Hi
The terms spoofing has different meaning and generally denotes systems
which send false acknowledgements to the end node to trick it into
sending data faster than what TCP specifies. In this case, spoofing
does not take into account the risk of congestion. Whereas when in
the case of protocol conversion at the same level a new congestion
mecanism is used. What is important in the split configuration (Flash
or Mentat) that there is a low throughput variation as a function of
the bit error rate for the new protocol.
-----Message d'origine-----
De: ketan bajaj [SMTP:[email protected]]
Date: vendredi 23 juillet 1999 09:12
�: [email protected]
Cc: [email protected]
Objet: RE: making satellite channel loss transparent
thanks for the feedback, i agree that techniques such as spoofing
(splitting
the tcp connection) from companies mentioned like flash-networks &
mentat,
and explict signalling NASA JPL's SCPS-TP, and other techniques given
in tcp
pep, improve performance.
topology : (sender)x------G1- - - - G2-------y(receiver)
where G1-G2 is the satellite hop
But, spoofing would violate the end-to-end tcp semantics, as the sender
could receive an ack from the proxy receiver(G1) at one of the
satellite
link ends, before the data packet reaches the actual receiver (y).
Calculating the correct round trip time could be crucial to some
applications. Moreover if data is lost over a terrestial link which is
after
the satellite hop, then the sender won't be informed of the congestion
loss,
as the other satellite link end (G2) would do the recovery.
Whereas in case of NASA's SCPS-TP, the hosts would require modification
to
process the corruption signal from the satellite link. Any protocol
which
requires the end hosts to be modified wouldn't be easy to deploy in the
existing network. If it is deployed partially at some hosts, other
hosts
using the same satellite channel would be at a loss.
Still we don't have a protocol which is completely transparent and
preserves
all existing semantics of tcp.
What i'm trying to say is why not attack the real problem. The root
cause
for tcp's poor performace over satellite channels is that it fast
retransmits on receiving 3 duplicate acks (reducing its window), and as
the
delay*bandwidth product over a satellite channel is large, the no. of
data
packets in the end-to-end pipe are large, so a loss due to corruption,
results in a number of duplicates acks being generated, moving the
sender
tcp to fast retranmission, and thus poor performance.
ketan
> > The issue i'm trying to raise here is why bother TCP to
> > distinguish between
> > packet loss due to corrution or due to congestion, why not make the
> > corruption loss transparent to sender tcp ? Some of the work
> > ive seen in
> > doing the above, are split-connection and berkeley snoop
> > designs. But they
> > are limited to a particular satellite network topologies, where the
> > satellite hop is the last hop of the end-to-end path. What about
more
> > generic topologies ?
>
______________________________________________________
Get Your Private, Free Email at http://www.hotmail.com
_________________________________________________________
Do You Yahoo!?
Get your free @yahoo.com address at http://mail.yahoo.com
This archive was generated by hypermail 2b29 : Mon Feb 14 2000 - 16:14:55 EST