[openib-general] [ANNOUNCE] Contribute RDS(ReliableDatagramSockets) to OpenIB

[Caitlin Bestler]

________________________________

	From: openib-general-bounces at openib.org
[mailto:openib-general-bounces at openib.org] On Behalf Of Michael Krause
	Sent: Wednesday, November 09, 2005 12:21 PM
	To: Rick Frank; Ranjit Pandit
	Cc: openib-general at openib.org
	Subject: Re: [openib-general] [ANNOUNCE] Contribute
RDS(ReliableDatagramSockets) to OpenIB
	
	

		One could be able to talk to the remote node across
other HCA but that does not mean one has an understanding of the state
at the remote node unless the failure is noted and a resync of state
occurs or the remote is able to deal with duplicates, etc.   This has
nothing to do with API or the transport involved but, as Caitlin noted,
the difference between knowing a send buffer is free vs. knowing that
the application received the data requested.  Therefore, one has only
reduced the reliability / robustness problem space to some extent but
has not solved it by the use of RDS.
		
		

Correct. When there are point-to-point credits (even if only
enforced/understood
at the ULP) then the application can correctly infer that message N was
successfully processed because the matching credit was restored. A
transport
neutral application can only communicate restoration of credits via ULP
messaging. When credits are shared across sessions then the ULP
has a much more complex task to properly communicate credits.
 
The proposal I presented at RAIT for multistreamed MPA had a
non-highlighted
option for a "wildcard" endpoint. Without the option multistream MPA is
essentially
the SCTP adaptation for RDMA running over plain MPA/TCP. It achieves the
same reduction in reliable transport layer connections that RDS does,
but
does not reduce the number of RDMA endpoints. The wildcard option 
reduces the number of RDMA endpoints as well, but greatly complicates
the RDMA state machines. RDS over IB faces similar problems, but solved
them slightly differently.
 
Over iWARP I believe these complexities favor keeping the point-to-point
logical connection between QP and only reducing the number of L4 
connections (from many TCP connections to a single TCP connection
or SCTP association). The advantage of that approach is that the API
from application to RDMA endpoint (QP) can be left totally unchanged.
But I do not see any such option over IB, unless RD is improved or a
new SCTP-like connection mode is defined.
 
In my opinion the multi-streaming is the most important feature here,
but over IB I do not think there is a natural adaptation that provides
multi-streaming without also adding the any-to-any endpoint semantics.
Multistream MPA and SCTP can both support the any-to-any endpoint
semantics by moving the source to payload information rather than
transport information (by invoking "wildcard status" in MS-MPA or
by duplicating the field for SCTP). So the RDS API strikes me as
the best option for a transport neutral application. MS-MPA and SCTP
reductions in transport overhead would be available without special
API support.
 
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