On Aug 22, 2006, at 11:22 AM, Pete Wyckoff wrote:
[EMAIL PROTECTED] wrote on Tue, 22 Aug 2006 09:52 -0400:
What is the purpose of the Request To Send (RTS) and Clear To Send
(CTS) messages in the IB and other BMI implementations? Is it to
check if resources are available on the receiver (i.e. to see if a
matching receive has been posted) or to explicitly tell the receiver
to post a receive?
The former. I'm sure Patrick would be happy to quote from his rant
on how much IB sucks in imposing this netowrk semantic. :)
He is always happy to rant. :-)
What should a BMI method do if the receive is not posted? Wait?
Cancel with failure?
From my discussions with Sam and Murali, they mentioned that in any
exchange the first message is always unexpected. These messages are
guaranteed to below a preset size (determined by the BMI method). Any
expected message should then find a matching receive on the peer. If
so, why use the RTS and CTS messages?
BMI does not guarantee that receives be preposted. It also does not
look into message contents to divine that, for instance, an
unexpected message implies a following expected message. Even if we
did integrate BMI into the server protocol, there are situations
where you wouldn't know the response size in advance.
-- Pete
I am still trying to have a clear understanding of how a send varies
from an unexpected send. The receiver will have some number of pre-
posted, "generic" receives (i.e. can receive from any peer) to catch
unexpected sends. If the send is _not_ unexpected (i.e. expected),
then it implies that the receiver will post a receive for the
expected send (either pre-posted or slightly late due to clock
drift), no? If not, what am I missing?
Is the purpose of the RTS/CTS messages then to stall the sending
until the receiver has posted the receive? If so, would the receiver
ever send a CTS to indicate that a match is not forthcoming? Or does
the receiver only send a CTS when the receive is posted? In the
latter case, the sender may time out waiting for a CTS and thus
cancel the send?
Thanks,
Scott
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