[Starlink] Starlink "beam spread"

[Sebastian Moeller]

Hi Ulrich,

focussing on the CDN part

> On Aug 31, 2022, at 15:41, Ulrich Speidel <u.speidel at auckland.ac.nz> wrote:
> [...]
> CDNs & Co - are NOT just dumb economic optimisations to lower bit miles. They actually improve performance, and significantly so. A lower RTT between you and a server that you grab data from via TCP allows a much faster opening of the congestion window. With initial TCP cwnd's being typically 10 packets or around 15 kB of data, having a server within 10 ms of your client means that you've transferred 15 kB after 5 ms, 45 kB after 10 ms, 105 kB after 15 ms, 225 kB after 20 ms, and 465 kB after 25 ms. Make your RTT 100 ms, and it takes half a second to get to your 465 kB. Having a CDN server in close topological proximity also generally reduces the number of queues between you and the server at which packets can die an untimely early death, and generally, by taking load off such links, reduces the probability of this happening at a lot of queues. Bottom line: Having a CDN keeps your users happier. Also, live streaming and video conferencing aside, most video is not multicast or broadcast, but unicast.
> [...]

Sure, that is a consequence of slow start*, but I argue that having 2ms or 20ms is not going to result in too noticeable a slow down, and bulk transfers like movies really do not care since DASH in all likelihood leaves slow-start for good after the initial ramp-up. Yes, 1ms versus 100ms makes a difference for interactive uses, but putting the CDN in space versus at the base station IMHO is a less clear improvement. Add to this that caches partly work by exploiting "locality", so e.g. for video streaming platforms I expect different countries to have different viewing profiles and hence different content needs to be cached, but satellites cover large "rings" around the world; meaning either you cache everything (or at least the content for your primary market) in space multiple times over so that your main service area is covered at all times or...

I am prepared to eat crow on this in the future, but I am highly skeptical about CDNs in space (in spite of it being a cool project from the technological side).


*) As it looks slow start is getting a bad rep from multiple sides, but I see not better alternative out there that solves the challenge slow-start tackles in a better way. Namely gradual ramping and probing of sending rates/congestion windows to avoid collapse, this in turn means that short flows will never reach capacity, the solution to which might well be, use longer flows then...