[Starlink] fiber IXPs in space

[Ulrich Speidel]

On 17/04/2023 10:03 am, David Lang wrote:
> On Mon, 17 Apr 2023, Ulrich Speidel via Starlink wrote:
>
>> On 17/04/2023 5:54 am, David Fernández via Starlink wrote:
>>> In case you put a DNS server in the satellite, so that it replies
>>> instead of a DNS server on ground, the RTT is reduced by half.
>>>
>>> The idea would be that the satellite inspects IP packets and when it
>>> detects a DNS query, instead of forwarding the packet to ground
>>> station, it just answers back to the sender of the query.
>> Understood - it's just that the gain you have from this is quite 
>> small. DNS queries only happen the first time a host needs to resolve 
>> a name, and then again after cache expiry much later, so they account 
>> for only a tiny fraction of the traffic, and also for only a small 
>> amount of the total delay in page loads. RTT isn't really the big 
>> issue in Starlink - yes it's larger than it perhaps needs to be, and 
>> bufferbloat seems to be present, but compared to GEO, it's now in the 
>> range seen for terrestrial Internet.
>
> DNS time is more significant than you think, due to the fact that so 
> many websites pull data from many different locations, you end up with 
> a lot of DNS queries when hitting a new site for the first time (and 
> many of these queries are serial not parallel) so it adds quite a bit 
> to the first rendering time of a page.
But most people don't hit new sites most of the time, and a lot of 
cascading loads hit the same CDNs you've seen previously.
>
>>> CDNs or even datacenters (Cloud) in GEO or LEO is even more complex.
>>
>> Indeed. In so many ways.
>>
>> Mind though that CDNs are generally tied in with DNS nowadays, and 
>> there's another snag: Take two users, Alice in the UK and Bob in New 
>> Zealand - pretty much antipodean, using Starlink in bent-pipe 
>> configuration, i.e., their traffic goes through, say, the London 
>> gateway in the UK and the Clevedon gateway in NZ. Now imagine both 
>> trying to resolve the same CDN hostname some time apart, but via the 
>> same satellite DNS as the satellite has moved from the UK to NZ in 
>> the interim. Say Alice resolves first and gets the IP address of a 
>> CDN server in the UK. If the satellite DNS now caches this, and Bob 
>> queries the same hostname, he gets directed to a server in the UK 
>> literally a world away instead of the Auckland one closest to him. So 
>> unless each satellite carries a geolocated copy of the world's DNS 
>> entries with it and makes a decision based on user location, you have 
>> a problem.
>
> This is true when the DNS answer is dynamic, but such cases also have 
> short cache timeouts. Even with a 90 min orbit, a 15 min timeout would 
> significantly lessen the impact (and I would expect that an orbital 
> DNS would detect short timeouts and treat them as a signal to shorten 
> the timeout even more)

Timeout where? At the end user client or at the satellite?

At the end user client, a short timeout makes no sense at all because 
their host-to-CDN-IP server mapping shouldn't really change in bent pipe 
- only the sat hop changes.

If the timeout is meant to be on the satellite, it means that the 
satellite knows nothing about anything when it arrives to assist you, 
and needs to query some sort of (probably ground-based) DNS server anyway.

Also, the assumption that a satellite will return to the same spot after 
a full orbital period (of say 90 minutes) only applies to satellites in 
equatorial orbits (or polar orbits, and then only to the poles). In all 
other cases, the Earth's rotation will assure that the satellite's 
return to the same location takes many orbital periods.

>
> David Lang
>
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-- 
****************************************************************
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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