Re: [Starlink] fiber IXPs in space

[Ulrich Speidel <u.speidel@auckland.ac.nz>]

On 17/04/2023 1:04 pm, David Lang wrote:
> I think it is going to be fairly common, but the beauty of the idea is 
> that you don't have to risk much to try it. Long lived DNS answers 
> (and especially root servers and TLD servers) can trivially be 
> mirrored to the satellites, and you can experiment with caching to see 
> what sort of hit rate you get. Even if you don't cache a lot of the 
> CDN type traffic, it should still be a win to have the longer term 
> stuff there.

Yes - but root servers and TLD servers also get cached a long time at 
the clients. If each of your clients needs a root server and a few TLD 
lookups a day, it's not a huge gain in terms of performance.

It is however a significant step up in terms of complexity. E.g., your 
satellite-based DNS would have to point you at the TLD server that is 
topologically closest to your Starlink gateway, or risk a potentially 
much longer RTT for the lookup. So you'd need to maintain a list of TLD 
instances on your satellite-based DNS and return the one that 
corresponds to what your gateway-based DNS would get. Sure possible but 
more complex than a bog standard DNS server in a fixed network.

>
>> Practically speaking, we know from various sources that each Starlink 
>> satellite provides - ballpark - a couple of dozen Gb/s in capacity, 
>> and that active users on a "busy" satellite see a couple of dozen 
>> Mb/s of that. "Busy" means most active users, and so we can conclude 
>> that the number of users per satellite who use any site is at most 
>> around 1000. The subset of users navigating to new sites among them 
>> is probably in the low 100's at best. If we're excluding new sites 
>> that aren't dynamic, we're probably down to a couple of dozen new 
>> static sites being queried per satellite pass. How many of these 
>> queries will be duplicates? Not a lot. If we're including sites that 
>> are dynamic, we're still not getting a huge probability of cache 
>> entry re-use.
>
> I think that each user is typically going to use a lot less than the 
> 'couple dozen MB' that is the limits that we see, so the number of 
> users in a cell would be much higher.
Yes, but these users aren't "active" in the sense that they will be 
firing off DNS queries during the pass...
>
>>> DNS data is not that large, getting enough storage into the 
>>> satellites to serve 90% of the non-dynamic data should not be a big 
>>> deal. The dynamic data expires fast enough (and can be detected as 
>>> being dynamic and expired faster in the satellite) that I'm not 
>>> worried about serving data from one side of the world to the other.
>> Yes, but the only advantage we'd get here is faster resolution for a 
>> very small subset of DNS queries.
>
> and while that's not as big a win as faster resolution of a larger 
> set, it's still a win.
Yes, but are we chasing diminishing margins here when there are much 
bigger fish to fry?
>
> There are various things that could be done if there is enough 
> interest in starlink users to improve the CDN traffic, and it's not 
> clear that misdirecting starlink users in some (or even many) cases 
> would be that horrible. Geolocation is very imprecise at best and 
> routinely misidentifies locations.
See my comments on geolocation in my response to David Reed's post - I 
shouldn't have used that term.

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

School of Computer Science

Room 303S.594 (City Campus)

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