Re: Réf. Re: Alcatel Space interest about IP/DVB

[Keith Hogie <Keith.Hogie@gsfc.nasa.gov>]

Kearney wrote:
> 
> Hello Stéphane>
> > Thanks a lot for your reviewing and comments !
> >
> let's keep the discussion cooking!
> > By "Ethernet-like" we only mean "connectionless layer 2 based on broadcast
> > medium".
> Question - why does it have to be connectionless?
> Most of the applications are anyhow sessio-oriented - TCP, HTTP, and in
> particulatr multicast applications are based on the concept of a session. If
> you "tune" your receiver to a PID you are basically opening a session - so
> why wouild you like to have the next higher layer (link/encapsulation)
> connection-less?
> 

  There may be various definitions of "connectionless" and "session"
going on here.  In one sense things like an ATM or Frame Relay 
Permanent Virtual Circuit (PVC) are a "connection" but that is a 
often a different definition than a TCP connection.

  I see the OSI terms "connection-oriented" and "connectionless" refer
more to upper layer protocols and not to things like a PVC.  Their
"connection" term relates to protocols that do some sort of handshaking
like X.25, PPP, or TCP.  The protocol actually initiates a "connection",
which may also be called a "session", by exchanging packets between
the two ends of the connection and creates the connected state. 

  I think the comment on "Ethernet-like" "connectionless" was aimed at
making sure you don't start running a connection-oriented lower layer
underneath a connection-oriented upper layer like TCP.  This relates
back to the days when people ran TCP over X.25 lower layers.  TCP is
fully prepared to wait for retransmissions and having X.25 underneath
it also doing retransmissions is redundant and causes other problems.

  When you ran multiple TCP connections over a single X.25 connection,
lost data packets could cause X.25 to slow down and wait for
retransmissions. This would delay all of the TCP connections running
over it even though the lost packets only affected one TCP connection.
This is where Frame Relay came from.  It removed the X.25 flow control
and retransmission and just accepted frames and relayed them on.

  You can call the lower layer data path a "connection" if you want
but the real point about "connectionless" is that the lower layers should
not be introducing any sort of flow control.  

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  Keith Hogie                   e-mail: Keith.Hogie@gsfc.nasa.gov
  Computer Sciences Corp.       office: 301-794-2999  fax: 301-794-9480
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