RE: A Transport Protocol Without ACK (resend)

["Douglas Otis" <dotis@sanlight.net>]

Matt,

You would not want an End-to-End scheme.  You would want a scheme that
prevents any node or sub-node from over-flowing.  That is accomplished by
Buffer-to-Buffer control.  The present scheme now in place is with a rough
approximation of command space with any excess data discarded as required.
This makes for a lossy channel and one that does not allow control at a
specific sub-node that may be at its limits.  Restrain the depth of the
sub-node with specific control.  Just 8M-byte of FIFO will represent 100
millisecond of delay at FC speeds.  A token credit scheme ensures no node
ever gets to a point of dropping frames in addition to allowing maintenance
of the FIFO depth.  Should a sub-node fail, only that node stops functioning
rather than everything being hung until resolution.  A sizable short-coming
among many others in aggregating everything into this common scheme.

Doug

> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Matt Wakeley
> Sent: Monday, October 02, 2000 10:44 AM
> To: ips@ece.cmu.edu
> Subject: Re: A Transport Protocol Without ACK (resend)
>
>
> Douglas Otis wrote:
>
> > I agree, XON/XOFF would not work.  A credit token scheme
> similar to FC Class
> > 3 would.
>
> FC class 3 does not have an end to end "credit token scheme".  It
> (class 3 bb
> credit) is simply a credit mechanism between a node and it's neighbor node
> (the
> switch port) on whether the node can receive a frame.  This means that the
> fabric has buffering inside it, and will hold onto a frame if the
> destination
> node does not have room for it.  Thus, if the receiving node
> temporarily runs
> out of buffers, it will withold credit from the switch port.  The
> sending node
> doesn't know that the receiving node has run out of buffers, so the fabric
> buffers fill up with undeliverable frames.
>
> -Matt
>