[e2e] TCP in outer space

Courtney, Bill Bill.Courtney at trw.com
Wed Apr 11 09:36:23 PDT 2001


I didn't mean to sound quite as discouraging as I obviously
sounded to you. My intent was to point out that while non-GEO 
space networks have the obvious, immediate appeal of promising
lower latency, this latency comes at the price of considerable
increase in operational complexity and some of this complexity
erodes the savings in latency. That said, there is work that
is being done toward the space network that you seem to envisage.

First, NASA has been studying space networks where each instrument
on board a satellite has an IP address and the space subnet(s)
would be connected to the terrestrial internet. If fact, some of
my colleagues here at TRW have worked on preliminary studies of
such network architectures. NASA has put a bit of documentation
on-line. The paper at 
is but one example.

Second, the IETF's work on ad-hoc mobile networks is addressing
many of the issues that I raised in my previous posting. I was
aware of this work and its relevance to non-GEO space networks,
but I wanted to be something of a devil's advocate by raising
some issues and concerns.

My opinion is that space networks will have to operate with non-
GEO elements, since many of the nodes will be non-GEO. I do not
believe that GEO nodes will necessarily anchor a space network,
but I do believe that any concept of operations that does not
depend on GEO anchors should be considered in depth before it
is championed. Else, it is all too easy to work oneself into a
corner that can be escaped only with costly and inelegant fixes.

Your enthusiasm is contagious. The space network is clearly an
idea whose time has come. It should exist today. It behooves us
to make it exist tomorrow.


> -----Original Message-----
> From: David P. Reed [mailto:dpreed at reed.com]
> Sent: Wednesday, April 11, 2001 5:58 AM
> To: Courtney, Bill; end2end-interest at postel.org
> Subject: RE: [e2e] TCP in outer space
> I hope this isn't boring the vast majority of e2e list 
> participants.  (send 
> complaints to me, not to the list!)
> But now I'm really intrigued.  Here's why:  a "space 
> Internet" is (should 
> be?) motivated by the same rationale as the original Internet 
> - a network 
> of heterogeneous networks can incorporate technological and 
> applications 
> innovation much more effectively than dedicated, 
> purpose-built Internet.
> TDRS is a dedicated, highly "optimized" technology (at a 
> point in time, for 
> a class of applications).  Sounds wonderfully well thought 
> out. But... Like 
> all "optimized" technologies, it is brittle with respect to 
> change.  Doesn't tolerate new infrastructures or new 
> applications very well.
> IP is, as we all know, "good enough" technology, optimized 
> for nothing in 
> particular, except for its ability to rapidly incorporate or 
> support new 
> stuff at the low levels and new apps and requirements at the 
> high levels.
> Now that there's enough going on in space, I'd say the time 
> is NOW to start 
> architecting a "space Internet".  Remember, the Internet is not the 
> hardware, and never was.  You can run "space Internet" over TDRS and 
> anything else out there.
> I bet there is a cadre of young designers who can carry this 
> torch, just as 
> the young students and others carried the Internet torch.  It 
> is clearly 
> "enabling" technology for space exploration, and even more 
> than ISS, it can 
> enable all kinds of stuff to happen.  But the key has to be a 
> net that 
> follows design principles like the Internet's.  End-to-end, 
> liberal on 
> inputs, conservative on outputs, no specialization to 
> underlying transport 
> "features", no privileged "applications".
> Comments below.
> At 02:01 PM 4/10/01 -0700, Courtney, Bill wrote:
> >One of the problems with using non-GEO satellites is that
> >the topology of the constellation is changing. True,
> >it changes in a predictable way, but the changes will,
> >nonetheless, require network reconfiguration. With non-GEO
> >satellites, not only might the inter-satellite links
> >be breaking and making, but also the earth-space access
> >links for each user will be continually changing.
> Yup.  The fact that links are independent of technology is 
> what makes it a 
> "network".  In the Internet, we said, since topology is 
> constantly being 
> broken because of reliability and autonomy, we need a routing 
> protocol that 
> can incorporate change, and a transport protocol that doesn't bind 
> end-to-end state into physical circuits.
> >In addition, satellite coverage footprints in a
> >constellation usually overlap, and for many
> >constellations the overlaps are also in constant
> >change. This means that uplink and downlink bandwidth
> >allocations among the satellites will be subject to
> >changing re-use constraints. This makes it difficult
> >to ensure that users in any particular area on the
> >ground will receive a constant amount of bandwidth. And
> >fluctuating access bandwidth leads to all sorts of
> >problems, as you can imagine.
> Fluctuating access bandwidth is no more a problem than fluctuating 
> application demand.  I suspect that fluctuating app demand is 
> "hidden" in 
> low average utilization of end-to-end fixed rate circuits.  
> To me, there 
> are very few applications (voice is one) that really have 
> "fixed rate" 
> needs.  The rest are kludged by assigning a fixed rate 
> circuit that can 
> handle worst case traffic, then leaving most of the capacity idle.
> It's hard to believe that computer-computer (and human-human 
> mediated by 
> com-puter) traffic is fixed rate.  So the matching of demand 
> to capacity by 
> buffering and dynamic allocation (e.g. datagrams and bursty 
> VCs) is going 
> to be an issue for space - why not admit it and design a 
> network that does 
> that.
> >These and other difficulties must be balanced against the
> >possibility that latency might be lowered with non-GEO
> >constellations. (I say "might," because some of the
> >non-GEO difficulties can be addressed at the cost
> >adding latency.) Non-GEO can be a bear!!
> It isn't just latency that might be lowered.  Like the Internet, by 
> allowing the incorporation of a wide variety of underlying 
> technologies, 
> new applications can be introduced at low cost.  Every time 
> we increase the 
> level of resource sharing by creating a network to mediate 
> access to a 
> resource (i.e. space), we boost innovation by lowering the 
> cost of entry.
> It would seem to me that a "space Internet" would be as big a 
> deal as the 
> "fast, cheap, out-of-control" mars robot design philosophy is.
> Of course, I hold no hope to sell this idea to the "Beltway 
> Space Bandits" 
> - the corporate biggies whose goal is to sell expensive 
> engineering time to 
> risk-averse gov'ts, rather than looking for ways to do stuff 
> cheaply and 
> innovatively.  But maybe there's an innovative scientist-type 
> at NASA or JPL?
> - David
> --------------------------------------------
> WWW Page: http://www.reed.com/dpr.html

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