From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Michael Krause
Sent: Wednesday, November 09, 2005 12:21 PM
To: Rick Frank; Ranjit Pandit
Cc: [email protected]
Subject: Re: [openib-general] [ANNOUNCE] Contribute RDS(ReliableDatagramSockets) to OpenIBOne 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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