[Sis-csi] Green book thoughts

[Scott Burleigh]

Scott, Keith L. wrote:

>  It seems to me that the points of contention here center on whether
>  or not an application using UDP might cause network congestion and
>  hence lose packets, and whether/how building reliability on top of
>  UDP is a Good Idea. The arguments seem to oscillate between
>  high-bandwidth downlinks where we want to use all of the available
>  capacity and the assertion that UDP flows from space (there's a
>  B-movie title in there somewhere...) simply can't congest the
>  "network" because the space link bandwidth is too low.
>
>  I would assert that for some (possibly many?) future missions,
>  bandwidths will be such that pure rate-controlled streams coming from
>  some space applications would have the ability to congest shared
>  (space and/or ground). A single HDTV stream competing with any other
>  appreciable flows in the ground or space portions of the network
>  could do this, e.g. I also don't think we can assert that streams
>  will not cross some portion of shared network, especially if there is
>  inter-agency cross-support. One must consider the possibility of
>  commercial ground stations, and also the possibility of shared
>  in-space crosslinks.
>
>  That said, do we all agree (at least among ourselves -- the case will
>  need to be made to external audiences) that: 1) Moving to IP
>  provides a large benefit to missions in that: o it decouples
>  applications from the data links o it facilitates multi-hop routing
>  over heterogeneous data links o it provides an efficient multiplexing
>  mechanism for numerous data types o traffic can be directly routed
>  from ground stations over closed networks or the Internet to its
>  destination(s) on the ground with commercial network equipment

Yes.

>  2) We don't really know how operators will want to use a networked
>  capability, except that they will probably want some mix of real-time
>  data that can take loss and reliable data that wants no loss. These
>  are supportable in continuously connected environments by TCP, UDP,
>  and NORM (the latter two supporting simplex environments, to some
>  extent); and in disconnected environments by overlays like CFDP, and
>  DTN.

Yes.  (And, as necessary, CFDP and DTN can be used in continuously 
connected environments as well.)

>  3) Building application-specific reliability mechanisms on top of UDP
>  is an option, but *in general*, new applications should first look to
>  standard transport mechanisms (exact list TBD from Red Books from
>  this WG) to fulfill their needs. Non-congestion controlled flows
>  that might cause significant network congestion are discouraged, but
>  not prohibited if circumstances require their use and they can be
>  designed to 'not-too- adversely' affect the network. Note that
>  'not-too-adversely' here is an overall system design trade -- a
>  particular application might need to simply blast bits without regard
>  to the rest of the network. Note also that the overlays mentioned
>  above may be part of the recommended set of standard transports.

That sounds right to me.

Scott
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