[Starlink] fiber IXPs in space

[Ulrich Speidel]

On 17/04/2023 2:34 pm, David Lang wrote:
> On Mon, 17 Apr 2023, Ulrich Speidel wrote:
>
>> 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.
>
> really? I don't think the root and TLD servers do any geolocation, I 
> think they have fixed answers and rely on anycast addresses to find 
> the closest one. Adding those anycast addresses to the satellites 
> would be transpoarent to all users (assuming the satellites are 
> operating at the IP layer, the old bent-pipe approach did not, but 
> once you have routing in space via the laser links...)

So what you're suggesting would be a generic anycast-based set of DNS 
records to be kept on all satellites?

Hm. My terrestrial RTT to the closest 8.8.8.8 and 8.8.4.4 anycast DNS 
servers is about 28 ms, indicating that they're over in Australia. No 
big surprise here, but that RTT isn't all too different from what I'd 
expect from a Starlink connection. That compares to < 1 ms to my local 
resolver.

Would an anycast-based generic DNS give us better RTT than a 
gateway-based DNS with full ability to cache local information?

Even with the laser links, it's worth repeating that most of Starlink's 
operation is still bent pipe, and likely to continue to be so in future. 
There is little evidence that the lasers are currently being used for 
anything but remote location access.

That said, bent pipe isn't the same as bent pipe. Analog telecom GEO 
sats used to do physical layer bent pipe with the satellite incapable of 
IP. If you have a satellite capable of IP, you can still call it a bent 
pipe as long as it downlinks traffic that has come in on the uplink. You 
could even run an IP-based tunnel between dishy and gateway that makes 
it appear like a single link for the next network layer up the stack. 
And do the same for lasers. There'd be a number of advantages in this.

>
>>>> 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...
>
> even users who are using the connection aren't going to be using 
> dozens of MB of bandwidth.
Yes - but that should reflect as a lower level of DNS usage in most 
cases, shouldn't it?
>
>>>>> 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?
>
> possibly. it's worth discussing and DNS is an easy thing to start with 
> compared to most others.
>
> David Lang
>
-- 
****************************************************************
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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