Re: TCP limitations (was Re: ISCSI: Urgent Flag requirement violates TCP.)

[Matt Wakeley <matt_wakeley@agilent.com>]

Jim Williams wrote:
 
> http://www.ietf.org/internet-drafts/draft-williams-rdmatcp-00.txt
> 
> is an example of a shim layer that provides a mechanism
> to recover framing upon loss of TCP segments.

This draft has the following text for recovering framing on packet loss:

"The recommended method for doing this is to assume that the RDMA
segment is aligned with a TCP segment, and verify the correctness of
the header fields of the RDMA segment."

Aligning the iSCSI headers to begin in a new TCP segment is exactly what we
want.  But that "would require changes to TCP blah blah blah" which is what we
can't do.

"If these header fields are not correct, then the NIC should fall back to
buffering the packet until it can be processed in order."

This then requires buffering on the NIC (store and forward) or data copies in
the host, which we are trying to avoid.

-Matt