Subject: [rddp] SCTP Multi-homing issues

From: Caitlin Bestler <[email-masked]>
Date: Tue, 29 Oct 2002 10:08:00 -0600

The multi-homing capabilities of SCTP raises some questions
for DDP and the accessibility of host memory across multiple
interfaces.

An SCTP endpoint on a multi-homed host is a collection of
eligible IP addresses. Presumably these are on different
physical interfaces, doing otherwise just slows down the
failover process that is supposed to provide fault tolerance
for network failures.

This raises a few possibilities:

-- An RDMA/DDP SCTP endpoint may only select IP addresses
   that are managed by the same RNIC. This would represent
   a restriction on RDMA/DDP SCTP associations that would
   not apply to other SCTP associations.
   
-- All STags enabled for any stream in the association must
    be enabled for ALL RNICs that are part of the SCTP
    endpoint. This would require that SCTP RNICs accept
    host processor assigned STags.

If my understanding of the feature is correct, an SCTP
endpoint must be ready to recieve packets on any of the
eligible interfaces at any time. This is a sharp contrast to
automatic path migration under InfiniBand. It also mandates
full real-time duplication of all STag related resources on
every interface that is part of an SCTP endpoint. This is
clearly undesirable if the alternate interface was intended
solely as a fallback (i.e., a primary interfacewould carry
the entire traffic, as opposed to having n interfaces load
balance the traffic and willingly reduce to n-1 interfaces
with a reduced capacity).

In my opinion, many to most RDMA ULPs would prefer to handle
path migration on their own. This is especially true of
those that are using RDMA to achieve very low predictable
latencies. For those ULPs, restricting an SCTP endpoint to
the scope where the hardware can pool STag resources "for
free" would be the optimal choice.

For others, fully conforming to standard SCTP behavior with
its fault tolerance and congestion management would be
preferred.

Does anyone else have any thoughts on this tradeoff?

Caitlin Bestler
http://asomi.com/CaitlinBestler/
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