be lower 
and the bandwidth demand per user to be less (hands up who takes their 
50" TV onto trains to watch Netflix in HD?). The other is that most 
places have 3+ networks serving the train line, which brings down user 
numbers, or you have in-train cells, which communicate with off-train 
POPs that have no extra users.

But yes, good question IMHO!

Cheers,

Ulrich

On 13/05/2023 11:20 pm, Sebastian Moeller wrote:
> Hi Ulrich,
>
> This situation is not completely different from say a train full of 
> LTE/5G users moving through a set of cells with already established 
> 'static' users, no?
>
>
> On 13 May 2023 12:10:17 CEST, Ulrich Speidel via Starlink 
> <starlink at lists.bufferbloat.net> wrote:
>
>     Here's a bit of a question to you all. See what you make of it.
>     I've been thinking a bit about the latencies we see in the
>     Starlink network. This is why this list exist (right, Dave?). So
>     what do we know? 1) We know that RTTs can be in the 100's of ms
>     even in what appear to be bent-pipe scenarios where the physical
>     one-way path should be well under 3000 km, with physical RTT under
>     20 ms. 2) We know from plenty of traceroutes that these RTTs
>     accrue in the Starlink network, not between the Starlink handover
>     point (POP) to the Internet. 3) We know that they aren't an
>     artifact of the Starlink WiFi router (our traceroutes were done
>     through their Ethernet adaptor, which bypasses the router), so
>     they must be delays on the satellites or the teleports. 4) We know
>     that processing delay isn't a huge factor because we also see RTTs
>     well under 30 ms. 5) That leaves queuing delays. This issue has
>     been known for a while now. Starlink have been innovating their
>     heart out around pretty much everything here - and yet, this
>     bufferbloat issue hasn't changed, despite Dave proposing what
>     appears to be an easy fix compared to a lot of other things they
>     have done. So what are we possibly missing here? Going back to
>     first principles: The purpose of a buffer on a network device is
>     to act as a shock absorber against sudden traffic bursts. If I
>     want to size that buffer correctly, I need to know at the very
>     least (paraphrasing queueing theory here) something about my
>     packet arrival process. If I look at conventional routers, then
>     that arrival process involves traffic generated by a user
>     population that changes relatively slowly: WiFi users come and go.
>     One at a time. Computers in a company get turned on and off and
>     rebooted, but there are no instantaneous jumps in load - you don't
>     suddenly have a hundred users in the middle of watching Netflix
>     turning up that weren't there a second ago. Most of what we know
>     about Internet traffic behaviour is based on this sort of network,
>     and this is what we've designed our queuing systems around, right?
>     Observation: Starlink potentially breaks that paradigm. Why?
>     Imagine a satellite X handling N users that are located closely
>     together in a fibre-less rural town watching a range of movies.
>     Assume that N is relatively large. Say these users are currently
>     handled through ground station teleport A some distance away to
>     the west (bent pipe with switching or basic routing on the
>     satellite). X is in view of both A and the N users, but with X
>     being a LEO satellite, that bliss doesn't last. Say X is moving to
>     the (south- or north-)east and out of A's range. Before connection
>     is lost, the N users migrate simultaneously to a new satellite Y
>     that has moved into view of both A and themselves. Y is doing so
>     from the west and is also catering to whatever users it can see
>     there, and let's suppose has been using A for a while already. The
>     point is that the user load on X and Y from users other than our N
>     friends could be quite different. E.g., one of them could be over
>     the ocean with few users, the other over countryside with a lot of
>     customers. The TCP stacks of our N friends are (hopefully)
>     somewhat adapted to the congestion situation on X with their cwnds
>     open to reasonable sizes, but they are now thrown onto a
>     completely different congestion scenario on Y. Similarly, say that
>     Y had less than N users before the handover. For existing users on
>     Y, there is now a huge surge of competing traffic that wasn't
>     there a second ago - surging far faster than we would expect this
>     to happen in a conventional network because there is no slow start
>     involved. This seems to explain the huge jumps you see on Starlink
>     in TCP goodput over time. But could this be throwing a few
>     spanners into the works in terms of queuing? Does it invalidate
>     what we know about queues and queue management? Would surges like
>     these justify larger buffers?
>
> -- 
> Sent from my Android device with K-9 Mail. Please excuse my brevity.

-- 
****************************************************************
Dr. Ulrich Speidel

School of Computer Science

Room 303S.594 (City Campus)

The University of Auckland
u.speidel at auckland.ac.nz  
http://www.cs.auckland.ac.nz/~ulrich/
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