[Sis-uce] Thoughts?

[Krupiarz, Christopher]

All,

The following is an exchange regarding Scott's concept that he suggested be
forwarded to the mailing list.  I'll put my NAK comments here so that it
doesn't get too garbled:

The NAK timer would serve pretty much the same thing, but it would send a
NAK that may be unnecessary in the event of late arriving PDUs.  Thus the
sender would end up using bandwidth to resend PDUs that the receiver was
probably shortly to receive.  Of course, this is all dependent upon someone
using acknowledged mode which, as you say, isn't likely.  Therefore it's
probably not worth introducing the added complexity to acknowledged mode.  

Chris

---------------------------------------------
  Christopher J. Krupiarz
  Space Department, Embedded Applications Group
  Johns Hopkins University Applied Physics Laboratory
  11100 Johns Hopkins Rd.
  Laurel, Maryland 20723

  christopher.krupiarz@jhuapl.edu
  Voice: 443-778-5056
  FAX: 443-778-3839
---------------------------------------------

> -----Original Message-----
> From: Scott Burleigh [mailto:Scott.Burleigh@jpl.nasa.gov]
> Sent: Thursday, May 22, 2003 10:57 AM
> To: Krupiarz, Christopher
> Subject: RE: [Sis-uce] Thoughts?
> 
> 
> At 09:16 AM 5/22/2003 -0400, you wrote:
> >Scott,
> >
> >FYI, I see no problem with the concept.  It sounds like it's 
> needed and this
> >looks like a good solution.  One question: do you see any 
> scenarios where
> >acknowledged mode would still be used even with a 
> retransmitting link layer?
> 
> Actually, no.  If the link layer is reliable, I think 
> acknowledged-mode 
> CFDP would be unnecessary overhead.
> 
> >I was trying to come up with some but none seemed terribly 
> realistic.  I was
> >thinking along the lines of maybe a sensor web or a rover 
> that couldn't
> >handle the extra complexity may communicate to some waypoint 
> that was using a
> >retransmitting link layer.
> 
> I suspect that the link-layer retransmission being proposed 
> at JPL will be 
> simpler than acknowledged-mode CFDP, but I agree that the 
> scenario you're 
> talking about is possible.  If the rover is using ack-mode 
> CFDP over a 
> non-retransmitted link, then the waypoint will have to be 
> doing the same 
> thing in order for the two to communicate.  The question then 
> is what the 
> waypoint-to-earth communication looks like.
> 
> If you're using Extended Procedures then you've got only a 
> single CFDP 
> transaction end-to-end; I'm pretty sure you'd have to stick with 
> acknowledged mode on the waypoint-to-earth link even if that 
> link layer had 
> its own retransmission.
> 
> I think that's in fact an argument against Extended Procedures that I 
> hadn't considered.  If you are instead using 
> Store-and-Forward Overlay, 
> then the waypoint-to-earth transmission is a separate 
> transaction with a 
> potentially different configuration: it could be 
> unacknowledged-mode CFDP 
> over the retransmitting link.  A little cleaner.
> 
> >The reason I ask is that if there were, it would
> >naturally be beneficial to have the Check Timer on 
> acknowledged EOFs in
> >order to reduce NAK traffic.
> 
> Can you tell me a little more about how you'd see this 
> operating?  I'm 
> thinking that in acknowledged mode we already have the NAK 
> timer doing 
> essentially the same thing (i.e., it starts when you receive 
> an EOF and not 
> all of the file data and metadata have been received).
> 


> Scott
> 
> P.S.: please don't feel inhibited about posting this kind of 
> exchange to 
> the sis-uce mailing list.  This stuff is highly relevant to 
> the charter of 
> the BOF and the future WG.
>