Re: [dtn-interest] Question

DTN is inappropriate for inter-stellar communication. One can only code
like crazy (FEC) and transmit more or less continuously. one would need
dedicated resources to receive as well.

v

On Sun, Jan 27, 2013 at 7:29 PM, <sitaraman@nmsworks.co.in> wrote:

> OK i never thought about FEC in this context.
> When you say positioning messages "closer to the destination" that is
> exactly my question, the fundamental problem of distance and possible
> intermediate disruption (the case that you mention as wierd) is not
> handled, it is worked around essentially by reducing the distance, that is
> by simply building a bridge that makes the fundamental problem non
> existant.
>  For star communication, we perhaps couldnt afford to wait for the round
> trip time, and what if there are severe packet loss...i was seeing one of
> those Nat Geo programs where terribly nasty intense radiation pulses
> floating around that could destroy anything in it path let alone frail
> packets....This problem intensifies as the distance become longer.
>  Should we look for ways then to address the fundamental distance +
> disruption challenge other than placing messages close and making the
> distance smaller, to address tougher challenges in DSN is the question i
> still seem to ask myself...Or should we think this problem is not
> addressable at all? Like those mathematician types who can prove
> intractability is this proven to be an intractible problem?
>  Sitaraman
> > If it is 10s of days and there are no intermediate nodes, you are talking
> > about a long-delay one-hop link. This is what the Deep Space Network has
> > to
> > deal with all the time. Forward error correction is your friend in these
> > cases. Repeated transmissions are possible but for systems like the DSN,
> > you only get to point to the destination at certain times (when "contact"
> > is feasible and when you are scheduled to transmit or receive - it is a
> > huge scheduling challenge since the DSN resource is potentially
> > over-subscribed). If the probability of loss is relatively low except for
> > noise, forward error correction may be enough to reduce loss probability
> > in
> > which case one would not retransmit until no response is obtained after
> > the
> > round trip time has expired. One of the reasons the IPN is attractive is
> > that it can position messages closer to destination and recover from
> > forward hop failures more quickly.
> >
> > Your scenario of "packet loss" in 4 days on a one hop link is a bit weird
> > because you can't conclude loss until the signal gets to the receiver.
> >
> > v
> >
> >
> >
> > On Sun, Jan 27, 2013 at 7:00 PM, <sitaraman@nmsworks.co.in> wrote:
> >
> >> Hi Scott,
> >>  Thanks for the help in understanding all this. I still have a question
> >> in
> >> mind. For arguments sake consider the scenario where it takes 10s of
> >> days
> >> for message to reach from source to destination and there is no
> >> intermediate node. (If i understood Vint right the other day in the talk
> >> at Chennai,  this is on the works for messages to a star ion pumps and
> >> all! so hoping this is still a relevant case)Say a packet loss happens 4
> >> days down the road. How long would the source wait to decide to resend
> >> from its store?
> >> Sitaraman
> >> > Hi, Sitaraman.  It's correct to state that DTN is based on potentially
> >> > prolonged, potentially non-volatile storage at forwarding points (the
> >> > original source and all routers along the end-to-end path) and on
> >> > potential retransmission at all forwarding points.
> >> >
> >> > But forwarding points other than the original source are features of
> >> > specific topologies, not mandatory elements of the architecture.
> >> > Disruption in communication between the endpoints of a long-delay
> >> > source-destination pair is handled by storage and retransmission, both
> >> at
> >> > the source and also at whatever other nodes may be involved in the
> >> > communication.  Intermediate nodes are not required in order to make
> >> DTN
> >> > work.
> >> >
> >> > For example, in the DINET experiment in 2008 we did DTN communication
> >> > between nodes on Earth and a node aboard the Deep Impact flyby
> >> spacecraft
> >> > in interplanetary space, 10-15 million miles away.  There were
> >> disruptions
> >> > -- lapses in connectivity -- lasting several days, but DTN had no
> >> > difficulty handling them despite the fact that there were no
> >> intermediate
> >> > nodes between the spacecraft and the data sources and sinks on Earth.
> >> >
> >> > Scott
> >> >
> >> > -----Original Message-----
> >> > From: dtn-interest-bounces@irtf.org [mailto:
> >> dtn-interest-bounces@irtf.org]
> >> > On Behalf Of sitaraman@nmsworks.co.in
> >> > Sent: Sunday, January 27, 2013 8:51 AM
> >> > To: dtn-interest@irtf.org
> >> > Subject: [dtn-interest] Question
> >> >
> >> > Hi,
> >> >  Is it correct to state that DTN adopts a store and forward mechansim
> >> and
> >> > gets around delay and disruption by having intermediate nodes
> >> retransmit
> >> > on failure indication from its immediate neighbor? If this is true is
> >> it
> >> > correct to state that fundamentally, the problem of disruption in long
> >> > delay source-destination pair is not handled, but is solved by
> >> > essentially reducing the said distance by having intermediate nodes
> >> > storing and forwarding thereby acting as a "virtual" or "proxy" source
> >> > and destinations?
> >> >  Thanks
> >> > Sitaraman
> >> >
> >> >
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