Re: [dtn-interest] Question

[Vint Cerf <[e-mail]>]

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, <[e-mail]> 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: [e-mail] [mailto:
> [e-mail]]
> > On Behalf Of [e-mail]
> > Sent: Sunday, January 27, 2013 8:51 AM
> > To: [e-mail]
> > 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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