Re: Sigtran extension for network management and association changeover.

Xiangsong,

Diversion of traffic does not always occur as a result of
association failure.  It can also result from:

 deactivation of an AS at an ASP (ASPIA, ASPDN)
 deactivation of an AS at an SGP (unsolicited ASPIA Ack or ASPDN Ack)
 activation of an AS at an ASP or SGP (ASPAC, ASPAC Ack)

In these cases where the association has not failed, and because
retrieving unsent messages is only defined for SCTP after
association failure, these approaches will risk loss and
duplication.

SCTP acknowledgements only acknowledge that the SCTP peer
received a portion of a message.  They do not acknowledge that
the UA peer received the message at all.  For example, the UA
peer could close a failed association, losing all messages in
the receive buffer.

Withholding SCTP acknowledgement until the message is actually
read from the receive buffer by the ULP is not something that
tsvwg wanted to do.  Putting another layer of end-to-end
acknowledgement in SCTP to acknowledge on a per-stream basis is
not necessary for most other applications.  Instead of
performing this layering, an application layer acknowledgement
from ULP-to-ULP is more appropriate.  This is the concensus that
was reached.

When SCTP detects a restart condition, it is permitted by the
SCTP specification to discard all queued (send and receive)
messages.  Once an SCTP restart has occurred, the SCTP can no
longer acknowledge messages from the previous association;
however, the UA can.

SCTP does not stop messages from being placed into the receive
buffer that cannot be acknowledged to the far end (e.g. because
the association has already failed).

A UA level acknowlegement (Correlation Id/Ack) and flushing
(BEAT/ACK) handles all these cases, and in particular the ones
where the association has not failed.

--brian

-- 
Brian F. G. Bidulock
[email protected]
http://www.openss7.org/