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auth=pass smtp.auth=CAU17A13 smtp.mailfrom=u.speidel@auckland.ac.nz X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: auckland.ac.nz Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable Subject: Re: [Starlink] thinking about the laser links again X-BeenThere: starlink@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: "Starlink has bufferbloat. Bad." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 28 Oct 2021 08:01:01 -0000 On 28/10/2021 7:29 am, Michael Richardson wrote: > I guess the real question is: have you written the Hollywood Security=20 > Theatre > script based upon this issues, and can I play the geek that explains=20 > this? :-) Sure! > > > - Tell satellites where to send packets (in something along the=20 > lines of a > > long header, as in AX.25 for example). Then a sending ground station=20 > would > > need a complete almanach of the constellation and an idea as to=20 > where the > > receiving ground station is, and which satellite it would use for the > > downlink. Pros: The sending ground station can do all the number=20 > crunching on > > ground rather than space power. Cons:=C2=A0 Header size costs bandwidth= . > > From what I understood, Starlink shipped some kind of comodity SDN capabl= e > chip. So MPLS, or SRv6 ought to be easy, costing only a few bytes > interpreted in hardware, and a path computation element on the ground=20 > should > be able to deal with the calculation. > > It's a challenging situation perhaps because the network effectively gets > rewired every few minutes, but ground based computation should be able to > deal with the problem. That presumes that the ground station has complete topology information=20 for the constellation, though. That includes knowing about defective=20 satellites and lasers etc., birds deviating from assigned orbit. But in principle, I can see how that could work, yes. > > > - Get the satellites to work out where stuff needs to be sent. If=20 > they were > > to use something like Bellman-Ford here, that would require an enormous > > amount of update traffic. Dijkstra would require complete topology > > information, which should in principle be computable from an=20 > almanach on the > > satellites. > > I think, but I might be wrong, that there is a pattern which repeats=20 > over and > over again. Just need to update the mapping of which satellite is in whic= h > position in the precomputed mesh. No need to send the entire mesh. Of course. Bellman-Ford & Co. all assume a network without such=20 regularities. But you need to make use of those patterns in order to=20 make things possible - whether you do source or hop-to-hop routing. And=20 while the configuration of the network is indeed predictable at least=20 for the near future, it's not simply repeating over and over again. The=20 current constellation (if viewed in isolation) more or less runs in 95=20 minute cycles. Earth rotates under the constellation, so the teleports=20 only return to the same position with respect to the constellation when=20 multiples of the length of a sidereal day coincide with multiples of 95=20 minutes. Plus you may find that the Starlink constellation isn't=20 perfectly regular either in its pattern. --=20 **************************************************************** Dr. Ulrich Speidel School of Computer Science Room 303S.594 (City Campus) Ph: (+64-9)-373-7599 ext. 85282 The University of Auckland ulrich@cs.auckland.ac.nz http://www.cs.auckland.ac.nz/~ulrich/ ****************************************************************